Comping tool "segment boundaries" frozen

Hi - I came back to a project after shutting down the computer for a few hours and found that I couldn’t adjust the Comping Tool segment boundaries any more. The “hand/finger” tool appropriately changed to the short double line tool when I hovered over the boundary, but clicking there did not engage the boundary as it usually did.

I had a zillion of those edits created, and was panicked that I would be up the creek if I couldn’t edit them (they were preliminary placements). Luckily, I was able to get by using the hand tool to make new edits.

Has anyone come across “frozen” comping tool segment boundaries before?