editor: Do not start scroll when hovering the scroll thumb during dragging events (#30782)

Closes #30756
Closes #30729
Follow-up to #28064

The issue arose because GPUI does still propagate mouse events to all
event handlers during dragging actions even if the dragging action does
not belong to the current handler. I forgot about this in the other PR.

This resulted in an incorrect hover being registered for the thumb,
which was sufficient to trigger scrolling in the next frame, since
`dragging_scrollbar_axis` did not consider the actual thumb state (this
was generally sufficient, but not with this incorrectly registered
hover).

Theoretically, either of the both commits would suffice for fixing the
issue. However, I think it is better to fix both issues at hand instead
of just one. Now, we will only start the scroll on actual scrollbar
clicks and not show a hover on the thumb if any other drag is currently
going on.


https://github.com/user-attachments/assets/6634ffa0-78fc-428f-99b2-7bc23a320676

Release Notes:

- Fixed an issue where editor scrollbars would start scrolling when
hovering over the thumb whilst already dragging something else.

crates/editor/src/element.rs

@@ -5538,7 +5538,9 @@ impl EditorElement {
 
                         editor.scroll_manager.show_scrollbars(window, cx);
                         cx.stop_propagation();
-                    } else if let Some((layout, axis)) = scrollbars_layout.get_hovered_axis(window)
+                    } else if let Some((layout, axis)) = scrollbars_layout
+                        .get_hovered_axis(window)
+                        .filter(|_| !event.dragging())
                     {
                         if layout
                             .thumb_bounds

crates/editor/src/scroll.rs

@@ -374,6 +374,7 @@ impl ScrollManager {
     pub fn dragging_scrollbar_axis(&self) -> Option<Axis> {
         self.active_scrollbar
             .as_ref()
+            .filter(|scrollbar| scrollbar.thumb_state == ScrollbarThumbState::Dragging)
             .map(|scrollbar| scrollbar.axis)
     }