Revert "Cleanup enableSyncDefaultUpdate flag (facebook#26236)" (facebook#26528)

This reverts commit b2ae9dd.

While the feature flag is fully rolled out, these tests are also testing
behavior set with an unstable flag on root, which for now we want to
preserve.

Not sure if there's a better way then adding a dynamic feature flag to
the www build?

Author: kassens

packages/react-art/src/__tests__/ReactART-test.js

@@ -33,12 +33,16 @@ const ReactTestRenderer = require('react-test-renderer');
 
 // Isolate the noop renderer
 jest.resetModules();
+const ReactNoop = require('react-noop-renderer');
+const Scheduler = require('scheduler');
 
 let Group;
 let Shape;
 let Surface;
 let TestComponent;
 
+let waitFor;
+
 const Missing = {};
 
 function testDOMNodeStructure(domNode, expectedStructure) {
@@ -76,6 +80,8 @@ describe('ReactART', () => {
     Shape = ReactART.Shape;
     Surface = ReactART.Surface;
 
+    ({waitFor} = require('internal-test-utils'));
+
     TestComponent = class extends React.Component {
       group = React.createRef();
 
@@ -357,6 +363,58 @@ describe('ReactART', () => {
     doClick(instance);
     expect(onClick2).toBeCalled();
   });
+
+  // @gate !enableSyncDefaultUpdates
+  it('can concurrently render with a "primary" renderer while sharing context', async () => {
+    const CurrentRendererContext = React.createContext(null);
+
+    function Yield(props) {
+      Scheduler.log(props.value);
+      return null;
+    }
+
+    let ops = [];
+    function LogCurrentRenderer() {
+      return (
+        <CurrentRendererContext.Consumer>
+          {currentRenderer => {
+            ops.push(currentRenderer);
+            return null;
+          }}
+        </CurrentRendererContext.Consumer>
+      );
+    }
+
+    // Using test renderer instead of the DOM renderer here because async
+    // testing APIs for the DOM renderer don't exist.
+    ReactNoop.render(
+      <CurrentRendererContext.Provider value="Test">
+        <Yield value="A" />
+        <Yield value="B" />
+        <LogCurrentRenderer />
+        <Yield value="C" />
+      </CurrentRendererContext.Provider>,
+    );
+
+    await waitFor(['A']);
+
+    ReactDOM.render(
+      <Surface>
+        <LogCurrentRenderer />
+        <CurrentRendererContext.Provider value="ART">
+          <LogCurrentRenderer />
+        </CurrentRendererContext.Provider>
+      </Surface>,
+      container,
+    );
+
+    expect(ops).toEqual([null, 'ART']);
+
+    ops = [];
+    await waitFor(['B', 'C']);
+
+    expect(ops).toEqual(['Test']);
+  });
 });
 
 describe('ReactARTComponents', () => {

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

@@ -312,7 +312,11 @@ describe('ReactDOMNativeEventHeuristic-test', () => {
       expect(container.textContent).toEqual('not hovered');
 
       await waitFor(['hovered']);
-      expect(container.textContent).toEqual('hovered');
+      if (gate(flags => flags.enableSyncDefaultUpdates)) {
+        expect(container.textContent).toEqual('hovered');
+      } else {
+        expect(container.textContent).toEqual('not hovered');
+      }
     });
     expect(container.textContent).toEqual('hovered');
   });

packages/react-dom/src/__tests__/ReactDOMServerPartialHydration-test.internal.js

@@ -2036,7 +2036,14 @@ describe('ReactDOMServerPartialHydration', () => {
     suspend = true;
 
     await act(async () => {
-      await waitFor(['Before', 'After']);
+      if (gate(flags => flags.enableSyncDefaultUpdates)) {
+        await waitFor(['Before', 'After']);
+      } else {
+        await waitFor(['Before']);
+        // This took a long time to render.
+        Scheduler.unstable_advanceTime(1000);
+        await waitFor(['After']);
+      }
 
       // This will cause us to skip the second row completely.
     });

packages/react-dom/src/events/__tests__/DOMPluginEventSystem-test.internal.js

@@ -1984,9 +1984,13 @@ describe('DOMPluginEventSystem', () => {
             log.length = 0;
 
             // Increase counter
-            React.startTransition(() => {
+            if (gate(flags => flags.enableSyncDefaultUpdates)) {
+              React.startTransition(() => {
+                root.render(<Test counter={1} />);
+              });
+            } else {
               root.render(<Test counter={1} />);
-            });
+            }
             // Yield before committing
             await waitFor(['Test']);
 

packages/react-reconciler/src/ReactFiber.js

@@ -33,6 +33,7 @@ import {
   enableProfilerTimer,
   enableScopeAPI,
   enableLegacyHidden,
+  enableSyncDefaultUpdates,
   allowConcurrentByDefault,
   enableTransitionTracing,
   enableDebugTracing,
@@ -458,9 +459,11 @@ export function createHostRootFiber(
       mode |= StrictLegacyMode | StrictEffectsMode;
     }
     if (
+      // We only use this flag for our repo tests to check both behaviors.
+      // TODO: Flip this flag and rename it something like "forceConcurrentByDefaultForTesting"
+      !enableSyncDefaultUpdates ||
       // Only for internal experiments.
-      allowConcurrentByDefault &&
-      concurrentUpdatesByDefaultOverride
+      (allowConcurrentByDefault && concurrentUpdatesByDefaultOverride)
     ) {
       mode |= ConcurrentUpdatesByDefaultMode;
     }