diff --git a/bun.lock b/bun.lock
index 854a2ea9ac0..fdbaca4a389 100644
--- a/bun.lock
+++ b/bun.lock
@@ -69,7 +69,7 @@
         "express": "^5.1.0",
         "fix-path": "5.0.0",
         "ghostty-web": "^0.4.0-next.7.g03ead6e",
-        "happy-dom": "^20.0.10",
+        "happy-dom": "20.0.11",
         "ignore": "^7.0.5",
         "json-schema-to-typescript": "^15.0.4",
         "jsonc-parser": "^3.3.1",
diff --git a/package.json b/package.json
index 7a6646864bb..730202b3176 100644
--- a/package.json
+++ b/package.json
@@ -111,7 +111,7 @@
     "express": "^5.1.0",
     "fix-path": "5.0.0",
     "ghostty-web": "^0.4.0-next.7.g03ead6e",
-    "happy-dom": "^20.0.10",
+    "happy-dom": "20.0.11",
     "ignore": "^7.0.5",
     "json-schema-to-typescript": "^15.0.4",
     "jsonc-parser": "^3.3.1",
diff --git a/src/browser/happyDomMutationObserver.test.ts b/src/browser/happyDomMutationObserver.test.ts
new file mode 100644
index 00000000000..35710bf68fb
--- /dev/null
+++ b/src/browser/happyDomMutationObserver.test.ts
@@ -0,0 +1,29 @@
+import { expect, test } from "bun:test";
+import { Window } from "happy-dom";
+
+/**
+ * happy-dom 20.0.11 holds each MutationObserver's report callback only through a WeakRef, so after
+ * a garbage collection the observer stops reporting without an error (a cause of the #5989
+ * flake). patches/happy-dom@20.0.11.patch holds it strongly. bun applies that patch only while
+ * the installed version matches its key, and says nothing when it does not (#6348), so this
+ * test fails loudly if happy-dom moves to an unpatched version. An upgrade to a version that
+ * holds the callback upstream (20.11.15 or later) passes without the patch.
+ */
+test("a happy-dom MutationObserver keeps reporting after a garbage collection", async () => {
+  const window = new Window();
+  const target = window.document.createElement("div");
+  const batches: number[] = [];
+  const observer = new window.MutationObserver((records) => batches.push(records.length));
+  observer.observe(target, { childList: true });
+  try {
+    // A WeakRef keeps its target alive until the current job ends, so collect after this one.
+    await new Promise((resolve) => setTimeout(resolve, 0));
+    Bun.gc(true);
+    target.appendChild(window.document.createElement("span"));
+    await new Promise((resolve) => setTimeout(resolve, 10));
+    expect(batches).toEqual([1]);
+  } finally {
+    observer.disconnect();
+    await window.happyDOM.close();
+  }
+});