message: after completion, set caret in next part at start
  instead of end of current part
diff: @@ -124,18 +124,24 @@ export default class EditorModel {
  }
  _onAutoComplete = ({replacePart, caretOffset, close}) => {
  + let pos;
  if (replacePart) {
  this._replacePart(this._autoCompletePartIdx, replacePart);
  + let index = this._autoCompletePartIdx;
  + if (caretOffset === undefined) {
  + caretOffset = 0;
  + index += 1;
  + }
  + pos = new DocumentPosition(index, caretOffset);
  }
  - const index = this._autoCompletePartIdx;
  if (close) {
  this._autoComplete = null;
  this._autoCompletePartIdx = null;
  }
  - if (caretOffset === undefined) {
  - caretOffset = replacePart.text.length;
  - }
  - this._updateCallback(new DocumentPosition(index, caretOffset));
  + // rerender even if editor contents didn't change
  + // to make sure the MessageEditor checks
  + // model.autoComplete being empty and closes it
  + this._updateCallback(pos);
  }
  /*
  