From 37e918e790fc2a414f368bb0a636c2e21649fce4 Mon Sep 17 00:00:00 2001
From: Sebastian Sebbie Silbermann <sebastian.silbermann@vercel.com>
Date: Tue, 10 Feb 2026 12:25:16 +0100
Subject: [PATCH 1/2] Current behavior

---
 .../src/__tests__/store-test.js               | 55 +++++++++++++++++++
 1 file changed, 55 insertions(+)

diff --git a/packages/react-devtools-shared/src/__tests__/store-test.js b/packages/react-devtools-shared/src/__tests__/store-test.js
index 871b44827e77..3309131a5ffb 100644
--- a/packages/react-devtools-shared/src/__tests__/store-test.js
+++ b/packages/react-devtools-shared/src/__tests__/store-test.js
@@ -3616,4 +3616,59 @@ describe('Store', () => {
               <div>
     `);
   });
+
+  // @reactVersion >= 19
+  it('cleans up host hoistables', async () => {
+    function Left() {
+      return (
+        <style href="test.css" precedence="medium">
+          {'* {color:black}'}
+        </style>
+      );
+    }
+
+    function Right() {
+      return (
+        <style href="test.css" precedence="medium">
+          {'* {color:black}'}
+        </style>
+      );
+    }
+
+    await actAsync(() => {
+      render(
+        <>
+          <Left />
+          <Right />
+        </>,
+      );
+    });
+
+    // Ensure we're still testing deduplicated hoistables.
+    expect(document.head.querySelectorAll('style')).toHaveLength(1);
+    expect(store).toMatchInlineSnapshot(`
+      [root]
+          <Left>
+          <Right>
+    `);
+    let style = document.head.querySelector('style');
+    let styleID = agent.getIDForHostInstance(style).id;
+    expect(store.containsElement(styleID)).toBe(true);
+
+    await actAsync(() => {
+      render(
+        <>
+          <Right />
+        </>,
+      );
+    });
+
+    expect(store).toMatchInlineSnapshot(`
+      [root]
+          <Right>
+    `);
+    style = document.head.querySelector('style');
+    styleID = agent.getIDForHostInstance(style).id;
+    expect(store.containsElement(styleID)).toBe(false);
+  });
 });

From 2ae2122e5ed0193f0c211a34c8169b9baaafa8fb Mon Sep 17 00:00:00 2001
From: Sebastian Sebbie Silbermann <sebastian.silbermann@vercel.com>
Date: Tue, 10 Feb 2026 12:27:28 +0100
Subject: [PATCH 2/2] [DevTools] Fix memory leak when unmounting hoistables

---
 packages/react-devtools-shared/src/__tests__/store-test.js   | 2 +-
 packages/react-devtools-shared/src/backend/fiber/renderer.js | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/packages/react-devtools-shared/src/__tests__/store-test.js b/packages/react-devtools-shared/src/__tests__/store-test.js
index 3309131a5ffb..ded9088869f4 100644
--- a/packages/react-devtools-shared/src/__tests__/store-test.js
+++ b/packages/react-devtools-shared/src/__tests__/store-test.js
@@ -3669,6 +3669,6 @@ describe('Store', () => {
     `);
     style = document.head.querySelector('style');
     styleID = agent.getIDForHostInstance(style).id;
-    expect(store.containsElement(styleID)).toBe(false);
+    expect(store.containsElement(styleID)).toBe(true);
   });
 });
diff --git a/packages/react-devtools-shared/src/backend/fiber/renderer.js b/packages/react-devtools-shared/src/backend/fiber/renderer.js
index 3214c8392285..916d69823285 100644
--- a/packages/react-devtools-shared/src/backend/fiber/renderer.js
+++ b/packages/react-devtools-shared/src/backend/fiber/renderer.js
@@ -991,8 +991,8 @@ function releaseHostResource(
         // eslint-disable-next-line no-for-of-loops/no-for-of-loops
         for (const firstInstance of resourceInstances) {
           publicInstanceToDevToolsInstanceMap.set(
+            publicInstance,
             firstInstance,
-            nearestInstance,
           );
           break;
         }