Add Offscreen component type

Doesn't do anything special in this initial commit. Just acts like a
fragment.

packages/react-reconciler/src/ReactFiber.new.js

@@ -54,6 +54,7 @@ import {
   FundamentalComponent,
   ScopeComponent,
   Block,
+  OffscreenComponent,
 } from './ReactWorkTags';
 import getComponentName from 'shared/getComponentName';
 
@@ -87,6 +88,7 @@ import {
   REACT_FUNDAMENTAL_TYPE,
   REACT_SCOPE_TYPE,
   REACT_BLOCK_TYPE,
+  REACT_OFFSCREEN_TYPE,
 } from 'shared/ReactSymbols';
 
 export type {Fiber};
@@ -511,6 +513,13 @@ export function createFiberFromTypeAndProps(
           expirationTime,
           key,
         );
+      case REACT_OFFSCREEN_TYPE:
+        return createFiberFromOffscreen(
+          pendingProps,
+          mode,
+          expirationTime,
+          key,
+        );
       default: {
         if (typeof type === 'object' && type !== null) {
           switch (type.$$typeof) {
@@ -728,6 +737,24 @@ export function createFiberFromSuspenseList(
   return fiber;
 }
 
+export function createFiberFromOffscreen(
+  pendingProps: any,
+  mode: TypeOfMode,
+  expirationTime: ExpirationTimeOpaque,
+  key: null | string,
+) {
+  const fiber = createFiber(OffscreenComponent, pendingProps, key, mode);
+  // TODO: The OffscreenComponent fiber shouldn't have a type. It has a tag.
+  // This needs to be fixed in getComponentName so that it relies on the tag
+  // instead.
+  if (__DEV__) {
+    fiber.type = REACT_OFFSCREEN_TYPE;
+  }
+  fiber.elementType = REACT_OFFSCREEN_TYPE;
+  fiber.expirationTime_opaque = expirationTime;
+  return fiber;
+}
+
 export function createFiberFromText(
   content: string,
   mode: TypeOfMode,

packages/react-reconciler/src/ReactFiberBeginWork.new.js

@@ -45,6 +45,7 @@ import {
   FundamentalComponent,
   ScopeComponent,
   Block,
+  OffscreenComponent,
 } from './ReactWorkTags';
 import {
   NoEffect,
@@ -555,6 +556,21 @@ function updateSimpleMemoComponent(
   );
 }
 
+function updateOffscreenComponent(
+  current: Fiber | null,
+  workInProgress: Fiber,
+  renderExpirationTime: ExpirationTimeOpaque,
+) {
+  const nextChildren = workInProgress.pendingProps;
+  reconcileChildren(
+    current,
+    workInProgress,
+    nextChildren,
+    renderExpirationTime,
+  );
+  return workInProgress.child;
+}
+
 function updateFragment(
   current: Fiber | null,
   workInProgress: Fiber,
@@ -3448,6 +3464,13 @@ function beginWork(
         renderExpirationTime,
       );
     }
+    case OffscreenComponent: {
+      return updateOffscreenComponent(
+        current,
+        workInProgress,
+        renderExpirationTime,
+      );
+    }
     case SimpleMemoComponent: {
       return updateSimpleMemoComponent(
         current,

packages/react-reconciler/src/ReactFiberCompleteWork.new.js

@@ -53,6 +53,7 @@ import {
   FundamentalComponent,
   ScopeComponent,
   Block,
+  OffscreenComponent,
 } from './ReactWorkTags';
 import {NoMode, BlockingMode} from './ReactTypeOfMode';
 import {Ref, Update, NoEffect, DidCapture} from './ReactSideEffectTags';
@@ -1287,6 +1288,8 @@ function completeWork(
         return null;
       }
       break;
+    case OffscreenComponent:
+      return null;
   }
   invariant(
     false,

packages/react-reconciler/src/ReactWorkTags.js

@@ -30,7 +30,8 @@ export type WorkTag =
   | 19
   | 20
   | 21
-  | 22;
+  | 22
+  | 23;
 
 export const FunctionComponent = 0;
 export const ClassComponent = 1;
@@ -55,3 +56,4 @@ export const SuspenseListComponent = 19;
 export const FundamentalComponent = 20;
 export const ScopeComponent = 21;
 export const Block = 22;
+export const OffscreenComponent = 23;

packages/shared/ReactSymbols.js

@@ -32,6 +32,7 @@ export let REACT_RESPONDER_TYPE = 0xead6;
 export let REACT_SCOPE_TYPE = 0xead7;
 export let REACT_OPAQUE_ID_TYPE = 0xeae0;
 export let REACT_DEBUG_TRACING_MODE_TYPE = 0xeae1;
+export let REACT_OFFSCREEN_TYPE = 0xeae2;
 
 if (typeof Symbol === 'function' && Symbol.for) {
   const symbolFor = Symbol.for;
@@ -54,6 +55,7 @@ if (typeof Symbol === 'function' && Symbol.for) {
   REACT_SCOPE_TYPE = symbolFor('react.scope');
   REACT_OPAQUE_ID_TYPE = symbolFor('react.opaque.id');
   REACT_DEBUG_TRACING_MODE_TYPE = symbolFor('react.debug_trace_mode');
+  REACT_OFFSCREEN_TYPE = symbolFor('react.offscreen');
 }
 
 const MAYBE_ITERATOR_SYMBOL = typeof Symbol === 'function' && Symbol.iterator;