Merge pull request #3038 from iced-rs/mutable-widget

Mutable `Widget` Methods

core/src/element.rs

@@ -9,7 +9,7 @@ use crate::{
     Vector, Widget,
 };
 
-use std::borrow::Borrow;
+use std::borrow::{Borrow, BorrowMut};
 
 /// A generic [`Widget`].
 ///
@@ -239,6 +239,37 @@ impl<'a, Message, Theme, Renderer>
     }
 }
 
+impl<'a, Message, Theme, Renderer>
+    Borrow<dyn Widget<Message, Theme, Renderer> + 'a>
+    for &mut Element<'a, Message, Theme, Renderer>
+{
+    fn borrow(&self) -> &(dyn Widget<Message, Theme, Renderer> + 'a) {
+        self.widget.borrow()
+    }
+}
+
+impl<'a, Message, Theme, Renderer>
+    BorrowMut<dyn Widget<Message, Theme, Renderer> + 'a>
+    for Element<'a, Message, Theme, Renderer>
+{
+    fn borrow_mut(
+        &mut self,
+    ) -> &mut (dyn Widget<Message, Theme, Renderer> + 'a) {
+        self.widget.borrow_mut()
+    }
+}
+
+impl<'a, Message, Theme, Renderer>
+    BorrowMut<dyn Widget<Message, Theme, Renderer> + 'a>
+    for &mut Element<'a, Message, Theme, Renderer>
+{
+    fn borrow_mut(
+        &mut self,
+    ) -> &mut (dyn Widget<Message, Theme, Renderer> + 'a) {
+        self.widget.borrow_mut()
+    }
+}
+
 struct Map<'a, A, B, Theme, Renderer> {
     widget: Box<dyn Widget<A, Theme, Renderer> + 'a>,
     mapper: Box<dyn Fn(A) -> B + 'a>,
@@ -278,7 +309,7 @@ where
         self.widget.children()
     }
 
-    fn diff(&self, tree: &mut Tree) {
+    fn diff(&mut self, tree: &mut Tree) {
         self.widget.diff(tree);
     }
 
@@ -291,7 +322,7 @@ where
     }
 
     fn layout(
-        &self,
+        &mut self,
         tree: &mut Tree,
         renderer: &Renderer,
         limits: &layout::Limits,
@@ -300,7 +331,7 @@ where
     }
 
     fn operate(
-        &self,
+        &mut self,
         tree: &mut Tree,
         layout: Layout<'_>,
         renderer: &Renderer,
@@ -421,12 +452,12 @@ where
         self.element.widget.children()
     }
 
-    fn diff(&self, tree: &mut Tree) {
+    fn diff(&mut self, tree: &mut Tree) {
         self.element.widget.diff(tree);
     }
 
     fn layout(
-        &self,
+        &mut self,
         tree: &mut Tree,
         renderer: &Renderer,
         limits: &layout::Limits,
@@ -435,7 +466,7 @@ where
     }
 
     fn operate(
-        &self,
+        &mut self,
         state: &mut Tree,
         layout: Layout<'_>,
         renderer: &Renderer,
@@ -554,7 +585,7 @@ where
             }
 
             fn layout(
-                &self,
+                &mut self,
                 _tree: &mut Tree,
                 _renderer: &Renderer,
                 _limits: &layout::Limits,

core/src/layout/flex.rs

@@ -68,7 +68,7 @@ pub fn resolve<Message, Theme, Renderer>(
     padding: Padding,
     spacing: f32,
     align_items: Alignment,
-    items: &[Element<'_, Message, Theme, Renderer>],
+    items: &mut [Element<'_, Message, Theme, Renderer>],
     trees: &mut [widget::Tree],
 ) -> Node
 where
@@ -95,7 +95,8 @@ where
     // We lay out non-fluid elements in the main axis.
     // If we need to compress the cross axis, then we skip any of these elements
     // that are also fluid in the cross axis.
-    for (i, (child, tree)) in items.iter().zip(trees.iter_mut()).enumerate() {
+    for (i, (child, tree)) in items.iter_mut().zip(trees.iter_mut()).enumerate()
+    {
         let (fill_main_factor, fill_cross_factor) = {
             let size = child.as_widget().size();
 
@@ -117,7 +118,7 @@ where
                 Limits::new(Size::ZERO, Size::new(max_width, max_height));
 
             let layout =
-                child.as_widget().layout(tree, renderer, &child_limits);
+                child.as_widget_mut().layout(tree, renderer, &child_limits);
             let size = layout.size();
 
             available -= axis.main(size);
@@ -141,7 +142,8 @@ where
     // We can defer the layout of any elements that have a fixed size in the main axis,
     // allowing them to use the cross calculations of the next pass.
     if cross_compress && some_fill_cross {
-        for (i, (child, tree)) in items.iter().zip(trees.iter_mut()).enumerate()
+        for (i, (child, tree)) in
+            items.iter_mut().zip(trees.iter_mut()).enumerate()
         {
             let (main_size, cross_size) = {
                 let size = child.as_widget().size();
@@ -161,7 +163,7 @@ where
                     Limits::new(Size::ZERO, Size::new(max_width, max_height));
 
                 let layout =
-                    child.as_widget().layout(tree, renderer, &child_limits);
+                    child.as_widget_mut().layout(tree, renderer, &child_limits);
                 let size = layout.size();
 
                 available -= axis.main(size);
@@ -186,7 +188,8 @@ where
     // THIRD PASS
     // We lay out the elements that are fluid in the main axis.
     // We use the remaining space to evenly allocate space based on fill factors.
-    for (i, (child, tree)) in items.iter().zip(trees.iter_mut()).enumerate() {
+    for (i, (child, tree)) in items.iter_mut().zip(trees.iter_mut()).enumerate()
+    {
         let (fill_main_factor, fill_cross_factor) = {
             let size = child.as_widget().size();
 
@@ -225,7 +228,7 @@ where
             );
 
             let layout =
-                child.as_widget().layout(tree, renderer, &child_limits);
+                child.as_widget_mut().layout(tree, renderer, &child_limits);
             cross = cross.max(axis.cross(layout.size()));
 
             nodes[i] = layout;
@@ -237,7 +240,7 @@ where
     // These are elements that must be compressed in their cross axis and have
     // a fixed length in the main axis.
     if cross_compress && some_fill_cross {
-        for (i, (child, tree)) in items.iter().zip(trees).enumerate() {
+        for (i, (child, tree)) in items.iter_mut().zip(trees).enumerate() {
             let (main_size, cross_size) = {
                 let size = child.as_widget().size();
 
@@ -255,7 +258,7 @@ where
                     Limits::new(Size::ZERO, Size::new(max_width, max_height));
 
                 let layout =
-                    child.as_widget().layout(tree, renderer, &child_limits);
+                    child.as_widget_mut().layout(tree, renderer, &child_limits);
                 let size = layout.size();
 
                 cross = cross.max(axis.cross(size));

core/src/layout/limits.rs

@@ -6,6 +6,7 @@ use crate::{Length, Size};
 pub struct Limits {
     min: Size,
     max: Size,
+    compress: Size<bool>,
 }
 
 impl Limits {
@@ -13,11 +14,16 @@ impl Limits {
     pub const NONE: Limits = Limits {
         min: Size::ZERO,
         max: Size::INFINITE,
+        compress: Size::new(false, false),
     };
 
     /// Creates new [`Limits`] with the given minimum and maximum [`Size`].
     pub const fn new(min: Size, max: Size) -> Limits {
-        Limits { min, max }
+        Limits {
+            min,
+            max,
+            compress: Size::new(false, false),
+        }
     }
 
     /// Returns the minimum [`Size`] of the [`Limits`].
@@ -33,13 +39,17 @@ impl Limits {
     /// Applies a width constraint to the current [`Limits`].
     pub fn width(mut self, width: impl Into<Length>) -> Limits {
         match width.into() {
-            Length::Shrink | Length::Fill | Length::FillPortion(_) => {}
+            Length::Shrink => {
+                self.compress.width = true;
+            }
             Length::Fixed(amount) => {
                 let new_width = amount.min(self.max.width).max(self.min.width);
 
                 self.min.width = new_width;
                 self.max.width = new_width;
+                self.compress.width = false;
             }
+            Length::Fill | Length::FillPortion(_) => {}
         }
 
         self
@@ -48,14 +58,18 @@ impl Limits {
     /// Applies a height constraint to the current [`Limits`].
     pub fn height(mut self, height: impl Into<Length>) -> Limits {
         match height.into() {
-            Length::Shrink | Length::Fill | Length::FillPortion(_) => {}
+            Length::Shrink => {
+                self.compress.height = true;
+            }
             Length::Fixed(amount) => {
                 let new_height =
                     amount.min(self.max.height).max(self.min.height);
 
                 self.min.height = new_height;
                 self.max.height = new_height;
+                self.compress.height = false;
             }
+            Length::Fill | Length::FillPortion(_) => {}
         }
 
         self
@@ -103,7 +117,11 @@ impl Limits {
             (self.max().height - size.height).max(0.0),
         );
 
-        Limits { min, max }
+        Limits {
+            min,
+            max,
+            compress: self.compress,
+        }
     }
 
     /// Removes the minimum width constraint for the current [`Limits`].
@@ -111,6 +129,7 @@ impl Limits {
         Limits {
             min: Size::ZERO,
             max: self.max,
+            compress: self.compress,
         }
     }
 
@@ -124,21 +143,23 @@ impl Limits {
         intrinsic_size: Size,
     ) -> Size {
         let width = match width.into() {
-            Length::Fill | Length::FillPortion(_) => self.max.width,
+            Length::Fill | Length::FillPortion(_) if !self.compress.width => {
+                self.max.width
+            }
             Length::Fixed(amount) => {
                 amount.min(self.max.width).max(self.min.width)
             }
-            Length::Shrink => {
+            _ => intrinsic_size.width.min(self.max.width).max(self.min.width),
-                intrinsic_size.width.min(self.max.width).max(self.min.width)
-            }
         };
 
         let height = match height.into() {
-            Length::Fill | Length::FillPortion(_) => self.max.height,
+            Length::Fill | Length::FillPortion(_) if !self.compress.height => {
+                self.max.height
+            }
             Length::Fixed(amount) => {
                 amount.min(self.max.height).max(self.min.height)
             }
-            Length::Shrink => intrinsic_size
+            _ => intrinsic_size
                 .height
                 .min(self.max.height)
                 .max(self.min.height),

core/src/widget.rs

@@ -58,7 +58,7 @@ where
     /// This [`layout::Node`] is used by the runtime to compute the [`Layout`] of the
     /// user interface.
     fn layout(
-        &self,
+        &mut self,
         tree: &mut Tree,
         renderer: &Renderer,
         limits: &layout::Limits,
@@ -96,13 +96,13 @@ where
     }
 
     /// Reconciles the [`Widget`] with the provided [`Tree`].
-    fn diff(&self, tree: &mut Tree) {
+    fn diff(&mut self, tree: &mut Tree) {
         tree.children.clear();
     }
 
     /// Applies an [`Operation`] to the [`Widget`].
     fn operate(
-        &self,
+        &mut self,
         _state: &mut Tree,
         _layout: Layout<'_>,
         _renderer: &Renderer,

core/src/widget/text.rs

@@ -207,7 +207,7 @@ where
     }
 
     fn layout(
-        &self,
+        &mut self,
         tree: &mut Tree,
         renderer: &Renderer,
         limits: &layout::Limits,
@@ -245,7 +245,7 @@ where
     }
 
     fn operate(
-        &self,
+        &mut self,
         _state: &mut Tree,
         layout: Layout<'_>,
         _renderer: &Renderer,

core/src/widget/tree.rs

@@ -2,7 +2,7 @@
 use crate::Widget;
 
 use std::any::{self, Any};
-use std::borrow::Borrow;
+use std::borrow::{Borrow, BorrowMut};
 use std::fmt;
 
 /// A persistent state widget tree.
@@ -56,12 +56,12 @@ impl Tree {
     /// [`Widget::diff`]: crate::Widget::diff
     pub fn diff<'a, Message, Theme, Renderer>(
         &mut self,
-        new: impl Borrow<dyn Widget<Message, Theme, Renderer> + 'a>,
+        mut new: impl BorrowMut<dyn Widget<Message, Theme, Renderer> + 'a>,
     ) where
         Renderer: crate::Renderer,
     {
-        if self.tag == new.borrow().tag() {
+        if self.tag == new.borrow_mut().tag() {
-            new.borrow().diff(self);
+            new.borrow_mut().diff(self);
         } else {
             *self = Self::new(new);
         }
@@ -70,13 +70,15 @@ impl Tree {
     /// Reconciles the children of the tree with the provided list of widgets.
     pub fn diff_children<'a, Message, Theme, Renderer>(
         &mut self,
-        new_children: &[impl Borrow<dyn Widget<Message, Theme, Renderer> + 'a>],
+        new_children: &mut [impl BorrowMut<
+            dyn Widget<Message, Theme, Renderer> + 'a,
+        >],
     ) where
         Renderer: crate::Renderer,
     {
         self.diff_children_custom(
             new_children,
-            |tree, widget| tree.diff(widget.borrow()),
+            |tree, widget| tree.diff(widget.borrow_mut()),
             |widget| Self::new(widget.borrow()),
         );
     }
@@ -85,8 +87,8 @@ impl Tree {
     /// logic both for diffing and creating new widget state.
     pub fn diff_children_custom<T>(
         &mut self,
-        new_children: &[T],
+        new_children: &mut [T],
-        diff: impl Fn(&mut Tree, &T),
+        diff: impl Fn(&mut Tree, &mut T),
         new_state: impl Fn(&T) -> Self,
     ) {
         if self.children.len() > new_children.len() {
@@ -94,7 +96,7 @@ impl Tree {
         }
 
         for (child_state, new) in
-            self.children.iter_mut().zip(new_children.iter())
+            self.children.iter_mut().zip(new_children.iter_mut())
         {
             diff(child_state, new);
         }
@@ -114,8 +116,8 @@ impl Tree {
 /// `maybe_changed` closure.
 pub fn diff_children_custom_with_search<T>(
     current_children: &mut Vec<Tree>,
-    new_children: &[T],
+    new_children: &mut [T],
-    diff: impl Fn(&mut Tree, &T),
+    diff: impl Fn(&mut Tree, &mut T),
     maybe_changed: impl Fn(usize) -> bool,
     new_state: impl Fn(&T) -> Tree,
 ) {
@@ -183,7 +185,7 @@ pub fn diff_children_custom_with_search<T>(
 
     // TODO: Merge loop with extend logic (?)
     for (child_state, new) in
-        current_children.iter_mut().zip(new_children.iter())
+        current_children.iter_mut().zip(new_children.iter_mut())
     {
         diff(child_state, new);
     }

examples/custom_quad/src/main.rs

@@ -175,7 +175,7 @@ mod quad {
         }
 
         fn layout(
-            &self,
+            &mut self,
             _tree: &mut widget::Tree,
             _renderer: &Renderer,
             _limits: &layout::Limits,

examples/custom_widget/src/main.rs

@@ -33,7 +33,7 @@ mod circle {
         }
 
         fn layout(
-            &self,
+            &mut self,
             _tree: &mut widget::Tree,
             _renderer: &Renderer,
             _limits: &layout::Limits,

examples/geometry/src/main.rs

@@ -27,7 +27,7 @@ mod rainbow {
         }
 
         fn layout(
-            &self,
+            &mut self,
             _tree: &mut widget::Tree,
             _renderer: &Renderer,
             limits: &layout::Limits,

examples/loading_spinners/src/circular.rs

@@ -253,7 +253,7 @@ where
     }
 
     fn layout(
-        &self,
+        &mut self,
         _tree: &mut Tree,
         _renderer: &Renderer,
         limits: &layout::Limits,

examples/loading_spinners/src/linear.rs

@@ -167,7 +167,7 @@ where
     }
 
     fn layout(
-        &self,
+        &mut self,
         _tree: &mut Tree,
         _renderer: &Renderer,
         limits: &layout::Limits,

examples/loupe/src/main.rs

@@ -87,8 +87,8 @@ mod loupe {
             self.content.as_widget().children()
         }
 
-        fn diff(&self, tree: &mut widget::Tree) {
+        fn diff(&mut self, tree: &mut widget::Tree) {
-            self.content.as_widget().diff(tree);
+            self.content.as_widget_mut().diff(tree);
         }
 
         fn size(&self) -> Size<Length> {
@@ -96,12 +96,12 @@ mod loupe {
         }
 
         fn layout(
-            &self,
+            &mut self,
             tree: &mut widget::Tree,
             renderer: &Renderer,
             limits: &layout::Limits,
         ) -> layout::Node {
-            self.content.as_widget().layout(tree, renderer, limits)
+            self.content.as_widget_mut().layout(tree, renderer, limits)
         }
 
         fn draw(

examples/pane_grid/Cargo.toml

@@ -7,4 +7,4 @@ publish = false
 
 [dependencies]
 iced.workspace = true
-iced.features = ["debug", "lazy"]
+iced.features = ["debug"]

examples/toast/src/main.rs

@@ -287,12 +287,12 @@ mod toast {
         }
 
         fn layout(
-            &self,
+            &mut self,
             tree: &mut Tree,
             renderer: &Renderer,
             limits: &layout::Limits,
         ) -> layout::Node {
-            self.content.as_widget().layout(
+            self.content.as_widget_mut().layout(
                 &mut tree.children[0],
                 renderer,
                 limits,
@@ -314,7 +314,7 @@ mod toast {
                 .collect()
         }
 
-        fn diff(&self, tree: &mut Tree) {
+        fn diff(&mut self, tree: &mut Tree) {
             let instants = tree.state.downcast_mut::<Vec<Option<Instant>>>();
 
             // Invalidating removed instants to None allows us to remove
@@ -336,21 +336,21 @@ mod toast {
             }
 
             tree.diff_children(
-                &std::iter::once(&self.content)
+                &mut std::iter::once(&mut self.content)
-                    .chain(self.toasts.iter())
+                    .chain(self.toasts.iter_mut())
                     .collect::<Vec<_>>(),
             );
         }
 
         fn operate(
-            &self,
+            &mut self,
             state: &mut Tree,
             layout: Layout<'_>,
             renderer: &Renderer,
             operation: &mut dyn Operation,
         ) {
             operation.container(None, layout.bounds(), &mut |operation| {
-                self.content.as_widget().operate(
+                self.content.as_widget_mut().operate(
                     &mut state.children[0],
                     layout,
                     renderer,
@@ -582,12 +582,12 @@ mod toast {
         ) {
             operation.container(None, layout.bounds(), &mut |operation| {
                 self.toasts
-                    .iter()
+                    .iter_mut()
                     .zip(self.state.iter_mut())
                     .zip(layout.children())
                     .for_each(|((child, state), layout)| {
                         child
-                            .as_widget()
+                            .as_widget_mut()
                             .operate(state, layout, renderer, operation);
                     });
             });

runtime/src/user_interface.rs

@@ -97,12 +97,12 @@ where
         cache: Cache,
         renderer: &mut Renderer,
     ) -> Self {
-        let root = root.into();
+        let mut root = root.into();
 
         let Cache { mut state } = cache;
-        state.diff(root.as_widget());
+        state.diff(root.as_widget_mut());
 
-        let base = root.as_widget().layout(
+        let base = root.as_widget_mut().layout(
             &mut state,
             renderer,
             &layout::Limits::new(Size::ZERO, bounds),
@@ -234,7 +234,7 @@ where
                     if shell.is_layout_invalid() {
                         drop(maybe_overlay);
 
-                        self.base = self.root.as_widget().layout(
+                        self.base = self.root.as_widget_mut().layout(
                             &mut self.state,
                             renderer,
                             &layout::Limits::new(Size::ZERO, self.bounds),
@@ -335,7 +335,7 @@ where
                 input_method.merge(shell.input_method());
 
                 shell.revalidate_layout(|| {
-                    self.base = self.root.as_widget().layout(
+                    self.base = self.root.as_widget_mut().layout(
                         &mut self.state,
                         renderer,
                         &layout::Limits::new(Size::ZERO, self.bounds),
@@ -539,7 +539,7 @@ where
     ) {
         let viewport = Rectangle::with_size(self.bounds);
 
-        self.root.as_widget().operate(
+        self.root.as_widget_mut().operate(
             &mut self.state,
             Layout::new(&self.base),
             renderer,

widget/src/button.rs

@@ -223,8 +223,8 @@ where
         vec![Tree::new(&self.content)]
     }
 
-    fn diff(&self, tree: &mut Tree) {
+    fn diff(&mut self, tree: &mut Tree) {
-        tree.diff_children(std::slice::from_ref(&self.content));
+        tree.diff_children(std::slice::from_mut(&mut self.content));
     }
 
     fn size(&self) -> Size<Length> {
@@ -235,7 +235,7 @@ where
     }
 
     fn layout(
-        &self,
+        &mut self,
         tree: &mut Tree,
         renderer: &Renderer,
         limits: &layout::Limits,
@@ -246,7 +246,7 @@ where
             self.height,
             self.padding,
             |limits| {
-                self.content.as_widget().layout(
+                self.content.as_widget_mut().layout(
                     &mut tree.children[0],
                     renderer,
                     limits,
@@ -256,14 +256,14 @@ where
     }
 
     fn operate(
-        &self,
+        &mut self,
         tree: &mut Tree,
         layout: Layout<'_>,
         renderer: &Renderer,
         operation: &mut dyn Operation,
     ) {
         operation.container(None, layout.bounds(), &mut |operation| {
-            self.content.as_widget().operate(
+            self.content.as_widget_mut().operate(
                 &mut tree.children[0],
                 layout.children().next().unwrap(),
                 renderer,

widget/src/canvas.rs

@@ -207,7 +207,7 @@ where
     }
 
     fn layout(
-        &self,
+        &mut self,
         _tree: &mut Tree,
         _renderer: &Renderer,
         limits: &layout::Limits,

widget/src/checkbox.rs

@@ -269,7 +269,7 @@ where
     }
 
     fn layout(
-        &self,
+        &mut self,
         tree: &mut Tree,
         renderer: &Renderer,
         limits: &layout::Limits,
@@ -447,7 +447,7 @@ where
     }
 
     fn operate(
-        &self,
+        &mut self,
         _state: &mut Tree,
         layout: Layout<'_>,
         _renderer: &Renderer,

widget/src/column.rs

@@ -194,8 +194,8 @@ where
         self.children.iter().map(Tree::new).collect()
     }
 
-    fn diff(&self, tree: &mut Tree) {
+    fn diff(&mut self, tree: &mut Tree) {
-        tree.diff_children(&self.children);
+        tree.diff_children(&mut self.children);
     }
 
     fn size(&self) -> Size<Length> {
@@ -206,7 +206,7 @@ where
     }
 
     fn layout(
-        &self,
+        &mut self,
         tree: &mut Tree,
         renderer: &Renderer,
         limits: &layout::Limits,
@@ -222,13 +222,13 @@ where
             self.padding,
             self.spacing,
             self.align,
-            &self.children,
+            &mut self.children,
             &mut tree.children,
         )
     }
 
     fn operate(
-        &self,
+        &mut self,
         tree: &mut Tree,
         layout: Layout<'_>,
         renderer: &Renderer,
@@ -236,12 +236,12 @@ where
     ) {
         operation.container(None, layout.bounds(), &mut |operation| {
             self.children
-                .iter()
+                .iter_mut()
                 .zip(&mut tree.children)
                 .zip(layout.children())
                 .for_each(|((child, state), layout)| {
                     child
-                        .as_widget()
+                        .as_widget_mut()
                         .operate(state, layout, renderer, operation);
                 });
         });

widget/src/combo_box.rs

@@ -471,7 +471,7 @@ where
     }
 
     fn layout(
-        &self,
+        &mut self,
         tree: &mut widget::Tree,
         renderer: &Renderer,
         limits: &layout::Limits,
@@ -509,7 +509,7 @@ where
         vec![widget::Tree::new(&self.text_input as &dyn Widget<_, _, _>)]
     }
 
-    fn diff(&self, _tree: &mut widget::Tree) {
+    fn diff(&mut self, _tree: &mut widget::Tree) {
         // do nothing so the children don't get cleared
     }
 

widget/src/container.rs

@@ -247,8 +247,8 @@ where
         self.content.as_widget().children()
     }
 
-    fn diff(&self, tree: &mut Tree) {
+    fn diff(&mut self, tree: &mut Tree) {
-        self.content.as_widget().diff(tree);
+        self.content.as_widget_mut().diff(tree);
     }
 
     fn size(&self) -> Size<Length> {
@@ -259,7 +259,7 @@ where
     }
 
     fn layout(
-        &self,
+        &mut self,
         tree: &mut Tree,
         renderer: &Renderer,
         limits: &layout::Limits,
@@ -273,12 +273,14 @@ where
             self.padding,
             self.horizontal_alignment,
             self.vertical_alignment,
-            |limits| self.content.as_widget().layout(tree, renderer, limits),
+            |limits| {
+                self.content.as_widget_mut().layout(tree, renderer, limits)
+            },
         )
     }
 
     fn operate(
-        &self,
+        &mut self,
         tree: &mut Tree,
         layout: Layout<'_>,
         renderer: &Renderer,
@@ -288,7 +290,7 @@ where
             self.id.as_ref().map(|id| &id.0),
             layout.bounds(),
             &mut |operation| {
-                self.content.as_widget().operate(
+                self.content.as_widget_mut().operate(
                     tree,
                     layout.children().next().unwrap(),
                     renderer,

widget/src/float.rs

@@ -103,8 +103,8 @@ where
         self.content.as_widget().children()
     }
 
-    fn diff(&self, tree: &mut widget::Tree) {
+    fn diff(&mut self, tree: &mut widget::Tree) {
-        self.content.as_widget().diff(tree);
+        self.content.as_widget_mut().diff(tree);
     }
 
     fn size(&self) -> Size<Length> {
@@ -116,12 +116,12 @@ where
     }
 
     fn layout(
-        &self,
+        &mut self,
         tree: &mut widget::Tree,
         renderer: &Renderer,
         limits: &layout::Limits,
     ) -> layout::Node {
-        self.content.as_widget().layout(tree, renderer, limits)
+        self.content.as_widget_mut().layout(tree, renderer, limits)
     }
 
     fn update(
@@ -197,14 +197,14 @@ where
     }
 
     fn operate(
-        &self,
+        &mut self,
         state: &mut widget::Tree,
         layout: Layout<'_>,
         renderer: &Renderer,
         operation: &mut dyn widget::Operation,
     ) {
         self.content
-            .as_widget()
+            .as_widget_mut()
             .operate(state, layout, renderer, operation);
     }
 

widget/src/grid.rs

@@ -158,8 +158,8 @@ where
         self.children.iter().map(Tree::new).collect()
     }
 
-    fn diff(&self, tree: &mut Tree) {
+    fn diff(&mut self, tree: &mut Tree) {
-        tree.diff_children(&self.children);
+        tree.diff_children(&mut self.children);
     }
 
     fn size(&self) -> Size<Length> {
@@ -176,7 +176,7 @@ where
     }
 
     fn layout(
-        &self,
+        &mut self,
         tree: &mut Tree,
         renderer: &Renderer,
         limits: &layout::Limits,
@@ -220,10 +220,10 @@ where
         let mut row_height = 0.0f32;
 
         for (i, (child, tree)) in
-            self.children.iter().zip(&mut tree.children).enumerate()
+            self.children.iter_mut().zip(&mut tree.children).enumerate()
         {
             let node = child
-                .as_widget()
+                .as_widget_mut()
                 .layout(tree, renderer, &cell_limits)
                 .move_to((x, y));
 
@@ -251,7 +251,7 @@ where
     }
 
     fn operate(
-        &self,
+        &mut self,
         tree: &mut Tree,
         layout: Layout<'_>,
         renderer: &Renderer,
@@ -259,12 +259,12 @@ where
     ) {
         operation.container(None, layout.bounds(), &mut |operation| {
             self.children
-                .iter()
+                .iter_mut()
                 .zip(&mut tree.children)
                 .zip(layout.children())
                 .for_each(|((child, state), layout)| {
                     child
-                        .as_widget()
+                        .as_widget_mut()
                         .operate(state, layout, renderer, operation);
                 });
         });

widget/src/helpers.rs

@@ -6,7 +6,7 @@ use crate::container::{self, Container};
 use crate::core;
 use crate::core::widget::operation::{self, Operation};
 use crate::core::window;
-use crate::core::{Element, Length, Pixels, Widget};
+use crate::core::{Element, Length, Pixels, Size, Widget};
 use crate::float::{self, Float};
 use crate::keyed;
 use crate::overlay;
@@ -25,7 +25,9 @@ use crate::text_input::{self, TextInput};
 use crate::toggler::{self, Toggler};
 use crate::tooltip::{self, Tooltip};
 use crate::vertical_slider::{self, VerticalSlider};
-use crate::{Column, Grid, MouseArea, Pin, Row, Sensor, Space, Stack, Themer};
+use crate::{
+    Column, Grid, MouseArea, Pin, Responsive, Row, Sensor, Space, Stack, Themer,
+};
 
 use std::borrow::Borrow;
 use std::ops::RangeInclusive;
@@ -596,8 +598,8 @@ where
             self.content.as_widget().children()
         }
 
-        fn diff(&self, tree: &mut Tree) {
+        fn diff(&mut self, tree: &mut Tree) {
-            self.content.as_widget().diff(tree);
+            self.content.as_widget_mut().diff(tree);
         }
 
         fn size(&self) -> Size<Length> {
@@ -609,12 +611,12 @@ where
         }
 
         fn layout(
-            &self,
+            &mut self,
             tree: &mut Tree,
             renderer: &Renderer,
             limits: &layout::Limits,
         ) -> layout::Node {
-            self.content.as_widget().layout(tree, renderer, limits)
+            self.content.as_widget_mut().layout(tree, renderer, limits)
         }
 
         fn draw(
@@ -633,14 +635,14 @@ where
         }
 
         fn operate(
-            &self,
+            &mut self,
             state: &mut Tree,
             layout: Layout<'_>,
             renderer: &Renderer,
             operation: &mut dyn operation::Operation,
         ) {
             self.content
-                .as_widget()
+                .as_widget_mut()
                 .operate(state, layout, renderer, operation);
         }
 
@@ -759,8 +761,8 @@ where
             vec![Tree::new(&self.base), Tree::new(&self.top)]
         }
 
-        fn diff(&self, tree: &mut Tree) {
+        fn diff(&mut self, tree: &mut Tree) {
-            tree.diff_children(&[&self.base, &self.top]);
+            tree.diff_children(&mut [&mut self.base, &mut self.top]);
         }
 
         fn size(&self) -> Size<Length> {
@@ -772,18 +774,18 @@ where
         }
 
         fn layout(
-            &self,
+            &mut self,
             tree: &mut Tree,
             renderer: &Renderer,
             limits: &layout::Limits,
         ) -> layout::Node {
-            let base = self.base.as_widget().layout(
+            let base = self.base.as_widget_mut().layout(
                 &mut tree.children[0],
                 renderer,
                 limits,
             );
 
-            let top = self.top.as_widget().layout(
+            let top = self.top.as_widget_mut().layout(
                 &mut tree.children[1],
                 renderer,
                 &layout::Limits::new(Size::ZERO, base.size()),
@@ -834,18 +836,20 @@ where
         }
 
         fn operate(
-            &self,
+            &mut self,
             tree: &mut Tree,
             layout: Layout<'_>,
             renderer: &Renderer,
             operation: &mut dyn operation::Operation,
         ) {
-            let children = [&self.base, &self.top]
+            let children = [&mut self.base, &mut self.top]
                 .into_iter()
                 .zip(layout.children().zip(&mut tree.children));
 
             for (child, (layout, tree)) in children {
-                child.as_widget().operate(tree, layout, renderer, operation);
+                child
+                    .as_widget_mut()
+                    .operate(tree, layout, renderer, operation);
             }
         }
 
@@ -2138,3 +2142,18 @@ where
 {
     Float::new(content)
 }
+
+/// Creates a new [`Responsive`] widget with a closure that produces its
+/// contents.
+///
+/// The `view` closure will receive the maximum available space for
+/// the [`Responsive`] during layout. You can use this [`Size`] to
+/// conditionally build the contents.
+pub fn responsive<'a, Message, Theme, Renderer>(
+    f: impl Fn(Size) -> Element<'a, Message, Theme, Renderer> + 'a,
+) -> Responsive<'a, Message, Theme, Renderer>
+where
+    Renderer: core::Renderer,
+{
+    Responsive::new(f)
+}

widget/src/image.rs

@@ -386,7 +386,7 @@ where
     }
 
     fn layout(
-        &self,
+        &mut self,
         _tree: &mut Tree,
         renderer: &Renderer,
         limits: &layout::Limits,

widget/src/image/viewer.rs

@@ -117,7 +117,7 @@ where
     }
 
     fn layout(
-        &self,
+        &mut self,
         _tree: &mut Tree,
         renderer: &Renderer,
         limits: &layout::Limits,

widget/src/keyed/column.rs

@@ -222,7 +222,7 @@ where
         self.children.iter().map(Tree::new).collect()
     }
 
-    fn diff(&self, tree: &mut Tree) {
+    fn diff(&mut self, tree: &mut Tree) {
         let Tree {
             state, children, ..
         } = tree;
@@ -231,8 +231,8 @@ where
 
         tree::diff_children_custom_with_search(
             children,
-            &self.children,
+            &mut self.children,
-            |tree, child| child.as_widget().diff(tree),
+            |tree, child| child.as_widget_mut().diff(tree),
             |index| {
                 self.keys.get(index).or_else(|| self.keys.last()).copied()
                     != Some(state.keys[index])
@@ -253,7 +253,7 @@ where
     }
 
     fn layout(
-        &self,
+        &mut self,
         tree: &mut Tree,
         renderer: &Renderer,
         limits: &layout::Limits,
@@ -272,13 +272,13 @@ where
             self.padding,
             self.spacing,
             self.align_items,
-            &self.children,
+            &mut self.children,
             &mut tree.children,
         )
     }
 
     fn operate(
-        &self,
+        &mut self,
         tree: &mut Tree,
         layout: Layout<'_>,
         renderer: &Renderer,
@@ -286,12 +286,12 @@ where
     ) {
         operation.container(None, layout.bounds(), &mut |operation| {
             self.children
-                .iter()
+                .iter_mut()
                 .zip(&mut tree.children)
                 .zip(layout.children())
                 .for_each(|((child, state), layout)| {
                     child
-                        .as_widget()
+                        .as_widget_mut()
                         .operate(state, layout, renderer, operation);
                 });
         });

widget/src/lazy.rs

@@ -2,11 +2,9 @@
 pub(crate) mod helpers;
 
 pub mod component;
-pub mod responsive;
 
 #[allow(deprecated)]
 pub use component::Component;
-pub use responsive::Responsive;
 
 mod cache;
 
@@ -127,7 +125,7 @@ where
         self.with_element(|element| vec![Tree::new(element.as_widget())])
     }
 
-    fn diff(&self, tree: &mut Tree) {
+    fn diff(&mut self, tree: &mut Tree) {
         let current = tree
             .state
             .downcast_mut::<Internal<Message, Theme, Renderer>>();
@@ -146,8 +144,8 @@ where
             current.element = Rc::new(RefCell::new(Some(element)));
 
             (*self.element.borrow_mut()) = Some(current.element.clone());
-            self.with_element(|element| {
+            self.with_element_mut(|element| {
-                tree.diff_children(std::slice::from_ref(&element.as_widget()));
+                tree.diff_children(std::slice::from_mut(element));
             });
         } else {
             (*self.element.borrow_mut()) = Some(current.element.clone());
@@ -166,27 +164,29 @@ where
     }
 
     fn layout(
-        &self,
+        &mut self,
         tree: &mut Tree,
         renderer: &Renderer,
         limits: &layout::Limits,
     ) -> layout::Node {
-        self.with_element(|element| {
+        self.with_element_mut(|element| {
-            element
+            element.as_widget_mut().layout(
-                .as_widget()
+                &mut tree.children[0],
-                .layout(&mut tree.children[0], renderer, limits)
+                renderer,
+                limits,
+            )
         })
     }
 
     fn operate(
-        &self,
+        &mut self,
         tree: &mut Tree,
         layout: Layout<'_>,
         renderer: &Renderer,
         operation: &mut dyn widget::Operation,
     ) {
-        self.with_element(|element| {
+        self.with_element_mut(|element| {
-            element.as_widget().operate(
+            element.as_widget_mut().operate(
                 &mut tree.children[0],
                 layout,
                 renderer,

widget/src/lazy/component.rs

@@ -147,13 +147,13 @@ where
     Renderer: renderer::Renderer,
 {
     fn diff_self(&self) {
-        self.with_element(|element| {
+        self.with_element_mut(|element| {
             self.tree
                 .borrow_mut()
                 .borrow_mut()
                 .as_mut()
                 .unwrap()
-                .diff_children(std::slice::from_ref(&element));
+                .diff_children(std::slice::from_mut(element));
         });
     }
 
@@ -279,7 +279,7 @@ where
         vec![]
     }
 
-    fn diff(&self, tree: &mut Tree) {
+    fn diff(&mut self, tree: &mut Tree) {
         let tree = tree.state.downcast_ref::<Rc<RefCell<Option<Tree>>>>();
         *self.tree.borrow_mut() = tree.clone();
         self.rebuild_element_if_necessary();
@@ -299,15 +299,15 @@ where
     }
 
     fn layout(
-        &self,
+        &mut self,
         tree: &mut Tree,
         renderer: &Renderer,
         limits: &layout::Limits,
     ) -> layout::Node {
         let t = tree.state.downcast_mut::<Rc<RefCell<Option<Tree>>>>();
 
-        self.with_element(|element| {
+        self.with_element_mut(|element| {
-            element.as_widget().layout(
+            element.as_widget_mut().layout(
                 &mut t.borrow_mut().as_mut().unwrap().children[0],
                 renderer,
                 limits,
@@ -378,7 +378,7 @@ where
     }
 
     fn operate(
-        &self,
+        &mut self,
         tree: &mut Tree,
         layout: Layout<'_>,
         renderer: &Renderer,
@@ -387,8 +387,8 @@ where
         self.rebuild_element_with_operation(layout, operation);
 
         let tree = tree.state.downcast_mut::<Rc<RefCell<Option<Tree>>>>();
-        self.with_element(|element| {
+        self.with_element_mut(|element| {
-            element.as_widget().operate(
+            element.as_widget_mut().operate(
                 &mut tree.borrow_mut().as_mut().unwrap().children[0],
                 layout,
                 renderer,

widget/src/lazy/helpers.rs

@@ -1,10 +1,10 @@
-use crate::core::{self, Element, Size};
+use crate::core::{self, Element};
 use crate::lazy::component;
 
 use std::hash::Hash;
 
 #[allow(deprecated)]
-pub use crate::lazy::{Component, Lazy, Responsive};
+pub use crate::lazy::{Component, Lazy};
 
 /// Creates a new [`Lazy`] widget with the given data `Dependency` and a
 /// closure that can turn this data into a widget tree.
@@ -41,19 +41,3 @@ where
 {
     component::view(component)
 }
-
-/// Creates a new [`Responsive`] widget with a closure that produces its
-/// contents.
-///
-/// The `view` closure will be provided with the current [`Size`] of
-/// the [`Responsive`] widget and, therefore, can be used to build the
-/// contents of the widget in a responsive way.
-#[cfg(feature = "lazy")]
-pub fn responsive<'a, Message, Theme, Renderer>(
-    f: impl Fn(Size) -> Element<'a, Message, Theme, Renderer> + 'a,
-) -> Responsive<'a, Message, Theme, Renderer>
-where
-    Renderer: core::Renderer,
-{
-    Responsive::new(f)
-}

widget/src/lazy/responsive.rs

@@ -1,461 +0,0 @@
-use crate::core::layout::{self, Layout};
-use crate::core::mouse;
-use crate::core::overlay;
-use crate::core::renderer;
-use crate::core::widget;
-use crate::core::widget::tree::{self, Tree};
-use crate::core::{
-    self, Clipboard, Element, Event, Length, Point, Rectangle, Shell, Size,
-    Vector, Widget,
-};
-use crate::horizontal_space;
-use crate::runtime::overlay::Nested;
-
-use ouroboros::self_referencing;
-use std::cell::{RefCell, RefMut};
-use std::marker::PhantomData;
-use std::ops::Deref;
-
-/// A widget that is aware of its dimensions.
-///
-/// A [`Responsive`] widget will always try to fill all the available space of
-/// its parent.
-#[cfg(feature = "lazy")]
-#[allow(missing_debug_implementations)]
-pub struct Responsive<
-    'a,
-    Message,
-    Theme = crate::Theme,
-    Renderer = crate::Renderer,
-> {
-    view: Box<dyn Fn(Size) -> Element<'a, Message, Theme, Renderer> + 'a>,
-    content: RefCell<Content<'a, Message, Theme, Renderer>>,
-}
-
-impl<'a, Message, Theme, Renderer> Responsive<'a, Message, Theme, Renderer>
-where
-    Renderer: core::Renderer,
-{
-    /// Creates a new [`Responsive`] widget with a closure that produces its
-    /// contents.
-    ///
-    /// The `view` closure will be provided with the current [`Size`] of
-    /// the [`Responsive`] widget and, therefore, can be used to build the
-    /// contents of the widget in a responsive way.
-    pub fn new(
-        view: impl Fn(Size) -> Element<'a, Message, Theme, Renderer> + 'a,
-    ) -> Self {
-        Self {
-            view: Box::new(view),
-            content: RefCell::new(Content {
-                size: Size::ZERO,
-                layout: None,
-                is_layout_invalid: true,
-                element: Element::new(horizontal_space().width(0)),
-            }),
-        }
-    }
-}
-
-struct Content<'a, Message, Theme, Renderer> {
-    size: Size,
-    layout: Option<layout::Node>,
-    is_layout_invalid: bool,
-    element: Element<'a, Message, Theme, Renderer>,
-}
-
-impl<'a, Message, Theme, Renderer> Content<'a, Message, Theme, Renderer>
-where
-    Renderer: core::Renderer,
-{
-    fn layout(&mut self, tree: &mut Tree, renderer: &Renderer) {
-        if self.layout.is_none() || self.is_layout_invalid {
-            self.layout = Some(self.element.as_widget().layout(
-                tree,
-                renderer,
-                &layout::Limits::new(Size::ZERO, self.size),
-            ));
-            self.is_layout_invalid = false;
-        }
-    }
-
-    fn update(
-        &mut self,
-        tree: &mut Tree,
-        new_size: Size,
-        view: &dyn Fn(Size) -> Element<'a, Message, Theme, Renderer>,
-    ) {
-        if self.size != new_size {
-            self.element = view(new_size);
-            self.size = new_size;
-            self.layout = None;
-
-            tree.diff(&self.element);
-        } else {
-            let is_tree_empty =
-                tree.tag == tree::Tag::stateless() && tree.children.is_empty();
-
-            if is_tree_empty {
-                self.layout = None;
-                tree.diff(&self.element);
-            }
-        }
-    }
-
-    fn resolve<R, T>(
-        &mut self,
-        tree: &mut Tree,
-        renderer: R,
-        layout: Layout<'_>,
-        view: &dyn Fn(Size) -> Element<'a, Message, Theme, Renderer>,
-        f: impl FnOnce(
-            &mut Tree,
-            R,
-            Layout<'_>,
-            &mut Element<'a, Message, Theme, Renderer>,
-        ) -> T,
-    ) -> T
-    where
-        R: Deref<Target = Renderer>,
-    {
-        self.update(tree, layout.bounds().size(), view);
-        self.layout(tree, renderer.deref());
-
-        let content_layout = Layout::with_offset(
-            layout.position() - Point::ORIGIN,
-            self.layout.as_ref().unwrap(),
-        );
-
-        f(tree, renderer, content_layout, &mut self.element)
-    }
-}
-
-struct State {
-    tree: RefCell<Tree>,
-}
-
-impl<Message, Theme, Renderer> Widget<Message, Theme, Renderer>
-    for Responsive<'_, Message, Theme, Renderer>
-where
-    Renderer: core::Renderer,
-{
-    fn tag(&self) -> tree::Tag {
-        tree::Tag::of::<State>()
-    }
-
-    fn state(&self) -> tree::State {
-        tree::State::new(State {
-            tree: RefCell::new(Tree::empty()),
-        })
-    }
-
-    fn size(&self) -> Size<Length> {
-        Size {
-            width: Length::Fill,
-            height: Length::Fill,
-        }
-    }
-
-    fn layout(
-        &self,
-        _tree: &mut Tree,
-        _renderer: &Renderer,
-        limits: &layout::Limits,
-    ) -> layout::Node {
-        layout::Node::new(limits.max())
-    }
-
-    fn operate(
-        &self,
-        tree: &mut Tree,
-        layout: Layout<'_>,
-        renderer: &Renderer,
-        operation: &mut dyn widget::Operation,
-    ) {
-        let state = tree.state.downcast_mut::<State>();
-        let mut content = self.content.borrow_mut();
-
-        content.resolve(
-            &mut state.tree.borrow_mut(),
-            renderer,
-            layout,
-            &self.view,
-            |tree, renderer, layout, element| {
-                element
-                    .as_widget()
-                    .operate(tree, layout, renderer, operation);
-            },
-        );
-    }
-
-    fn update(
-        &mut self,
-        tree: &mut Tree,
-        event: &Event,
-        layout: Layout<'_>,
-        cursor: mouse::Cursor,
-        renderer: &Renderer,
-        clipboard: &mut dyn Clipboard,
-        shell: &mut Shell<'_, Message>,
-        viewport: &Rectangle,
-    ) {
-        let state = tree.state.downcast_mut::<State>();
-        let mut content = self.content.borrow_mut();
-
-        let mut local_messages = vec![];
-        let mut local_shell = Shell::new(&mut local_messages);
-
-        content.resolve(
-            &mut state.tree.borrow_mut(),
-            renderer,
-            layout,
-            &self.view,
-            |tree, renderer, layout, element| {
-                element.as_widget_mut().update(
-                    tree,
-                    event,
-                    layout,
-                    cursor,
-                    renderer,
-                    clipboard,
-                    &mut local_shell,
-                    viewport,
-                );
-            },
-        );
-
-        if local_shell.is_layout_invalid() {
-            content.layout = None;
-        }
-
-        shell.merge(local_shell, std::convert::identity);
-    }
-
-    fn draw(
-        &self,
-        tree: &Tree,
-        renderer: &mut Renderer,
-        theme: &Theme,
-        style: &renderer::Style,
-        layout: Layout<'_>,
-        cursor: mouse::Cursor,
-        viewport: &Rectangle,
-    ) {
-        let state = tree.state.downcast_ref::<State>();
-        let mut content = self.content.borrow_mut();
-
-        content.resolve(
-            &mut state.tree.borrow_mut(),
-            renderer,
-            layout,
-            &self.view,
-            |tree, renderer, layout, element| {
-                element.as_widget().draw(
-                    tree, renderer, theme, style, layout, cursor, viewport,
-                );
-            },
-        );
-    }
-
-    fn mouse_interaction(
-        &self,
-        tree: &Tree,
-        layout: Layout<'_>,
-        cursor: mouse::Cursor,
-        viewport: &Rectangle,
-        renderer: &Renderer,
-    ) -> mouse::Interaction {
-        let state = tree.state.downcast_ref::<State>();
-        let mut content = self.content.borrow_mut();
-
-        content.resolve(
-            &mut state.tree.borrow_mut(),
-            renderer,
-            layout,
-            &self.view,
-            |tree, renderer, layout, element| {
-                element
-                    .as_widget()
-                    .mouse_interaction(tree, layout, cursor, viewport, renderer)
-            },
-        )
-    }
-
-    fn overlay<'b>(
-        &'b mut self,
-        tree: &'b mut Tree,
-        layout: Layout<'b>,
-        renderer: &Renderer,
-        viewport: &Rectangle,
-        translation: Vector,
-    ) -> Option<overlay::Element<'b, Message, Theme, Renderer>> {
-        use std::ops::DerefMut;
-
-        let state = tree.state.downcast_ref::<State>();
-
-        let overlay = OverlayBuilder {
-            content: self.content.borrow_mut(),
-            tree: state.tree.borrow_mut(),
-            types: PhantomData,
-            overlay_builder: |content: &mut RefMut<
-                '_,
-                Content<'_, _, _, _>,
-            >,
-                              tree| {
-                content.update(tree, layout.bounds().size(), &self.view);
-                content.layout(tree, renderer);
-
-                let Content {
-                    element,
-                    layout: content_layout_node,
-                    is_layout_invalid,
-                    ..
-                } = content.deref_mut();
-
-                let content_layout = Layout::with_offset(
-                    layout.bounds().position() - Point::ORIGIN,
-                    content_layout_node.as_ref().unwrap(),
-                );
-
-                (
-                    element
-                        .as_widget_mut()
-                        .overlay(
-                            tree,
-                            content_layout,
-                            renderer,
-                            viewport,
-                            translation,
-                        )
-                        .map(|overlay| RefCell::new(Nested::new(overlay))),
-                    is_layout_invalid,
-                )
-            },
-        }
-        .build();
-
-        if overlay.with_overlay(|(overlay, _layout)| overlay.is_some()) {
-            Some(overlay::Element::new(Box::new(overlay)))
-        } else {
-            None
-        }
-    }
-}
-
-impl<'a, Message, Theme, Renderer>
-    From<Responsive<'a, Message, Theme, Renderer>>
-    for Element<'a, Message, Theme, Renderer>
-where
-    Message: 'a,
-    Theme: 'a,
-    Renderer: core::Renderer + 'a,
-{
-    fn from(responsive: Responsive<'a, Message, Theme, Renderer>) -> Self {
-        Self::new(responsive)
-    }
-}
-
-#[self_referencing]
-struct Overlay<'a, 'b, Message, Theme, Renderer> {
-    content: RefMut<'a, Content<'b, Message, Theme, Renderer>>,
-    tree: RefMut<'a, Tree>,
-    types: PhantomData<Message>,
-
-    #[borrows(mut content, mut tree)]
-    #[not_covariant]
-    overlay: (
-        Option<RefCell<Nested<'this, Message, Theme, Renderer>>>,
-        &'this mut bool,
-    ),
-}
-
-impl<Message, Theme, Renderer> Overlay<'_, '_, Message, Theme, Renderer> {
-    fn with_overlay_maybe<T>(
-        &self,
-        f: impl FnOnce(&mut Nested<'_, Message, Theme, Renderer>) -> T,
-    ) -> Option<T> {
-        self.with_overlay(|(overlay, _layout)| {
-            overlay.as_ref().map(|nested| (f)(&mut nested.borrow_mut()))
-        })
-    }
-
-    fn with_overlay_mut_maybe<T>(
-        &mut self,
-        f: impl FnOnce(&mut Nested<'_, Message, Theme, Renderer>) -> T,
-    ) -> Option<T> {
-        self.with_overlay_mut(|(overlay, _layout)| {
-            overlay.as_mut().map(|nested| (f)(nested.get_mut()))
-        })
-    }
-}
-
-impl<Message, Theme, Renderer> overlay::Overlay<Message, Theme, Renderer>
-    for Overlay<'_, '_, Message, Theme, Renderer>
-where
-    Renderer: core::Renderer,
-{
-    fn layout(&mut self, renderer: &Renderer, bounds: Size) -> layout::Node {
-        self.with_overlay_maybe(|overlay| overlay.layout(renderer, bounds))
-            .unwrap_or_default()
-    }
-
-    fn draw(
-        &self,
-        renderer: &mut Renderer,
-        theme: &Theme,
-        style: &renderer::Style,
-        layout: Layout<'_>,
-        cursor: mouse::Cursor,
-    ) {
-        let _ = self.with_overlay_maybe(|overlay| {
-            overlay.draw(renderer, theme, style, layout, cursor);
-        });
-    }
-
-    fn mouse_interaction(
-        &self,
-        layout: Layout<'_>,
-        cursor: mouse::Cursor,
-        renderer: &Renderer,
-    ) -> mouse::Interaction {
-        self.with_overlay_maybe(|overlay| {
-            overlay.mouse_interaction(layout, cursor, renderer)
-        })
-        .unwrap_or_default()
-    }
-
-    fn update(
-        &mut self,
-        event: &Event,
-        layout: Layout<'_>,
-        cursor: mouse::Cursor,
-        renderer: &Renderer,
-        clipboard: &mut dyn Clipboard,
-        shell: &mut Shell<'_, Message>,
-    ) {
-        let mut is_layout_invalid = false;
-
-        let _ = self.with_overlay_mut_maybe(|overlay| {
-            overlay.update(event, layout, cursor, renderer, clipboard, shell);
-
-            is_layout_invalid = shell.is_layout_invalid();
-        });
-
-        if is_layout_invalid {
-            self.with_overlay_mut(|(_overlay, layout)| {
-                **layout = true;
-            });
-        }
-    }
-
-    fn operate(
-        &mut self,
-        layout: Layout<'_>,
-        renderer: &Renderer,
-        operation: &mut dyn widget::Operation,
-    ) {
-        let _ = self.with_overlay_mut_maybe(|overlay| {
-            overlay.operate(layout, renderer, operation);
-        });
-    }
-}

widget/src/lib.rs

@@ -12,6 +12,7 @@ mod action;
 mod column;
 mod mouse_area;
 mod pin;
+mod responsive;
 mod space;
 mod stack;
 mod themer;
@@ -78,6 +79,8 @@ pub use progress_bar::ProgressBar;
 #[doc(no_inline)]
 pub use radio::Radio;
 #[doc(no_inline)]
+pub use responsive::Responsive;
+#[doc(no_inline)]
 pub use row::Row;
 #[doc(no_inline)]
 pub use rule::Rule;

widget/src/mouse_area.rs

@@ -181,8 +181,8 @@ where
         vec![Tree::new(&self.content)]
     }
 
-    fn diff(&self, tree: &mut Tree) {
+    fn diff(&mut self, tree: &mut Tree) {
-        tree.diff_children(std::slice::from_ref(&self.content));
+        tree.diff_children(std::slice::from_mut(&mut self.content));
     }
 
     fn size(&self) -> Size<Length> {
@@ -190,24 +190,26 @@ where
     }
 
     fn layout(
-        &self,
+        &mut self,
         tree: &mut Tree,
         renderer: &Renderer,
         limits: &layout::Limits,
     ) -> layout::Node {
-        self.content
+        self.content.as_widget_mut().layout(
-            .as_widget()
+            &mut tree.children[0],
-            .layout(&mut tree.children[0], renderer, limits)
+            renderer,
+            limits,
+        )
     }
 
     fn operate(
-        &self,
+        &mut self,
         tree: &mut Tree,
         layout: Layout<'_>,
         renderer: &Renderer,
         operation: &mut dyn Operation,
     ) {
-        self.content.as_widget().operate(
+        self.content.as_widget_mut().operate(
             &mut tree.children[0],
             layout,
             renderer,

widget/src/overlay/menu.rs

@@ -205,7 +205,7 @@ where
             class,
         } = menu;
 
-        let list = Scrollable::new(List {
+        let mut list = Scrollable::new(List {
             options,
             hovered_option,
             on_selected,
@@ -218,7 +218,7 @@ where
             class,
         });
 
-        state.tree.diff(&list as &dyn Widget<_, _, _>);
+        state.tree.diff(&mut list as &mut dyn Widget<_, _, _>);
 
         Self {
             position,
@@ -369,7 +369,7 @@ where
     }
 
     fn layout(
-        &self,
+        &mut self,
         _tree: &mut Tree,
         renderer: &Renderer,
         limits: &layout::Limits,

widget/src/pane_grid.rs

@@ -378,7 +378,7 @@ where
         self.contents.iter().map(Content::state).collect()
     }
 
-    fn diff(&self, tree: &mut Tree) {
+    fn diff(&mut self, tree: &mut Tree) {
         let Memory { order, .. } = tree.state.downcast_ref();
 
         // `Pane` always increments and is iterated by Ord so new
@@ -401,7 +401,7 @@ where
         });
 
         tree.diff_children_custom(
-            &self.contents,
+            &mut self.contents,
             |state, content| content.diff(state),
             Content::state,
         );
@@ -418,7 +418,7 @@ where
     }
 
     fn layout(
-        &self,
+        &mut self,
         tree: &mut Tree,
         renderer: &Renderer,
         limits: &layout::Limits,
@@ -432,20 +432,19 @@ where
 
         let children = self
             .panes
-            .iter()
+            .iter_mut()
-            .copied()
+            .zip(&mut self.contents)
-            .zip(&self.contents)
             .zip(tree.children.iter_mut())
             .filter_map(|((pane, content), tree)| {
                 if self
                     .internal
                     .maximized()
-                    .is_some_and(|maximized| maximized != pane)
+                    .is_some_and(|maximized| maximized != *pane)
                 {
                     return Some(layout::Node::new(Size::ZERO));
                 }
 
-                let region = regions.get(&pane)?;
+                let region = regions.get(pane)?;
                 let size = Size::new(region.width, region.height);
 
                 let node = content.layout(
@@ -462,7 +461,7 @@ where
     }
 
     fn operate(
-        &self,
+        &mut self,
         tree: &mut Tree,
         layout: Layout<'_>,
         renderer: &Renderer,
@@ -470,15 +469,14 @@ where
     ) {
         operation.container(None, layout.bounds(), &mut |operation| {
             self.panes
-                .iter()
+                .iter_mut()
-                .copied()
+                .zip(&mut self.contents)
-                .zip(&self.contents)
                 .zip(&mut tree.children)
                 .zip(layout.children())
                 .filter(|(((pane, _), _), _)| {
                     self.internal
                         .maximized()
-                        .is_none_or(|maximized| *pane == maximized)
+                        .is_none_or(|maximized| **pane == maximized)
                 })
                 .for_each(|(((_, content), state), layout)| {
                     content.operate(state, layout, renderer, operation);

widget/src/pane_grid/content.rs

@@ -91,13 +91,13 @@ where
         }
     }
 
-    pub(super) fn diff(&self, tree: &mut Tree) {
+    pub(super) fn diff(&mut self, tree: &mut Tree) {
         if tree.children.len() == 2 {
-            if let Some(title_bar) = self.title_bar.as_ref() {
+            if let Some(title_bar) = self.title_bar.as_mut() {
                 title_bar.diff(&mut tree.children[1]);
             }
 
-            tree.children[0].diff(&self.body);
+            tree.children[0].diff(&mut self.body);
         } else {
             *tree = self.state();
         }
@@ -165,12 +165,12 @@ where
     }
 
     pub(crate) fn layout(
-        &self,
+        &mut self,
         tree: &mut Tree,
         renderer: &Renderer,
         limits: &layout::Limits,
     ) -> layout::Node {
-        if let Some(title_bar) = &self.title_bar {
+        if let Some(title_bar) = &mut self.title_bar {
             let max_size = limits.max();
 
             let title_bar_layout = title_bar.layout(
@@ -181,7 +181,7 @@ where
 
             let title_bar_size = title_bar_layout.size();
 
-            let body_layout = self.body.as_widget().layout(
+            let body_layout = self.body.as_widget_mut().layout(
                 &mut tree.children[0],
                 renderer,
                 &layout::Limits::new(
@@ -201,7 +201,7 @@ where
                 ],
             )
         } else {
-            self.body.as_widget().layout(
+            self.body.as_widget_mut().layout(
                 &mut tree.children[0],
                 renderer,
                 limits,
@@ -210,13 +210,13 @@ where
     }
 
     pub(crate) fn operate(
-        &self,
+        &mut self,
         tree: &mut Tree,
         layout: Layout<'_>,
         renderer: &Renderer,
         operation: &mut dyn widget::Operation,
     ) {
-        let body_layout = if let Some(title_bar) = &self.title_bar {
+        let body_layout = if let Some(title_bar) = &mut self.title_bar {
             let mut children = layout.children();
 
             title_bar.operate(
@@ -231,7 +231,7 @@ where
             layout
         };
 
-        self.body.as_widget().operate(
+        self.body.as_widget_mut().operate(
             &mut tree.children[0],
             body_layout,
             renderer,

widget/src/pane_grid/title_bar.rs

@@ -128,17 +128,17 @@ where
         }
     }
 
-    pub(super) fn diff(&self, tree: &mut Tree) {
+    pub(super) fn diff(&mut self, tree: &mut Tree) {
         if tree.children.len() == 3 {
-            if let Some(controls) = self.controls.as_ref() {
+            if let Some(controls) = self.controls.as_mut() {
-                if let Some(compact) = controls.compact.as_ref() {
+                if let Some(compact) = controls.compact.as_mut() {
                     tree.children[2].diff(compact);
                 }
 
-                tree.children[1].diff(&controls.full);
+                tree.children[1].diff(&mut controls.full);
             }
 
-            tree.children[0].diff(&self.content);
+            tree.children[0].diff(&mut self.content);
         } else {
             *tree = self.state();
         }
@@ -274,7 +274,7 @@ where
     }
 
     pub(crate) fn layout(
-        &self,
+        &mut self,
         tree: &mut Tree,
         renderer: &Renderer,
         limits: &layout::Limits,
@@ -282,7 +282,7 @@ where
         let limits = limits.shrink(self.padding);
         let max_size = limits.max();
 
-        let title_layout = self.content.as_widget().layout(
+        let title_layout = self.content.as_widget_mut().layout(
             &mut tree.children[0],
             renderer,
             &layout::Limits::new(Size::ZERO, max_size),
@@ -290,8 +290,8 @@ where
 
         let title_size = title_layout.size();
 
-        let node = if let Some(controls) = &self.controls {
+        let node = if let Some(controls) = &mut self.controls {
-            let controls_layout = controls.full.as_widget().layout(
+            let controls_layout = controls.full.as_widget_mut().layout(
                 &mut tree.children[1],
                 renderer,
                 &layout::Limits::new(Size::ZERO, max_size),
@@ -300,8 +300,8 @@ where
             if title_layout.bounds().width + controls_layout.bounds().width
                 > max_size.width
             {
-                if let Some(compact) = controls.compact.as_ref() {
+                if let Some(compact) = controls.compact.as_mut() {
-                    let compact_layout = compact.as_widget().layout(
+                    let compact_layout = compact.as_widget_mut().layout(
                         &mut tree.children[2],
                         renderer,
                         &layout::Limits::new(Size::ZERO, max_size),
@@ -369,7 +369,7 @@ where
     }
 
     pub(crate) fn operate(
-        &self,
+        &mut self,
         tree: &mut Tree,
         layout: Layout<'_>,
         renderer: &Renderer,
@@ -382,16 +382,16 @@ where
         let title_layout = children.next().unwrap();
         let mut show_title = true;
 
-        if let Some(controls) = &self.controls {
+        if let Some(controls) = &mut self.controls {
             let controls_layout = children.next().unwrap();
 
             if title_layout.bounds().width + controls_layout.bounds().width
                 > padded.bounds().width
             {
-                if let Some(compact) = controls.compact.as_ref() {
+                if let Some(compact) = controls.compact.as_mut() {
                     let compact_layout = children.next().unwrap();
 
-                    compact.as_widget().operate(
+                    compact.as_widget_mut().operate(
                         &mut tree.children[2],
                         compact_layout,
                         renderer,
@@ -400,7 +400,7 @@ where
                 } else {
                     show_title = false;
 
-                    controls.full.as_widget().operate(
+                    controls.full.as_widget_mut().operate(
                         &mut tree.children[1],
                         controls_layout,
                         renderer,
@@ -408,7 +408,7 @@ where
                     );
                 }
             } else {
-                controls.full.as_widget().operate(
+                controls.full.as_widget_mut().operate(
                     &mut tree.children[1],
                     controls_layout,
                     renderer,
@@ -418,7 +418,7 @@ where
         };
 
         if show_title {
-            self.content.as_widget().operate(
+            self.content.as_widget_mut().operate(
                 &mut tree.children[0],
                 title_layout,
                 renderer,

widget/src/pick_list.rs

@@ -348,7 +348,7 @@ where
     }
 
     fn layout(
-        &self,
+        &mut self,
         tree: &mut Tree,
         renderer: &Renderer,
         limits: &layout::Limits,

widget/src/pin.rs

@@ -127,8 +127,8 @@ where
         self.content.as_widget().children()
     }
 
-    fn diff(&self, tree: &mut widget::Tree) {
+    fn diff(&mut self, tree: &mut widget::Tree) {
-        self.content.as_widget().diff(tree);
+        self.content.as_widget_mut().diff(tree);
     }
 
     fn size(&self) -> Size<Length> {
@@ -139,7 +139,7 @@ where
     }
 
     fn layout(
-        &self,
+        &mut self,
         tree: &mut widget::Tree,
         renderer: &Renderer,
         limits: &layout::Limits,
@@ -151,7 +151,7 @@ where
 
         let node = self
             .content
-            .as_widget()
+            .as_widget_mut()
             .layout(tree, renderer, &layout::Limits::new(Size::ZERO, available))
             .move_to(self.position);
 
@@ -160,13 +160,13 @@ where
     }
 
     fn operate(
-        &self,
+        &mut self,
         tree: &mut widget::Tree,
         layout: Layout<'_>,
         renderer: &Renderer,
         operation: &mut dyn widget::Operation,
     ) {
-        self.content.as_widget().operate(
+        self.content.as_widget_mut().operate(
             tree,
             layout.children().next().unwrap(),
             renderer,

widget/src/progress_bar.rs

@@ -157,7 +157,7 @@ where
     }
 
     fn layout(
-        &self,
+        &mut self,
         _tree: &mut Tree,
         _renderer: &Renderer,
         limits: &layout::Limits,

widget/src/qr_code.rs

@@ -136,7 +136,7 @@ where
     }
 
     fn layout(
-        &self,
+        &mut self,
         _tree: &mut Tree,
         _renderer: &Renderer,
         _limits: &layout::Limits,

widget/src/radio.rs

@@ -290,7 +290,7 @@ where
     }
 
     fn layout(
-        &self,
+        &mut self,
         tree: &mut Tree,
         renderer: &Renderer,
         limits: &layout::Limits,

widget/src/responsive.rs

@@ -0,0 +1,213 @@
+use crate::core::layout::{self, Layout};
+use crate::core::mouse;
+use crate::core::overlay;
+use crate::core::renderer;
+use crate::core::widget;
+use crate::core::widget::tree::{self, Tree};
+use crate::core::{
+    self, Clipboard, Element, Event, Length, Rectangle, Shell, Size, Vector,
+    Widget,
+};
+use crate::horizontal_space;
+
+/// A widget that is aware of its dimensions.
+///
+/// A [`Responsive`] widget will always try to fill all the available space of
+/// its parent.
+#[allow(missing_debug_implementations)]
+pub struct Responsive<
+    'a,
+    Message,
+    Theme = crate::Theme,
+    Renderer = crate::Renderer,
+> {
+    view: Box<dyn Fn(Size) -> Element<'a, Message, Theme, Renderer> + 'a>,
+    width: Length,
+    height: Length,
+    content: Element<'a, Message, Theme, Renderer>,
+}
+
+impl<'a, Message, Theme, Renderer> Responsive<'a, Message, Theme, Renderer>
+where
+    Renderer: core::Renderer,
+{
+    /// Creates a new [`Responsive`] widget with a closure that produces its
+    /// contents.
+    ///
+    /// The `view` closure will receive the maximum available space for
+    /// the [`Responsive`] during layout. You can use this [`Size`] to
+    /// conditionally build the contents.
+    pub fn new(
+        view: impl Fn(Size) -> Element<'a, Message, Theme, Renderer> + 'a,
+    ) -> Self {
+        Self {
+            view: Box::new(view),
+            width: Length::Fill,
+            height: Length::Fill,
+            content: Element::new(horizontal_space().width(0)),
+        }
+    }
+
+    /// Sets the width of the [`Responsive`].
+    pub fn width(mut self, width: impl Into<Length>) -> Self {
+        self.width = width.into();
+        self
+    }
+
+    /// Sets the height of the [`Responsive`].
+    pub fn height(mut self, height: impl Into<Length>) -> Self {
+        self.height = height.into();
+        self
+    }
+}
+
+impl<Message, Theme, Renderer> Widget<Message, Theme, Renderer>
+    for Responsive<'_, Message, Theme, Renderer>
+where
+    Renderer: core::Renderer,
+{
+    fn tag(&self) -> tree::Tag {
+        struct Marker;
+        tree::Tag::of::<Marker>()
+    }
+
+    fn diff(&mut self, _tree: &mut Tree) {
+        // Diff is deferred to layout
+    }
+
+    fn size(&self) -> Size<Length> {
+        Size {
+            width: self.width,
+            height: self.height,
+        }
+    }
+
+    fn layout(
+        &mut self,
+        tree: &mut Tree,
+        renderer: &Renderer,
+        limits: &layout::Limits,
+    ) -> layout::Node {
+        let limits = limits.width(self.width).height(self.height);
+        let size = limits.max();
+
+        self.content = (self.view)(size);
+        tree.diff_children(std::slice::from_mut(&mut self.content));
+
+        let node = self.content.as_widget_mut().layout(
+            &mut tree.children[0],
+            renderer,
+            &limits.loose(),
+        );
+
+        let size = limits.resolve(self.width, self.height, node.size());
+
+        layout::Node::with_children(size, vec![node])
+    }
+
+    fn update(
+        &mut self,
+        tree: &mut Tree,
+        event: &Event,
+        layout: Layout<'_>,
+        cursor: mouse::Cursor,
+        renderer: &Renderer,
+        clipboard: &mut dyn Clipboard,
+        shell: &mut Shell<'_, Message>,
+        viewport: &Rectangle,
+    ) {
+        self.content.as_widget_mut().update(
+            &mut tree.children[0],
+            event,
+            layout.children().next().unwrap(),
+            cursor,
+            renderer,
+            clipboard,
+            shell,
+            viewport,
+        );
+    }
+
+    fn draw(
+        &self,
+        tree: &Tree,
+        renderer: &mut Renderer,
+        theme: &Theme,
+        style: &renderer::Style,
+        layout: Layout<'_>,
+        cursor: mouse::Cursor,
+        viewport: &Rectangle,
+    ) {
+        self.content.as_widget().draw(
+            &tree.children[0],
+            renderer,
+            theme,
+            style,
+            layout.children().next().unwrap(),
+            cursor,
+            viewport,
+        );
+    }
+
+    fn mouse_interaction(
+        &self,
+        tree: &Tree,
+        layout: Layout<'_>,
+        cursor: mouse::Cursor,
+        viewport: &Rectangle,
+        renderer: &Renderer,
+    ) -> mouse::Interaction {
+        self.content.as_widget().mouse_interaction(
+            &tree.children[0],
+            layout.children().next().unwrap(),
+            cursor,
+            viewport,
+            renderer,
+        )
+    }
+
+    fn operate(
+        &mut self,
+        tree: &mut Tree,
+        layout: Layout<'_>,
+        renderer: &Renderer,
+        operation: &mut dyn widget::Operation,
+    ) {
+        self.content.as_widget_mut().operate(
+            &mut tree.children[0],
+            layout.children().next().unwrap(),
+            renderer,
+            operation,
+        );
+    }
+
+    fn overlay<'a>(
+        &'a mut self,
+        tree: &'a mut Tree,
+        layout: Layout<'a>,
+        renderer: &Renderer,
+        viewport: &Rectangle,
+        translation: Vector,
+    ) -> Option<overlay::Element<'a, Message, Theme, Renderer>> {
+        self.content.as_widget_mut().overlay(
+            &mut tree.children[0],
+            layout.children().next().unwrap(),
+            renderer,
+            viewport,
+            translation,
+        )
+    }
+}
+
+impl<'a, Message, Theme, Renderer>
+    From<Responsive<'a, Message, Theme, Renderer>>
+    for Element<'a, Message, Theme, Renderer>
+where
+    Message: 'a,
+    Theme: 'a,
+    Renderer: core::Renderer + 'a,
+{
+    fn from(responsive: Responsive<'a, Message, Theme, Renderer>) -> Self {
+        Self::new(responsive)
+    }
+}

widget/src/row.rs

@@ -196,8 +196,8 @@ where
         self.children.iter().map(Tree::new).collect()
     }
 
-    fn diff(&self, tree: &mut Tree) {
+    fn diff(&mut self, tree: &mut Tree) {
-        tree.diff_children(&self.children);
+        tree.diff_children(&mut self.children);
     }
 
     fn size(&self) -> Size<Length> {
@@ -208,7 +208,7 @@ where
     }
 
     fn layout(
-        &self,
+        &mut self,
         tree: &mut Tree,
         renderer: &Renderer,
         limits: &layout::Limits,
@@ -222,13 +222,13 @@ where
             self.padding,
             self.spacing,
             self.align,
-            &self.children,
+            &mut self.children,
             &mut tree.children,
         )
     }
 
     fn operate(
-        &self,
+        &mut self,
         tree: &mut Tree,
         layout: Layout<'_>,
         renderer: &Renderer,
@@ -236,12 +236,12 @@ where
     ) {
         operation.container(None, layout.bounds(), &mut |operation| {
             self.children
-                .iter()
+                .iter_mut()
                 .zip(&mut tree.children)
                 .zip(layout.children())
                 .for_each(|((child, state), layout)| {
                     child
-                        .as_widget()
+                        .as_widget_mut()
                         .operate(state, layout, renderer, operation);
                 });
         });
@@ -398,7 +398,7 @@ where
         self.row.children()
     }
 
-    fn diff(&self, tree: &mut Tree) {
+    fn diff(&mut self, tree: &mut Tree) {
         self.row.diff(tree);
     }
 
@@ -407,7 +407,7 @@ where
     }
 
     fn layout(
-        &self,
+        &mut self,
         tree: &mut Tree,
         renderer: &Renderer,
         limits: &layout::Limits,
@@ -449,8 +449,8 @@ where
             }
         };
 
-        for (i, child) in self.row.children.iter().enumerate() {
+        for (i, child) in self.row.children.iter_mut().enumerate() {
-            let node = child.as_widget().layout(
+            let node = child.as_widget_mut().layout(
                 &mut tree.children[i],
                 renderer,
                 &limits,
@@ -528,7 +528,7 @@ where
     }
 
     fn operate(
-        &self,
+        &mut self,
         tree: &mut Tree,
         layout: Layout<'_>,
         renderer: &Renderer,

widget/src/rule.rs

@@ -112,7 +112,7 @@ where
     }
 
     fn layout(
-        &self,
+        &mut self,
         _tree: &mut Tree,
         _renderer: &Renderer,
         limits: &layout::Limits,

widget/src/scrollable.rs

@@ -409,8 +409,8 @@ where
         vec![Tree::new(&self.content)]
     }
 
-    fn diff(&self, tree: &mut Tree) {
+    fn diff(&mut self, tree: &mut Tree) {
-        tree.diff_children(std::slice::from_ref(&self.content));
+        tree.diff_children(std::slice::from_mut(&mut self.content));
     }
 
     fn size(&self) -> Size<Length> {
@@ -421,7 +421,7 @@ where
     }
 
     fn layout(
-        &self,
+        &mut self,
         tree: &mut Tree,
         renderer: &Renderer,
         limits: &layout::Limits,
@@ -453,7 +453,7 @@ where
                         ),
                     );
 
-                    self.content.as_widget().layout(
+                    self.content.as_widget_mut().layout(
                         &mut tree.children[0],
                         renderer,
                         &child_limits,
@@ -527,7 +527,7 @@ where
     }
 
     fn operate(
-        &self,
+        &mut self,
         tree: &mut Tree,
         layout: Layout<'_>,
         renderer: &Renderer,
@@ -553,7 +553,7 @@ where
             self.id.as_ref().map(|id| &id.0),
             bounds,
             &mut |operation| {
-                self.content.as_widget().operate(
+                self.content.as_widget_mut().operate(
                     &mut tree.children[0],
                     layout.children().next().unwrap(),
                     renderer,

widget/src/sensor.rs

@@ -184,8 +184,8 @@ where
         vec![Tree::new(&self.content)]
     }
 
-    fn diff(&self, tree: &mut Tree) {
+    fn diff(&mut self, tree: &mut Tree) {
-        tree.diff_children(&[&self.content]);
+        tree.diff_children(std::slice::from_mut(&mut self.content));
     }
 
     fn update(
@@ -287,14 +287,16 @@ where
     }
 
     fn layout(
-        &self,
+        &mut self,
         tree: &mut Tree,
         renderer: &Renderer,
         limits: &layout::Limits,
     ) -> layout::Node {
-        self.content
+        self.content.as_widget_mut().layout(
-            .as_widget()
+            &mut tree.children[0],
-            .layout(&mut tree.children[0], renderer, limits)
+            renderer,
+            limits,
+        )
     }
 
     fn draw(
@@ -319,13 +321,13 @@ where
     }
 
     fn operate(
-        &self,
+        &mut self,
         tree: &mut Tree,
         layout: core::Layout<'_>,
         renderer: &Renderer,
         operation: &mut dyn widget::Operation,
     ) {
-        self.content.as_widget().operate(
+        self.content.as_widget_mut().operate(
             &mut tree.children[0],
             layout,
             renderer,

widget/src/shader.rs

@@ -77,7 +77,7 @@ where
     }
 
     fn layout(
-        &self,
+        &mut self,
         _tree: &mut Tree,
         _renderer: &Renderer,
         limits: &layout::Limits,

widget/src/slider.rs

@@ -234,7 +234,7 @@ where
     }
 
     fn layout(
-        &self,
+        &mut self,
         _tree: &mut Tree,
         _renderer: &Renderer,
         limits: &layout::Limits,

widget/src/space.rs

@@ -65,7 +65,7 @@ where
     }
 
     fn layout(
-        &self,
+        &mut self,
         _tree: &mut Tree,
         _renderer: &Renderer,
         limits: &layout::Limits,

widget/src/stack.rs

@@ -146,8 +146,8 @@ where
         self.children.iter().map(Tree::new).collect()
     }
 
-    fn diff(&self, tree: &mut Tree) {
+    fn diff(&mut self, tree: &mut Tree) {
-        tree.diff_children(&self.children);
+        tree.diff_children(&mut self.children);
     }
 
     fn size(&self) -> Size<Length> {
@@ -158,7 +158,7 @@ where
     }
 
     fn layout(
-        &self,
+        &mut self,
         tree: &mut Tree,
         renderer: &Renderer,
         limits: &layout::Limits,
@@ -173,7 +173,7 @@ where
             ));
         }
 
-        let base = self.children[0].as_widget().layout(
+        let base = self.children[0].as_widget_mut().layout(
             &mut tree.children[0],
             renderer,
             &limits,
@@ -183,18 +183,21 @@ where
         let limits = layout::Limits::new(Size::ZERO, size);
 
         let nodes = std::iter::once(base)
-            .chain(self.children[1..].iter().zip(&mut tree.children[1..]).map(
+            .chain(
-                |(layer, tree)| {
+                self.children[1..]
-                    layer.as_widget().layout(tree, renderer, &limits)
+                    .iter_mut()
-                },
+                    .zip(&mut tree.children[1..])
-            ))
+                    .map(|(layer, tree)| {
+                        layer.as_widget_mut().layout(tree, renderer, &limits)
+                    }),
+            )
             .collect();
 
         layout::Node::with_children(size, nodes)
     }
 
     fn operate(
-        &self,
+        &mut self,
         tree: &mut Tree,
         layout: Layout<'_>,
         renderer: &Renderer,
@@ -202,12 +205,12 @@ where
     ) {
         operation.container(None, layout.bounds(), &mut |operation| {
             self.children
-                .iter()
+                .iter_mut()
                 .zip(&mut tree.children)
                 .zip(layout.children())
                 .for_each(|((child, state), layout)| {
                     child
-                        .as_widget()
+                        .as_widget_mut()
                         .operate(state, layout, renderer, operation);
                 });
         });

widget/src/svg.rs

@@ -162,7 +162,7 @@ where
     }
 
     fn layout(
-        &self,
+        &mut self,
         _tree: &mut Tree,
         renderer: &Renderer,
         limits: &layout::Limits,

widget/src/table.rs

@@ -227,12 +227,12 @@ where
             .collect()
     }
 
-    fn diff(&self, state: &mut widget::Tree) {
+    fn diff(&mut self, state: &mut widget::Tree) {
-        state.diff_children(&self.cells);
+        state.diff_children(&mut self.cells);
     }
 
     fn layout(
-        &self,
+        &mut self,
         tree: &mut widget::Tree,
         renderer: &Renderer,
         limits: &layout::Limits,
@@ -265,7 +265,7 @@ where
         let mut y = self.padding_y;
 
         for (i, (cell, state)) in
-            self.cells.iter().zip(&mut tree.children).enumerate()
+            self.cells.iter_mut().zip(&mut tree.children).enumerate()
         {
             let row = i / columns;
             let column = i % columns;
@@ -306,7 +306,7 @@ where
             )
             .width(width);
 
-            let layout = cell.as_widget().layout(state, renderer, &limits);
+            let layout = cell.as_widget_mut().layout(state, renderer, &limits);
             let size = limits.resolve(width, Length::Shrink, layout.size());
 
             metrics.columns[column] = metrics.columns[column].max(size.width);
@@ -344,7 +344,7 @@ where
         let mut y = self.padding_y;
 
         for (i, (cell, state)) in
-            self.cells.iter().zip(&mut tree.children).enumerate()
+            self.cells.iter_mut().zip(&mut tree.children).enumerate()
         {
             let row = i / columns;
             let column = i % columns;
@@ -396,7 +396,7 @@ where
             )
             .width(width);
 
-            let layout = cell.as_widget().layout(state, renderer, &limits);
+            let layout = cell.as_widget_mut().layout(state, renderer, &limits);
             let size = limits.resolve(
                 if let Length::Fixed(_) = width {
                     width
@@ -581,7 +581,7 @@ where
     }
 
     fn operate(
-        &self,
+        &mut self,
         tree: &mut widget::Tree,
         layout: Layout<'_>,
         renderer: &Renderer,
@@ -589,11 +589,12 @@ where
     ) {
         for ((cell, state), layout) in self
             .cells
-            .iter()
+            .iter_mut()
             .zip(&mut tree.children)
             .zip(layout.children())
         {
-            cell.as_widget().operate(state, layout, renderer, operation);
+            cell.as_widget_mut()
+                .operate(state, layout, renderer, operation);
         }
     }
 

widget/src/text/rich.rs

@@ -225,7 +225,7 @@ where
     }
 
     fn layout(
-        &self,
+        &mut self,
         tree: &mut Tree,
         renderer: &Renderer,
         limits: &layout::Limits,

widget/src/text_editor.rs

@@ -597,7 +597,7 @@ where
     }
 
     fn layout(
-        &self,
+        &mut self,
         tree: &mut widget::Tree,
         renderer: &Renderer,
         limits: &layout::Limits,
@@ -1051,7 +1051,7 @@ where
     }
 
     fn operate(
-        &self,
+        &mut self,
         tree: &mut widget::Tree,
         layout: Layout<'_>,
         _renderer: &Renderer,

widget/src/text_input.rs

@@ -297,7 +297,7 @@ where
     ///
     /// [`Renderer`]: text::Renderer
     pub fn layout(
-        &self,
+        &mut self,
         tree: &mut Tree,
         renderer: &Renderer,
         limits: &layout::Limits,
@@ -654,7 +654,7 @@ where
         tree::State::new(State::<Renderer::Paragraph>::new())
     }
 
-    fn diff(&self, tree: &mut Tree) {
+    fn diff(&mut self, tree: &mut Tree) {
         let state = tree.state.downcast_mut::<State<Renderer::Paragraph>>();
 
         // Stop pasting if input becomes disabled
@@ -671,7 +671,7 @@ where
     }
 
     fn layout(
-        &self,
+        &mut self,
         tree: &mut Tree,
         renderer: &Renderer,
         limits: &layout::Limits,
@@ -680,7 +680,7 @@ where
     }
 
     fn operate(
-        &self,
+        &mut self,
         tree: &mut Tree,
         layout: Layout<'_>,
         _renderer: &Renderer,

widget/src/themer.rs

@@ -81,8 +81,8 @@ where
         self.content.as_widget().children()
     }
 
-    fn diff(&self, tree: &mut Tree) {
+    fn diff(&mut self, tree: &mut Tree) {
-        self.content.as_widget().diff(tree);
+        self.content.as_widget_mut().diff(tree);
     }
 
     fn size(&self) -> Size<Length> {
@@ -90,23 +90,23 @@ where
     }
 
     fn layout(
-        &self,
+        &mut self,
         tree: &mut Tree,
         renderer: &Renderer,
         limits: &layout::Limits,
     ) -> layout::Node {
-        self.content.as_widget().layout(tree, renderer, limits)
+        self.content.as_widget_mut().layout(tree, renderer, limits)
     }
 
     fn operate(
-        &self,
+        &mut self,
         tree: &mut Tree,
         layout: Layout<'_>,
         renderer: &Renderer,
         operation: &mut dyn Operation,
     ) {
         self.content
-            .as_widget()
+            .as_widget_mut()
             .operate(tree, layout, renderer, operation);
     }
 

widget/src/toggler.rs

@@ -270,7 +270,7 @@ where
     }
 
     fn layout(
-        &self,
+        &mut self,
         tree: &mut Tree,
         renderer: &Renderer,
         limits: &layout::Limits,

widget/src/tooltip.rs

@@ -155,10 +155,10 @@ where
         ]
     }
 
-    fn diff(&self, tree: &mut widget::Tree) {
+    fn diff(&mut self, tree: &mut widget::Tree) {
-        tree.diff_children(&[
+        tree.diff_children(&mut [
-            self.content.as_widget(),
+            self.content.as_widget_mut(),
-            self.tooltip.as_widget(),
+            self.tooltip.as_widget_mut(),
         ]);
     }
 
@@ -179,14 +179,16 @@ where
     }
 
     fn layout(
-        &self,
+        &mut self,
         tree: &mut widget::Tree,
         renderer: &Renderer,
         limits: &layout::Limits,
     ) -> layout::Node {
-        self.content
+        self.content.as_widget_mut().layout(
-            .as_widget()
+            &mut tree.children[0],
-            .layout(&mut tree.children[0], renderer, limits)
+            renderer,
+            limits,
+        )
     }
 
     fn update(
@@ -294,7 +296,7 @@ where
         let tooltip = if let State::Hovered { cursor_position } = *state {
             Some(overlay::Element::new(Box::new(Overlay {
                 position: layout.position() + translation,
-                tooltip: &self.tooltip,
+                tooltip: &mut self.tooltip,
                 state: children.next().unwrap(),
                 cursor_position,
                 content_bounds: layout.bounds(),
@@ -366,7 +368,7 @@ where
     Renderer: text::Renderer,
 {
     position: Point,
-    tooltip: &'b Element<'a, Message, Theme, Renderer>,
+    tooltip: &'b mut Element<'a, Message, Theme, Renderer>,
     state: &'b mut widget::Tree,
     cursor_position: Point,
     content_bounds: Rectangle,
@@ -386,7 +388,7 @@ where
     fn layout(&mut self, renderer: &Renderer, bounds: Size) -> layout::Node {
         let viewport = Rectangle::with_size(bounds);
 
-        let tooltip_layout = self.tooltip.as_widget().layout(
+        let tooltip_layout = self.tooltip.as_widget_mut().layout(
             self.state,
             renderer,
             &layout::Limits::new(

widget/src/vertical_slider.rs

@@ -238,7 +238,7 @@ where
     }
 
     fn layout(
-        &self,
+        &mut self,
         _tree: &mut Tree,
         _renderer: &Renderer,
         limits: &layout::Limits,