segmented_button: add on_double_click callback

Fires in addition to on_activate when the same entity is left-clicked
twice within 400 ms. Lets applications bind quick actions (e.g. rename
a tab) without blocking the normal single-click activation path.

- New Widget::on_double_click builder mirroring on_activate/on_close.
- last_click field on LocalState for timestamp tracking.
- Detection branch in the mouse handler after on_activate fires, so the
  target entity is already focused when the callback runs.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/widget/segmented_button/widget.rs

@@ -173,6 +173,10 @@ where
     pub(super) on_context: Option<Box<dyn Fn(Entity) -> Message + 'static>>,
     #[setters(skip)]
     pub(super) on_middle_press: Option<Box<dyn Fn(Entity) -> Message + 'static>>,
+    /// Emits the ID of the item that was double-clicked with the left button.
+    /// Fires in addition to `on_activate` (which keeps firing on each click).
+    #[setters(skip)]
+    pub(super) on_double_click: Option<Box<dyn Fn(Entity) -> Message + 'static>>,
     #[setters(skip)]
     pub(super) on_dnd_drop:
         Option<Box<dyn Fn(Entity, Vec<u8>, String, DndAction) -> Message + 'static>>,
@@ -232,6 +236,7 @@ where
             on_close: None,
             on_context: None,
             on_middle_press: None,
+            on_double_click: None,
             on_dnd_drop: None,
             on_dnd_enter: None,
             on_dnd_leave: None,
@@ -354,6 +359,16 @@ where
         self
     }
 
+    /// Emitted when a tab is double-clicked with the left mouse button.
+    /// Fires in addition to `on_activate`, which keeps firing on each click.
+    pub fn on_double_click<T>(mut self, on_double_click: T) -> Self
+    where
+        T: Fn(Entity) -> Message + 'static,
+    {
+        self.on_double_click = Some(Box::new(on_double_click));
+        self
+    }
+
     /// Enable drag-and-drop support for tabs using the provided payload builder.
     pub fn enable_tab_drag(mut self, mime: String) -> Self {
         self.tab_drag = Some(TabDragSource::new(mime));
@@ -912,6 +927,7 @@ where
             hovered: Default::default(),
             known_length: Default::default(),
             middle_clicked: Default::default(),
+            last_click: None,
             internal_layout: Default::default(),
             context_cursor: Point::default(),
             show_context: Default::default(),
@@ -1383,6 +1399,33 @@ where
                                 state.set_focused();
                                 state.focused_item = Item::Tab(key);
                                 state.pressed_item = None;
+
+                                // Double-click detection on the same entity
+                                // within 400 ms — fires after on_activate so
+                                // the tab is already focused when the handler
+                                // runs.
+                                if let Some(on_double_click) =
+                                    self.on_double_click.as_ref()
+                                {
+                                    let now = Instant::now();
+                                    let is_double = match state.last_click {
+                                        Some((prev, t)) => {
+                                            prev == key
+                                                && now.duration_since(t)
+                                                    < Duration::from_millis(400)
+                                        }
+                                        None => false,
+                                    };
+                                    state.last_click = if is_double {
+                                        None
+                                    } else {
+                                        Some((key, now))
+                                    };
+                                    if is_double {
+                                        shell.publish(on_double_click(key));
+                                    }
+                                }
+
                                 shell.capture_event();
                                 return;
                             }
@@ -2387,6 +2430,9 @@ pub struct LocalState {
     hovered: Item,
     /// The ID of the button that was middle-clicked, but not yet released.
     middle_clicked: Option<Item>,
+    /// Entity and timestamp of the most recent left-click activation, used
+    /// to detect double-clicks on the same tab.
+    last_click: Option<(Entity, Instant)>,
     /// Last known length of the model.
     pub(super) known_length: usize,
     /// Dimensions of internal buttons when shrinking
@@ -2532,6 +2578,7 @@ mod tests {
             hovered: Item::default(),
             known_length: 0,
             middle_clicked: None,
+            last_click: None,
             internal_layout: Vec::new(),
             context_cursor: Point::ORIGIN,
             show_context: None,