I've done an application doing complex calculations (it lasts several minutes or much more) and I've noticed that if the app looses the focus, it freezes. If I keep the focus on it, it's working well. 
It's just a UI with one button that I've done with qt designer. There are 2 progressBars who stops when the QMainWindow looses the focus.
Have you ever heard about this problem? Maybe it comes from one of the QMainWindow property?  

Thank you for your help

EDIT: Even if UI freezes, calculation is still running. But it's boring for the user.