Steps to reproduce:

1. Open any Qt gui app, place a breakpoint in QWindowSystemInterface::handlePrimaryScreenChanged.
2. Open display settings, change main display (you need at least two monitors for this to work).
3. Breakpoint is never hit (and thus QGuiApplication:: primaryScreen is not changed, and QGuiApplication:: primaryScreenChanged is not emitted).

I'm testing this on Win10 19041.928.

I did some digging and it looks like the right place to call QWindowSystemInterface::handlePrimaryScreenChanged would be from inside the QWindowsScreen::handleChanges. I've attached a patch.