When doing an external diff with FileMerge (may happen with other apps as well), SourceTree will hang if FileMerge isn't already running. This is 100% reproducible:

1. In Preferences, select FileMerge as the visual diff tool.
2. Make sure FileMerge is not already running.
3. Do an external diff, which will launch FileMerge.

Result: SourceTree will hang (beachball) until you quit FileMerge – closing the file window will not suffice.

Expected result: SourceTree should continue to respond and operate independently of FileMerge.

Workaround: In Xcode, launch FileMerge manually (Xcode->Open Developer Tool->FileMerge) before doing an external diff. If it's already running, the hang won't occur.

Suspected cause: SourceTree launches the external diff application synchronously and must therefore wait until it exits. It should be launched asynchronously.