I edit people's posts occasionally on ServerFault when the grammar/punctuation/coherence goes below a threshold that my OCD can handle. However, on some of the thornier posts I would like to be able to compare my edits to the original text, especially when doing a final proof-reading of my edits.
At the moment, my only options are to either copy the original text into a text editor or copy a duplicate of the text into the editing field. From there I can compare my edits to the original text and make sure I'm preserving the original meaning as I attempt to untangle less than optimal communication.
This request is somewhat similar to this Meta post: "Display the original question beneath the answer edit form"