Recently all of the scrolling hacks for iOS in MobileFrontend were removed (https://gerrit.wikimedia.org/r/c/mediawiki/extensions/MobileFrontend/+/496827) because they were causing multiple different scrolling problems in several different interfaces.
The purpose of the hacks was to allow the editing toolbar (and other sticky headers in other overlays) to stay on the screen while the keyboard is open. Without them, it can be just scrolled away, because of iOS Safari's reckless disregard for position: fixed.
Here's a video demonstrating the problem (on iPhone SE running iOS 12.1.14, testing on https://en.m.wikipedia.beta.wmflabs.org/wiki/Division this morning). When the keyboard opens, the toolbar scrolls out of the viewport. If you want to scroll the toolbar back in, you have to scroll away from your selection. If you want to view the selection again, you have to scroll away from the toolbar. This makes any formatting changes very awkward.
This task is to figure out a new set of hacks to fix this. The new approach should only apply to the VisualEditor toolbar, and not any other sticky headers (see T218415 about that).