QMdiSubWindow does not respect size constraints

Setting a minimum size to a widget inside QMdiSubwidget is not respected by the layout manager. This leads to a missing scrollbar when the mdi window is shown in fullscreen
Original bug report is from the forum: https://forum.qt.io/topic/134094/bug-of-qmdiarea-showing-scrollbar

When a subwidget has a minimum size, then the top level widget has a proper minimum size set. This is not the case for a QMdiSubWidget because it's not a topLevel widget as you can see in the testcase. Therefore it's possible to set a smaller size than the (expected) minimum size with e.g. resize() or setGeometry().