Qt 5.5.x flickering by dragging QSizeGrip (but ok on 5.4.2 and less) on (at least) Mac OS X 10.10

Hi Qt-Team,
I tried to find a fix for this problem at stackoverflow, but got no answers yet. Now I try it here. The question was (I am not sure if it's really a bug, but I think it is):
Previous question:
"I see flickering in my window content when I drag an added QSizeGrip. If I resize the window using the window borders, everything is okay. The problem occurs only on Qt 5.5.x (0/1); on Qt 5.4.2 earlier, there is no problem. Currently I can only build and test on Mac OS X Yosemite (10.10.5).

I attached an example app that reproduces the problem. Could someone check that and can confirm whether that's a bug, or only my fault? The QSizeGrip is a red rounded rectangle in the test app.

I also see that the resizing in Qt 5.4.2 is not as bad in Qt 5.5.x, but still not as smooth as in 4.8.7 (no flickering but some ugly moving). Maybe there is something known what have to be set in Qt 5.x.x to fix that problem."

After I posted that I could already check this on Windows too, but I could not see that problem there in none of the above mentioned versions. So I think it's a Mac dependent problem.

Thank you very much.
Best regards.