How to properly set QMainWindow fullscreen across dual monitors when monitors aligned bottom and left monitor has smaller resolution?

Question

My application is using Qt5.14 single QMainWIndow. I am on X11 Debian based Linux. I need to make App window fullscreen but multimonitors must be supported and fullscreen means cover all monitors and Tint2 bar.

I use 2 lines of code to make app Fullscreen:

move(0,0);
showFullScreen();

And this works perfectly but not all always. It does not work if coord [0,0] is not visible in dualscreen. For example when left monitor has smaller resolution and aligned bottom. Or it does not work when Above monitor has smaller resolution and aligned right.

This picture explains what I get:

I tried many different ways but without any success. How to make it properly? There many discussions in internet about dual monitor fullscreen Qt5 app but I cannot find solution yet.