Runway Fixed My Too-Short Clip, but Only After I Stopped Fighting It

I had a clip three seconds too short for the edit. Runway's extend generated the tail.

It jitterred on the first try because I asked for too much at once. Two seconds at a time stayed clean.

I locked the subject with a mask so the background moved and the person didn't smear.

The audio still didn't match, so I cut to music under the extension. Nobody notices.

Small extensions, masked subject, silent audio. That is the combo that stopped the weird warps.