This can be worked around until a true client side patch is made for both World of Warcraft and Diablo 3.
In the Terminal application (found in your /Applications/Utilities/ folder), type the following line and hit Enter:
defaults write com.blizzard.diablo3 DesktopModeIsDefault 1
With this command entered, you should set your Desktop resolution to 1920x1080@60p (1080p in the Displays control panel/menubar pulldown) before you launch Diablo 3. Once set to this resolution, you can launch Diablo 3 and the screen should remain in 1080p if you are using Fullscreen mode.
To undo this change once a fix is in place, open the Terminal application again and type the following line, then hit Enter:
defaults delete com.blizzard.diablo3 DesktopModeIsDefault
This command will revert your Diablo 3 client to the default resolutions handling mode.
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