Fix hovering through custom menu button (#5555)

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This change discards widgets which are fully covered by another widget
in a higher layer from the hit test algorithm.

* Closes <#5498>
* [x] I have followed the instructions in the PR template

---------

Co-authored-by: Emil Ernerfeldt <[email protected]>

crates/egui/src/hit_test.rs

@@ -2,7 +2,7 @@ use ahash::HashMap;
 
 use emath::TSTransform;
 
-use crate::{ahash, emath, LayerId, Pos2, Rect, Sense, WidgetRect, WidgetRects};
+use crate::{ahash, emath, id::IdSet, LayerId, Pos2, Rect, Sense, WidgetRect, WidgetRects};
 
 /// Result of a hit-test against [`WidgetRects`].
 ///
@@ -133,6 +133,23 @@ pub fn hit_test(
         }
     }
 
+    // Find widgets which are hidden behind another widget and discard them.
+    // This is the case when a widget fully contains another widget and is on a different layer.
+    // It prevents "hovering through" widgets when there is a clickable widget behind.
+
+    let mut hidden = IdSet::default();
+    for (i, current) in close.iter().enumerate().rev() {
+        for next in &close[i + 1..] {
+            if next.interact_rect.contains_rect(current.interact_rect)
+                && current.layer_id != next.layer_id
+            {
+                hidden.insert(current.id);
+            }
+        }
+    }
+
+    close.retain(|c| !hidden.contains(&c.id));
+
     let mut hits = hit_test_on_close(&close, pos);
 
     hits.contains_pointer = close