message: preventDefault when moving autocomplete selection with vertical arrow
  Otherwise the composer selection updates, in turn hiding the autocomplete box - fixes vector-im/riot-web#4790
diff: @@ -890,6 +890,7 @@ export default class MessageComposerInput extends React.Component {
  }
  } else {
  this.moveAutocompleteSelection(up);
  + e.preventDefault();
  }
  };
  