Screen slides off to the right and disappears

Please describe in much more detail exactly what you mean by "main screen being used will often slide off to the right out of sight and I can't figure out how to stop or retrieve it. "

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You may have (accidentally?) selected a different desktop through Mission Control. Are you the only user of the computer? Someone else may have set up another desktop that you are not aware of. I use a different desktop to to run a Firefox browser window and then run a Safari window in the main Desktop window (cuz my wife can not [will not?] remember to leave my browser tabs open). Apple has provided instructions on how use Mission Control at this link. There are several ways to switch desktops. I simply hold down the control key and hit the right or left arrow depending on which desktop I need to use. Try this last step to see if that is what is happening.

Try resetting the NVRAM and restarting in Safe Mode, then retest. Instructions for each are in the links below:

NVRAM Reset & Safe Mode Reset instructions

Don't thank me too soon. Please reply back here and let us know if that helps.