When editing on mobile, the keyboard sits "on top of" the text, rather than pushing it up the screen. The result is that it is impossible to edit the end of any section of text (e.g. add a sentence to the end of a paragraph, reply to a discussion on a talkpage...) because the cursor is "hidden" by the keyboard.
In the following screenshots, I am on the edit page of the en.wp article "national library of australia", editing the opening section: https://en.wikipedia.org/w/index.php?title=National_Library_of_Australia&action=edit§ion=0
The first image shows the editing screen, scrolled to the bottom of the section, but before I press my finger to place the cursor. So far so good:
The second image is after I have pressed my finger at the end of the last line (as if I was about to add another sentence to the end of the paragraph, for example). You can see that the keyboard has risen up:
Note, however, that the keyboard completely obscures the last 8 lines of text - and the cursor with it. The browser believes that it is already showing the "end" of the page, and therefore it is impossible to scroll further down the screen. In effect, the keyboard is "sitting in top of" the last 8 lines of text, rather than "pushing" those lines up the screen.
I am using the default browser (Safari) on an iPhone 6, running the latest operating system (iOS 9.1).