Warnings and flicker when moving windows between screens on Mac

Move a QQuickWindow from one screen to another. When one is retina, the other is not, the result is a warning like

QSGThreadedRenderLoop: expose event received for window with invalid geometry.

and an ugly moment of flickering where the scaled/non-scaled content (i.e. the content suitable for the old screen) is visible for a moment. This is especially visible for stuff that is animating. Non-animating apps feature a less-visible white flicker.