Wrong behaviour of popup context menu in Qt5 (at least on Windows)

1. Do a right click to invoke context menu on a widget. Do not close the popup menu
2. Do a second right click on the same widget but in other place
Result: the first context menu disappears, but the second menu didn't pop out.

The bug can be easily reproduced in
C:\qt5\qtbase\examples\widgets\mainwindows\menus

The same demo from Qt 4.8.4 doesn't have this bug
C:\qt\examples\mainwindows\menus

PS. I tested in Windows 7 x64, Visual Studio 2008 SP1, though it's easy to reproduce in any environment.