hello anda_skoa, thank you for your answer.

The app should run on windows and linux.

The new widget needs to be a child of the window it should over, just not be in any of its layouts.
I want to grey out the main window. Following the proposed approach, my dialog needs a pointer to his "backgrounder-widget", which should be a child of a main window. Thus, I need to propagate this pointer from the main window to the dialog widget and call show()/hide() on the pointer when the dialog opens/closes. There are a lot of intermideate widgets between the main window and the dialog, each of them has to take over this pointer to one of its children. Is there are some shortcut how to make it easier?