MobileFrontend provides an InlineDifferenceEngine which rather than displaying the diff in a 2 column layout displays it in a single column (e.g. unified). This seems like it would be useful for core as most websites these days offer unified as well as 2 column layout diffs and the former is extremely useful on mobile.
The Mobilefrontend diff has been a nuisance point in recent changes to revisions (e.g. Multi content revisions) and it would be helpful to make unified diffs a first class citizen in some form. This inconsistency causes confusion due to the completely different implementations: (see T123413)
[x] Put the MobileDiff code in core with any necessary modifications (The code for includes/diff/DifferenceEngine.php is pretty messy and is likely to need heavy refactoring to achieve this).
 Update core diff page to allow switching to the inline diff and surface this in the desktop UI. (T240608)
 Remove the code in MobileFrontend that has been upstreamed to core. (T240622)
 It should be possible that https://en.wikipedia.org/w/index.php?title=San_Francisco&type=revision&diff=687927333&oldid=687926997&diffonly=1&useskin=minerva&mode=inline renders a single column diff and replicates the functionality of https://en.m.wikipedia.org/wiki/Special:MobileDiff/687926997
 Update MobileFrontend to use this link and retire Special:MobileDiff page
 Consider hiding or surfacing side by side diffs in mobile
* What other solutions provide inline diffs? Is MobileFrontend implementation the best one?
* What refactors are necessary in core / to mobile code to support such a change if the mobile implementation is favoured.