Reflowing the current line won't work. If I delete a word, I need the next line's text to be flowed up to join with the current line. And that might require reflowing of subsequent lines as well.

Reflowing the current sentence won't work, because this style involves inserting semantic line breaks in certain places within a single sentence.

Reformatting a user selected portion is going to take more effort on my part. Currently my editor reflows the entire file every time the file is saved; I don't have to think about which part to reflow.