In the writing modes, Typewriter keeps the line you write on in the middle of the screen; Focus keeps the sentence or paragraph bright and dims the rest.
While you write, the header steps aside and leaves the page alone; it comes back as soon as you touch the screen or move the pointer. In Settings › Interface you can keep it in view, remove the F. at the end of the text and read comments in full in the text, instead of their sign alone.