* [x] I have followed the instructions in the PR template
* Closes https://github.com/emilk/egui/issues/3734
## Info
This PR addresses an issue where resizing a scroll handle can lead to
unwanted overlap.
It is also happening on the egui-demo.
## Cause
*Note: The following explanation assumes a vertical scroll; however, the
logic applies equally to horizontal scrolling.*
When the scroll handle is positioned at the top or bottom of the scroll
area and the handle is resized to fit the minimum handle size, there is
a risk of overlap. This occurs if the handle’s new size extends beyond
the bounds of the scroll area.
## Proposed Solution
1. Check whether increasing the handle size will cause it to overlap
with the scroll area.
2. If an overlap is detected, adjust the handle’s center position by the
overlap amount, moving it towards the center of the scroll area.
1dea8fac9b