in src/CSSTransition.js [186:205]
addClass(node, type, phase) {
let className = this.getClassNames(type)[`${phase}ClassName`];
const { doneClassName } = this.getClassNames('enter');
if (type === 'appear' && phase === 'done' && doneClassName) {
className += ` ${doneClassName}`;
}
// This is to force a repaint,
// which is necessary in order to transition styles when adding a class name.
if (phase === 'active') {
/* eslint-disable no-unused-expressions */
node && node.scrollTop;
}
if (className) {
this.appliedClasses[type][phase] = className;
addClass(node, className);
}
}