ui: Change scrollbar hitbox insertion (#40008)

Closes #39974

Since the thumb hitboxes themselves do not propagate events, we need to
paint the normal parent hitbox on top of the other ones. This also
caused hover detection to fail, which caused the issue linked.

Release Notes:

- Fixed an issue where hovering scrollbars in hovers would dismiss
these.

crates/ui/src/components/scrollbar.rs

@@ -1102,7 +1102,6 @@ impl<T: ScrollableHandle> Element for ScrollbarElement<T> {
             .disabled()
             .not()
             .then(|| ScrollbarPrepaintState {
-                parent_bounds_hitbox: window.insert_hitbox(bounds, HitboxBehavior::Normal),
                 thumbs: {
                     let thumb_ranges = self.state.read(cx).thumb_ranges().collect::<Vec<_>>();
                     let width = self.state.read(cx).width.to_pixels();
@@ -1178,6 +1177,7 @@ impl<T: ScrollableHandle> Element for ScrollbarElement<T> {
                         })
                         .collect()
                 },
+                parent_bounds_hitbox: window.insert_hitbox(bounds, HitboxBehavior::Normal),
             });
         if prepaint_state
             .as_ref()