I have applied the Kvantum Layan theme and for some reason when I have a program on full screen, the transparency for something open on top no longer works. Let me demonstrate:
With both windows being in windowed mode transparency works fine:
Theming the … out of a system usually causes more trouble than one like.
You are displaying the diffs between a GTK and Qt app. Accept the difference.
Yes I know - theming is a matter of taste - I won’t argue on that.
The best result - long term - personal experience - comes from using the defaults for the platform and use the default theme.
You can use qt5ct and qt6ct to smoothen the differences between gtk and qt. Using kvantum - my personal opinion - is making things more complicated - again - very personal.