From e360cc5023092e765f01e482c9c9bf5c2fd9546e Mon Sep 17 00:00:00 2001
From: James Tu <j.tu@salesforce.com>
Date: Fri, 5 Sep 2025 20:00:12 -0700
Subject: [PATCH 1/6] fix: check for connectedness before scheduling
 rehydration

---
 packages/@lwc/engine-core/src/framework/vm.ts | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/packages/@lwc/engine-core/src/framework/vm.ts b/packages/@lwc/engine-core/src/framework/vm.ts
index 54db244bc1..edeadbbd79 100644
--- a/packages/@lwc/engine-core/src/framework/vm.ts
+++ b/packages/@lwc/engine-core/src/framework/vm.ts
@@ -673,7 +673,14 @@ function flushRehydrationQueue() {
     for (let i = 0, len = vms.length; i < len; i += 1) {
         const vm = vms[i];
         try {
-            rehydrate(vm);
+            // We want to prevent rehydration from occurring when nodes are detached from the DOM as this can trigger
+            // unintended side effects, like lifecycle methods being called multiple times.
+            // For backwards compatibility, we use a flag to control the check.
+            // 1. When flag is disabled always rehydrate (legacy behavior)
+            // 2. When flag is enabled only rehydrate when the VM state is connected (fixed behavior)
+            if (!lwcRuntimeFlags.DISABLE_DETACHED_REHYDRATION || vm.state === VMState.connected) {
+                rehydrate(vm);
+            }
         } catch (error) {
             if (i + 1 < len) {
                 // pieces of the queue are still pending to be rehydrated, those should have priority

From f42948b1cf9b938d5755a6074f11bbb1bf516bd5 Mon Sep 17 00:00:00 2001
From: James Tu <j.tu@salesforce.com>
Date: Fri, 5 Sep 2025 20:01:02 -0700
Subject: [PATCH 2/6] chore: add new feature flags for fix

---
 packages/@lwc/features/src/index.ts | 1 +
 packages/@lwc/features/src/types.ts | 6 ++++++
 2 files changed, 7 insertions(+)

diff --git a/packages/@lwc/features/src/index.ts b/packages/@lwc/features/src/index.ts
index 2c4608d7b0..59dc9624f2 100644
--- a/packages/@lwc/features/src/index.ts
+++ b/packages/@lwc/features/src/index.ts
@@ -23,6 +23,7 @@ const features: FeatureFlagMap = {
     DISABLE_SCOPE_TOKEN_VALIDATION: null,
     LEGACY_LOCKER_ENABLED: null,
     DISABLE_LEGACY_VALIDATION: null,
+    DISABLE_DETACHED_REHYDRATION: null,
 };
 
 if (!(globalThis as any).lwcRuntimeFlags) {
diff --git a/packages/@lwc/features/src/types.ts b/packages/@lwc/features/src/types.ts
index ab7984b7a8..f66032f010 100644
--- a/packages/@lwc/features/src/types.ts
+++ b/packages/@lwc/features/src/types.ts
@@ -93,6 +93,12 @@ export interface FeatureFlagMap {
      * If false or unset, then the value of the `LEGACY_LOCKER_ENABLED` flag is used.
      */
     DISABLE_LEGACY_VALIDATION: FeatureFlagValue;
+
+    /**
+     * If true, skips rehydration of DOM elements that are not connected.
+     * Applies to rehydration performed while flushing the rehydration queue.
+     */
+    DISABLE_DETACHED_REHYDRATION: FeatureFlagValue;
 }
 
 export type FeatureFlagName = keyof FeatureFlagMap;

From fc014b563a7b8b7fd018bd68d88ea7d122b3ff8d Mon Sep 17 00:00:00 2001
From: James Tu <j.tu@salesforce.com>
Date: Fri, 5 Sep 2025 20:02:10 -0700
Subject: [PATCH 3/6] test: add flags for tests

---
 packages/@lwc/integration-not-karma/configs/base.js    | 6 +++++-
 packages/@lwc/integration-not-karma/helpers/options.js | 3 +++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/packages/@lwc/integration-not-karma/configs/base.js b/packages/@lwc/integration-not-karma/configs/base.js
index 4b08ac144f..1a7340646f 100644
--- a/packages/@lwc/integration-not-karma/configs/base.js
+++ b/packages/@lwc/integration-not-karma/configs/base.js
@@ -18,6 +18,7 @@ const env = {
         'ENGINE_SERVER',
         'FORCE_NATIVE_SHADOW_MODE_FOR_TEST',
         'NATIVE_SHADOW',
+        'DISABLE_DETACHED_REHYDRATION',
     ]),
     LWC_VERSION,
     NODE_ENV: options.NODE_ENV_FOR_TEST,
@@ -61,7 +62,10 @@ export default {
             <script type="module">
             globalThis.process = ${JSON.stringify({ env })};
             globalThis.lwcRuntimeFlags = ${JSON.stringify(
-                pluck(options, ['DISABLE_NATIVE_CUSTOM_ELEMENT_LIFECYCLE'])
+                pluck(options, [
+                    'DISABLE_NATIVE_CUSTOM_ELEMENT_LIFECYCLE',
+                    'DISABLE_DETACHED_REHYDRATION',
+                ])
             )};
 
             ${maybeImport('@lwc/synthetic-shadow', !options.DISABLE_SYNTHETIC)}
diff --git a/packages/@lwc/integration-not-karma/helpers/options.js b/packages/@lwc/integration-not-karma/helpers/options.js
index a18ae6df4a..bf22b7b96f 100644
--- a/packages/@lwc/integration-not-karma/helpers/options.js
+++ b/packages/@lwc/integration-not-karma/helpers/options.js
@@ -34,6 +34,8 @@ export const DISABLE_NATIVE_CUSTOM_ELEMENT_LIFECYCLE = Boolean(
     process.env.DISABLE_NATIVE_CUSTOM_ELEMENT_LIFECYCLE
 );
 
+export const DISABLE_DETACHED_REHYDRATION = Boolean(process.env.DISABLE_DETACHED_REHYDRATION);
+
 export const ENGINE_SERVER = Boolean(process.env.ENGINE_SERVER);
 
 // --- Test config --- //
@@ -62,6 +64,7 @@ export const COVERAGE_DIR_FOR_OPTIONS =
         NODE_ENV_FOR_TEST,
         DISABLE_NATIVE_CUSTOM_ELEMENT_LIFECYCLE,
         ENGINE_SERVER,
+        DISABLE_DETACHED_REHYDRATION,
     })
         .filter(([, val]) => val)
         .map(([key, val]) => `${key}=${val}`)

From ea515d665cea37823658beb2fc3e9e9fc8f225b9 Mon Sep 17 00:00:00 2001
From: James Tu <j.tu@salesforce.com>
Date: Fri, 5 Sep 2025 20:05:54 -0700
Subject: [PATCH 4/6] test: add fixtures

---
 .../lifecycle-callbacks/index.spec.js         | 136 +++++++++++++++++-
 .../mutations/child/child.html                |   5 +
 .../mutations/child/child.js                  |  23 +++
 .../mutations/childLight/childLight.html      |   5 +
 .../mutations/childLight/childLight.js        |  24 ++++
 .../mutations/grandChild/grandChild.html      |   1 +
 .../mutations/grandChild/grandChild.js        |  18 +++
 .../grandChildLight/grandChildLight.html      |   1 +
 .../grandChildLight/grandChildLight.js        |  19 +++
 .../mutations/parent/parent.html              |   5 +
 .../mutations/parent/parent.js                |  27 ++++
 .../mutations/parentLight/parentLight.html    |   5 +
 .../mutations/parentLight/parentLight.js      |  28 ++++
 .../scheduled-rehydration/index.spec.js       |  31 ++++
 .../scheduled-rehydration/x/child/child.html  |   3 +
 .../scheduled-rehydration/x/child/child.js    |  42 ++++++
 .../x/parent/parent.html                      |   7 +
 .../scheduled-rehydration/x/parent/parent.js  |  31 ++++
 18 files changed, 406 insertions(+), 5 deletions(-)
 create mode 100644 packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/child/child.html
 create mode 100644 packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/child/child.js
 create mode 100644 packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/childLight/childLight.html
 create mode 100644 packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/childLight/childLight.js
 create mode 100644 packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/grandChild/grandChild.html
 create mode 100644 packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/grandChild/grandChild.js
 create mode 100644 packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/grandChildLight/grandChildLight.html
 create mode 100644 packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/grandChildLight/grandChildLight.js
 create mode 100644 packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/parent/parent.html
 create mode 100644 packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/parent/parent.js
 create mode 100644 packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/parentLight/parentLight.html
 create mode 100644 packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/parentLight/parentLight.js
 create mode 100644 packages/@lwc/integration-not-karma/test/reactivity/scheduled-rehydration/index.spec.js
 create mode 100644 packages/@lwc/integration-not-karma/test/reactivity/scheduled-rehydration/x/child/child.html
 create mode 100644 packages/@lwc/integration-not-karma/test/reactivity/scheduled-rehydration/x/child/child.js
 create mode 100644 packages/@lwc/integration-not-karma/test/reactivity/scheduled-rehydration/x/parent/parent.html
 create mode 100644 packages/@lwc/integration-not-karma/test/reactivity/scheduled-rehydration/x/parent/parent.js

diff --git a/packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/index.spec.js b/packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/index.spec.js
index 48b6f2d306..483d5914e8 100644
--- a/packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/index.spec.js
+++ b/packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/index.spec.js
@@ -11,6 +11,10 @@ import TimingParentLight from 'timing/parentLight';
 import ReorderingList from 'reordering/list';
 import ReorderingListLight from 'reordering/listLight';
 import Details from 'x/details';
+import MutationsParent from 'mutations/parent';
+import MutationsParentLight from 'mutations/parentLight';
+
+import { extractDataIds } from 'test-utils';
 
 function resetTimingBuffer() {
     window.timingBuffer = [];
@@ -319,11 +323,18 @@ describe('connectedCallback/renderedCallback timing when reconnected', () => {
                 resetTimingBuffer();
 
                 document.body.appendChild(elm);
-                expect(window.timingBuffer).toEqual(
-                    !lwcRuntimeFlags.DISABLE_NATIVE_CUSTOM_ELEMENT_LIFECYCLE
-                        ? ['parent:connectedCallback', 'child:connectedCallback']
-                        : ['parent:connectedCallback']
-                );
+
+                const expected = ['parent:connectedCallback'];
+
+                if (lwcRuntimeFlags.DISABLE_DETACHED_REHYDRATION) {
+                    expected.push('parent:renderedCallback');
+                }
+
+                if (!lwcRuntimeFlags.DISABLE_NATIVE_CUSTOM_ELEMENT_LIFECYCLE) {
+                    expected.push('child:connectedCallback');
+                }
+
+                expect(window.timingBuffer).toEqual(expected);
             });
         });
     });
@@ -436,3 +447,118 @@ describe('attributeChangedCallback', () => {
         expect(details.getAttribute('open')).toBeNull();
     });
 });
+
+describe('child mutations - scheduled rehydration', () => {
+    const scenarios = [
+        {
+            testName: 'shadow',
+            tagName: 'mutations-parent',
+            Ctor: MutationsParent,
+        },
+        {
+            testName: 'light',
+            tagName: 'mutations-parent-light',
+            Ctor: MutationsParentLight,
+        },
+    ];
+
+    scenarios.forEach(({ testName, tagName, Ctor }) => {
+        describe(testName, () => {
+            it('connect', async () => {
+                const elm = createElement(tagName, { is: Ctor });
+                document.body.appendChild(elm);
+
+                expect(window.timingBuffer).toEqual([
+                    'parent:connectedCallback',
+                    'child1:connectedCallback',
+                    'grand:child1:connectedCallback',
+                    'grand:child1:renderedCallback',
+                    'child1:renderedCallback',
+                    'parent:renderedCallback',
+                ]);
+            });
+
+            it('connect/mutate-child', async () => {
+                const elm = createElement(tagName, { is: Ctor });
+                document.body.appendChild(elm);
+                resetTimingBuffer();
+
+                const ids = extractDataIds(elm);
+                ids.child1.addChild();
+
+                await Promise.resolve();
+
+                expect(window.timingBuffer).toEqual([
+                    'grand:child2:connectedCallback',
+                    'grand:child2:renderedCallback',
+                    'child1:renderedCallback',
+                ]);
+            });
+
+            it('connect/mutate-child/disconnect-child', async () => {
+                const elm = createElement(tagName, { is: Ctor });
+                document.body.appendChild(elm);
+                resetTimingBuffer();
+
+                const ids = extractDataIds(elm);
+                // Mutate child - grand child 2
+                ids.child1.addChild();
+                // Disconnect the child that was just mutated
+                elm.disconnectLastChild();
+
+                await Promise.resolve();
+
+                const expected = [
+                    'child1:disconnectedCallback',
+                    'grand:child1:disconnectedCallback',
+                    'parent:renderedCallback',
+                ];
+
+                if (
+                    lwcRuntimeFlags.DISABLE_NATIVE_CUSTOM_ELEMENT_LIFECYCLE &&
+                    !lwcRuntimeFlags.DISABLE_DETACHED_REHYDRATION
+                ) {
+                    // These are fired in the children of the disconnected child
+                    expected.push(
+                        'grand:child2:connectedCallback',
+                        'grand:child2:renderedCallback'
+                    );
+                }
+
+                expect(window.timingBuffer).toEqual(expected);
+            });
+
+            it('connect/disconnect-child/mutate-child', async () => {
+                const elm = createElement(tagName, { is: Ctor });
+                document.body.appendChild(elm);
+                resetTimingBuffer();
+
+                const ids = extractDataIds(elm);
+                elm.disconnectLastChild();
+                // Mutate child that was just disconnected
+                ids.child1.addChild();
+
+                await Promise.resolve();
+
+                const expected = [
+                    'child1:disconnectedCallback',
+                    'grand:child1:disconnectedCallback',
+                    'parent:renderedCallback',
+                ];
+
+                if (
+                    lwcRuntimeFlags.DISABLE_NATIVE_CUSTOM_ELEMENT_LIFECYCLE &&
+                    !lwcRuntimeFlags.DISABLE_DETACHED_REHYDRATION
+                ) {
+                    // These are fired in the children of the disconnected child
+                    expected.push(
+                        'grand:child2:connectedCallback',
+                        'grand:child2:renderedCallback'
+                    );
+                }
+
+                expect(window.timingBuffer).toEqual(expected);
+            });
+        });
+    });
+});
diff --git a/packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/child/child.html b/packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/child/child.html
new file mode 100644
index 0000000000..56287ac3f4
--- /dev/null
+++ b/packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/child/child.html
@@ -0,0 +1,5 @@
+<template>
+    <template for:each={children} for:item="child">
+        <mutations-grand-child key={child.uid} uid={child.uid}></mutations-grand-child>
+    </template>
+</template>
diff --git a/packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/child/child.js b/packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/child/child.js
new file mode 100644
index 0000000000..6b47d503fa
--- /dev/null
+++ b/packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/child/child.js
@@ -0,0 +1,23 @@
+import { LightningElement, api } from 'lwc';
+
+export default class extends LightningElement {
+    @api uid;
+    @api children = [{ uid: '1' }];
+    connectedCallback() {
+        window.timingBuffer.push(`child${this.uid}:connectedCallback`);
+    }
+    renderedCallback() {
+        window.timingBuffer.push(`child${this.uid}:renderedCallback`);
+    }
+    disconnectedCallback() {
+        // This component could get disconnected by our Karma `test-setup.js` after `window.timingBuffer` has
+        // already been cleared; we don't care about the `disconnectedCallback`s in that case.
+        if (window.timingBuffer) {
+            window.timingBuffer.push(`child${this.uid}:disconnectedCallback`);
+        }
+    }
+    @api
+    addChild() {
+        this.children = [...this.children, { uid: `${this.children.length + 1}` }];
+    }
+}
diff --git a/packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/childLight/childLight.html b/packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/childLight/childLight.html
new file mode 100644
index 0000000000..8b55d200e9
--- /dev/null
+++ b/packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/childLight/childLight.html
@@ -0,0 +1,5 @@
+<template lwc:render-mode="light">
+    <template for:each={children} for:item="child">
+        <mutations-grand-child key={child.uid} uid={child.uid}></mutations-grand-child>
+    </template>
+</template>
diff --git a/packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/childLight/childLight.js b/packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/childLight/childLight.js
new file mode 100644
index 0000000000..0c95ed4be3
--- /dev/null
+++ b/packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/childLight/childLight.js
@@ -0,0 +1,24 @@
+import { LightningElement, api } from 'lwc';
+
+export default class extends LightningElement {
+    static renderMode = 'light';
+    @api uid;
+    @api children = [{ uid: '1' }];
+    connectedCallback() {
+        window.timingBuffer.push(`child${this.uid}:connectedCallback`);
+    }
+    renderedCallback() {
+        window.timingBuffer.push(`child${this.uid}:renderedCallback`);
+    }
+    disconnectedCallback() {
+        // This component could get disconnected by our Karma `test-setup.js` after `window.timingBuffer` has
+        // already been cleared; we don't care about the `disconnectedCallback`s in that case.
+        if (window.timingBuffer) {
+            window.timingBuffer.push(`child${this.uid}:disconnectedCallback`);
+        }
+    }
+    @api
+    addChild() {
+        this.children = [...this.children, { uid: `${this.children.length + 1}` }];
+    }
+}
diff --git a/packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/grandChild/grandChild.html b/packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/grandChild/grandChild.html
new file mode 100644
index 0000000000..41a40c8d47
--- /dev/null
+++ b/packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/grandChild/grandChild.html
@@ -0,0 +1 @@
+<template></template>
\ No newline at end of file
diff --git a/packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/grandChild/grandChild.js b/packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/grandChild/grandChild.js
new file mode 100644
index 0000000000..9ce1e25309
--- /dev/null
+++ b/packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/grandChild/grandChild.js
@@ -0,0 +1,18 @@
+import { LightningElement, api } from 'lwc';
+
+export default class extends LightningElement {
+    @api uid;
+    connectedCallback() {
+        window.timingBuffer.push(`grand:child${this.uid}:connectedCallback`);
+    }
+    renderedCallback() {
+        window.timingBuffer.push(`grand:child${this.uid}:renderedCallback`);
+    }
+    disconnectedCallback() {
+        // This component could get disconnected by our Karma `test-setup.js` after `window.timingBuffer` has
+        // already been cleared; we don't care about the `disconnectedCallback`s in that case.
+        if (window.timingBuffer) {
+            window.timingBuffer.push(`grand:child${this.uid}:disconnectedCallback`);
+        }
+    }
+}
diff --git a/packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/grandChildLight/grandChildLight.html b/packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/grandChildLight/grandChildLight.html
new file mode 100644
index 0000000000..7f2416f70f
--- /dev/null
+++ b/packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/grandChildLight/grandChildLight.html
@@ -0,0 +1 @@
+<template lwc:render-mode="light"></template>
\ No newline at end of file
diff --git a/packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/grandChildLight/grandChildLight.js b/packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/grandChildLight/grandChildLight.js
new file mode 100644
index 0000000000..2d9ab130ea
--- /dev/null
+++ b/packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/grandChildLight/grandChildLight.js
@@ -0,0 +1,19 @@
+import { LightningElement, api } from 'lwc';
+
+export default class extends LightningElement {
+    static renderMode = 'light';
+    @api uid;
+    connectedCallback() {
+        window.timingBuffer.push(`grand:child${this.uid}:connectedCallback`);
+    }
+    renderedCallback() {
+        window.timingBuffer.push(`grand:child${this.uid}:renderedCallback`);
+    }
+    disconnectedCallback() {
+        // This component could get disconnected by our Karma `test-setup.js` after `window.timingBuffer` has
+        // already been cleared; we don't care about the `disconnectedCallback`s in that case.
+        if (window.timingBuffer) {
+            window.timingBuffer.push(`grand:child${this.uid}:disconnectedCallback`);
+        }
+    }
+}
diff --git a/packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/parent/parent.html b/packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/parent/parent.html
new file mode 100644
index 0000000000..1d7c36ddfd
--- /dev/null
+++ b/packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/parent/parent.html
@@ -0,0 +1,5 @@
+<template>
+    <template for:each={children} for:item="child">
+        <mutations-child key={child.uid} uid={child.uid} data-id={child.name}></mutations-child>
+    </template>
+</template>
diff --git a/packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/parent/parent.js b/packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/parent/parent.js
new file mode 100644
index 0000000000..ee69cd1bbf
--- /dev/null
+++ b/packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/parent/parent.js
@@ -0,0 +1,27 @@
+import { api, LightningElement } from 'lwc';
+
+export default class extends LightningElement {
+    @api children = [{ uid: '1', name: 'child1' }];
+    connectedCallback() {
+        window.timingBuffer.push('parent:connectedCallback');
+    }
+    renderedCallback() {
+        window.timingBuffer.push('parent:renderedCallback');
+    }
+    disconnectedCallback() {
+        // This component could get disconnected by our Karma `test-setup.js` after `window.timingBuffer` has
+        // already been cleared; we don't care about the `disconnectedCallback`s in that case.
+        if (window.timingBuffer) {
+            window.timingBuffer.push('parent:disconnectedCallback');
+        }
+    }
+    @api
+    addChild() {
+        const uid = this.children.length + 1;
+        this.children = [...this.children, { uid, name: `child${uid}` }];
+    }
+    @api
+    disconnectLastChild() {
+        this.children = this.children.slice(0, this.children.length - 1);
+    }
+}
diff --git a/packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/parentLight/parentLight.html b/packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/parentLight/parentLight.html
new file mode 100644
index 0000000000..6d3f69d45f
--- /dev/null
+++ b/packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/parentLight/parentLight.html
@@ -0,0 +1,5 @@
+<template lwc:render-mode="light">
+    <template for:each={children} for:item="child">
+        <mutations-child key={child.uid} uid={child.uid} data-id={child.name}></mutations-child>
+    </template>
+</template>
diff --git a/packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/parentLight/parentLight.js b/packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/parentLight/parentLight.js
new file mode 100644
index 0000000000..2d934dd86e
--- /dev/null
+++ b/packages/@lwc/integration-not-karma/test/component/lifecycle-callbacks/mutations/parentLight/parentLight.js
@@ -0,0 +1,28 @@
+import { api, LightningElement } from 'lwc';
+
+export default class extends LightningElement {
+    static renderMode = 'light';
+    @api children = [{ uid: '1', name: 'child1' }];
+    connectedCallback() {
+        window.timingBuffer.push('parent:connectedCallback');
+    }
+    renderedCallback() {
+        window.timingBuffer.push('parent:renderedCallback');
+    }
+    disconnectedCallback() {
+        // This component could get disconnected by our Karma `test-setup.js` after `window.timingBuffer` has
+        // already been cleared; we don't care about the `disconnectedCallback`s in that case.
+        if (window.timingBuffer) {
+            window.timingBuffer.push('parent:disconnectedCallback');
+        }
+    }
+    @api
+    addChild() {
+        const uid = this.children.length + 1;
+        this.children = [...this.children, { uid, name: `child${uid}` }];
+    }
+    @api
+    disconnectLastChild() {
+        this.children = this.children.slice(0, this.children.length - 1);
+    }
+}
diff --git a/packages/@lwc/integration-not-karma/test/reactivity/scheduled-rehydration/index.spec.js b/packages/@lwc/integration-not-karma/test/reactivity/scheduled-rehydration/index.spec.js
new file mode 100644
index 0000000000..212c0bddc7
--- /dev/null
+++ b/packages/@lwc/integration-not-karma/test/reactivity/scheduled-rehydration/index.spec.js
@@ -0,0 +1,31 @@
+import { createElement } from 'lwc';
+
+import Parent from 'x/parent';
+
+describe('detached rehydration', () => {
+    it(`should${lwcRuntimeFlags.DISABLE_DETACHED_REHYDRATION ? ' not ' : ' '}schedule rehydration for detached nodes`, async () => {
+        const parent = createElement('x-parent', { is: Parent });
+        document.body.appendChild(parent);
+
+        const children = parent.shadowRoot.querySelectorAll('x-child');
+        expect(children.length).toEqual(3);
+        children.forEach((child) => expect(child.called).toEqual(1));
+
+        // Schedule child nodes for rehydration
+        children[1].value = 'new value';
+        children[2].value = 'new value';
+
+        // Schedule parent node for rehydration and disconnect 2nd & 3rd children
+        parent.items = [{ id: 'item1', name: 'Item 4' }];
+
+        await Promise.resolve();
+
+        // Use calls to the getters to determine if rehydration was processed.
+        // Rehydration will invoke the tmpl function -> calls getters for values
+        // Note that this means the entire diffing algo was traversed in the disconnected node
+        const expectedDetachedGetterCalls = lwcRuntimeFlags.DISABLE_DETACHED_REHYDRATION ? 1 : 2;
+        expect(children[0].called).toEqual(2);
+        expect(children[1].called).toEqual(expectedDetachedGetterCalls);
+        expect(children[2].called).toEqual(expectedDetachedGetterCalls);
+    });
+});
diff --git a/packages/@lwc/integration-not-karma/test/reactivity/scheduled-rehydration/x/child/child.html b/packages/@lwc/integration-not-karma/test/reactivity/scheduled-rehydration/x/child/child.html
new file mode 100644
index 0000000000..ca1bd19056
--- /dev/null
+++ b/packages/@lwc/integration-not-karma/test/reactivity/scheduled-rehydration/x/child/child.html
@@ -0,0 +1,3 @@
+<template>
+    <span>{value}</span>
+</template>
\ No newline at end of file
diff --git a/packages/@lwc/integration-not-karma/test/reactivity/scheduled-rehydration/x/child/child.js b/packages/@lwc/integration-not-karma/test/reactivity/scheduled-rehydration/x/child/child.js
new file mode 100644
index 0000000000..98f45d15dd
--- /dev/null
+++ b/packages/@lwc/integration-not-karma/test/reactivity/scheduled-rehydration/x/child/child.js
@@ -0,0 +1,42 @@
+import { api, LightningElement } from 'lwc';
+
+export default class extends LightningElement {
+    _called = 0;
+    _value = 'default';
+
+    @api
+    set value(val) {
+        this._value = val;
+    }
+
+    get value() {
+        this._called++;
+        return this._value;
+    }
+
+    @api
+    get called() {
+        return this._called;
+    }
+
+    @api
+    connectedCallbackCalled = 0;
+
+    @api
+    disconnectedCallbackCalled = 0;
+
+    @api
+    renderedCallbackCalled = 0;
+
+    connectedCallback() {
+        this.connectedCallbackCalled++;
+    }
+
+    disconnectedCallback() {
+        this.disconnectedCallbackCalled++;
+    }
+
+    renderedCallback() {
+        this.renderedCallbackCalled++;
+    }
+}
diff --git a/packages/@lwc/integration-not-karma/test/reactivity/scheduled-rehydration/x/parent/parent.html b/packages/@lwc/integration-not-karma/test/reactivity/scheduled-rehydration/x/parent/parent.html
new file mode 100644
index 0000000000..885bef215b
--- /dev/null
+++ b/packages/@lwc/integration-not-karma/test/reactivity/scheduled-rehydration/x/parent/parent.html
@@ -0,0 +1,7 @@
+<template>
+    <div class="items-container">
+        <template for:each={items} for:item="item">
+            <x-child key={item.id} value={item.value}></x-child>
+        </template>
+    </div>
+</template>
\ No newline at end of file
diff --git a/packages/@lwc/integration-not-karma/test/reactivity/scheduled-rehydration/x/parent/parent.js b/packages/@lwc/integration-not-karma/test/reactivity/scheduled-rehydration/x/parent/parent.js
new file mode 100644
index 0000000000..7ef9173b11
--- /dev/null
+++ b/packages/@lwc/integration-not-karma/test/reactivity/scheduled-rehydration/x/parent/parent.js
@@ -0,0 +1,31 @@
+import { api, LightningElement } from 'lwc';
+
+export default class extends LightningElement {
+    @api
+    items = [
+        { id: 'item1', value: 'Item 1' },
+        { id: 'item2', value: 'Item 2' },
+        { id: 'item3', value: 'Item 3' },
+    ];
+
+    @api
+    connectedCallbackCalled = 0;
+
+    @api
+    disconnectedCallbackCalled = 0;
+
+    @api
+    renderedCallbackCalled = 0;
+
+    connectedCallback() {
+        this.connectedCallbackCalled++;
+    }
+
+    disconnectedCallback() {
+        this.disconnectedCallbackCalled++;
+    }
+
+    renderedCallback() {
+        this.renderedCallbackCalled++;
+    }
+}

From 0bd84c74d29a3857877f5e510e283f617ca098c0 Mon Sep 17 00:00:00 2001
From: James Tu <j.tu@salesforce.com>
Date: Fri, 5 Sep 2025 20:16:56 -0700
Subject: [PATCH 5/6] chore: bump bundle size limit

---
 scripts/bundlesize/bundlesize.config.json | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/scripts/bundlesize/bundlesize.config.json b/scripts/bundlesize/bundlesize.config.json
index 941a42f360..a4fa1b488c 100644
--- a/scripts/bundlesize/bundlesize.config.json
+++ b/scripts/bundlesize/bundlesize.config.json
@@ -2,7 +2,7 @@
     "files": [
         {
             "path": "packages/@lwc/engine-dom/dist/index.js",
-            "maxSize": "24.68KB"
+            "maxSize": "25.0KB"
         },
         {
             "path": "packages/@lwc/synthetic-shadow/dist/index.js",

From e8fc09a1c741ea3a2bd5dd97e6dd5e21880cc6a1 Mon Sep 17 00:00:00 2001
From: James Tu <j.tu@salesforce.com>
Date: Fri, 5 Sep 2025 20:29:59 -0700
Subject: [PATCH 6/6] chore: update ci workflows

---
 .github/workflows/karma.yml | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/.github/workflows/karma.yml b/.github/workflows/karma.yml
index 841704d518..51fc27b5e8 100644
--- a/.github/workflows/karma.yml
+++ b/.github/workflows/karma.yml
@@ -58,7 +58,9 @@ jobs:
             - run: LEGACY_BROWSERS=1 yarn sauce:ci
             - run: FORCE_NATIVE_SHADOW_MODE_FOR_TEST=1 yarn sauce:ci
             - run: DISABLE_NATIVE_CUSTOM_ELEMENT_LIFECYCLE=1 yarn sauce:ci
+            - run: DISABLE_DETACHED_REHYDRATION=1 yarn sauce:ci
             - run: DISABLE_NATIVE_CUSTOM_ELEMENT_LIFECYCLE=1 DISABLE_SYNTHETIC=1 yarn sauce:ci
+            - run: DISABLE_NATIVE_CUSTOM_ELEMENT_LIFECYCLE=1 DISABLE_DETACHED_REHYDRATION=1 yarn sauce:ci
 
             - name: Upload coverage results
               uses: actions/upload-artifact@v4
@@ -187,6 +189,8 @@ jobs:
             - run: NODE_ENV_FOR_TEST=production yarn hydration:sauce:ci:engine-server
             - run: DISABLE_NATIVE_CUSTOM_ELEMENT_LIFECYCLE=1 yarn hydration:sauce:ci:engine-server
             - run: DISABLE_STATIC_CONTENT_OPTIMIZATION=1 yarn hydration:sauce:ci:engine-server
+            - run: DISABLE_DETACHED_REHYDRATION=1 yarn hydration:sauce:ci:engine-server
+            - run: DISABLE_NATIVE_CUSTOM_ELEMENT_LIFECYCLE=1 DISABLE_DETACHED_REHYDRATION=1 yarn hydration:sauce:ci:engine-server
             - run: yarn hydration:sauce:ci
             - run: ENABLE_SYNTHETIC_SHADOW_IN_HYDRATION=1 yarn hydration:sauce:ci
             - run: NODE_ENV_FOR_TEST=production yarn hydration:sauce:ci