billscroll should now remember user's position across devices

src/functional_tests/test_billboard.py

@@ -1,3 +1,5 @@
+import time
+
 from selenium.webdriver.common.by import By
 
 from .base import FunctionalTest
@@ -123,6 +125,63 @@ class BillboardScrollTest(FunctionalTest):
         self.assertIn("other", theirs_events[0].text)
 
 
+class BillscrollPositionTest(FunctionalTest):
+    """
+    FT: the user's scroll position in a billscroll is saved to the server
+    and restored when they return to the same scroll from any device/session.
+    """
+
+    def setUp(self):
+        super().setUp()
+        self.founder = User.objects.create(email="founder@scrollpos.io")
+        self.room = Room.objects.create(name="Persistent Chamber", owner=self.founder)
+        # Enough events to make #id_drama_scroll scrollable
+        for i in range(20):
+            record(
+                self.room, GameEvent.SLOT_FILLED, actor=self.founder,
+                slot_number=(i % 6) + 1, token_type="coin",
+                token_display=f"Coin-{i}", renewal_days=7,
+            )
+
+    def test_scroll_position_persists_across_sessions(self):
+        # 1. Log in and navigate to the room's billscroll
+        self.create_pre_authenticated_session("founder@scrollpos.io")
+        self.browser.get(
+            self.live_server_url + f"/billboard/room/{self.room.id}/scroll/"
+        )
+        scroll_el = self.wait_for(
+            lambda: self.browser.find_element(By.ID, "id_drama_scroll")
+        )
+
+        # 2. Force the element scrollable (CSS not served by StaticLiveServerTestCase),
+        #    set position, and dispatch scroll event to trigger the debounced save
+        target = 100
+        self.browser.execute_script("""
+            var el = arguments[0];
+            el.style.overflow = 'auto';
+            el.style.height = '150px';
+            el.scrollTop = arguments[1];
+            el.dispatchEvent(new Event('scroll'));
+        """, scroll_el, target)
+
+        # 3. Wait for debounce (800ms) + fetch to complete
+        time.sleep(3)
+
+        # 4. Navigate away and back in a fresh session
+        self.browser.get(self.live_server_url + "/billboard/")
+        self.create_pre_authenticated_session("founder@scrollpos.io")
+        self.browser.get(
+            self.live_server_url + f"/billboard/room/{self.room.id}/scroll/"
+        )
+
+        # 5. The saved position is reflected in the page's data attribute
+        scroll_el = self.wait_for(
+            lambda: self.browser.find_element(By.ID, "id_drama_scroll")
+        )
+        restored = int(scroll_el.get_attribute("data-scroll-position"))
+        self.assertEqual(restored, target)
+
+
 class BillboardAppletsTest(FunctionalTest):
     """
     FT: billboard page renders three applets in the grid — My Scrolls,