Mutter: Areas outside of damage are being repainted with wrong data

Reproducible in GNOME Shell 3.22 and 3.24, with Qt 5.7.1, 5.8.0 and 5.9.1 Beta with the Wayland-EGL backend. Best reproducible with KCalc with the adwaita-qt widget style (or any other style animating hover and press).

When clicking the buttons in a rapid succession, when clicking one of the buttons, you can see parts of the already ended animations on the other buttons appearing briefly, making most of the previously pressed buttons flash wildly with shadows and highlights when they should remain still. This description may be confusing, therefore I've put a recording here: https://mbriza.fedorapeople.org/Screencast_01-24-2017_10:44:53%20AM.webm .

Seems the multiple buffering comes to play and the areas which were not marked as damaged get repainted too. I discussed this on #wayland and the general consensus seems to be the apps can't rely on the compositor repainting only the changed areas. However, after tests in Weston (where it works alright) it's not clear where this behavior is coming from.

I'm attaching a log from the channel in the bugreport: wayland.log A similar bug is reported against GNOME Shell here: https://bugzilla.gnome.org/show_bug.cgi?id=782381  as there still could be a problem in Mutter, not in Qt.

If you have any other questions, hit me either here or on #wayland.