I'm not that fluent in python, but

layout_final.addWidget(self.widgetGL) layout_final.addWidget(self.widget)

Horizontal layouts add stuff left to right, so, if anywhere, the GL widget would be on the left, not on the right.

I don't think a GL widget has any minimum size. It might be that the other widgets have "greedy" horizontal size policies (I think a label does by default). Try setting a minimum width or an expanding horizontal size policy on the GL widget.

I think I was finally able to solve the problem.

As the solution suggested by @hskoglund is Windows-specific, I decided to first investigate a bit deeper the other options presented by @J-Hilk

It turned out that the Qt::WindowStaysOnTopHint flag was not set for our main window. I am now setting that flag in our main windows's constructor, and afterwards show it full screen. This seems to work.

Thanks again for your suggestions!