I have a 4 hour .mov (A in the pic), I trimmed it to start at 03:37:57.190, put it on the time line. I put 5 more clips on a 2nd video track (B). I synced up A and B (you can see the audio matches)
Now I realize I want A to start at 03:37:45:00
I expected to be able to grab the left side of A and drag it, but it just moves all of A. The mouse cursor changes and the edge turns green, but the behaviour is the same as if I grab the clip in the middle and move it.
If I grab the right side of C, it turns red and I can extend it (change the duration)
(this image is constructed so you can see all the things I am talking about, like what mouse icon I get when I am over the left edge)
This is intentional right now because redrawing the waveform to expose new portions (makes it appear static on timeline) is too compute intensive and interferes with the interactivity while drag-trimming. I do not consider it a defect, just a UX shortcoming.