diff --git a/src/functional_tests/test_room_tray.py b/src/functional_tests/test_room_tray.py
index 496755c..1058617 100644
--- a/src/functional_tests/test_room_tray.py
+++ b/src/functional_tests/test_room_tray.py
@@ -104,12 +104,29 @@ class TrayTest(FunctionalTest):
         tray = self.browser.find_element(By.ID, "id_tray")
         self.assertFalse(tray.is_displayed())
 
-        # Clicking the closed btn adds a wobble class to the wrap
+        # Clicking the closed btn adds a wobble class to the wrap.
+        # Use a MutationObserver to capture the transient class change — in CI
+        # headless Firefox the 0.45s animation may complete before the first
+        # wait_for poll (0.5s), causing a false miss.
+        self.browser.execute_script("""
+            window._trayWobbled = false;
+            var wrap = document.getElementById('id_tray_wrap');
+            var obs = new MutationObserver(function(muts) {
+                muts.forEach(function(m) {
+                    if (m.type === 'attributes' && m.attributeName === 'class') {
+                        if (m.target.classList.contains('wobble')) {
+                            window._trayWobbled = true;
+                            obs.disconnect();
+                        }
+                    }
+                });
+            });
+            obs.observe(wrap, {attributes: true, attributeFilter: ['class']});
+        """)
         self.browser.find_element(By.ID, "id_tray_btn").click()
         self.wait_for(
-            lambda: self.assertIn(
-                "wobble",
-                self.browser.find_element(By.ID, "id_tray_wrap").get_attribute("class"),
+            lambda: self.assertTrue(
+                self.browser.execute_script("return window._trayWobbled;")
             )
         )
         # Tray still not visible — a click alone must not open it