First of all, what a great piece of software. Does everything that I used to use Scrivener for but without any activation/licensing concerns. I am astonished and grateful.
One small suggestion: when you Ctrl-Z to undo a change while the viewport is not currently displaying the affected text, then the undo happens off-screen. I believe a more conventional approach would be to scroll back to the previous insertion point so the user can see what’s happening. This also has the benefit of being easily able to return to the place you’re working on, if you inadvertently scroll to the beginning/end of your document.