Fix scroll handle overlap

[gilbertoalexsantos]

• I have followed the instructions in the PR template
• Closes ScrollArea's scrollbar goes past frame's borders #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.

[github-actions]

Preview available at https://egui-pr-preview.github.io/pr/5286-fix-scroll-handle-overlap
Note that it might take a couple seconds for the update to show up after the preview_build workflow has completed.

[lucasmerlin]

While this is a improvement, now it will look like you're at the top while already scrolled done by some amount:

Bildschirmaufnahme.2024-10-20.um.17.22.48.mov

Ideally, the scrollbar position would just be calculated correctly, taking the min size into account.

[gilbertoalexsantos]

@lucasmerlin

Oh, you're totally right. I didn't notice this behaviour 🥲

I'll work on a fix for this ☺️