The scrolling list in the UI example doesn't handle resizing correctly

## Bevy version

main [0174d63](https://github.com/bevyengine/bevy/commit/0174d632a56964494a5edfaf3612b8dc0b2c9a44)

## What you did

1. cargo run --example ui
2. Resize the window vertically so that there isn't room for the entire list in the box.
3. Scroll the list down to the last item, as far as it will go.
3. Resize the window vertically again so that there is room for more list elements.

## What went wrong

The list doesn't move down to fill the available space.