It sounds like you mock, but: you can do this, or near enough, and you can do it already, and it works really well!
0. Pick suitable tool. Firefox works. Notepad++ works. GIMP works. Emacs, not so much, but if you use Emacs, you know that you can fix this
1. Load file A into a tab
2. Load file B into a tab
3. Close all other tabs
4. Hold down the tab switch shortcut key and note the result
For images this is actually pretty decent and I've used it a lot. Good for figuring out what the differences actually are when your image-based tests fail, and similarly after making a speculative change. Let your eyes do the difference operation for you. That's what they're there for.
For text: you'd be better served by some other kind of tool. But text is just an image with letters in it, so the same principle applies. It does work!
(I've previously read a blog post, link to which I of course now can't find, about how old-style Photoshop undo was designed with this sort of thing in mind. Instead of working through the operation queue like normal people, it simply switched repeatedly between previous state and current state - the idea being that you'd make a change that you weren't certain of, then press the key repeatedly to see before and after. No need to think. Thinking isn't appropriate here anyway. Just let your eyes look at what they're seeing.)
