Fix Sense::drag detecting drags when clicking widget above it (#8396)

Co-authored-by: Lucas Meurer <lucas@rerun.io>
Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

crates/egui/src/interaction.rs

@@ -198,17 +198,22 @@ pub(crate) fn interact(
                 // When the mouse first is pressed, it could be either,
                 // so we postpone the decision until we know.
                 //
-                // …unless a click is no longer possible at all: a click has to be
-                // released on the widget, and `hits.click` tells us whether the
-                // pointer is still somewhere a release would land on this widget.
-                // Note that this is not the same as being inside `interact_rect`:
-                // the hit-test also picks up widgets within `interact_radius`, and
-                // lets a widget on top take the hit.
+                // …unless the pointer has left the widget: a click has to be
+                // released on the widget, so once the pointer is outside there is
+                // nothing left to wait for.
                 //
                 // Deciding here means a thin drag handle (narrower than
                 // `max_click_dist`) doesn't spend the decision window as neither
                 // hovered nor dragged, which would make its highlight blink out.
-                let could_still_be_clicked = hits.click.is_some_and(|hit| hit.id == widget.id);
+                // The hit-test picks up widgets within `interact_radius`, so
+                // `hits.click` can name such a handle even when the pointer is a
+                // few points outside it.
+                //
+                // A widget on top might "steal" the click hit, but then the pointer is still inside
+                // us, and pressing that button must not start a drag. So we check both.
+                let pointer_is_inside = hits.contains_pointer.iter().any(|w| w.id == widget.id);
+                let could_still_be_clicked =
+                    pointer_is_inside || hits.click.is_some_and(|hit| hit.id == widget.id);
                 input.pointer.is_decidedly_dragging() || !could_still_be_clicked
             } else {
                 // This widget is just sensitive to drags, so we can mark it as dragged right away:

tests/egui_tests/tests/test_click_or_drag.rs

@@ -185,3 +185,50 @@ fn click_inside_a_widget_still_clicks() {
         "press and release without moving should be a click"
     );
 }
+
+/// A button inside a draggable row takes the click hit, because it is on top.
+/// The pointer is still inside the row though, so the press must stay undecided —
+/// otherwise the row starts dragging the moment the user touches the button.
+#[test]
+fn press_on_a_button_inside_a_draggable_row_stays_undecided() {
+    let button_id = Id::new("button");
+    let button_size = Vec2::new(50.0, 20.0);
+
+    let mut harness = Harness::builder()
+        .with_step_dt(1.0 / 60.0)
+        .with_size(Vec2::new(300.0, 200.0))
+        .build_ui(move |ui| {
+            let row_rect = ui.max_rect();
+            ui.interact(row_rect, widget_id(), Sense::click_and_drag());
+
+            // Allocated after the row, so it ends up _on top_ of it.
+            let button_rect = Rect::from_min_size(row_rect.min, button_size);
+            ui.interact(button_rect, button_id, Sense::click());
+        });
+    harness.step();
+
+    let grab = Rect::from_min_size(widget_rect(&harness).min, button_size).center();
+    press_at(&mut harness, grab);
+
+    let (_hovered, dragged) = widget_state(&harness);
+    assert!(
+        !dragged,
+        "pressing a button inside the row must not start dragging the row"
+    );
+
+    harness.event(egui::Event::PointerButton {
+        pos: grab,
+        button: egui::PointerButton::Primary,
+        pressed: false,
+        modifiers: egui::Modifiers::NONE,
+    });
+    harness.step();
+
+    assert!(
+        harness
+            .ctx
+            .read_response(button_id)
+            .is_some_and(|r| r.clicked()),
+        "the button should have been clicked"
+    );
+}