Z-index and rendering bugs with native MDI subwindows and "Stay on top"

In an MDI area, if there is a native sub-window and a sub-window set to "Stay on top", then visual glitches and z-order issues can occur when sub-windows are moved over each other.

In particular, making a non-native sub-window "stay on top" and moving the native sub-window over a different non-native sub-window causes parts of the moved window to render incorrectly, and clicking on the moved window above the non-native window instead clicks on the non-native window below.

This behaviour occurs on Windows; I have also tested with Ubuntu, where the z-order bug is present but without any visual glitches.

Attached is a Python script using PyQt5 to demonstrate the issue, along with a video of the behaviour (the window is rendered correctly upon taking a screenshot, so none of the behaviour can be demonstrated that way).