While investigating T76274, I found crash scenario when playhead is near end frame and moving a strip. It is not as easy to reproduce, about 5% success rate, and it will be even harder after rB4066c921fbe5. Exact cause wasn't identified yet. I wanted to disable prefetching during modal operator execution in VSE, but currently I don't have any signalling method in place. Checking for G.moving seems to resolve this problem, but it doesn't adress root cause of bug. Reviewed By: brecht Differential Revision: https://developer.blender.org/D7820