QMdiSubWindow title bar buttons do not work when focus comes from other program

0) Open third party program like Konsole or Firefox.

1) Open the sample QMdiArea program that comes with Qt (do not maximize the window).

2) Open a QMdiSubWindow.

3) Test all the buttons in the subwindow title bar (close, maximize, minimize) (all works perfect with the left mouse button single click).

4) Keep the QMdiSubWindow in nomal state.

5) Open external program clicking on KDE task bar. The program gains the active window focus. You can see both programs in the screen.

6) Then click on one button of the QMdiSubWindow of the QMdiArea program. The expected behavior is than the subwindow gains the active window focus and do the button action. But the action is not done. You need an additional second click in the button.

I can see this bug in other cases but is not working all the time and I can't guarantee to be replicated.