bailoutOnLowPriority correct pushes context for ClassComponents to mirror complete phase context pop

scripts/fiber/tests-passing.txt

@@ -1148,6 +1148,7 @@ src/renderers/shared/fiber/__tests__/ReactIncremental-test.js
 * provides context when reusing work
 * reads context when setState is below the provider
 * reads context when setState is above the provider
+* maintains the correct context index when context proviers are bailed out due to low priority
 
 src/renderers/shared/fiber/__tests__/ReactIncrementalErrorHandling-test.js
 * catches render error in a boundary during full deferred mounting

src/renderers/shared/fiber/ReactFiberBeginWork.js

@@ -462,8 +462,17 @@ module.exports = function<T, P, I, TI, C, CX>(
   }
 
   function bailoutOnLowPriority(current, workInProgress) {
-    if (workInProgress.tag === HostPortal) {
-      pushHostContainer(workInProgress.stateNode.containerInfo);
+    // TODO: Handle HostComponent tags here as well and call pushHostContext()?
+    // See PR 8590 discussion for context
+    switch (workInProgress.tag) {
+      case ClassComponent:
+        if (isContextProvider(workInProgress)) {
+          pushContextProvider(workInProgress, false);
+        }
+        break;
+      case HostPortal:
+        pushHostContainer(workInProgress.stateNode.containerInfo);
+        break;
     }
     // TODO: What if this is currently in progress?
     // How can that happen? How is this not being cloned?

src/renderers/shared/fiber/__tests__/ReactIncremental-test.js

@@ -1932,4 +1932,47 @@ describe('ReactIncremental', () => {
       'ShowLocaleFn:read {"locale":"gr"}',
     ]);
   });
+
+  it('maintains the correct context index when context proviers are bailed out due to low priority', () => {
+    class Root extends React.Component {
+      render() {
+        return <Middle {...this.props} />;
+      }
+    }
+
+    let instance;
+
+    class Middle extends React.Component {
+      constructor(props, context) {
+        super(props, context);
+        instance = this;
+      }
+      shouldComponentUpdate() {
+        // Return false so that our child will get a NoWork priority (and get bailed out)
+        return false;
+      }
+      render() {
+        return <Child />;
+      }
+    }
+
+    // Child must be a context provider to trigger the bug
+    class Child extends React.Component {
+      static childContextTypes = {};
+      getChildContext() {
+        return {};
+      }
+      render() {
+        return <div />;
+      }
+    }
+
+    // Init
+    ReactNoop.render(<Root />);
+    ReactNoop.flush();
+
+    // Trigger an update in the middle of the tree
+    instance.setState({});
+    ReactNoop.flush();
+  });
 });