I'm using an iPad Pro running iOS 12.1 and Safari, without a physical keyboard attached.
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 touch some text near the bottom of the editing area. The iOS virtual keyboard appears and the text is scrolled up so that the cursor remains visible, which is correct behavior. However, at this point it is impossible to scroll further up in the editing area. (Scrolling down remains possible.)
It's as if the editing area believes that it is showing the top of the text, forgetting that the appearance of the virtual keyboard caused the text to scroll.