I have a 2009 Mac Pro (4,1) that has had the "unofficial" 5,1 firmware update applied (to enable additional CPU support), CPUs upgraded to dual 3.46 GHz 6-core Xeons and GPU replaced with an AMD Radeon R9 280x flashed for Mac. It's running Yosemite 10.10.3. The graphics card uses the AMD drivers that ship with OS X.
I am having graphical glitches - a couple of different ones, but this is the one that's bugging me the most right now. (Link is to a screen recording made with QuickTime Player.)
The system upgrades mentioned above happened in February, and I have only noticed this issue within the last month-ish, so I don't think they are related. I did notice this issue before upgrading OS X from 10.10.2 to .3, so I don't think that's related either.
It only appears to happen in Chrome, although oddly, one of the glitches that happens (not caught in the video, sadly) is a black rectangle appearing over the middle of the menu bar, outside of Chrome's window - but only when Chrome is in the foreground. Switching to any other application when that happens causes the black box to disappear instantly. Another manifestation places a gray rectangle over the middle of the tab bar. Clicking on a couple tabs usually makes it disappear, as does, again, switching applications.
I found what may be a related bug in the Chromium bug tracker, but I am not seeing the extensive corruption/glitches shown in the screenshot attached to the bug (maybe I'm lucky?!?), nor am I seeing any issues in webpages themselves, only the tab bar, address bar and bookmark bar. I did add comments on that bug though.
I was using Chrome beta (stable version + 1 basically) but have reverted to Chrome v42.0.2311.135 stable and am experiencing the same problems. I have disabled all extensions and Chrome hardware acceleration and the problem persists.