From 206e8cff82f3157d9f2ac1f3b038ba42a9af6dc3 Mon Sep 17 00:00:00 2001
From: Hiroshi Ogawa <hi.ogawa.zz@gmail.com>
Date: Thu, 4 Jun 2026 16:10:23 +0900
Subject: [PATCH] fix!: allow mutating happy-dom/jsdom window object (#10373)

Co-authored-by: Hiroshi Ogawa <4232207+hi-ogawa@users.noreply.github.com>
Co-authored-by: Codex <noreply@openai.com>
---
 docs/guide/migration.md                       |  4 +++
 packages/vitest/src/integrations/env/utils.ts |  4 +++
 test/unit/test/environments/happy-dom.spec.ts | 25 +++++++++++++++++++
 test/unit/test/environments/jsdom.spec.ts     | 13 ++++++++++
 4 files changed, 46 insertions(+)

diff --git a/docs/guide/migration.md b/docs/guide/migration.md
index 1bc18c249..ea3892bcb 100644
--- a/docs/guide/migration.md
+++ b/docs/guide/migration.md
@@ -134,6 +134,10 @@ $ cd subdir && vitest # [!code --]
 $ cd subdir && vitest --config ../vitest.config.ts # [!code ++]
 ```
 
+### DOM Environment Global Assignments Now Update the Underlying Window
+
+Assignments to properties on `globalThis` or `window` in `jsdom` and `happy-dom` environments are now propagated to the underlying DOM implementation. Mutable properties such as `innerWidth` can affect APIs implemented by the DOM environment, for example `happy-dom`'s `matchMedia`.
+
 ## Migrating to Vitest 4.0 {#vitest-4}
 
 ::: warning Prerequisites
diff --git a/packages/vitest/src/integrations/env/utils.ts b/packages/vitest/src/integrations/env/utils.ts
index bbbeee5fb..f8c4a75ef 100644
--- a/packages/vitest/src/integrations/env/utils.ts
+++ b/packages/vitest/src/integrations/env/utils.ts
@@ -78,6 +78,10 @@ export function populateGlobal(
       },
       set(v) {
         overrideObject.set(key, v)
+        // propagate changes to underlying window implementation,
+        // which can affect other window API behavior internally, e.g.
+        // updating `innerWidth` affects `matchMedia("(max-width: *)")` on happy-dom.
+        win[key] = v
       },
       configurable: true,
     })
diff --git a/test/unit/test/environments/happy-dom.spec.ts b/test/unit/test/environments/happy-dom.spec.ts
index 47bea78d8..6d99b3219 100644
--- a/test/unit/test/environments/happy-dom.spec.ts
+++ b/test/unit/test/environments/happy-dom.spec.ts
@@ -39,3 +39,28 @@ test('can pass down a simple form data', async () => {
     await req.formData()
   })()).resolves.not.toThrow()
 })
+
+test('innerWidth and matchMedia', () => {
+  expect(window.innerWidth).toBe(1024)
+  expect(window.matchMedia('(max-width: 100px)').matches).toBe(false)
+  window.innerWidth = 50
+  expect(window.matchMedia('(max-width: 100px)').matches).toBe(true)
+})
+
+test('readonly window assignment throws', ({ task }) => {
+  // happy-dom's vmThreads setup returns Window as a Node VM context directly.
+  // Node contextification reports this getter-only assignment as successful,
+  // unlike the populateGlobal facade used by threads/forks.
+  if (task.file.pool === 'vmThreads') {
+    expect(() => {
+      Object.assign(window, { navigator: {} })
+    }).not.toThrow()
+    return
+  }
+
+  expect(() => {
+    Object.assign(window, { navigator: {} })
+  }).toThrowErrorMatchingInlineSnapshot(
+    `[TypeError: Cannot set property navigator of #<GlobalWindow> which has only a getter]`,
+  )
+})
diff --git a/test/unit/test/environments/jsdom.spec.ts b/test/unit/test/environments/jsdom.spec.ts
index c2d8ab6c5..bac68eea1 100644
--- a/test/unit/test/environments/jsdom.spec.ts
+++ b/test/unit/test/environments/jsdom.spec.ts
@@ -335,3 +335,16 @@ test('jsdom global is exposed', () => {
 test('ssr is disabled', () => {
   expect(import.meta.env.SSR).toBe(false)
 })
+
+test('innerWidth and matchMedia', () => {
+  expect(window.innerWidth).toBe(1024)
+  expect(window.matchMedia).toBe(undefined)
+})
+
+test('readonly window assignment throws', () => {
+  expect(() => {
+    Object.assign(window, { navigator: {} })
+  }).toThrowErrorMatchingInlineSnapshot(
+    `[TypeError: Cannot set property navigator of [object Window] which has only a getter]`,
+  )
+})
-- 
2.51.2