Aaaaack! Another problem, but it turned into a “new feature” of Mountain Lion.
It’s in Pages and Safari; maybe other programs, too? Looking at a Google news page on Safari I saw some little icons at the top. Pointing at one, a tiny box comes up, naming it. Clicking on the unnamed one with two little arrows suddenly made my open page in Google expand to fill the entire screen. There was no indication of how to go back to the previously normal-sized page.
Trying various keys on the keyboard: the Function 3 key brought up the Dashboard. At least I could now get to other icons in the Dock. I tried looking up Help, without finding any help. Perhaps I just didn’t word my search correctly?
In desperation, I emailed the helpful tech down at the Peoria MacMedia Inc. store. Amazingly, I soon got a reply from Kyle Smith, sent from his iPhone. “This is a feature of Apple, and you can do that from any program, but if you hold your cursor in the top of the screen it will drop down a menu and then you can click the blue icon that has the 2 arrows pointing towards each other. That should fix it.”
....................... Here's the arrows at the right side, top.
..................Here's what happened.
Here's the view of the blue arrows at the top.