Regression, part of the statusbar with the sizegrip is not redrawn properly

I am pretty sure, that this is a regression.
In this example I used the servman binary to demonstrate the bug, but it does affect some other applications as well, which have that sizegrip, but have no other parts within the statusbar, which may enforce redraws explicitly (on their own?).
I am sure, that the bug is not within servman, but maybe either in Win32SS or (maybe comctl32)
BISECT_NEEDED

Observed result
0.4.15-dev-7842-g558ab78_drawingDirtInStatusbar.webm
0.4.15-dev-7842-g558ab78_drawingDirtInStatusbar.log the log shows nothing while it misbehvaes like that