Status update:

• on macOS, it works
• on Linux, I can't check but I think it works
• on Windows, CI builds (almost) work, but local builds not, because they can't find Qt DLLs.
  • There are two solutions:
    • letting everyone add Qt library folders to their PATH (for normal targets, Qt Creator does this automatically, but not for "Custom executable" targets; and we have to use the latter, because we always need to use the executable from the install folder)
    • making windeployqt part of the CMake install step (might take long-ish)
      I'm not sure yet which one I'll choose.
• we finally have VTests to look at!
  • I see no major disasters at a first glance
  • It looks like they refined the BBox calculation for text, seemingly for the better. This causes some texts to land on a different pixel.
    • At lyrics-9-1, you can see that the bbox is now less overestimated.
    • It is beneficial for center/right alignment, see frametext-1.
    • Sometimes, this causes slightly more drastic layout changes, because now some items suddenly fit 'before' something else instead of trying to avoid it by moving up vertically. See for example musejazz-10-1.
  • There are some disappearing texts though:
    • the 'full' text on (old?) bends (bend-1-1)
    • the fret number for fretboard diagrams (harmony-align-1)
    • (in lyrics-11-1, the gliss text appears, because with the new bbox calculation there is now enough space)
• And then we have the fact that the version of KDDockWidgets that we use is not really compatible with Qt 6.5+, see Qt 6.5+: docks do not open after the last dock on a panel is closed #24866
• And we'll doubtlessly find more bugs. I already found two:
  • on Windows, the text on the Spash Screen seems to be partially missing
  • the positioning on those big icon+text buttons looks off
    

Next thing I'm going to try is what happens when we switch to DirectWrite.

I noticed that the nightlies (at least on Linux) are still compiled with Qt 6.2.4, even after this has been merged. Is this intended? Do you have plans to change this so that Linux releases with Qt 6.9 built into the AppImage? What would be the downsides of doing so?

It's intentional that we're still using 6.2 for nightly builds, because there are still some important bugs to fix with 6.9. However, the plan is to do that in time for the next release. But we found it too scary to make the switch definitively now.