message: add comment about positioning the format bar
diff: @@ -274,6 +274,7 @@ export default class BasicMessageEditor extends React.Component {
  }
  this._formatBarRef.style.left = `${selectionRect.left - leftOffset}px`;
  + // 12 is half the height of the bar (e.g. to center it) and 16 is an offset that felt ok.
  this._formatBarRef.style.top = `${selectionRect.top - topOffset - 16 - 12}px`;
  }
  }
  