only warn on unacted effects for strict / non sync modes

(basically, when `fiber.mode !== 0b0000`)

Warnings on unacted effects may be too noisy, especially for legacy apps. This PR fires the warning only when in a non sync mode (concurrent/batched), or when in strict mode. This should make updating easier.

I also added batched mode tests to the act() suite.

packages/react-dom/src/__tests__/ReactDOMHooks-test.js

@@ -53,32 +53,23 @@ describe('ReactDOMHooks', () => {
       return 3 * n;
     }
 
-    // we explicitly catch the missing act() warnings
-    // to simulate this tricky repro
-    expect(() => {
-      ReactDOM.render(<Example1 n={1} />, container);
-      expect(container.textContent).toBe('1');
-      expect(container2.textContent).toBe('');
-      expect(container3.textContent).toBe('');
-      Scheduler.unstable_flushAll();
-      expect(container.textContent).toBe('1');
-      expect(container2.textContent).toBe('2');
-      expect(container3.textContent).toBe('3');
-
-      ReactDOM.render(<Example1 n={2} />, container);
-      expect(container.textContent).toBe('2');
-      expect(container2.textContent).toBe('2'); // Not flushed yet
-      expect(container3.textContent).toBe('3'); // Not flushed yet
-      Scheduler.unstable_flushAll();
-      expect(container.textContent).toBe('2');
-      expect(container2.textContent).toBe('4');
-      expect(container3.textContent).toBe('6');
-    }).toWarnDev([
-      'An update to Example1 ran an effect',
-      'An update to Example2 ran an effect',
-      'An update to Example1 ran an effect',
-      'An update to Example2 ran an effect',
-    ]);
+    ReactDOM.render(<Example1 n={1} />, container);
+    expect(container.textContent).toBe('1');
+    expect(container2.textContent).toBe('');
+    expect(container3.textContent).toBe('');
+    Scheduler.unstable_flushAll();
+    expect(container.textContent).toBe('1');
+    expect(container2.textContent).toBe('2');
+    expect(container3.textContent).toBe('3');
+
+    ReactDOM.render(<Example1 n={2} />, container);
+    expect(container.textContent).toBe('2');
+    expect(container2.textContent).toBe('2'); // Not flushed yet
+    expect(container3.textContent).toBe('3'); // Not flushed yet
+    Scheduler.unstable_flushAll();
+    expect(container.textContent).toBe('2');
+    expect(container2.textContent).toBe('4');
+    expect(container3.textContent).toBe('6');
   });
 
   it('should not bail out when an update is scheduled from within an event handler', () => {

packages/react-dom/src/__tests__/ReactTestUtilsAct-test.js

@@ -48,6 +48,20 @@ describe('ReactTestUtils.act()', () => {
     ReactDOM.unmountComponentAtNode(dom);
   }
   runActTests('legacy sync mode', renderSync, unmountSync);
+
+  // and then in batched mode
+  let batchedRoot;
+  function renderBatched(el, dom) {
+    batchedRoot = ReactDOM.unstable_createSyncRoot(dom);
+    batchedRoot.render(el);
+  }
+  function unmountBatched(dom) {
+    if (batchedRoot !== null) {
+      batchedRoot.unmount();
+      batchedRoot = null;
+    }
+  }
+  runActTests('batched mode', renderBatched, unmountBatched);
 });
 
 function runActTests(label, render, unmount) {
@@ -68,6 +82,26 @@ function runActTests(label, render, unmount) {
       document.body.removeChild(container);
     });
     describe('sync', () => {
+      it('warns if an effect is queued outside an act scope, except in legacy sync+non-strict mode', () => {
+        function App() {
+          React.useEffect(() => {}, []);
+          return null;
+        }
+        expect(() => {
+          render(<App />, container);
+          // flush all queued work
+          Scheduler.unstable_flushAll();
+        }).toWarnDev(
+          label !== 'legacy sync mode'
+            ? [
+                // warns twice because we're in strict+dev mode
+                'An update to App ran an effect, but was not wrapped in act(...)',
+                'An update to App ran an effect, but was not wrapped in act(...)',
+              ]
+            : [],
+        );
+      });
+
       it('can use act to flush effects', () => {
         function App() {
           React.useEffect(() => {

packages/react-reconciler/src/ReactFiberWorkLoop.js

@@ -2453,6 +2453,7 @@ export function warnIfNotCurrentlyActingEffectsInDEV(fiber: Fiber): void {
   if (__DEV__) {
     if (
       warnsIfNotActing === true &&
+      fiber.mode &&
       IsSomeRendererActing.current === false &&
       IsThisRendererActing.current === false
     ) {

packages/react-reconciler/src/__tests__/ReactHooks-test.internal.js

@@ -1806,7 +1806,6 @@ describe('ReactHooks', () => {
       globalListener();
       globalListener();
     }).toWarnDev([
-      'An update to C ran an effect',
       'An update to C inside a test was not wrapped in act',
       'An update to C inside a test was not wrapped in act',
       // Note: should *not* warn about updates on unmounted component.