When trimming or expanding the clip, put your mouse to the beginning or end of a clip and the mouse will change to a double-arrow resize cursor, then as you push down the left mouse button and drag, the clip will expand or be trimmed accordingly.
Vertical dotted lines in the clip indicate the start point and end point of the original clip, if the clip has been expanded beyond its original length. As a clip is expanded beyond its original length, the clip contents will be looped.
You may trim or expand a clip many times, but if you want to expand or trim the clip based on the current clip, not the original one from file, you can use the context menu "Set as Trim/Expand base", then the current clip will be treated as base for the later trimming and expanding.
After Set as Trim/Expand base has been selected, any vertical dotted lines in the clip will disappear for that clip, meaning the visible portion of the clip is now the base for later trimming or expanding.