bug#63187: 30.0.50; Tail of longer lines painted after end of nearby lines on macOS

[Eli Zaretskii]

> From: Aaron Jensen <aaronjensen@gmail.com>
> Date: Sat, 24 Jun 2023 10:52:08 -0400
> 
> If I recall correctly, there's some code in Emacs that optimizes which
> areas of the screen glyphs are drawn to. Maybe it remembers what was
> the background color already and doesn't clear it again... I don't
> remember where I saw it, but I think it was outside of nsterm. Does
> that ring a bell? I'm thinking about how this manifests. For me, it's
> always whole characters that are painted, they are always painted in
> what would otherwise be whitespace, and they tend to get "copied" to
> the next line in the whitespace as scrolling happens. In other words,
> it doesn't just seem like a fluke write during the transfer to VRAM.
> It seems like something in the engine is writing them, that the state
> is getting "stuck" somehow. Does any of that ring a bell/jog anything?

If you are talking about the frame's scroll_run_hook, then this is
also called by xdisp.c as part of some redisplay optimizations.