the widget of the overlapped window is rendered in the window that overlays it

I have three windows. Window1 is at the bottom and contains a QTreeWidget with its header. Then I have window2 which partially overlaps this header and window3 which is on top. The content of the QTreeWidget is periodically updated / redrawn every 300 ms. If I move window3, the header from window1 comes into view. The whole width of the widget is rendered, even the part that is below window2.

i have tried setting -graphicssystem [opengl, vulkan, software, d3d11] but no fix