Combination of unset initial viewport scale and CSS override hack prevented proper initial viewport width calculation on pages containing super-wide elements.
Note that this does not fix the upstream "graphs-not-being-mobile-friendly" issues from T161405, but it does make issues like this far less explode-y on iOS.
Examples:
enwiki > Lagrangian point
frwiki > Liste de sondages sur l'élection présidentielle française de 2017
(Edit: see the PR for before/after images.)