Alternate fix for "Fix a crash in Suspense with findDOMNode"

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

@@ -233,4 +233,48 @@ describe('ReactDOMSuspensePlaceholder', () => {
     await Lazy;
     expect(log).toEqual(['cDU first', 'cDU second']);
   });
+
+  // Regression test for https://github.com/facebook/react/issues/14188
+  it('can call findDOMNode() in a suspended component commit phase (#2)', () => {
+    let suspendOnce = Promise.resolve();
+    function Suspend() {
+      if (suspendOnce) {
+        let promise = suspendOnce;
+        suspendOnce = null;
+        throw promise;
+      }
+      return null;
+    }
+
+    const log = [];
+    class Child extends React.Component {
+      componentDidMount() {
+        log.push('cDM');
+        ReactDOM.findDOMNode(this);
+      }
+
+      componentDidUpdate() {
+        log.push('cDU');
+        ReactDOM.findDOMNode(this);
+      }
+
+      render() {
+        return null;
+      }
+    }
+
+    function App() {
+      return (
+        <Suspense fallback="Loading">
+          <Suspend />
+          <Child />
+        </Suspense>
+      );
+    }
+
+    ReactDOM.render(<App />, container);
+    expect(log).toEqual(['cDM']);
+    ReactDOM.render(<App />, container);
+    expect(log).toEqual(['cDM', 'cDU']);
+  });
 });

packages/react-reconciler/src/ReactFiberTreeReflection.js

@@ -113,15 +113,28 @@ export function findCurrentFiberUsingSlowPath(fiber: Fiber): Fiber | null {
   // If we have two possible branches, we'll walk backwards up to the root
   // to see what path the root points to. On the way we may hit one of the
   // special cases and we'll deal with them.
-  let a = fiber;
-  let b = alternate;
+  let a: Fiber = fiber;
+  let b: Fiber = alternate;
   while (true) {
     let parentA = a.return;
-    let parentB = parentA ? parentA.alternate : null;
-    if (!parentA || !parentB) {
+    if (parentA === null) {
       // We're at the root.
       break;
     }
+    let parentB = parentA.alternate;
+    if (parentB === null) {
+      // There is no alternate. This is an unusual case. Currently, it only
+      // happens when a Suspense component is hidden. An extra fragment fiber
+      // is inserted in between the Suspense fiber and its children. Skip
+      // over this extra fragment fiber and proceed to the next parent.
+      const nextParent = parentA.return;
+      if (nextParent !== null) {
+        a = b = nextParent;
+        continue;
+      }
+      // If there's no parent, we're at the root.
+      break;
+    }
 
     // If both copies of the parent fiber point to the same child, we can
     // assume that the child is current. This happens when we bailout on low