I found myself thinking I could see the answer as you were describing the problem. That's good writing! My guess would be that you'd need to have a new data s

I found myself thinking I could see the answer as you were describing the problem. That's good writing! My guess would be that you'd need to have a new data structure for the text (or pointing into the straight text) that tracked the order of editing, then a corresponding set of new lexemes and reductions "sitting on top" of the existing parse tree, until the new edits + intervening test/parse tree reduces into something acceptable for the node of the parse tree in which the new text lives. Fun stuff.