From e6cf23ceee14dfdc952baedc75e8058da1f143e7 Mon Sep 17 00:00:00 2001
From: Sebastian Sebbie Silbermann <sebastian.silbermann@vercel.com>
Date: Tue, 16 Sep 2025 15:00:31 +0200
Subject: [PATCH 1/2] [DevTools] Use alternate as previous Fiber when
 reconciling root

---
 packages/react-devtools-shared/src/backend/fiber/renderer.js | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/packages/react-devtools-shared/src/backend/fiber/renderer.js b/packages/react-devtools-shared/src/backend/fiber/renderer.js
index 37e1c0a87d15b..ca01ca2135133 100644
--- a/packages/react-devtools-shared/src/backend/fiber/renderer.js
+++ b/packages/react-devtools-shared/src/backend/fiber/renderer.js
@@ -5398,7 +5398,7 @@ export function attach(
       rootToFiberInstanceMap.set(root, rootInstance);
       idToDevToolsInstanceMap.set(rootInstance.id, rootInstance);
     } else {
-      prevFiber = rootInstance.data;
+      prevFiber = rootInstance.data.alternate;
     }
     currentRoot = rootInstance;
 

From a387a3fd22fc2c0858c2ec789372e8efa23ae235 Mon Sep 17 00:00:00 2001
From: Sebastian Sebbie Silbermann <sebastian.silbermann@vercel.com>
Date: Tue, 16 Sep 2025 15:41:40 +0200
Subject: [PATCH 2/2] [DevTools] Don't keep stale root instances around we
 never mounted

---
 .../src/backend/fiber/renderer.js                 | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/packages/react-devtools-shared/src/backend/fiber/renderer.js b/packages/react-devtools-shared/src/backend/fiber/renderer.js
index ca01ca2135133..22300ec5f7f99 100644
--- a/packages/react-devtools-shared/src/backend/fiber/renderer.js
+++ b/packages/react-devtools-shared/src/backend/fiber/renderer.js
@@ -1516,7 +1516,7 @@ export function attach(
       currentRoot = rootInstance;
       unmountInstanceRecursively(rootInstance);
       rootToFiberInstanceMap.delete(root);
-      flushPendingEvents(root);
+      flushPendingEvents();
       currentRoot = (null: any);
     });
 
@@ -1541,7 +1541,7 @@ export function attach(
       currentRoot = newRoot;
       setRootPseudoKey(currentRoot.id, root.current);
       mountFiberRecursively(root.current, false);
-      flushPendingEvents(root);
+      flushPendingEvents();
       currentRoot = (null: any);
     });
 
@@ -2099,7 +2099,7 @@ export function attach(
     }
   }
 
-  function flushPendingEvents(root: Object): void {
+  function flushPendingEvents(): void {
     if (shouldBailoutWithPendingOperations()) {
       // If we aren't profiling, we can just bail out here.
       // No use sending an empty update over the bridge.
@@ -5349,7 +5349,7 @@ export function attach(
 
         mountFiberRecursively(root.current, false);
 
-        flushPendingEvents(root);
+        flushPendingEvents();
 
         needsToFlushComponentLogs = false;
         currentRoot = (null: any);
@@ -5398,7 +5398,7 @@ export function attach(
       rootToFiberInstanceMap.set(root, rootInstance);
       idToDevToolsInstanceMap.set(rootInstance.id, rootInstance);
     } else {
-      prevFiber = rootInstance.data.alternate;
+      prevFiber = rootInstance.data;
     }
     currentRoot = rootInstance;
 
@@ -5452,6 +5452,9 @@ export function attach(
       unmountInstanceRecursively(rootInstance);
       removeRootPseudoKey(currentRoot.id);
       rootToFiberInstanceMap.delete(root);
+    } else if (!prevWasMounted && !nextIsMounted) {
+      // We don't need this root anymore.
+      rootToFiberInstanceMap.delete(root);
     }
 
     if (isProfiling && isProfilingSupported) {
@@ -5475,7 +5478,7 @@ export function attach(
     }
 
     // We're done here.
-    flushPendingEvents(root);
+    flushPendingEvents();
 
     needsToFlushComponentLogs = false;