From b867f5c765b2a4a639066a41d5cd2a549e72f15d Mon Sep 17 00:00:00 2001
From: Andrew Clark <git@andrewclark.io>
Date: Thu, 21 Nov 2019 14:20:47 -0800
Subject: [PATCH] Failing test: Updates "un-committed" when rebasing

Adds a failing test case where an update that was committed is
later skipped over during a rebase. This should never happen.
---
 .../ReactIncrementalUpdates-test.internal.js  | 64 +++++++++++++++++++
 1 file changed, 64 insertions(+)

diff --git a/packages/react-reconciler/src/__tests__/ReactIncrementalUpdates-test.internal.js b/packages/react-reconciler/src/__tests__/ReactIncrementalUpdates-test.internal.js
index 7ae9874105dce..095f3d8979a6a 100644
--- a/packages/react-reconciler/src/__tests__/ReactIncrementalUpdates-test.internal.js
+++ b/packages/react-reconciler/src/__tests__/ReactIncrementalUpdates-test.internal.js
@@ -653,4 +653,68 @@ describe('ReactIncrementalUpdates', () => {
       expect(Scheduler).toFlushAndYield(['Commit: goodbye']);
     });
   });
+
+  it('when rebasing, does not exclude updates that were already committed, regardless of priority', async () => {
+    const {useState, useLayoutEffect} = React;
+
+    let pushToLog;
+    function App() {
+      const [log, setLog] = useState('');
+      pushToLog = msg => {
+        setLog(prevLog => prevLog + msg);
+      };
+
+      useLayoutEffect(
+        () => {
+          Scheduler.unstable_yieldValue('Committed: ' + log);
+          if (log === 'B') {
+            // Right after B commits, schedule additional updates.
+            Scheduler.unstable_runWithPriority(
+              Scheduler.unstable_UserBlockingPriority,
+              () => {
+                pushToLog('C');
+              },
+            );
+            setLog(prevLog => prevLog + 'D');
+          }
+        },
+        [log],
+      );
+
+      return log;
+    }
+
+    const root = ReactNoop.createRoot();
+    await ReactNoop.act(async () => {
+      root.render(<App />);
+    });
+    expect(Scheduler).toHaveYielded(['Committed: ']);
+    expect(root).toMatchRenderedOutput('');
+
+    await ReactNoop.act(async () => {
+      pushToLog('A');
+      Scheduler.unstable_runWithPriority(
+        Scheduler.unstable_UserBlockingPriority,
+        () => {
+          pushToLog('B');
+        },
+      );
+    });
+    expect(Scheduler).toHaveYielded([
+      // A and B are pending. B is higher priority, so we'll render that first.
+      'Committed: B',
+      // Because A comes first in the queue, we're now in rebase mode. B must
+      // be rebased on top of A. Also, in a layout effect, we received two new
+      // updates: C and D. C is user-blocking and D is synchronous.
+      //
+      // First render the synchronous update. What we're testing here is that
+      // B *is not dropped* even though it has lower than sync priority. That's
+      // because we already committed it. However, this render should not
+      // include C, because that update wasn't already committed.
+      'Committed: BD',
+      'Committed: BCD',
+      'Committed: ABCD',
+    ]);
+    expect(root).toMatchRenderedOutput('ABCD');
+  });
 });