Fix: ScrollArea

[rustbasic]

This is needed to resolve the following explanation, which cannot be resolved by #4754.

            // Move the scrollbar so it is visible. This is needed in some cases.
            // For instance:
            // * When we have a vertical-only scroll area in a top level panel,
            //   and that panel is not wide enough for the contents.
            // * When one ScrollArea is nested inside another, and the outer
            //   is scrolled so that the scroll-bars of the inner ScrollArea (us)
            //   is outside the clip rectangle.
            // Really this should use the tighter clip_rect that ignores clip_rect_margin, but we don't store that.
            // clip_rect_margin is quite a hack. It would be nice to get rid of it.

[emilk]

I feel we need to add some proper tests for ScrollArea, or we will just regress old bugs.

Also: please explain what a PR is supposed to fix.