Note it only looks fine as long as you're not using fractional
scaling. Once framebuffer/fractional scaling is enabled in Mutter, it
looks absolutely terrible, because X11 applications are not scaled
properly. There are also other glitches if Qt5 applications run as X11
apps, and I'm not sure if some prominent Qt5 applications expose the
correct functionality in X11 mode (e.g. OBS Studio uses PipeWire for
screencasting when it is in Wayland mode).
In general, I'd like to see if we can figure out *why* Mutter isn't
doing the right thing for Qt applications and why they're breaking
this way, because the fact that they're fine in Plasma Wayland and not
on GNOME Wayland points to Mutter needing work here.
That doesn't make any more logical sense than "the fact that GTK applications work fine with Mutter points to Qt needing work here." Without a diagnosis, it's impossible to say.
If we can put someone who understands Qt together with someone who understands Mutter it should be possible to figure out what is going on. If the fix is hard, then reverting the Qt change until F35 might be needed, but patching a reversion to X into Qt apps one-by-one doesn't seem like something we'd want to do: it would leave unpatched apps affected, we might forget to remove some of the patches, and it's going to make things altogether more confusing.
Owen