The normal install sets it up as a screensaver. You can change the screensaver to (none) or something else to prevent this from happening. The energysaver feature of the monitor will turn the monitor off after a set time. The computer continues to crunch. Most of us disable the screensaver anyway to give most of the computer cycles to the crunching.

If you want to see what is going on, rollover the light blue WCG icon in the lower right to see a text summary or click on it to see the large display.

The computer is still crunching, and that is an active image of the fragment you are working on. It takes some computer power to run, so the crunching will be a few percent slower and finish slightly later.