I have a laptop running Windows 7, 64-bits, with an Intel HD graphics card, and an external display (which is the main display). If I disconnect the main display, to similate undocking the laptop, my laptop changes to be the main display.

I then start my Qt application, which appears in the middle of the laptop display, and connect the external display. The external display is changed back to be the main display again and the application appears in the middle of the external display. All is fine so far. 

However, If I press the applications menu (QMenu), they appear on the right edge of the laptop display. So, for some reason QMenu doesn't know that the primary display has changed. If I move the application just a tiny bit, the problem is solved.