Scaling issue with Qt6 apps under Debian Testing

[danboid]

Expected behaviour

I want qtractor and qjackctl to launch with non-scaled GUIs.

Actual behaviour

The GUI of qtractor and qjackctl is always scaled 2X. This causes drawing issues from qtractors MIDI editor.

Steps to reproduce the behaviour

First, I checked MATE wasn't in HiDPI mode:

MATE tweak -> Windows -> HiDPI set to Regular

Apparently these settings are relevant too:

dan@zbook:~$ gsettings get org.mate.interface window-scaling-factor-qt-sync
false
dan@zbook:~$ gsettings get org.mate.interface window-scaling-factor 
1

Then, for extra belts and braces, I've even tried launching qtractor and qjackctl from the terminal like:

QT_SCALE_FACTOR=1 /usr/bin/qtractor

but even then, it still gets launched scaled. I do not have this problem running these apps under XFCE on the same system.

MATE general version

1.26

Package version

mate-session-manager 1.26.0-1

Linux Distribution

Debian Testing

Link to bugreport of your Distribution (requirement)

[lukefromdc]

Since this only occurs in MATE, mate-session-manager's way of talking to QT5 might not work with QT6 yet. I don't have QT6 or any QT6 apps installed yet. Can't use it for Kdenlive (though Kdenlive now optionally supports it) unless and until it gets something ike QT5ctl so I can force it to use the GTK2 theme