isShowingAnyFallbacks -> findFirstFallback

This lets us reuse the found fallback so we can transfer the update queue
from it.

packages/react-reconciler/src/ReactFiberBeginWork.js

@@ -125,7 +125,7 @@ import {
   addSubtreeSuspenseContext,
   setShallowSuspenseContext,
 } from './ReactFiberSuspenseContext';
-import {isShowingAnyFallbacks} from './ReactFiberSuspenseComponent';
+import {findFirstFallback} from './ReactFiberSuspenseComponent';
 import {
   pushProvider,
   propagateContextChange,
@@ -2000,7 +2000,7 @@ function findLastContentRow(firstChild: null | Fiber): null | Fiber {
   while (row !== null) {
     let currentRow = row.alternate;
     // New rows can't be content rows.
-    if (currentRow !== null && !isShowingAnyFallbacks(currentRow)) {
+    if (currentRow !== null && findFirstFallback(currentRow) === null) {
       lastContentRow = row;
     }
     row = row.sibling;
@@ -2281,7 +2281,7 @@ function updateSuspenseListComponent(
         while (row !== null) {
           let currentRow = row.alternate;
           // New rows can't be content rows.
-          if (currentRow !== null && !isShowingAnyFallbacks(currentRow)) {
+          if (currentRow !== null && findFirstFallback(currentRow) === null) {
             // This is the beginning of the main content.
             workInProgress.child = row;
             break;

packages/react-reconciler/src/ReactFiberCompleteWork.js

@@ -92,7 +92,7 @@ import {
   ForceSuspenseFallback,
   setDefaultShallowSuspenseContext,
 } from './ReactFiberSuspenseContext';
-import {isShowingAnyFallbacks} from './ReactFiberSuspenseComponent';
+import {findFirstFallback} from './ReactFiberSuspenseComponent';
 import {
   isContextProvider as isLegacyContextProvider,
   popContext as popLegacyContext,
@@ -1061,7 +1061,8 @@ function completeWork(
       } else {
         // Append the rendered row to the child list.
         if (!didSuspendAlready) {
-          if (isShowingAnyFallbacks(renderedTail)) {
+          let fallback = findFirstFallback(renderedTail);
+          if (fallback !== null) {
             workInProgress.effectTag |= DidCapture;
             didSuspendAlready = true;
             cutOffTailIfNeeded(renderState, true);
@@ -1072,9 +1073,11 @@ function completeWork(
             ) {
               // We need to delete the row we just rendered.
               // Ensure we transfer the update queue to the parent.
-              // TODO: This just reuses the code from the other path but could
-              // be optimized better.
-              hasSuspendedChildrenAndNewContent(workInProgress, renderedTail);
+              let fallbackThennables = fallback.updateQueue;
+              if (fallbackThennables !== null) {
+                workInProgress.updateQueue = fallbackThennables;
+                workInProgress.effectTag |= Update;
+              }
               // Reset the effect list to what it w as before we rendered this
               // child. The nested children have already appended themselves.
               let lastEffect = (workInProgress.lastEffect =

packages/react-reconciler/src/ReactFiberSuspenseComponent.js

@@ -61,30 +61,30 @@ export function shouldCaptureSuspense(
   return true;
 }
 
-export function isShowingAnyFallbacks(row: Fiber): boolean {
+export function findFirstFallback(row: Fiber): null | Fiber {
   let node = row;
   while (node !== null) {
     if (node.tag === SuspenseComponent) {
       const state: SuspenseState | null = node.memoizedState;
       if (state !== null) {
-        return true;
+        return node;
       }
     } else if (node.child !== null) {
       node.child.return = node;
       node = node.child;
       continue;
     }
     if (node === row) {
-      return false;
+      return null;
     }
     while (node.sibling === null) {
       if (node.return === null || node.return === row) {
-        return false;
+        return null;
       }
       node = node.return;
     }
     node.sibling.return = node.return;
     node = node.sibling;
   }
-  return false;
+  return null;
 }