The specific circumstance for this issue is that I was merging a branch which reported a conflict.  While the merge changes were still uncommitted, I was unable to view differences on files that were merged successfully.  The screenshot before the merge commit shows one of these files selected, and ST will not display diff info, nor would it allow me to open an external diff.  The screenshot after the commit shows the same file selected, and I am able to view differences both in ST and externally without issue.

I had this happen at least once before (in an earlier 2.x beta), and if I remember correctly, that was also during a merge or rebase where there were conflicts to resolve.  In that instance, I really needed to confirm the merged changes, so my only solution was close the ST beta and open the earlier release version.  I never encountered this problem prior to the 2.x betas.

This might be related to SRCTREEWIN-6967, though the circumstances described there are different.

[UPDATE] I replicated this issue twice this morning.  Both times, I was doing a merge, and there were merge conflicts to be resolved.  I could view diff info for the files that HAVE a conflict, but I could not view a diff for the files that do NOT have a conflict.