I have tried the recently released 4.5 update of VSTGUI. Everything seems to work fine so far except for bitmap drawing on Mac OS when running on a Retina display and the CFrame::setZoom() is set to anything != 1.0 (see image below). The example contains a CAnimKnob and the CFrame zoom is set to a value <1.The black border sometimes appears on the left, sometimes on the top or bottom or on both edges of the bounding box. The control itself is positioned on integral coordinates and has integral and even dimensions when created. The described behaviour can be reproduced in 10.11 and 10.12, but probably also affects older versions of Mac OS.
I was diff’ing lib/platform/mac/cgdrawcontext.cpp against the 4.3 revision and had the impression that it might have to do something with line 621 in CGDrawContext::drawBitmap() where the integralOffset argument for beginCGContext() was changed to true. But alas that didn’t help.
It also doesn’t make any difference whether the bitmap sits in a layered or non layered view container, except for the erroneousthe border being drawn in white instead of black when layered.