Along the same lines as #134, I'd like to have a way to preview comments before submitting them and/or to edit comments after submitting them.
Fix formatting (especially Markdown). This function should be available in a preview.
Fix wording (for example, sentences I noticed are ambigious). This would be a reason for editing because sometimes you notice only a bit later that something isn't as easy to understand as you initially thought. I'm aware that you can always add another comment to clarify, but for many cases, this seems overkill, so the clarification doesn't happen.
Adding a part of program output that you omitted when submitting the comment (maybe because you thought it's not relevant). Adding this output to the comment is much more concise than repeating the previously listed output together with the previously omitted output.
So in my opinion neither of preview or edit is a substitute for the other. For Markdown fixes, you'd want a preview, for text changes you'd want editing. That said, editing is the more
universal of the two features, so if you implement only one I'd prefer edit over preview.