Bugfix: nextFlushedRoot should always be set when performing work

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

@@ -2001,4 +2001,16 @@ describe('ReactErrorBoundaries', () => {
     // Error should be the first thrown
     expect(caughtError.message).toBe('child sad');
   });
+
+  it('propagates uncaught error inside unbatched initial mount', () => {
+    function Foo() {
+      throw new Error('foo error');
+    }
+    const container = document.createElement('div');
+    expect(() => {
+      ReactDOM.unstable_batchedUpdates(() => {
+        ReactDOM.render(<Foo />, container);
+      });
+    }).toThrow('foo error');
+  });
 });

packages/react-reconciler/src/ReactFiberScheduler.js

@@ -1330,7 +1330,9 @@ export default function<T, P, I, TI, HI, PI, C, CC, CX, PL>(
       if (isUnbatchingUpdates) {
         // ...unless we're inside unbatchedUpdates, in which case we should
         // flush it now.
-        performWorkOnRoot(root, Sync);
+        nextFlushedRoot = root;
+        nextFlushedExpirationTime = Sync;
+        performWorkOnRoot(nextFlushedRoot, nextFlushedExpirationTime);
       }
       return;
     }