Fix bugs in undo/redo and paste for JSON settings.

* Normalize in the `input` event that follows a `beforeinput`
  event, rather than in the later-queued microtask. This could be
  important if the microtask runs after *multiple* events that
  install multiple commands.

* Improve normalization, which did not always return the correct
  first element. (normalize should return the first line element
  in the normalized range; but if the first line element was a
  multi-line <div> -- e.g., <div><div>a<br>b<br>c</div></div> --
  it did not.)

* Track more DOM modifications in the window selection object.

* Do not combine modifications that have already been undone.

scripts/script.js

@@ -8024,7 +8024,7 @@ function suggest() {
             repl += " ";
             spacestate = hintinfo.smart_punctuation ? outPos : -1;
         }
-        if (endPos < 0) {
+        if (startPos < 0 || endPos < 0 || startPos > endPos || endPos > val.length) {
             let x = Object.assign({}, hintinfo);
             delete x.items;
             log_jserror(JSON.stringify({value:val,hint:x,repl:repl,startPos:startPos,endPos:endPos}));