Updates at the same priority should not interrupt current render

packages/react-noop-renderer/src/ReactNoop.js

@@ -455,7 +455,11 @@ var ReactNoop = {
 
   unbatchedUpdates: NoopRenderer.unbatchedUpdates,
 
-  flushSync: NoopRenderer.flushSync,
+  flushSync(fn: () => mixed) {
+    yieldedValues = [];
+    NoopRenderer.flushSync(fn);
+    return yieldedValues;
+  },
 
   // Logs the current state of the tree.
   dumpTree(rootID: string = DEFAULT_ROOT_ID) {

packages/react-reconciler/src/ReactFiberScheduler.js

@@ -1202,7 +1202,7 @@ export default function<T, P, I, TI, HI, PI, C, CC, CX, PL>(
           if (
             !isWorking &&
             root === nextRoot &&
-            expirationTime <= nextRenderExpirationTime
+            expirationTime < nextRenderExpirationTime
           ) {
             // Restart the root from the top.
             if (nextUnitOfWork !== null) {

packages/react-reconciler/src/__tests__/ReactIncremental-test.js

@@ -168,6 +168,7 @@ describe('ReactIncremental', () => {
     ops = [];
 
     // This will abort the previous work and restart
+    ReactNoop.flushSync(() => ReactNoop.render(null));
     ReactNoop.render(<Foo text="baz" />);
 
     // Flush part of the new work
@@ -221,6 +222,7 @@ describe('ReactIncremental', () => {
     expect(ops).toEqual(['setState1']);
 
     // This will abort the previous work and restart
+    ReactNoop.flushSync(() => ReactNoop.render(<Foo />));
     inst.setState(
       () => {
         ops.push('setState2');
@@ -1877,6 +1879,8 @@ describe('ReactIncremental', () => {
     );
     ReactNoop.flush();
     expect(ops).toEqual([
+      'ShowLocale {"locale":"sv"}',
+      'ShowBoth {"locale":"sv"}',
       'Intl {}',
       'ShowLocale {"locale":"en"}',
       'Router {}',
@@ -2648,4 +2652,67 @@ describe('ReactIncremental', () => {
       'count:1, name:not brian',
     ]);
   });
+
+  it('does not interrupt for update at same priority', () => {
+    function Parent(props) {
+      ReactNoop.yield('Parent: ' + props.step);
+      return <Child step={props.step} />;
+    }
+
+    function Child(props) {
+      ReactNoop.yield('Child: ' + props.step);
+      return null;
+    }
+
+    ReactNoop.render(<Parent step={1} />);
+    ReactNoop.flushThrough(['Parent: 1']);
+
+    // Interrupt at same priority
+    ReactNoop.render(<Parent step={2} />);
+
+    expect(ReactNoop.flush()).toEqual(['Child: 1', 'Parent: 2', 'Child: 2']);
+  });
+
+  it('does not interrupt for update at lower priority', () => {
+    function Parent(props) {
+      ReactNoop.yield('Parent: ' + props.step);
+      return <Child step={props.step} />;
+    }
+
+    function Child(props) {
+      ReactNoop.yield('Child: ' + props.step);
+      return null;
+    }
+
+    ReactNoop.render(<Parent step={1} />);
+    ReactNoop.flushThrough(['Parent: 1']);
+
+    // Interrupt at lower priority
+    ReactNoop.expire(2000);
+    ReactNoop.render(<Parent step={2} />);
+
+    expect(ReactNoop.flush()).toEqual(['Child: 1', 'Parent: 2', 'Child: 2']);
+  });
+
+  it('does interrupt for update at higher priority', () => {
+    function Parent(props) {
+      ReactNoop.yield('Parent: ' + props.step);
+      return <Child step={props.step} />;
+    }
+
+    function Child(props) {
+      ReactNoop.yield('Child: ' + props.step);
+      return null;
+    }
+
+    ReactNoop.render(<Parent step={1} />);
+    ReactNoop.flushThrough(['Parent: 1']);
+
+    // Interrupt at higher priority
+    expect(
+      ReactNoop.flushSync(() => ReactNoop.render(<Parent step={2} />)),
+    ).toEqual(['Parent: 2', 'Child: 2']);
+
+    expect(ReactNoop.flush()).toEqual([]);
+  });
 });