Make useFlushSync do sync flushes

Author: JoviDeCroock

compat/src/index.js

@@ -5,7 +5,8 @@ import {
 	createRef,
 	Component,
 	createContext,
-	Fragment
+	Fragment,
+	options
 } from 'preact';
 import {
 	useState,
@@ -139,7 +140,13 @@ const unstable_batchedUpdates = (callback, arg) => callback(arg);
  * @param {Arg} [arg] Optional argument that can be passed to the callback
  * @returns
  */
-const flushSync = (callback, arg) => callback(arg);
+const flushSync = (callback, arg) => {
+	const prevDebounce = options.debounceRendering;
+	options.debounceRendering = cb => cb();
+	const res = callback(arg);
+	options.debounceRendering = prevDebounce;
+	return res;
+};
 
 /**
  * Strict Mode is not implemented in Preact, so we provide a stand-in for it

compat/test/browser/useSyncExternalStore.test.js

@@ -379,11 +379,9 @@ describe('useSyncExternalStore', () => {
 
 			// Switch stores and update in the same batch
 			await act(() => {
-				ReactDOM.flushSync(() => {
-					// This update will be disregarded
-					storeA.set(2);
-					setStore(storeB);
-				});
+				// This update will be disregarded
+				storeA.set(2);
+				setStore(storeB);
 			});
 			// Now reading from B instead of A
 			assertLog([0]);
@@ -500,7 +498,7 @@ describe('useSyncExternalStore', () => {
 			// Should flip back to 0
 			expect(container.textContent).to.equal('0');
 
-			// Preact: Wait for 'Passive effect: 0' to flush from the rAF so it doesn't impact other tests
+			// Preact: Wait for 'Passive effect: 0' to y from the rAF so it doesn't impact other tests
 			await new Promise(r => setTimeout(r, 32));
 		});
 
@@ -686,10 +684,8 @@ describe('useSyncExternalStore', () => {
 
 			// Update the store and getSnapshot at the same time
 			await act(() => {
-				ReactDOM.flushSync(() => {
-					setGetSnapshot(() => getSnapshotB);
-					store.set({ a: 1, b: 2 });
-				});
+				setGetSnapshot(() => getSnapshotB);
+				store.set({ a: 1, b: 2 });
 			});
 			// It should read from B instead of A
 			assertLog([2]);