QMdiSubWindows do not properly restore their positions using the inherited restoreGeometry() function (X11 only)

I'm trying to save the positions and states of all QMdiSubWindows living on a QMdiArea. Therefore I use the inherited saveGeometry() and restoreGeometry() functions of each mdi child window.

When restoring children formerly living close to the top of the QMdiArea (with an y-coordinate lower than about 20 (y=<20)), they are restored a few pixels below their expected position. As soon as they are saved with y>20, their sizes and positions are restored as expected. The problem does not affect the X-coordinate at all. The problem also does not occur when running the application on windows.

I seems to me, that the non-working area (y in about [0,20]) matches the height of the X11 window decoration...