lp1850729: Disable QtWayland in desktop launcher

[uklotzde]

Workaround for https://bugs.launchpad.net/mixxx/+bug/1850729

Mixxx has massive usability issues and a CPU utilization leak on QtWayland that is now used as the default in Fedora 31 on Wayland if QT_QPA_PLATFORM is unset. By explicitly setting it to "xcb" Qt uses XWayland as a fallback.

[daschuer]

I wonder if this does not harm at all distros. Do we need to install libx11-xcb1 for that?

[Be-ing]

I don't think so. Fedora patched Qt to make this change: https://fedoraproject.org/wiki/Changes/Qt_Wayland_By_Default_On_Gnome

[uklotzde]

The change should work on all distros and only makes a formerly implicit assumption explicit.

My first attempt to patch the launcher failed, because I got confused by the redundant occurrences. New builds with an effective patch are available in the testing repositories. I will remove the patch with the release of 2.2.3.

http://koji.rpmfusion.org/koji/packageinfo?packageID=148

[Holzhaus]

LGTM. Do you mind adding some info to the Wiki's troubleshooting page? Not everybody launches Mixxx via the *.desktop entry (e.g. users of Sway WM might just run the mixxx executable via dmenu/rofi/bemenu)

[uklotzde]

Done: https://mixxx.org/wiki/doku.php/troubleshooting#mixxx_on_wayland

[Holzhaus]

Merge?

[daschuer]

Yes. I have just tested it on Ubuntu Xenial and it still works.
Thank you. LGTM

[Be-ing]

I'm okay with merging this for the stable branch, but I think we should try to get Mixxx working with the Qt Wayland platform plugin for 2.3. That may require risky changes that wouldn't be appropriate for the stable branch.

[daschuer]

We should not add more requirements, that prevents releasing 2.3. IMHO it is overdue since long.

[Be-ing]

Yes, if we don't fix it by the time we release 2.3 that is okay. But I think we should try to fix it soon.