How is the rearrangement implemented?

[PachVerb]

https://github.com/reactjs/react-transition-group/blob/5007303e729a74be66a21c3e2205e4916821524b/src/CSSTransition.js#L208-L215

CSS Transition-API source-code:

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);
  }