QDockWidget: When a dockwidget which was tabbed is undocked or it is docked on its own in the mainwindow then it still leaves space for the tabbar

QDockWidget: When a dockwidget which was tabbed is undocked or it is docked on its own in the mainwindow then it still leaves space for the tabbar. This space should be reclaimed as its not needed anymore.

Attached is an example to reproduce this problem, to reproduce double click on the dock widgets and see that they both have space at the bottom but can be resized, double click on one of them again to dock it and see the space is still there.