Fit destop view to screen size

Twice in the last 10 days, all of a sudden the size of my desktop view changes so that it does not fit within the confines of the iMac screen size. Either the menu bar at the top of the screen or the dock at the bottom are not visible as are the left and right hand edge of the screen. I can scroll to get them to come into view individually but I'm not able to see the whole desktop at any one moment.

The first time that this happened I had to reload Mountain Lion to get the whole desktop view to fit within the confines of the viewable area of the screen.

It happened again today. This time I noticed that I was pressing the "control" key slightly as I was preparing to use the "right click" function with my wireless mouse. In todays instance I had to leave my mac for a while so I placed it into the screen saver mode by placing the cursor in the lower right hand corner to active the screen saver. When I came back to my mac after 15 or 20 minutes - the desktop view had spontaneously corrected itself.

1) Has this problem been previously reported by anyone?

2) Is there a known cause of the problem?

3) Is there some keyboard command that will cause the desktop view to completely fit the screen? I seem to vaguely remember instances of the desktop view not fitting the screen and being fixed by some keyboard fix for the problem.

Please advise.

M. Thomas