Overlay completely repaints changing surfaces
When opening the overview, at least on Wayland Mutter damages the surfaces of all shown applications constantly, making them repaint all their content. I noticed this when working on !323 (merged), where it turned out to be a great help during debugging surface damage, but forgot to open an issue about it. I believe this is partly responsible for the sluggishness many people observe when opening the overview.
As the painting will get heavily overhauled in the ongoing clutter content work (!409 (merged), https://gitlab.gnome.org/GNOME/mutter/commits/gbsneto/content-part2, https://gitlab.gnome.org/GNOME/gnome-shell/commits/gbsneto/more-content), there is a great opportunity to fix this on the way.
Edit: I just checked more precisely using CLUTTER_PAINT=damage-region
. The issue only affects updating windows, which then get repainted completely. I still need to double check if only the paint is affected or if the surface gets damaged as well and we trigger a reupload of the texture. If it's only a repaint, then this issue might not be such a great deal.
To reproduce, start a session with CLUTTER_PAINT=damage-region
and then run weston-simple-damage