Hmm, right thanks. That was mostly meant to fix the tests, but I see now that it breaks things. Still not clear to me why or how.
It's because the win32u side does things the Windows way, and assumes that non-DPI aware windows will be scaled by the driver.
If the driver doesn't actually scale the bits, it means window coordinates between win32u and x11drv get out of sync. So we have to force per-monitor awareness to disable coordinates scaling on the win32u side.