Add optimization to inline all boundaries at once if possible

Author: sebmarkbage

packages/react-dom/src/__tests__/ReactDOMServerPartialHydration-test.internal.js

@@ -2444,14 +2444,16 @@ describe('ReactDOMServerPartialHydration', () => {
 
     function App({showMore}) {
       return (
-        <SuspenseList revealOrder="together">
-          {a}
-          {showMore ? (
-            <Suspense fallback="Loading B">
-              <AlwaysSuspend />
-            </Suspense>
-          ) : null}
-        </SuspenseList>
+        <div>
+          <SuspenseList revealOrder="together">
+            {a}
+            {showMore ? (
+              <Suspense fallback="Loading B">
+                <AlwaysSuspend />
+              </Suspense>
+            ) : null}
+          </SuspenseList>
+        </div>
       );
     }
 

packages/react-server/src/ReactFizzServer.js

@@ -5394,14 +5394,39 @@ function flushSegment(
     return flushSubtree(request, destination, segment, hoistableState);
   }
 
+  const row = boundary.row;
+  if (
+    boundary.status === PENDING &&
+    row !== null &&
+    row.together &&
+    row.pendingTasks === row.boundaries.length
+  ) {
+    // If we have a "together" row and all the pendingTasks are really the boundaries themselves,
+    // and we won't outline any of them then we can unblock this row early so that we can inline
+    // all the boundaries at once.
+    let allComplete = true;
+    for (let i = 0; i < row.boundaries.length; i++) {
+      const rowBoundary = row.boundaries[i];
+      if (
+        rowBoundary.pendingTasks !== 1 ||
+        isEligibleForOutlining(request, rowBoundary)
+      ) {
+        allComplete = false;
+        break;
+      }
+    }
+    if (allComplete) {
+      unblockSuspenseListRow(request, row);
+    }
+  }
+
   boundary.parentFlushed = true;
   // This segment is a Suspense boundary. We need to decide whether to
   // emit the content or the fallback now.
   if (boundary.status === CLIENT_RENDERED) {
     // Emit a client rendered suspense boundary wrapper.
     // We never queue the inner boundary so we'll never emit its content or partial segments.
 
-    const row = boundary.row;
     if (row !== null) {
       // Since this boundary end up client rendered, we can unblock future suspense list rows.
       // This means that they may appear out of order if the future rows succeed but this is
@@ -5500,7 +5525,6 @@ function flushSegment(
       hoistHoistables(hoistableState, boundary.contentState);
     }
 
-    const row = boundary.row;
     if (row !== null && isEligibleForOutlining(request, boundary)) {
       // Once we have written the boundary, we can unblock the row and let future
       // rows be written. This may schedule new completed boundaries.