Regression: Doubleclick on sizegrip on the lower right corner of application main window doesn't grab the grip

This is a regression from Qt4.

Simple to repro with e.g. sample application from QTBUG-27639:
Just doubleclick and hold on sizegrip of the application main window -> Sizegrip mouse cursor is left on as long as mouse button is pressed down and the cursor remains within the main window, but moving the mouse doesn't resize the window.