Have non-resizable Areas ignore mouse input just beyond their bounds (#3039)

As described in #576, the method `Memory::layer_id_at` expands the
hit-testing region for every `Area` slightly. This is necessary so that,
when the user clicks to resize a `Window`, the mouse input isn't routed
to a different `Window`.

For non-resizable `Area`s (such as dropdown menus, context menus, date
pickers, and any non-resizable `Window`), this causes them to be
surrounded by a "dead zone" where any underlying widgets can't be
hovered or clicked. The effect is particularly noticeable in menu bars.

This commit adds a persisted `edges_padded_for_resize` property to
`Area`, which is `true` when the `Area` belongs to a resizable `Window`
and `false` for all other `Area`s. The hit-testing region is only
expanded when this property is `true`.

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/egui/src/containers/area.rs

@@ -20,6 +20,10 @@ pub(crate) struct State {
     /// If false, clicks goes straight through to what is behind us.
     /// Good for tooltips etc.
     pub interactable: bool,
+
+    /// When `true`, this `Area` belongs to a resizable window, so it needs to
+    /// receive mouse input which occurs a short distance beyond its bounding rect.
+    pub edges_padded_for_resize: bool,
 }
 
 impl State {
@@ -71,6 +75,7 @@ pub struct Area {
     pivot: Align2,
     anchor: Option<(Align2, Vec2)>,
     new_pos: Option<Pos2>,
+    edges_padded_for_resize: bool,
 }
 
 impl Area {
@@ -87,6 +92,7 @@ impl Area {
             new_pos: None,
             pivot: Align2::LEFT_TOP,
             anchor: None,
+            edges_padded_for_resize: false,
         }
     }
 
@@ -217,6 +223,14 @@ impl Area {
             Align2::LEFT_TOP
         }
     }
+
+    /// When `true`, this `Area` belongs to a resizable window, so it needs to
+    /// receive mouse input which occurs a short distance beyond its bounding rect.
+    #[inline]
+    pub(crate) fn edges_padded_for_resize(mut self, edges_padded_for_resize: bool) -> Self {
+        self.edges_padded_for_resize = edges_padded_for_resize;
+        self
+    }
 }
 
 pub(crate) struct Prepared {
@@ -261,6 +275,7 @@ impl Area {
             anchor,
             constrain,
             constrain_rect,
+            edges_padded_for_resize,
         } = self;
 
         let layer_id = LayerId::new(order, id);
@@ -281,9 +296,11 @@ impl Area {
             pivot,
             size: Vec2::ZERO,
             interactable,
+            edges_padded_for_resize,
         });
         state.pivot_pos = new_pos.unwrap_or(state.pivot_pos);
         state.interactable = interactable;
+        state.edges_padded_for_resize = edges_padded_for_resize;
 
         if let Some((anchor, offset)) = anchor {
             let screen = ctx.available_rect();

crates/egui/src/containers/window.rs

@@ -48,7 +48,9 @@ impl<'open> Window<'open> {
     /// If you need a changing title, you must call `window.id(…)` with a fixed id.
     pub fn new(title: impl Into<WidgetText>) -> Self {
         let title = title.into().fallback_text_style(TextStyle::Heading);
-        let area = Area::new(Id::new(title.text())).constrain(true);
+        let area = Area::new(Id::new(title.text()))
+            .constrain(true)
+            .edges_padded_for_resize(true);
         Self {
             title,
             open: None,
@@ -117,6 +119,9 @@ impl<'open> Window<'open> {
     #[inline]
     pub fn resize(mut self, mutate: impl Fn(Resize) -> Resize) -> Self {
         self.resize = mutate(self.resize);
+        self.area = self
+            .area
+            .edges_padded_for_resize(self.resize.is_resizable());
         self
     }
 
@@ -273,6 +278,7 @@ impl<'open> Window<'open> {
     #[inline]
     pub fn fixed_size(mut self, size: impl Into<Vec2>) -> Self {
         self.resize = self.resize.fixed_size(size);
+        self.area = self.area.edges_padded_for_resize(false);
         self
     }
 
@@ -294,6 +300,7 @@ impl<'open> Window<'open> {
     #[inline]
     pub fn resizable(mut self, resizable: bool) -> Self {
         self.resize = self.resize.resizable(resizable);
+        self.area = self.area.edges_padded_for_resize(resizable);
         self
     }
 
@@ -319,6 +326,7 @@ impl<'open> Window<'open> {
     pub fn auto_sized(mut self) -> Self {
         self.resize = self.resize.auto_sized();
         self.scroll = ScrollArea::neither();
+        self.area = self.area.edges_padded_for_resize(false);
         self
     }
 

crates/egui/src/context.rs

@@ -317,6 +317,7 @@ impl ContextImpl {
                 pivot: Align2::LEFT_TOP,
                 size: screen_rect.size(),
                 interactable: true,
+                edges_padded_for_resize: false,
             },
         );
 

crates/egui/src/memory.rs

@@ -862,8 +862,11 @@ impl Areas {
                 if let Some(state) = self.areas.get(&layer.id) {
                     let mut rect = state.rect();
                     if state.interactable {
-                        // Allow us to resize by dragging just outside the window:
-                        rect = rect.expand(resize_interact_radius_side);
+                        if state.edges_padded_for_resize {
+                            // Allow us to resize by dragging just outside the window:
+                            rect = rect.expand(resize_interact_radius_side);
+                        }
+
                         if rect.contains(pos) {
                             return Some(*layer);
                         }