After digging for 1d I found that a few super strange things:

• it happens only if Qt & the application are built in release mode
• it happens on rare apps/occasions
• it hangs in QtActivityDelegate::createSurface when we want to remove the dummy view same problem as here http://stackoverflow.com/questions/27440951/android-lock-freeze-layout-for-the-time-changes-are-made-programmatically