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There are two reasons for this problem. Either the resolution setting is corrupted or the Zoom feature is turned on. Let's try turning off Zoom. Go to Apple > System Preferences > Accessibility. Then turn off Zoom in there. Or try pressing Command, Option and 8 at same time to turn off Zoom.
If that didn't work, then then go to System Preferences > Displays and select a different resolution. If that didn't work, then reset the PRAM and NVRAM of your Mac.
Shut down your Mac
Restart and immediately hold down Command(Windows)-Option(alt)-P-R keys at the same time
Hold the keys down until the computer restarts and you hear the startup sound for two or three times.
Release the keys and let the Mac bootup.
Please let me know if that didn't work, so we can continue troubleshooting