gpui: Fix Interactivity prepaint to update scroll_handle bounds (#35013)

It took a long time to check this problem.

Finally, I found that due to a detail missing when changing #34832, the
bounds of `ScrollHandle` was not updated in the Interactivity `prepaint`
phase.

```diff
- scroll_handle_state.padded_content_size = padded_content_size;
+ scroll_handle_state.max_offset = scroll_max;
```

It was correct before the change, because the `padded_content_size`
(including `bounds.size`) was saved before, and the bounds was missing
after changing to `max_offset`, but the bounds were not updated
anywhere.

So when `scroll_handle.bounds()` is obtained outside, it is always 0px
here.

@MrSubidubi

Release Notes:

- N/A

crates/gpui/src/elements/div.rs

@@ -1334,7 +1334,6 @@ impl Element for Div {
         } else if let Some(scroll_handle) = self.interactivity.tracked_scroll_handle.as_ref() {
             let mut state = scroll_handle.0.borrow_mut();
             state.child_bounds = Vec::with_capacity(request_layout.child_layout_ids.len());
-            state.bounds = bounds;
             for child_layout_id in &request_layout.child_layout_ids {
                 let child_bounds = window.layout_bounds(*child_layout_id);
                 child_min = child_min.min(&child_bounds.origin);
@@ -1706,6 +1705,7 @@ impl Interactivity {
 
             if let Some(mut scroll_handle_state) = tracked_scroll_handle {
                 scroll_handle_state.max_offset = scroll_max;
+                scroll_handle_state.bounds = bounds;
             }
 
             *scroll_offset
@@ -3007,11 +3007,6 @@ impl ScrollHandle {
         self.0.borrow().bounds
     }
 
-    /// Set the bounds into which this child is painted
-    pub(super) fn set_bounds(&self, bounds: Bounds<Pixels>) {
-        self.0.borrow_mut().bounds = bounds;
-    }
-
     /// Get the bounds for a specific child.
     pub fn bounds_for_item(&self, ix: usize) -> Option<Bounds<Pixels>> {
         self.0.borrow().child_bounds.get(ix).cloned()

crates/gpui/src/elements/uniform_list.rs

@@ -295,9 +295,8 @@ impl Element for UniformList {
                     bounds.bottom_right() - point(border.right + padding.right, border.bottom),
                 );
 
-                let y_flipped = if let Some(scroll_handle) = self.scroll_handle.as_mut() {
-                    let mut scroll_state = scroll_handle.0.borrow_mut();
-                    scroll_state.base_handle.set_bounds(bounds);
+                let y_flipped = if let Some(scroll_handle) = &self.scroll_handle {
+                    let scroll_state = scroll_handle.0.borrow();
                     scroll_state.y_flipped
                 } else {
                     false