Toggle snapping off, then select the event you want to bounce and bounce it, you will see it will not lengthen it after bouncing.
Or you can also create range with range tool (with the event you want to bounce selected) and then bounce it, you will see it will bounce it to a new file with the length of selected range. Snapping must off for this too.
Selecting with range tool:
Bouncing it with Ctrl + B: