QWindow::screenChanged signal not received by child window when switching parent

When using a multi-monitor window setup, we encountered an issue with the QWindow::screenChanged signal. The issue occurs when there is a parent-child window hierarchy, where window A is positioned on a secondary monitor and has a linear parent-child relationship with windows B and C (A->B->C).

The specific problem arises when window B's parent is set to nullptr, and subsequently set back to A. In this situation, window C, which has registered for the QWindow::screenChanged signal, is expected to receive the signal with the screen set to the secondary monitor, as that is where window A is initially positioned. However, no signal is received by window C.

Steps to Reproduce

• Set up a multi-monitor window setup with at least two monitors.
• Create a linear parent-child window hierarchy, where window A is positioned on a secondary monitor, and window C is a child of window B, which is a child of window A (A->B->C).
• Ensure that window C has registered for the screenChanged signal.
• Set the parent of window B to nullptr.
• Set the parent of window B back to window A.

Expected Results

After step 5, window C should have received screenChanged signal with the screen set to the secondary screen, as that is where window A is initially positioned.

Actual Results

After step 5, no screenChanged signal is received by window C. However, C's screen() API would return it as the secondary screen as expected. 

Attachments

qtbug-120928.zip is a sample test app that reproduces this issue.

Configuring the project and running the app should cause the application to assert with an error message.