Now we have a nice side-by-side diff in the 'suggested edits' review pages, and as an option in edit history, could something similar be made available as an option when editing a post?
It wouldn't need to be interactive - a button to get the side-by-side diff of the current state of your edit, versus the original would do. This would be useful when shuffling lots of whitespace around - a final check that you haven't broken some embedded code.
A workaround at the moment is to submit the edit, then check the side-by-side view in the edit history.
This is one of the features I find missing most often.
There are several reasons I think this feature would make a big difference:
At the end of an extensive edit I want to make sure I captured the OP's original meaning and intent. While I can open the original post in a new tab, and compare the original with the edited version, that method is far from satisfactory. It is not easy to see the changes, and it is easy to overlook things. If, on the other hand, I could see the diffs side by side, I could easily and accurately check my edits against the author's original meaning.
Sometimes when moving large pieces of text around, it is easy to drop some of it. Occasionally I have nearly deleted parts of a post unknowingly. Being able to check the diffs would make it super easy to recognise and correct this.
Some other good reasons:
As a less than 2k rep user, I want to be able to see what the reviewer sees, making it easy to see if my edit would appear satisfactory to the reviewer.
As Paŭlo Ebermann metioned, seeing the diffs would make it easier to write the edit description for long edits.
I want to be able to see how much the post has changed, making it easy for me to tell if an edit is a worthwhile or not. Sometimes I start editing a post, only to eventually decide that I can't improve the post that much without essentially starting from scratch. If I could see the difference early on, though, I might save some time.
It would allow me to check my writing style against the OP's, making sure that they are not too different.
These are just a few reasons to implement this, and I am sure there are many more. I believe this feature would help me edit better, and I wish that the dev team would revisit this feature-request.
If you know of any other good reasons, please let me know in the comments.
My current workaround for lack of this feature results in the following workflow:
Do the edit, make sure it looks as you intended, not submitting yet
Ctrl+A, copy & paste into a text editor, save as file like new.md
Discard the edit by clicking the button in the web UI
Start editing again, save the original text into another file, like old.md
Use your favorite local diff viewer (I use kdiff3) to check the diffs, fix new version as necessary
Copy & paste the new version from new.md to the edit box in the web UI
Submit the edit.
A more forward-looking approach would be to save old.md before starting to edit, of course. Then you won't need to discard the edit to get original text. Or use other ways of getting original text after you've changed that in the edit box – e.g. holding Ctrl+Z to go back via repeated undoings, or navigating to a URL like https://meta.stackexchange.com/posts/<YourPostId>/revisions, where <YourPostId> is the Id of the post you can find after /a/ part in the share link.