This will be fixed in an upcoming patch, and I'm sorry I can't give an ETA for when this will happen.
In the meantime, the workarounds posted in this thread can help - either going into Options, unchecking Fullscreen to go into Windowed Mode, and then checking Fullscreen again, or hitting Cmd + F, should enable Cmd + Tabbing, and Control + Left / Right Arrowing between Hearthstone and the Desktop.
Thanks for all of your reports :)
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