Check if we're in sync mode before deprioritizing hidden subtrees

scripts/fiber/tests-passing.txt

@@ -854,6 +854,7 @@ src/renderers/__tests__/ReactUpdates-test.js
 * handles reentrant mounting in synchronous mode
 * mounts and unmounts are sync even in a batch
 * does not re-render if state update is null
+* synchronously renders hidden subtrees
 
 src/renderers/__tests__/refs-destruction-test.js
 * should remove refs when destroying the parent

src/renderers/__tests__/ReactUpdates-test.js

@@ -1160,15 +1160,16 @@ describe('ReactUpdates', () => {
     expect(mounts).toBe(1);
   });
 
-  it('mounts and unmounts are sync even in a batch', done => {
+  it('mounts and unmounts are sync even in a batch', () => {
+    var ops = [];
     var container = document.createElement('div');
     ReactDOM.unstable_batchedUpdates(() => {
       ReactDOM.render(<div>Hello</div>, container);
-      expect(container.textContent).toEqual('Hello');
+      ops.push(container.textContent);
       ReactDOM.unmountComponentAtNode(container);
-      expect(container.textContent).toEqual('');
-      done();
+      ops.push(container.textContent);
     });
+    expect(ops).toEqual(['Hello', '']);
   });
 
   it('does not re-render if state update is null', () => {
@@ -1192,7 +1193,7 @@ describe('ReactUpdates', () => {
 
   // Will change once we switch to async by default
   it('synchronously renders hidden subtrees', () => {
-    let container = document.createElement('div');
+    const container = document.createElement('div');
     let ops = [];
 
     function Baz() {

src/renderers/shared/fiber/ReactFiberBeginWork.js

@@ -77,7 +77,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
   getPriorityContext : () => PriorityLevel,
 ) {
 
-  const { shouldSetTextContent } = config;
+  const { shouldSetTextContent, useSyncScheduling } = config;
 
   const {
     pushHostContext,
@@ -357,7 +357,8 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
         );
       }
     } else if (nextProps === null || memoizedProps === nextProps) {
-      if (memoizedProps.hidden &&
+      if (!useSyncScheduling &&
+          memoizedProps.hidden &&
           workInProgress.pendingWorkPriority !== OffscreenPriority) {
         // This subtree still has work, but it should be deprioritized so we need
         // to bail out and not do any work yet.
@@ -398,7 +399,8 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
 
     markRef(current, workInProgress);
 
-    if (nextProps.hidden &&
+    if (!useSyncScheduling &&
+        nextProps.hidden &&
         workInProgress.pendingWorkPriority !== OffscreenPriority) {
       // If this host component is hidden, we can bail out on the children.
       // We'll rerender the children later at the lower priority.