[posted this solution on Reddit]
I had the same problem. While searching for a solution I ran into a similar issue mentioned with ‘Heart Stone’ on another forum. There was a suggestion of it being the Graphics card and I got an idea. After a fresh restart.
The fix was: Go to Library -> Application Support -> GW2 -> ‘p_drive’ -> User -> Application Data -> GW2 -> then DELETE ‘GFXSettings.gw2.exe.xml’.
It should repopulate and reset to the last successful setting. Thought it would reset to default but it went to the last successful windowed setting I had. Hope this helps the tons of GW2 Mac players out there!
this worked for me! thanks