When you type in a text editor, you always type out of some range. When your insertion point or cursor is blinking at the end of a word you just typed, you expect to still be “in touch” with the word, and that the next key you press will for example add a character to that word. This is a useful deception for us human users. It’s not actually part of the technological underpinnings.
Typewriter modes depend on the feature that you can scroll farther up and down than usual. You need extra whitespace, most of the time in both directions. Let’s start with “overscrolling” to understand what we need. Regular text views show additional bottom whitespace only until you fill it with text. Take TextEdit, for example. You can start to type at the topmost edge of the text view and the rest of the window is blank.
I am currently working on a typewriter mode text view. Even though this is a very popular trend for a couple of years, I couldn’t find any open source component for this feature. I assume those who figure this out keep it a secret. If that is the case, it doesn’t make that much sense nowadays anymore since every 3rd note taking app or so has such a feature already.