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