Initially, I've encountered this issue in my program that uses QGLWidget, but then I found out that simply setting AA_NativeWidgets attribute to QApplication triggers this behavior (see the attached sample project).
Steps to reproduce and symptoms:
1. Switch the window to fullscreen mode. Note that the window does not maximize completely, there is a stripe at the very top of the screen filled with toolbar contents (or some part of it).
2. Move the mouse to the very top of the screen to display the menu and title bar. The menu shows up fine, but the title bar is invisible - it is impossible to exit fullscreen mode.