This would be the best way to show up the composer, as seen on reply:
(Notice, that the adress bar and bookmarks are hiding automatically by starting a new reply by Safari. It seems the page is expanding and therefore iOS can resolve the issue…)
This is how it looks like at me (with tags): Terrible…
And here without tags, at your instance:
Overall, I think the best way to get more available space is, to bring the buttons and dropdown fields in a separate dialog field before/after editing.
@sam Specs: iPad mini 4; iOS 10.3.1