diff --git a/package-lock.json b/package-lock.json
index e3d1877..34f8352 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -29,7 +29,7 @@
         "@vue/compiler-sfc": "^3.5.0",
         "@vue/test-utils": "^2.5.1",
         "eslint": "^10.13.0",
-        "happy-dom": "^20.14.6",
+        "happy-dom": "^21.0.3",
         "sass": "^1.105.2",
         "typescript": "^6.0.3",
         "vite": "^7.3.7",
@@ -5279,9 +5279,9 @@
       "license": "ISC"
     },
     "node_modules/happy-dom": {
-      "version": "20.14.6",
-      "resolved": "https://registry.npmjs.org/happy-dom/-/happy-dom-20.14.6.tgz",
-      "integrity": "sha512-5DDsf4QmEIlRL6NkCv/fxSjYHHkZceL/IaYQdJc2h3G7DRTEai++VTjbfL0LB9geW2VrA3+vrjF3Efx+OTtX0Q==",
+      "version": "21.0.3",
+      "resolved": "https://registry.npmjs.org/happy-dom/-/happy-dom-21.0.3.tgz",
+      "integrity": "sha512-PesNgu5BnIEmAwzKGewB3K1ORFeznFZJ4JrRz89ea1wkHnxskdpzX9sN/+zb4KO6k8oSMhyJSj/cwXAFrYBBNA==",
       "dev": true,
       "license": "MIT",
       "dependencies": {
diff --git a/package.json b/package.json
index ec2e121..28125e9 100644
--- a/package.json
+++ b/package.json
@@ -37,7 +37,7 @@
     "@vue/compiler-sfc": "^3.5.0",
     "@vue/test-utils": "^2.5.1",
     "eslint": "^10.13.0",
-    "happy-dom": "^20.14.6",
+    "happy-dom": "^21.0.3",
     "sass": "^1.105.2",
     "typescript": "^6.0.3",
     "vite": "^7.3.7",
diff --git a/vitest.config.ts b/vitest.config.ts
index 018b301..549d063 100644
--- a/vitest.config.ts
+++ b/vitest.config.ts
@@ -7,6 +7,11 @@ export default defineConfig({
 	test: {
 		globals: true,
 		environment: 'happy-dom',
+		// happy-dom 21 makes iframe page loading opt-in, which leaves
+		// iframes without a contentWindow; the editor-frame tests need one.
+		environmentOptions: {
+			happyDOM: { settings: { enableIframePageLoading: true } },
+		},
 		// @nextcloud/files@3 (aliased as -legacy) imports a named export from
 		// the CJS-only `cancelable-promise`; let Vite transform it instead of
 		// loading it as native ESM.