reflowAndAddClass(node, className) {
// This is for to force a repaint,
// which is necessary in order to transition styles when adding a class name.
/* eslint-disable no-unused-expressions */
node && node.scrollTop;
/* eslint-enable no-unused-expressions */
addClass(node, className);
}
https://github.com/reactjs/react-transition-group/blob/5007303e729a74be66a21c3e2205e4916821524b/src/CSSTransition.js#L208-L215
CSS Transition-API source-code: