T41273 uncovered actually an interesting possibility to explore - to have both live preview and live diff preview at the same time.
This would probably require adding another button to have downstairs:
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Version: 1.21.x
Severity: enhancement
Users interested in such a feature might want to look at the user script that I have been working on: https://en.wikipedia.org/wiki/User:TheDJ/Actual_Live_Preview
TheDJ's script does something different. Here's some js that does what we're talking about:
mw.loader.load("https://en.wikipedia.org/w/index.php?title=User:Writ_Keeper/Scripts/previewAndDiff.js&action=raw&ctype=text/javascript");
Removing as a subtask of T41272: Bugs in "Live preview" feature (tracking) since this is a feature request, not a bug.