As long as we don't support multiple window scenes our primary scene with role UIWindowSceneSessionRoleApplication is the one where you can effectively place new windows, so it should be the primary screen. This handles the case where the primary scene is placed on an external screen at startup, as well as when the primary scene is moved between screens. In the former case we also make sure to move windows on to this screen, as they may have been created when the primary screen was the built in display, where we don't have an associated window scene showing. Fixes: QTBUG-137544 Pick-to: 6.10 6.9 6.8 Change-Id: Iae87479996419ea315cece5beb88de5ccf7089ba Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>