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I would say most likely it's a resolution problem from an external display. If not, I would say an Archive & Install is your next option.
 
Don't archive and install yet.

What computer are you running? Is the monitor a standard computer monitor or a tv that might have overscan?

Restart. Or command-option-escape and restart finder.

Next, system preferences > displays. There should be a button that says only use the displays native resolutions. Click it and then pick one.

After that it's a guessing game... Repair permissions?
 
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