lib/assets/scripts/vendor/codemirror/codemirror.js
function updateDoc(cm, from, to, newText, selUpdate, origin) {
    // Possibly split or suppress the update based on the presence
    // of read-only spans in its range.
    var split = sawReadOnlySpans &&
      removeReadOnlyRanges(cm.view.doc, from, to);
    if (split) {
      for (var i = split.length - 1; i >= 1; --i)
        updateDocInner(cm, split[i].from, split[i].to, [""], origin);
      if (split.length)
        return updateDocInner(cm, split[0].from, split[0].to, newText, selUpdate, origin);
    } else {
      return updateDocInner(cm, from, to, newText, selUpdate, origin);
    }
  }
function updateDoc(cm, from, to, newText, selUpdate, origin) {
    // Possibly split or suppress the update based on the presence
    // of read-only spans in its range.
    var split = sawReadOnlySpans &&
      removeReadOnlyRanges(cm.view.doc, from, to);
    if (split) {
      for (var i = split.length - 1; i >= 1; --i)
        updateDocInner(cm, split[i].from, split[i].to, [""], origin);
      if (split.length)
        return updateDocInner(cm, split[0].from, split[0].to, newText, selUpdate, origin);
    } else {
      return updateDocInner(cm, from, to, newText, selUpdate, origin);
    }
  }
Replace the range from from to to by the strings in newText. Afterwards, set the selection to selFrom, selTo.