After encountering SRCTREE-4574 I changed branches and did a merge. This merge resulted in a number of conflicts, which I had to resolve by hand. I used Cmd-D to perform an external diff, then went back and forth between SourceTree-Beta and BBEdit, comparing and merging lines. At some point, SourceTree-Beta hanged and Activity Monitor showed it using 100% CPU.

I used the Sample button in Activity Monitor to capture a sample of the running process, for what good that does.