fix: perform sync update when exiting animation

reactjs#885

When running in React 18 concurrent mode some state updates are batched, which results in inconsistent timing of events compared to the legacy mode. For example when using animations, after animationend event fires, the onExited event is not fired immediately, so there is a brief period of time when animation is finished and the styles are reset back to normal, which may cause a flash or a jump. One of these scenarios is described in reactjs#816.

This change makes sure that the updates are performed synchronously, in order to make sure that events fire consistently.

Author: otomad

src/Transition.tsx

@@ -1,6 +1,6 @@
 import React, { useRef } from "react";
 import type { ContextType, ReactNode } from "react";
-import ReactDOM from "react-dom";
+import ReactDOM, { flushSync } from "react-dom";
 
 import config from "./config";
 import TransitionGroupContext from "./TransitionGroupContext";
@@ -225,8 +225,10 @@ class TransitionComponent extends React.Component<
 			this.props.onExiting(node);
 
 			this.onTransitionEnd(timeouts.exit, () => {
-				this.safeSetState({ status: EXITED }, () => {
-					this.props.onExited(node);
+				flushSync(() => {
+					this.safeSetState({ status: EXITED }, () => {
+						this.props.onExited(node);
+					});
 				});
 			});
 		});