Repro: Backspace over a newline character. Then a few lines down, hit Enter at the end of an expression. You will find that the autoindent level is chosen as if you had hit Enter one line up from where you are.
Was resetting diffindex within the context of a single file, which is clearly wrong--diffindexes need to be unique within a file's diffs. The result was that if multiple chunks were showing in the review window, Stage All would not work.