Scrollbars have visual noise at the ends

[waywardmonkeys]

This is apparently a known issue, but needs resolution.

When you have a scrollbar in a portal, there's some strangeness going on with the ends where sometimes there is flickering or visual artifacts.

[DJMcNab]

Example (from #1092)

[DJMcNab]

I wonder if this is related to linebender/vello#1115.

Edit: Based on #1389, that seems unlikely.