I saw it in express version of photoshop which located in www.photoshop.com/tools?wf=editor and found that useful. I think it would be great when manipulating photos. When holding it, it will show the original image like in the link.
Hello, pHeR-d (Ferdy?)
You'll be happy, as it already exists: go to your History panel, and click on the Snapshot that was created when the document was opened. CTRL+Z to toggle.
When you open your document, save it immediately, so that you can select a name, and proper location. After that Ctrl+S will be a click and forget thing, and...
by hitting F12, or going to File>Revert, you can revert to the saved (original) version!
Then, CTRL+Z to undo/redo and compare.
Hello and yes you approximately find it, it's Ferdi. I know that ways you mentioned. But i thought it would be useful as in editor. Otherwise, duplicating original layer and working on that duplicate will also prevent original file from changes.