I encountered this problem in the application I'm writing but wasn't able to reproduce exactly the same issue. In the sample I provided I managed to recreate a similar scenario (though not the same).

The original main window in my application has lots of toolbars and when triggering an action it goes through lots of changes, also hiding all its toolbars. What happens is that the space for the toolbar is not reassigned to the central widget, it just remains empty space in the main window.

In the test application i sent you, there's a similar scenario:

After 3 seconds all toolbars are hidden programmatically. If you drag out one of them before the hiding process and drop it into the same area where it was after the hiding process, the main window will remain with the space originally reserved for the toolbars.

There are also 2 screenshots to demonstrate the issue: toolbar1.jpg (before) and toolbar2.jpg (after).

While the scenarios are different, the main problem is that in some cases the layout is not correctly recalculated.

I'm sorry I can't reproduce the other scenario.