Ergonomy - provide editing context
A good rule of thum is to always provide proper context for the user. In case of a text editor, it's the de facto standard to only scroll the screen when the text cursor would leave the visible window. However, consider the use case of composing an answer to a letter. It feels more comfortable to be able to read a bit ahead, as well as back in this case. Toggling full screen via scroll lock was a working strategy in the past, though only a few editors support it, and nowadays less and less keyboards have a button for it... There exist another breed of editors which, on automatic page scroll, advance only a small proportion of the editing window at a time. A third possible implementation would scroll in such a way as to always position the cursor in the middle of the editing window. I have constructed a few prototypes on this paradigm way back in primary school which worked nicely to me.