Regression: Right dock area in QMainWindow won't resize in dev

In `dev` branch of `qtbase` - in a QMainWindow with docked widgets, dragging the external separator of the right dock area only doesn't work. The left, bottom and top areas resize as expected.

This appears to be a regression from 6.9.1

For convenience, please find attached a reproducer - although any QMainWindow example will show this.

P.S the reproducer should be used with the `xcb` platform plugin. The problem exists with Wayland too (and probably all other platforms), but there appear to be some other issues there as well which I have not had the chance to distill yet.