Those buttons also don't work once the system loses track of the separate windows or maps them to a location off the screen. I found that only resetting the normal.mxt file works in this case. In my case, I suspect this is from poor handling of a high-resolution display by some configuration setting in Arc (2880 x 1620, 10.2, Win7), but I haven't been able to get a response on this issue yet that fixes it permanently. It's a real nuisance to be totally locked out and have to reset the normal.mxt to get control back.
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