Down-prioritize children of hidden host components

To make sure we don't reset the priority of a down-prioritized fiber,
we compare the priority we're currently rendering at to the fiber's
work priority. If the work priority is lower, then we know not to reset
the work priority.

scripts/fiber/tests-passing.txt

@@ -2023,6 +2023,8 @@ src/renderers/shared/fiber/__tests__/ReactIncremental-test.js
 * updates a previous render
 * can cancel partially rendered work and restart
 * should call callbacks even if updates are aborted
+* can deprioritize unfinished work and resume it later
+* can deprioritize a tree from without dropping work
 * memoizes work even if shouldComponentUpdate returns false
 * can update in the middle of a tree using setState
 * can queue multiple state updates
@@ -2035,6 +2037,7 @@ src/renderers/shared/fiber/__tests__/ReactIncremental-test.js
 * can handle if setState callback throws
 * merges and masks context
 * does not leak own context into context provider
+* 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 when providers bail out due to low priority
@@ -2078,6 +2081,7 @@ src/renderers/shared/fiber/__tests__/ReactIncrementalPerf-test.js
 * warns on cascading renders from top-level render
 * does not treat setState from cWM or cWRP as cascading
 * captures all lifecycles
+* measures deprioritized work
 * measures deferred work in chunks
 * recovers from fatal errors
 * recovers from caught errors
@@ -2117,6 +2121,7 @@ src/renderers/shared/fiber/__tests__/ReactIncrementalSideEffects-test.js
 * can reuse side-effects after being preempted, if shouldComponentUpdate is false
 * can update a completed tree before it has a chance to commit
 * updates a child even though the old props is empty
+* deprioritizes setStates that happens within a deprioritized tree
 * calls callback after update is flushed
 * calls setState callback even if component bails out
 * calls componentWillUnmount after a deletion, even if nested

src/renderers/shared/fiber/ReactChildFiber.js

@@ -1463,7 +1463,7 @@ exports.cloneChildFibers = function(
   let currentChild = workInProgress.child;
   let newChild = createWorkInProgress(currentChild, renderPriority);
   // TODO: Pass this as an argument, since it's easy to forget.
-  newChild.pendingProps = null;
+  newChild.pendingProps = currentChild.pendingProps;
   workInProgress.child = newChild;
 
   newChild.return = workInProgress;
@@ -1473,7 +1473,7 @@ exports.cloneChildFibers = function(
       currentChild,
       renderPriority,
     );
-    newChild.pendingProps = null;
+    newChild.pendingProps = currentChild.pendingProps;
     newChild.return = workInProgress;
   }
   newChild.sibling = null;

src/renderers/shared/fiber/ReactFiber.js

@@ -451,3 +451,10 @@ exports.createFiberFromPortal = function(
   };
   return fiber;
 };
+
+exports.largerPriority = function(
+  p1: PriorityLevel,
+  p2: PriorityLevel,
+): PriorityLevel {
+  return p1 !== NoWork && (p2 === NoWork || p2 > p1) ? p1 : p2;
+};

src/renderers/shared/fiber/ReactFiberBeginWork.js

@@ -49,7 +49,7 @@ var {
   YieldComponent,
   Fragment,
 } = ReactTypeOfWork;
-var {NoWork} = require('ReactPriorityLevel');
+var {NoWork, OffscreenPriority} = require('ReactPriorityLevel');
 var {
   PerformedWork,
   Placement,
@@ -76,7 +76,11 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
   scheduleUpdate: (fiber: Fiber, priorityLevel: PriorityLevel) => void,
   getPriorityContext: (fiber: Fiber, forceAsync: boolean) => PriorityLevel,
 ) {
-  const {shouldSetTextContent} = config;
+  const {
+    shouldSetTextContent,
+    useSyncScheduling,
+    shouldDeprioritizeSubtree,
+  } = config;
 
   const {pushHostContext, pushHostContainer} = hostContext;
 
@@ -375,28 +379,29 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
     return bailoutOnAlreadyFinishedWork(current, workInProgress);
   }
 
-  function updateHostComponent(current, workInProgress) {
+  function updateHostComponent(current, workInProgress, renderPriority) {
     pushHostContext(workInProgress);
 
     if (current === null) {
       tryToClaimNextHydratableInstance(workInProgress);
     }
 
-    let nextProps = workInProgress.pendingProps;
     const type = workInProgress.type;
-    const prevProps = current !== null ? current.memoizedProps : null;
     const memoizedProps = workInProgress.memoizedProps;
+    let nextProps = workInProgress.pendingProps;
+    if (nextProps === null) {
+      nextProps = memoizedProps;
+      invariant(
+        nextProps !== null,
+        'We should always have pending or current props. This error is ' +
+          'likely caused by a bug in React. Please file an issue.',
+      );
+    }
+    const prevProps = current !== null ? current.memoizedProps : null;
+
     if (hasContextChanged()) {
       // Normally we can bail out on props equality but if context has changed
       // we don't do the bailout and we have to reuse existing props instead.
-      if (nextProps === null) {
-        nextProps = memoizedProps;
-        invariant(
-          nextProps !== null,
-          'We should always have pending or current props. This error is ' +
-            'likely caused by a bug in React. Please file an issue.',
-        );
-      }
     } else if (nextProps === null || memoizedProps === nextProps) {
       return bailoutOnAlreadyFinishedWork(current, workInProgress);
     }
@@ -418,6 +423,18 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
 
     markRef(current, workInProgress);
 
+    // Check the host config to see if the children are offscreen/hidden.
+    if (
+      renderPriority !== OffscreenPriority &&
+      !useSyncScheduling &&
+      shouldDeprioritizeSubtree(type, nextProps)
+    ) {
+      // Down-prioritize the children.
+      workInProgress.pendingWorkPriority = OffscreenPriority;
+      // Bailout and come back to this fiber later at OffscreenPriority.
+      return null;
+    }
+
     reconcileChildren(current, workInProgress, nextChildren);
     memoizeProps(workInProgress, nextProps);
     return workInProgress.child;
@@ -686,10 +703,9 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
     return null;
   }
 
+  // TODO: Delete memoizeProps/State and move to reconcile/bailout instead
   function memoizeProps(workInProgress: Fiber, nextProps: any) {
     workInProgress.memoizedProps = nextProps;
-    // Reset the pending props
-    workInProgress.pendingProps = null;
   }
 
   function memoizeState(workInProgress: Fiber, nextState: any) {
@@ -728,7 +744,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
       case HostRoot:
         return updateHostRoot(current, workInProgress, priorityLevel);
       case HostComponent:
-        return updateHostComponent(current, workInProgress);
+        return updateHostComponent(current, workInProgress, priorityLevel);
       case HostText:
         return updateHostText(current, workInProgress);
       case CoroutineHandlerPhase:
@@ -793,13 +809,17 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
 
     // Unmount the current children as if the component rendered null
     const nextChildren = null;
-    reconcileChildren(current, workInProgress, nextChildren);
+    reconcileChildrenAtPriority(
+      current,
+      workInProgress,
+      nextChildren,
+      priorityLevel,
+    );
 
     if (workInProgress.tag === ClassComponent) {
       const instance = workInProgress.stateNode;
       workInProgress.memoizedProps = instance.props;
       workInProgress.memoizedState = instance.state;
-      workInProgress.pendingProps = null;
     }
 
     return workInProgress.child;

src/renderers/shared/fiber/ReactFiberCompleteWork.js

@@ -14,6 +14,7 @@
 
 import type {ReactCoroutine} from 'ReactTypes';
 import type {Fiber} from 'ReactFiber';
+import type {PriorityLevel} from 'ReactPriorityLevel';
 import type {HostContext} from 'ReactFiberHostContext';
 import type {HydrationContext} from 'ReactFiberHydrationContext';
 import type {FiberRoot} from 'ReactFiberRoot';
@@ -23,6 +24,7 @@ var {reconcileChildFibers} = require('ReactChildFiber');
 var {popContextProvider} = require('ReactFiberContext');
 var ReactTypeOfWork = require('ReactTypeOfWork');
 var ReactTypeOfSideEffect = require('ReactTypeOfSideEffect');
+var ReactPriorityLevel = require('ReactPriorityLevel');
 var {
   IndeterminateComponent,
   FunctionalComponent,
@@ -37,6 +39,7 @@ var {
   Fragment,
 } = ReactTypeOfWork;
 var {Placement, Ref, Update} = ReactTypeOfSideEffect;
+var {OffscreenPriority} = ReactPriorityLevel;
 
 if (__DEV__) {
   var ReactDebugCurrentFiber = require('ReactDebugCurrentFiber');
@@ -181,11 +184,24 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
   function completeWork(
     current: Fiber | null,
     workInProgress: Fiber,
+    renderPriority: PriorityLevel,
   ): Fiber | null {
     if (__DEV__) {
       ReactDebugCurrentFiber.current = workInProgress;
     }
 
+    // Get the latest props.
+    let newProps = workInProgress.pendingProps;
+    if (newProps === null) {
+      newProps = workInProgress.memoizedProps;
+    } else if (
+      workInProgress.pendingWorkPriority !== OffscreenPriority ||
+      renderPriority === OffscreenPriority
+    ) {
+      // Reset the pending props, unless this was a down-prioritization.
+      workInProgress.pendingProps = null;
+    }
+
     switch (workInProgress.tag) {
       case FunctionalComponent:
         return null;
@@ -216,7 +232,6 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
         popHostContext(workInProgress);
         const rootContainerInstance = getRootHostContainer();
         const type = workInProgress.type;
-        const newProps = workInProgress.memoizedProps;
         if (current !== null && workInProgress.stateNode != null) {
           // If we have an alternate, that means this is an update and we need to
           // schedule a side-effect to do the updates.
@@ -311,7 +326,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
         return null;
       }
       case HostText: {
-        let newText = workInProgress.memoizedProps;
+        let newText = newProps;
         if (current && workInProgress.stateNode != null) {
           const oldText = current.memoizedProps;
           // If we have an alternate, that means this is an update and we need

src/renderers/shared/fiber/ReactFiberScheduler.js

@@ -48,7 +48,7 @@ var ReactFiberHydrationContext = require('ReactFiberHydrationContext');
 var {ReactCurrentOwner} = require('ReactGlobalSharedState');
 var getComponentName = require('getComponentName');
 
-var {createWorkInProgress} = require('ReactFiber');
+var {createWorkInProgress, largerPriority} = require('ReactFiber');
 var {onCommitRoot} = require('ReactFiberDevToolsHook');
 
 var {
@@ -551,7 +551,18 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
     priorityContext = previousPriorityContext;
   }
 
-  function resetWorkPriority(workInProgress: Fiber) {
+  function resetWorkPriority(
+    workInProgress: Fiber,
+    renderPriority: PriorityLevel,
+  ) {
+    if (
+      workInProgress.pendingWorkPriority !== NoWork &&
+      workInProgress.pendingWorkPriority > renderPriority
+    ) {
+      // This was a down-prioritization. Don't bubble priority from children.
+      return;
+    }
+
     // Check for pending update priority.
     let newPriority = getUpdatePriority(workInProgress);
 
@@ -560,12 +571,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
     let child = workInProgress.child;
     while (child !== null) {
       // Ensure that remaining work priority bubbles up.
-      if (
-        child.pendingWorkPriority !== NoWork &&
-        (newPriority === NoWork || newPriority > child.pendingWorkPriority)
-      ) {
-        newPriority = child.pendingWorkPriority;
-      }
+      newPriority = largerPriority(newPriority, child.pendingWorkPriority);
       child = child.sibling;
     }
     workInProgress.pendingWorkPriority = newPriority;
@@ -578,12 +584,12 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
       // means that we don't need an additional field on the work in
       // progress.
       const current = workInProgress.alternate;
-      const next = completeWork(current, workInProgress);
+      const next = completeWork(current, workInProgress, nextPriorityLevel);
 
       const returnFiber = workInProgress.return;
       const siblingFiber = workInProgress.sibling;
 
-      resetWorkPriority(workInProgress);
+      resetWorkPriority(workInProgress, nextPriorityLevel);
 
       if (next !== null) {
         if (__DEV__) {

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

@@ -244,7 +244,7 @@ describe('ReactIncremental', () => {
     expect(inst.state).toEqual({text: 'bar', text2: 'baz'});
   });
 
-  xit('can deprioritize unfinished work and resume it later', () => {
+  it('can deprioritize unfinished work and resume it later', () => {
     var ops = [];
 
     function Bar(props) {
@@ -296,7 +296,7 @@ describe('ReactIncremental', () => {
     expect(ops).toEqual(['Middle', 'Middle']);
   });
 
-  xit('can deprioritize a tree from without dropping work', () => {
+  it('can deprioritize a tree from without dropping work', () => {
     var ops = [];
 
     function Bar(props) {
@@ -1962,7 +1962,7 @@ describe('ReactIncremental', () => {
     ]);
   });
 
-  xit('provides context when reusing work', () => {
+  it('provides context when reusing work', () => {
     var ops = [];
 
     class Intl extends React.Component {

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

@@ -280,7 +280,7 @@ describe('ReactDebugFiberPerf', () => {
     expect(getFlameChart()).toMatchSnapshot();
   });
 
-  xit('measures deprioritized work', () => {
+  it('measures deprioritized work', () => {
     addComment('Flush the parent');
     ReactNoop.syncUpdates(() => {
       ReactNoop.render(