Skip emitting entire hidden Activity boundary including comments from SSR

Then we handle this fully in the Activity wrapper. That's what determines
whether the offscreen content is SSR:ed or not.

This also optimizes by avoiding an emit commit pass that used to hydrate
the hidden Activity and then client renders the hidden content.

Author: sebmarkbage

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

@@ -3746,11 +3746,7 @@ describe('ReactDOMServerPartialHydration', () => {
         <span>
           Visible
         </span>
-        <!--&-->
-        <!--/&-->
         <!--$-->
-        <!--&-->
-        <!--/&-->
         <!--/$-->
       </div>
     `);
@@ -3767,10 +3763,6 @@ describe('ReactDOMServerPartialHydration', () => {
       await waitForPaint([]);
     }
 
-    // Commit just the Activity boundary
-    // TODO: Optimize this
-    await waitForPaint([]);
-
     // Subsequently, the hidden child is prerendered on the client
     // along with hydrating the Suspense boundary outside the Activity.
     await waitForPaint(['HiddenChild']);
@@ -3779,11 +3771,7 @@ describe('ReactDOMServerPartialHydration', () => {
         <span>
           Visible
         </span>
-        <!--&-->
-        <!--/&-->
         <!--$-->
-        <!--&-->
-        <!--/&-->
         <!--/$-->
         <span
           style="display: none;"
@@ -3801,11 +3789,7 @@ describe('ReactDOMServerPartialHydration', () => {
         <span>
           Visible
         </span>
-        <!--&-->
-        <!--/&-->
         <!--$-->
-        <!--&-->
-        <!--/&-->
         <!--/$-->
         <span
           style="display: none;"

packages/react-reconciler/src/ReactFiberBeginWork.js

@@ -730,12 +730,7 @@ function updateOffscreenComponent(
       reuseHiddenContextOnStack(workInProgress);
       pushOffscreenSuspenseHandler(workInProgress);
     } else if (
-      !includesSomeLane(renderLanes, (OffscreenLane: Lane)) ||
-      // SSR doesn't render hidden content (except legacy hidden) so it shouldn't hydrate,
-      // even at offscreen lane. Defer to a client rendered offscreen lane.
-      (getIsHydrating() &&
-        (!enableLegacyHidden ||
-          nextProps.mode !== 'unstable-defer-without-hiding'))
+      !includesSomeLane(renderLanes, (OffscreenLane: Lane))
     ) {
       // We're hidden, and we're not rendering at Offscreen. We will bail out
       // and resume this tree later.
@@ -1113,17 +1108,26 @@ function updateActivityComponent(
 
     // Special path for hydration
     // If we're currently hydrating, try to hydrate this boundary.
+    // Hidden Activity boundaries are not emitted on the server.
     if (getIsHydrating()) {
-      // We must push the suspense handler context *before* attempting to
-      // hydrate, to avoid a mismatch in case it errors.
-      pushDehydratedActivitySuspenseHandler(workInProgress);
-      const dehydrated: ActivityInstance =
-        claimNextHydratableActivityInstance(workInProgress);
-      return mountDehydratedActivityComponent(
-        workInProgress,
-        dehydrated,
-        renderLanes,
-      );
+      if (nextProps.mode === 'hidden') {
+        // SSR doesn't render hidden Activity so it shouldn't hydrate,
+        // even at offscreen lane. Defer to a client rendered offscreen lane.
+        mountActivityChildren(workInProgress, nextProps, renderLanes);
+        workInProgress.lanes = laneToLanes(OffscreenLane);
+        return null;
+      } else {
+        // We must push the suspense handler context *before* attempting to
+        // hydrate, to avoid a mismatch in case it errors.
+        pushDehydratedActivitySuspenseHandler(workInProgress);
+        const dehydrated: ActivityInstance =
+          claimNextHydratableActivityInstance(workInProgress);
+        return mountDehydratedActivityComponent(
+          workInProgress,
+          dehydrated,
+          renderLanes,
+        );
+      }
     }
 
     return mountActivityChildren(workInProgress, nextProps, renderLanes);

packages/react-server/src/ReactFizzServer.js

@@ -2227,24 +2227,24 @@ function renderActivity(
     }
   } else {
     // Render
-    // An Activity boundary is delimited so that we can hydrate it separately.
-    pushStartActivityBoundary(segment.chunks, request.renderState);
-    segment.lastPushedText = false;
     const mode = props.mode;
     if (mode === 'hidden') {
       // A hidden Activity boundary is not server rendered. Prerendering happens
       // on the client.
     } else {
+      // An Activity boundary is delimited so that we can hydrate it separately.
+      pushStartActivityBoundary(segment.chunks, request.renderState);
+      segment.lastPushedText = false;
       // A visible Activity boundary has its children rendered inside the boundary.
       const prevKeyPath = task.keyPath;
       task.keyPath = keyPath;
       // We use the non-destructive form because if something suspends, we still
       // need to pop back up and finish the end comment.
       renderNode(request, task, props.children, -1);
       task.keyPath = prevKeyPath;
+      pushEndActivityBoundary(segment.chunks, request.renderState);
+      segment.lastPushedText = false;
     }
-    pushEndActivityBoundary(segment.chunks, request.renderState);
-    segment.lastPushedText = false;
   }
 }