Nsoutlineview cell background changes color during animation on Yosemite

I have an application that works well under OS X 10.7 to 10.9. When it works under 10.10 there is a visual problem that I have tracking issues.

Basically, it appears that the background color of the cell changes as the outline expands and contracts. Here is a very short (slo-mo) video showing the problem:

http://www.thirdstreetsoftware.com/downloads/Misc/paletteclip.mp4

The problem only appears when running on OS X 10.10 no matter if it was built with Xcode 5 or 6, 10.9 or 10.10.

Any idea where I should be looking for the problem?