Add test for flash message persistence on login

Author: mjackson

demos/bookstore/app/auth.test.ts

@@ -47,6 +47,48 @@ describe('auth handlers', () => {
     assertContains(html, 'Invalid email or password')
   })
 
+  it('flash error message is cleared after being displayed once', async () => {
+    // POST invalid credentials to trigger flash message
+    let response = await router.fetch('https://remix.run/login', {
+      method: 'POST',
+      body: new URLSearchParams({
+        email: '[email protected]',
+        password: 'wrongpassword',
+      }),
+      redirect: 'manual',
+    })
+
+    assert.equal(response.status, 302)
+    assert.equal(response.headers.get('Location'), '/login')
+
+    // Follow redirect to see the error message (first request)
+    let sessionCookie = getSessionCookie(response)
+    let firstFollowUp = await router.fetch('https://remix.run/login', {
+      headers: {
+        Cookie: `session=${sessionCookie}`,
+      },
+    })
+
+    let firstHtml = await firstFollowUp.text()
+    assertContains(firstHtml, 'Invalid email or password')
+
+    // Get updated session cookie (session should be updated to clear flash)
+    let updatedSessionCookie = getSessionCookie(firstFollowUp) || sessionCookie
+
+    // Refresh the page (second request) - error should NOT be shown
+    let secondFollowUp = await router.fetch('https://remix.run/login', {
+      headers: {
+        Cookie: `session=${updatedSessionCookie}`,
+      },
+    })
+
+    let secondHtml = await secondFollowUp.text()
+    assert.ok(
+      !secondHtml.includes('Invalid email or password'),
+      'Expected flash error to be cleared after first display',
+    )
+  })
+
   it('POST /register creates new user and sets session', async () => {
     let uniqueEmail = `newuser-${Date.now()}@example.com`