fix(dock): keep auto-hiding panels shown while the pointer is on their dropdowns

iced gives the layer under an overlay a levitating cursor, so the dock
edge's mouse_area read the pointer on any dropdown or combo list in a
pallet as leaving, and auto-hide collapsed the panel mid-pick. Track
enter/leave with a small hover_zone wrapper that lands the cursor before
its bounds test, on docked edges and floating pallets alike.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_014ihuqzxdUhd9UbioPn5MGu

src/ui/hover_zone.rs

@@ -0,0 +1,175 @@
+//! A pointer-enter / leave area that keeps counting the pointer as inside
+//! while it rests on an overlay (a dropdown, a combo box list) drawn over
+//! its bounds.
+//!
+//! `mouse_area` reads such a pointer as gone: iced hands the layer below an
+//! overlay a levitating cursor, which has no position. An auto-hiding dock
+//! edge then hides as soon as one of its own dropdowns opens under the
+//! pointer, taking the dropdown with it. Here the cursor is landed before the
+//! bounds test, so only a pointer that really leaves the area exits it.
+
+use iced::advanced::layout::{self, Layout};
+use iced::advanced::widget::{self, tree, Tree, Widget};
+use iced::advanced::{mouse, overlay, renderer, Shell};
+use iced::{Element, Event, Length, Rectangle, Renderer, Size, Theme, Vector};
+
+use crate::app::Message;
+
+/// Wrap `content`, publishing `on_enter` / `on_exit` as the pointer enters
+/// and leaves its bounds, overlays drawn over them included.
+pub fn hover_zone<'a>(
+    content: impl Into<Element<'a, Message>>,
+    on_enter: Message,
+    on_exit: Message,
+) -> Element<'a, Message> {
+    Element::new(HoverZone {
+        content: content.into(),
+        on_enter,
+        on_exit,
+    })
+}
+
+struct HoverZone<'a> {
+    content: Element<'a, Message>,
+    on_enter: Message,
+    on_exit: Message,
+}
+
+#[derive(Default)]
+struct State {
+    hovered: bool,
+}
+
+impl Widget<Message, Theme, Renderer> for HoverZone<'_> {
+    fn tag(&self) -> tree::Tag {
+        tree::Tag::of::<State>()
+    }
+
+    fn state(&self) -> tree::State {
+        tree::State::new(State::default())
+    }
+
+    fn children(&self) -> Vec<Tree> {
+        vec![Tree::new(&self.content)]
+    }
+
+    fn diff(&mut self, tree: &mut Tree) {
+        tree.diff_children(std::slice::from_mut(&mut self.content));
+    }
+
+    fn size(&self) -> Size<Length> {
+        self.content.as_widget().size()
+    }
+
+    fn layout(
+        &mut self,
+        tree: &mut Tree,
+        renderer: &Renderer,
+        limits: &layout::Limits,
+    ) -> layout::Node {
+        self.content
+            .as_widget_mut()
+            .layout(&mut tree.children[0], renderer, limits)
+    }
+
+    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, renderer, operation);
+    }
+
+    fn update(
+        &mut self,
+        tree: &mut Tree,
+        event: &Event,
+        layout: Layout<'_>,
+        cursor: mouse::Cursor,
+        renderer: &Renderer,
+        shell: &mut Shell<'_, Message>,
+        viewport: &Rectangle,
+    ) {
+        self.content.as_widget_mut().update(
+            &mut tree.children[0],
+            event,
+            layout,
+            cursor,
+            renderer,
+            shell,
+            viewport,
+        );
+        // Unlike `mouse_area`, this runs for captured events too: a pointer
+        // move over a dropdown is captured by the dropdown but must still
+        // not read as leaving.
+        let hovered = cursor.land().is_over(layout.bounds());
+        let state = tree.state.downcast_mut::<State>();
+        if hovered != state.hovered {
+            state.hovered = hovered;
+            shell.publish(if hovered {
+                self.on_enter.clone()
+            } else {
+                self.on_exit.clone()
+            });
+        }
+    }
+
+    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,
+            cursor,
+            viewport,
+            renderer,
+        )
+    }
+
+    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,
+            cursor,
+            viewport,
+        );
+    }
+
+    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>> {
+        self.content.as_widget_mut().overlay(
+            &mut tree.children[0],
+            layout,
+            renderer,
+            viewport,
+            translation,
+        )
+    }
+}

src/ui/mod.rs

@@ -5,6 +5,7 @@ pub const ROW_H: f32 = 26.0;
 pub mod color_select;
 pub mod command_line;
 pub mod dock;
+pub mod hover_zone;
 pub mod icons;
 pub mod menu_bar;
 pub mod modal;