Windows OS: QMainWindow dock widget area cannot resize, while floating dock widget is under Windows taskbar.

Step to reproduce: 

1. Open standard "Dock Widget Example" from Qt Examples 
2. Undock one of dock widgets (make it floating)
3. Drag this widget down, out of screeen(hid it, for example behind Windows taskbar)
4. Maximize application window and observe issue.

This is not the only one example of this issue. You may reproduce it on QtDesighner application. Also, there are other graphical artifacts apears after that.