Always override priority level when visiting children

It is not possible for a child to have a higher priority level
than we're reconciling at, unless we intentionally want to
down-prioritize it.

src/renderers/shared/fiber/ReactChildFiber.js

@@ -148,10 +148,10 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
       clone.sibling = null;
       return clone;
     } else {
-      if (fiber.pendingWorkPriority === NoWork ||
-          fiber.pendingWorkPriority > priority) {
-        fiber.pendingWorkPriority = priority;
-      }
+      // We override the pending priority even if it is higher, because if
+      // we're reconciling at a lower priority that means that this was
+      // down-prioritized.
+      fiber.pendingWorkPriority = priority;
       fiber.effectTag = NoWork;
       fiber.index = 0;
       fiber.sibling = null;

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

@@ -417,6 +417,24 @@ describe('ReactIncremental', () => {
     // after them which is not correct.
     ReactNoop.flush();
     expect(ops).toEqual(['Bar', 'Middle', 'Bar']);
+
+    ops = [];
+
+    // Let us try this again without fully finishing the first time. This will
+    // create a hanging subtree that is reconciling at the normal priority.
+    ReactNoop.render(<Foo text="foo" />);
+    ReactNoop.flushDeferredPri(40);
+
+    expect(ops).toEqual(['Foo', 'Bar']);
+
+    ops = [];
+
+    // This update will create a tree that aborts that work and down-prioritizes
+    // it. If the priority levels aren't down-prioritized correctly this may
+    // abort rendering of the down-prioritized content.
+    ReactNoop.render(<Foo text="bar" />);
+    ReactNoop.flush();
+    expect(ops).toEqual(['Foo', 'Bar', 'Bar']);
   });
 
   it('can reuse work done after being preempted', () => {