External Diff option not available in context menu

When viewing a list of files in the staging area of the working copy (Uncommitted changes) You can right-click a file and select External Diff.

When viewing an existing commit, or selecting a range of existing commits to see the diff, right-clicking a file in the list of files doesn't show the External Diff option, yet pressing Ctrl-D or selecting Actions > External Diff still works.

The External Diff context menu option should be available anytime it's possible.