Dialog appears briefly then disappears, but FileMerge does not launch. Settings in SourceTree Diff section all look right. Is there a path setting I need to change.
See this StackOverFlow question for a workaround. However, SourceTree should definitely provide an error to the user telling them what to do and how to fix.
Try deleting your user config file and restart SourceTree.
sudo rm ~/.gitconfig
I have always solved this problem by ensuring the license agreement is agreed to. However, that didn't seem to be the problem anymore. I tried the solution provided by Tom Kraina in the link provided above (https://stackoverflow.com/questions/32729049/filemerge-quits-immediately-after-launching-from-sourcetree/41490804). This worked after I deleted my ~/.gitconfig file. But then selecting FileMerge as my merge tool in the preferences worked again.
Apologies for the late reply - it turned out that installing Xcode fixed it for me. Not sure specifically the reason behind this as I'm not aware of Xcode being a pre-requisite for Sourcetree, but since Xcode was mentioned in the title of this report I thought I'd give it a go... and it worked.
I am having the same issue with the new SourceTree Version 2.2.2 (51) after updating (though I swear I thought I had used this feature after updating.
I have mine setup to use Kaleidoscope for diffs. nothing very telling is populating into the syslog at this point (that I can tell) when it happens.