message: dont jump to next part when inserting at *start* of uneditable part
diff: @@ -232,8 +232,9 @@ export default class EditorModel {
  index += 1;
  this._insertPart(index, splitPart);
  }
  - } else {
  - // not-editable, insert str after this part
  + } else if (offset !== 0) {
  + // not-editable part, caret is not at start,
  + // so insert str after this part
  addLen += part.text.length - offset;
  index += 1;
  }
  