diff --git a/config/scripts/happy-dom-mutation-observer-retention.test.ts b/config/scripts/happy-dom-mutation-observer-retention.test.ts
new file mode 100644
index 000000000..475a84c9b
--- /dev/null
+++ b/config/scripts/happy-dom-mutation-observer-retention.test.ts
@@ -0,0 +1,72 @@
+/** @vitest-environment happy-dom */
+import { describe, expect, it } from 'vitest'
+
+import {
+  installHappyDomMutationObserverRetention,
+  retainedMutationCallbackCount
+} from './happy-dom-mutation-observer-retention'
+
+function readListenerCallbacks(target: Node): unknown[] {
+  const listenersSymbol = Object.getOwnPropertySymbols(target).find(
+    (candidate) => candidate.description === 'mutationListeners'
+  )
+  const listeners = listenersSymbol
+    ? (target as unknown as Record<symbol, unknown>)[listenersSymbol]
+    : []
+  if (!Array.isArray(listeners)) {
+    return []
+  }
+  return listeners.map((listener: { callback?: { deref: () => unknown } }) =>
+    listener.callback?.deref()
+  )
+}
+
+describe('happy-dom MutationObserver retention', () => {
+  it('pins the internal callback that happy-dom only holds weakly', () => {
+    expect(installHappyDomMutationObserverRetention()).toBe(true)
+    const target = document.createElement('div')
+    document.body.append(target)
+    const observer = new MutationObserver(() => {})
+    observer.observe(target, { childList: true, subtree: true })
+
+    const callbacks = readListenerCallbacks(target)
+    expect(callbacks.length).toBe(1)
+    expect(callbacks[0]).toBeTypeOf('function')
+    expect(retainedMutationCallbackCount(observer)).toBe(1)
+
+    observer.disconnect()
+    expect(retainedMutationCallbackCount(observer)).toBe(0)
+    target.remove()
+  })
+
+  it('keeps delivering records after the weak callback would have been collected', async () => {
+    installHappyDomMutationObserverRetention()
+    const target = document.createElement('div')
+    document.body.append(target)
+    let deliveries = 0
+    const observer = new MutationObserver(() => {
+      deliveries += 1
+    })
+    observer.observe(target, { childList: true, subtree: true })
+
+    target.replaceChildren(document.createElement('div'))
+    await Promise.resolve()
+    expect(deliveries).toBe(1)
+
+    const collectGarbage = (globalThis as { gc?: () => void }).gc
+    if (collectGarbage) {
+      for (let round = 0; round < 5; round += 1) {
+        await new Promise((resolve) => setTimeout(resolve, 1))
+        collectGarbage()
+      }
+      await new Promise((resolve) => setTimeout(resolve, 1))
+    }
+
+    target.replaceChildren(document.createElement('span'))
+    await Promise.resolve()
+    expect(deliveries).toBe(2)
+
+    observer.disconnect()
+    target.remove()
+  })
+})
diff --git a/config/scripts/happy-dom-mutation-observer-retention.ts b/config/scripts/happy-dom-mutation-observer-retention.ts
new file mode 100644
index 000000000..a315b3d52
--- /dev/null
+++ b/config/scripts/happy-dom-mutation-observer-retention.ts
@@ -0,0 +1,81 @@
+// Why: happy-dom stores each MutationObserver's internal callback in a WeakRef, so any GC pause
+// under parallel test load silently and permanently stops a still-connected observer. Browsers
+// keep that callback reachable for as long as the observer observes; mirror that lifetime here so
+// DOM-driven tests never lose mutation records mid-run.
+
+type HappyDomMutationListener = {
+  callback?: { deref: () => unknown }
+}
+
+type PatchableMutationObserver = {
+  observe: (target: Node, options?: MutationObserverInit) => void
+  disconnect: () => void
+}
+
+const MUTATION_LISTENERS_SYMBOL_DESCRIPTION = 'mutationListeners'
+const RETENTION_INSTALLED = Symbol.for('orca.happyDomMutationObserverRetention')
+
+const retainedCallbacks = new WeakMap<object, Set<unknown>>()
+
+function readMutationListeners(target: Node): HappyDomMutationListener[] {
+  const listenersSymbol = Object.getOwnPropertySymbols(target).find(
+    (candidate) => candidate.description === MUTATION_LISTENERS_SYMBOL_DESCRIPTION
+  )
+  if (!listenersSymbol) {
+    return []
+  }
+  const listeners = (target as unknown as Record<symbol, unknown>)[listenersSymbol]
+  return Array.isArray(listeners) ? (listeners as HappyDomMutationListener[]) : []
+}
+
+/** Number of internal callbacks pinned for `observer`; drops to 0 once it disconnects. */
+export function retainedMutationCallbackCount(observer: MutationObserver): number {
+  return retainedCallbacks.get(observer)?.size ?? 0
+}
+
+export function installHappyDomMutationObserverRetention(): boolean {
+  const observerClass = (globalThis as { MutationObserver?: typeof MutationObserver })
+    .MutationObserver
+  if (!observerClass) {
+    return false
+  }
+  const prototype = observerClass.prototype as unknown as PatchableMutationObserver &
+    Record<symbol, unknown>
+  if (prototype[RETENTION_INSTALLED] === true) {
+    return true
+  }
+  const observe = prototype.observe
+  const disconnect = prototype.disconnect
+
+  prototype.observe = function patchedObserve(
+    this: object,
+    target: Node,
+    options?: MutationObserverInit
+  ): void {
+    const existing = new Set(readMutationListeners(target))
+    observe.call(this as unknown as PatchableMutationObserver, target, options)
+    const pinned = retainedCallbacks.get(this) ?? new Set<unknown>()
+    for (const listener of readMutationListeners(target)) {
+      if (existing.has(listener)) {
+        continue
+      }
+      const callback = listener.callback?.deref()
+      if (callback) {
+        pinned.add(callback)
+      }
+    }
+    if (pinned.size > 0) {
+      retainedCallbacks.set(this, pinned)
+    }
+  }
+
+  prototype.disconnect = function patchedDisconnect(this: object): void {
+    disconnect.call(this as unknown as PatchableMutationObserver)
+    retainedCallbacks.delete(this)
+  }
+
+  prototype[RETENTION_INSTALLED] = true
+  return true
+}
+
+installHappyDomMutationObserverRetention()
diff --git a/config/vitest.config.ts b/config/vitest.config.ts
index 1146cd4a5..735e1b2a0 100644
--- a/config/vitest.config.ts
+++ b/config/vitest.config.ts
@@ -17,6 +17,8 @@ export default defineConfig({
     environment: 'node',
     // Why: Node 26's undefined Web Storage globals prevent Vitest from installing happy-dom's.
     execArgv: ['--no-experimental-webstorage'],
+    // Why: happy-dom drops MutationObserver callbacks on GC; keep them alive like a browser does.
+    setupFiles: [resolve('config/scripts/happy-dom-mutation-observer-retention.ts')],
     include: [
       'src/**/*.test.ts',
       'src/**/*.test.tsx',