SourceTree hangs in a manner very consistent with ----SRCTREE-2068---- in which user believed they had to force quit, but this bug is definitely not resolved and involves External Diff

The hangs can seem periodic/arbitrary due to a delay effect, but bug is reliably reproduced by:
1. Right click file in pending or posted changeset and select External Diff from menu
2. FileMerge is launched (default merge tool, hasn't been changed - no others installed to test with)
3. Within about 10 to 15 seconds but no sooner than 5, SourceTree will become unresponsive, showing the pinwheel icon.
4. SourceTree will remain unresponsive until immediately after FileMerge is fully exited. Closing the individual diff view has no effect. Opening and/or closing multiple diffs has no effect. Without quitting FileMerge, SourceTree has to be killed.

I think due to the delay, and given that users are very likely to move from the diff to editing code, and later coming back to SourceTree, it is easy to see how the issue could be diagnosed as intermittent or attributed to random other factors.