I use the saveGeometry/restoreGeometry functions of the QMdiSubWindow.
Unfortunately there are some issues around the usage of the save/restore feature combined with QMdiSubWindow and should be reviewed from Qt-Development again.
One defect happens in a scenario where two QMdiSubWindow are arranged with the tile (or cascade) pattern of the QMdiArea. If the MainWindow is placed on a second screen then the saveGeometry/restoreGeometry fail and both QMdiSubWindow are not displayed within the visible region of the QMdiArea.
I suppose that the computation of the widgets positions is not correct. Please see below the output of the widget attributes before and after restorGeometry:
QMdiSubWindow(137010 )(visible: true): 0, 0, 289, 572
QMdiSubWindow(2561548)(visible: true): 289, 0, 288, 572
QMdiSubWindow(137010 )(visible: true): -289, 0, 289, 572
QMdiSubWindow(2561548)(visbile: true): -288, 0, 288, 572