diff --git a/lib/__tests__/app-update.test.ts b/lib/__tests__/app-update.test.ts
index ede111d70..4adbb77bc 100644
--- a/lib/__tests__/app-update.test.ts
+++ b/lib/__tests__/app-update.test.ts
@@ -83,4 +83,16 @@ describe('app update', () => {
     adoptWaitingWorker({ waiting: other, addEventListener: () => {} } as unknown as ServiceWorkerRegistration);
     expect(other.postMessage).not.toHaveBeenCalled();
   });
+
+  it('also when the worker of this tab’s build was still installing', () => {
+    const listeners: Array<() => void> = [];
+    const installing = { scriptURL: 'https://mail.example.com/sw.js?v=build-a', state: 'installing', postMessage: vi.fn(), addEventListener: (_: string, fn: () => void) => listeners.push(fn) };
+    const registration = { waiting: null as unknown, installing, addEventListener: () => {} };
+    adoptWaitingWorker(registration as unknown as ServiceWorkerRegistration);
+    expect(installing.postMessage).not.toHaveBeenCalled();
+    installing.state = 'installed';
+    registration.waiting = installing;
+    listeners.forEach((fn) => fn());
+    expect(installing.postMessage).toHaveBeenCalledWith({ type: 'skip-waiting' });
+  });
 });
diff --git a/lib/app-update.ts b/lib/app-update.ts
index dab926309..c2d7db144 100644
--- a/lib/app-update.ts
+++ b/lib/app-update.ts
@@ -91,13 +91,15 @@ export function adoptWaitingWorker(registration: ServiceWorkerRegistration): voi
   const adopt = () => {
     if (ours(registration.waiting)) registration.waiting!.postMessage({ type: 'skip-waiting' });
   };
-  adopt();
-  registration.addEventListener('updatefound', () => {
-    const installing = registration.installing;
-    installing?.addEventListener('statechange', () => {
-      if (installing.state === 'installed') adopt();
+  const watch = (worker: ServiceWorker | null) => {
+    worker?.addEventListener('statechange', () => {
+      if (worker.state === 'installed') adopt();
     });
-  });
+  };
+  adopt();
+  // Still installing when the registration resolved, or installed later.
+  watch(registration.installing);
+  registration.addEventListener('updatefound', () => watch(registration.installing));
 }
 
 /** Tells the service worker which build this tab runs, so it keeps that build's files while the tab is open. */