[Joel_Mckay]

The theme overrides were not working, so the issue is likely in how the Qt5 template layer was built/integrated in the universe MATE build (Qt6ct seems unaffected, but will not help with code built on Qt5). i.e. the applications may not crash, but will look garbled and exhibit undefined GUI behaviors on legacy desktops. Overriding the themes and other documented workarounds will only suppress standard warnings, but have not had any impact on the core Qt5 issues.

For an LTS OS intended to be around for 12 years, I am not sure things have turned out well in user space and Qt5.

Hopefully, this saves someone a few hours of chasing this external issue in their own projects. =3

[anthk]

Uhm, I use Mate under Trisquel 11 (bound to Ubuntu 22 releases, but libre), it's the default desktop DE and I have no issues with neither qt5ct nor GTK theme overriding, which is more apt for Gnome3/OSTree based desktops than normal setups.

[Joel_Mckay]

This issue appeared in Canonical's Ubuntu 24.04.1 Noble Numbat MATE Desktop iso and universe repository meta packages ubuntu-mate-desktop and mate-desktop .

The meta package ubuntucinnamon-desktop does not seem to impact the Qt5 Apps/installation in a detrimental way.

If I could offer any advice about this mystery issue... probably wiser to defer upgrading for a bit =3