I'm using an iPad Pro running iOS 12.1 and Safari.
As a logged-out user, access a mobile Wikipedia page such as https://en.m.wikipedia.org/wiki/Pope_Innocent_III , click the edit icon in the upper right, choose "Edit without logging in" and proceed to the (default) source editing mode. Now scroll down about one screenful and touch some text near the top of the editing area. The iOS virtual keyboard and the blue blinking cursor appear. If you now type a character, the text inappropriately scrolls up and the cursor is not visible anymore (the entered character is inserted at the correct position however).
I see the same broken behavior, whether a physical keyboard is attached or not.