Comment 86 for bug 1953685

So when triggering the race (i.e. without the patch from above and after trying a couple of times) I can manage to get Firefox into all kinds of odd vsync states. In a setup with one 60Hz and on 120Hz display, testufo.com (less heavy than vsynctester) sometimes gives 90FPS, sometimes ~108, sometimes 60 (on the 120Hz display).
Trying with a build with the patch applied (i.e. ~nightly from tomorrow), I reliably get the right refresh rates*. Also setting `widget.wayland.vsync.enabled:false` does not trigger the bug again, but instead gives me solid 60FPS.
So without digging deeper I assume what we see here something that can only happen in a mixed state where some components use `WaylandVsyncSource` and others `SoftwareVsyncSource`. And the patch should avoid that.
I also looked into `VsyncParent` again but couldn't find anything really wrong. Thus declaring this fixed once the patch lands.

*: note: this is with https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2169 applied to not get wrong frame callbacks from Mutter.