None of the above Qt environment variables are set:

$ env | grep QT
QT_ACCESSIBILITY=1

 
The Qt version is 5.14.2 and I observed it using Xfce as DM.

Sorry, misread the ticket. Actually it was never really confirmed what was causing this and the Wayland thing was pure speculation. What I see in common with the debian bug report is that likely Qt 5.14.2 is being used in both cases.

But again, this is most likely a Qt issue. But without knowing what exactly is causing the issue and hence not being able to reproduce it makes it difficult to open an upstream bug report.

Sebastian, can you please open a new ticket for your issue, as this one was specifically about scaling issues on Wayland?

Please provide some background on used Qt version and desktop environment. Also please check the values of the above mentioned Qt environment variables (if they are set and to what value they are set).

It is more than likely that this issue needs to be reported against Qt5

This issue was now also reported for the Debian package: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=964277

I can confirm the issue without any scaling activated running under X.

Setting QT_AUTO_SCREEN_SCALE_FACTOR to 0 fixed the issue.
Thank you !

And if this is a Wayland compositor, try running Picard using XWayland instead of the native Qt Wayland backend and see if this fixes the display issues. Qt's wayland backend has quite a few issues still.

That looks either like a Qt theme issue or some issue with scaling. Do you have UI scaling activated? What desktop environment or WM are you running?

I would recommend you check if any of Qt's environment variables which control High DPI rendering are set:

QT_AUTO_SCREEN_SCALE_FACTOR
QT_SCALE_FACTOR
QT_SCREEN_SCALE_FACTORS

See also https://doc.qt.io/qt-5/highdpi.html