long overdue fix to last pipeline push, where scroll position did not persist across sessions

src/functional_tests/test_billboard.py

@@ -154,16 +154,19 @@ class BillscrollPositionTest(FunctionalTest):
         )
 
         # 2. Force the element scrollable (CSS not served by StaticLiveServerTestCase),
-        #    set position, and dispatch scroll event to trigger the debounced save.
-        #    JS saves scrollTop + clientHeight (bottom-of-viewport); forced height is 150px.
+        #    set position, dispatch scroll event, and capture the position value the
+        #    JS listener will save — all in one synchronous script so the layout
+        #    snapshot is identical to what the scroll handler sees.
         scroll_top = 100
-        forced_height = 150
-        self.browser.execute_script("""
+        saved_pos = self.browser.execute_script("""
             var el = arguments[0];
+            var remPx = parseFloat(getComputedStyle(document.documentElement).fontSize);
             el.style.overflow = 'auto';
             el.style.height = '150px';
             el.scrollTop = arguments[1];
+            var pos = Math.round(el.scrollTop + el.clientHeight + remPx * 2.5);
             el.dispatchEvent(new Event('scroll'));
+            return pos;
         """, scroll_el, scroll_top)
 
         # 3. Wait for debounce (800ms) + fetch to complete
@@ -180,11 +183,8 @@ class BillscrollPositionTest(FunctionalTest):
         scroll_el = self.wait_for(
             lambda: self.browser.find_element(By.ID, "id_drama_scroll")
         )
-        buffer_px = self.browser.execute_script(
-            "return Math.round(parseFloat(getComputedStyle(document.documentElement).fontSize) * 2.5)"
-        )
         restored = int(scroll_el.get_attribute("data-scroll-position"))
-        self.assertEqual(restored, scroll_top + forced_height + buffer_px)
+        self.assertEqual(restored, saved_pos)
 
 
 class BillboardAppletsTest(FunctionalTest):

src/templates/apps/billboard/_partials/_applet-billboard-scroll.html

@@ -22,8 +22,12 @@
             }
         }
 
-        // Restore: position stored is bottom-of-viewport; subtract clientHeight to align it
+        // Only restore if there's a meaningful saved position — avoids a
+        // no-op scrollTop assignment (0→0) that can fire a spurious scroll
+        // event and reset the debounce timer in tests / headless browsers.
+        if ({{ scroll_position }} > 0) {
             scroll.scrollTop = Math.max(0, {{ scroll_position }} - scroll.clientHeight);
+        }
     });
 
     // Animate "What happens next. . . ?" buffer dots — 4th span shows '?'
@@ -37,21 +41,24 @@
         }, 400);
     }
 
-    // Debounced save on scroll — store bottom-of-viewport so the last-read line is restored
+    // Debounced save on scroll — store bottom-of-viewport so the last-read line is restored.
+    // Position is captured at event time so layout changes during the debounce window
+    // (e.g. rAF adjusting marginTop) don't produce a stale clientHeight.
+    var remPx = parseFloat(getComputedStyle(document.documentElement).fontSize);
     var saveTimer;
     scroll.addEventListener('scroll', function() {
+        var pos = Math.round(scroll.scrollTop + scroll.clientHeight + remPx * 2.5);
         clearTimeout(saveTimer);
         saveTimer = setTimeout(function() {
             var csrfToken = document.querySelector('[name=csrfmiddlewaretoken]');
             var token = csrfToken ? csrfToken.value : '';
-            var remPx = parseFloat(getComputedStyle(document.documentElement).fontSize);
             fetch("{% url 'billboard:save_scroll_position' room.id %}", {
                 method: 'POST',
                 headers: {
                     'Content-Type': 'application/x-www-form-urlencoded',
                     'X-CSRFToken': token,
                 },
-                body: 'position=' + Math.round(scroll.scrollTop + scroll.clientHeight + remPx * 2.5),
+                body: 'position=' + pos,
             });
         }, 800);
     });