Hide toplevel from toplevel list when it is being dragged

Uses a custom wrapper widget, that makes `draw` do nothing when visible
is set to false.

This works well sometimes, but can cause weird bugs...

src/widgets/mod.rs

@@ -19,8 +19,8 @@ mod mouse_interaction_wrapper;
 pub use mouse_interaction_wrapper::mouse_interaction_wrapper;
 mod toplevels;
 pub use toplevels::toplevels;
-
-trait Foo {}
+mod visibility_wrapper;
+pub use visibility_wrapper::visibility_wrapper;
 
 pub fn layout_wrapper<'a, Msg, T: Into<cosmic::Element<'a, Msg>>>(
     inner: T,

src/widgets/visibility_wrapper.rs

@@ -0,0 +1,110 @@
+//! If `visible` is set to `true`, behaves exactly as wrapped widget. If `false`,
+//! takes the same space but does not draw.
+
+use cosmic::iced::{
+    advanced::{
+        layout, mouse, overlay, renderer,
+        widget::{tree, Id, Operation, OperationOutputWrapper, Tree},
+        Clipboard, Layout, Shell, Widget,
+    },
+    event::{self, Event},
+    Length, Rectangle, Size,
+};
+use std::marker::PhantomData;
+
+pub fn visibility_wrapper<'a, Msg, T: Into<cosmic::Element<'a, Msg>>>(
+    inner: T,
+    visible: bool,
+) -> VisibilityWrapper<'a, Msg> {
+    VisibilityWrapper {
+        content: inner.into(),
+        visible,
+        _msg: PhantomData,
+    }
+}
+
+pub struct VisibilityWrapper<'a, Msg> {
+    content: cosmic::Element<'a, Msg>,
+    visible: bool,
+    _msg: PhantomData<Msg>,
+}
+
+impl<'a, Msg> Widget<Msg, cosmic::Theme, cosmic::Renderer> for VisibilityWrapper<'a, Msg> {
+    delegate::delegate! {
+        to self.content.as_widget() {
+            fn tag(&self) -> tree::Tag;
+            fn state(&self) -> tree::State;
+            fn children(&self) -> Vec<Tree>;
+            fn size(&self) -> Size<Length>;
+            fn size_hint(&self) -> Size<Length>;
+            fn layout(
+                    &self,
+                    tree: &mut Tree,
+                    renderer: &cosmic::Renderer,
+                    limits: &layout::Limits,
+                ) -> layout::Node;
+            fn operate(
+                    &self,
+                    tree: &mut Tree,
+                    layout: Layout<'_>,
+                    renderer: &cosmic::Renderer,
+                    operation: &mut dyn Operation<OperationOutputWrapper<Msg>>,
+                );
+            fn mouse_interaction(
+                &self,
+                _tree: &Tree,
+                _layout: Layout<'_>,
+                _cursor: mouse::Cursor,
+                _viewport: &Rectangle,
+                _renderer: &cosmic::Renderer,
+            ) -> mouse::Interaction;
+            fn id(&self) -> Option<Id>;
+        }
+
+        to self.content.as_widget_mut() {
+            fn diff(&mut self, tree: &mut Tree);
+            fn on_event(
+                &mut self,
+                tree: &mut Tree,
+                event: Event,
+                layout: Layout<'_>,
+                cursor: mouse::Cursor,
+                renderer: &cosmic::Renderer,
+                clipboard: &mut dyn Clipboard,
+                shell: &mut Shell<'_, Msg>,
+                viewport: &Rectangle,
+            ) -> event::Status;
+            fn overlay<'b>(
+                &'b mut self,
+                tree: &'b mut Tree,
+                layout: Layout<'_>,
+                renderer: &cosmic::Renderer,
+            ) -> Option<overlay::Element<'b, Msg, cosmic::Theme, cosmic::Renderer>>;
+            fn set_id(&mut self, id: Id);
+        }
+    }
+
+    fn draw(
+        &self,
+        state: &Tree,
+        renderer: &mut cosmic::Renderer,
+        theme: &cosmic::Theme,
+        style: &renderer::Style,
+        layout: Layout<'_>,
+        cursor: mouse::Cursor,
+        viewport: &Rectangle,
+    ) {
+        if self.visible {
+            self.content
+                .as_widget()
+                .draw(state, renderer, theme, style, layout, cursor, viewport);
+        }
+    }
+}
+
+impl<'a, Msg: 'a> From<VisibilityWrapper<'a, Msg>> for cosmic::Element<'a, Msg> {
+    fn from(widget: VisibilityWrapper<'a, Msg>) -> Self {
+        cosmic::Element::new(widget)
+    }
+
+}