Unsolved Display QWidgets behind a QToolBar

[dublin19]

I try to create an application with a transparent QToolBar in the titlebar. This works with some modifications to the window itself by using some Objective-C. Also with setUnifiedTitleAndToolBarOnMac() it looks just like what I wanted. Now there is a problem. I want to add a QGridLayout later on. And just like in the new Photos app on iPadOS I want that the widgets go behind the toolbar. The transparent style would be probably achievable by styling the QToolBar (but this is a problem I can work on). My question is now, is there any possible way to overlap two widgets or send widgets behind any other widget? I could also work with a QVBoxLayout, but I don't know how to set some widgets behind any other widget (or layout).

What I try to achieve is the following:

My current approach is this:

I heard about stackUnder() but this does not work.

I hope I got my question clear, its my first time posting here.

Thanks!