message: clear height flexbox properties when collapsing
  so item doesn't maintain height given by resizing
diff: @@ -344,7 +344,7 @@ const RoomSubList = React.createClass({
  "mx_RoomSubList_nonEmpty": len && !this.state.hidden,
  });
  if (this.state.hidden) {
  - return <div className={subListClasses}>
  + return <div className={subListClasses} style={{flexBasis: "unset", flexGrow: "unset"}}>
  {this._getHeaderJsx()}
  </div>;
  } else {
  