Q: While using a window, it moves to the background

I am having a strange problem.  I can be typing in a dialog box, like this one here in Safari, and all of a sudden my computer will start beeping at each keystroke (and these keystrokes are not recorded in the dialog box).  It's the beep that says you're trying to enter text and there's nowhere to input.  Also at this time, the red, yellow, and green buttons on the top left of the window turn grey.

If I click on the window, everything returns to normal, for a little while.  What is going on?