If your goal is to shorten a certain drum loop and then place a different region right next to it, try this shorthand: Drag the second region right on top of the one you want to shorten, so that they overlap. When you release the mouse, you’ll see that you’ve vaporized the overlapped portion of the stationary region.
In some cases, you’ll just leave the dragged region where it is now, so that it plays side-by-side with the shortened one. But once the chopping is done, you don’t have to leave it where you dropped it. You can drag it right back where it started from, having used it as only a temporary chopper-offer.