Improved Behavior of Nested ScrollArea

[rustbasic]

Issue: When a ScrollArea is nested with both Horizontal and Vertical scrolling, the outer_rect can become very large due to the content, or when the content is moved. This can cause some issues.

Improvement:
There's a part that handles this with a comment, which we've simplified and made to work better.

• Related #4658
• Related #4754
• Related #4770

We could modify clip_max as follows, but we've omitted it since the ScrollArea's position changes.

let clip_max = ui.clip_rect().max[1 - d] - ui.style().spacing.item_spacing[1 - d];

[emilk]

As I said in https://github.com/emilk/egui/pull/4770 and https://github.com/emilk/egui/pull/4658:

We need a proper test for this, otherwise we will just go around in circles. ScrollArea is getting way to complex.

The test could be a manual one (in https://github.com/emilk/egui/blob/master/tests/) for now.

[rustbasic]

As I said in #4770 and #4658:

We need a proper test for this, otherwise we will just go around in circles. ScrollArea is getting way to complex.

The test could be a manual one (in https://github.com/emilk/egui/blob/master/tests/) for now.

This is as simplified as possible. I tested it for a long time without any problems. Please review.

[rustbasic]

completed.