feat(views): keep group visible as header for the running event

In the countdown view a user can subscribe to a group and to events
inside it. Once an event within the group started, the auto scroll
pushed the group card above the viewport, losing the context of which
group the running event belongs to. The operator view had the same
problem.

Group and its events are now wrapped in a section, and the group card is
pinned with position sticky, so it stays above the running event however
far down the group that event sits. The follow scroll offsets the sticky
header, and the follow button threshold discounts it so it keeps its
meaning.

A group holding the running event is now marked as active. In the
countdown the card keeps its background and gains a green outline, the
green fill stays reserved for the running event itself. In the operator
the card is already filled with the group colour, so the ring uses the
lighter active indicator over a dark frame to stay legible against any
group colour. Neither state was visible before: .sub--group overrode
both .sub--live and .sub--armed at equal specificity.

Also fixes the countdown handing the same selectedRef to both a group
row and its running child. React detaches refs before attaching them,
so the child unsetting the ref left it null while the group never
re-attached, silently disabling auto scroll.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GjX7D56AW8cL3FXjvtnjtL

apps/client/src/features/operator/Operator.tsx

@@ -1,5 +1,5 @@
 import { OntimeView, isOntimeEvent, isOntimeGroup } from 'ontime-types';
-import { Fragment, useCallback, useEffect, useMemo, useRef, useState } from 'react';
+import { useCallback, useEffect, useMemo, useRef, useState } from 'react';
 
 import EmptyFill from '../../common/components/state/EmptyFill';
 import EmptyPage from '../../common/components/state/EmptyPage';
@@ -54,11 +54,21 @@ function Operator({ rundown, rundownMetadata, customFields, settings }: Operator
   const [lockAutoScroll, setLockAutoScroll] = useState(false);
   const selectedRef = useRef<HTMLDivElement | null>(null);
   const scrollRef = useRef<HTMLDivElement | null>(null);
+  const stickyHeaderRef = useRef<HTMLDivElement | null>(null);
+
+  // the group of the running event is pinned to the top of the list, the running event needs to clear it
+  const getStickyOffset = useCallback(() => {
+    const header = stickyHeaderRef.current;
+    // account for the list gap, so that the rows do not touch
+    return header ? header.offsetHeight + 2 : 0;
+  }, []);
+  const getTopOffset = useCallback(() => selectedOffset + getStickyOffset(), [getStickyOffset]);
+
   const scrollToComponent = useFollowComponent({
     followRef: selectedRef,
     scrollRef,
     doFollow: !lockAutoScroll,
-    topOffset: selectedOffset,
+    getTopOffset,
     followTrigger: selectedEventId,
   });
 
@@ -86,7 +96,8 @@ function Operator({ rundown, rundownMetadata, customFields, settings }: Operator
       const selectedRect = selectedRef.current.getBoundingClientRect();
       const scrollerRect = scrollRef.current.getBoundingClientRect();
       if (selectedRect && scrollerRect) {
-        const distanceFromTop = selectedRect.top - scrollerRect.top;
+        // discount the pinned group header, so that the threshold keeps its meaning when a header is stuck
+        const distanceFromTop = selectedRect.top - scrollerRect.top - getStickyOffset();
         const hasScrolledOutOfThreshold = distanceFromTop < -8 || distanceFromTop > selectedOffset;
         setLockAutoScroll(hasScrolledOutOfThreshold);
       }
@@ -186,13 +197,14 @@ function Operator({ rundown, rundownMetadata, customFields, settings }: Operator
               }
 
               return (
-                <Fragment key={entry.id}>
+                <div className={style.groupSection} key={entry.id}>
                   <OperatorGroup
-                    key={entry.id}
+                    ref={isCurrentParent ? stickyHeaderRef : undefined}
                     title={entry.title}
                     colour={entry.colour}
                     count={entry.entries.length}
                     duration={entry.duration}
+                    isLive={isCurrentParent}
                   />
                   {entry.entries.map((nestedEntryId) => {
                     const nestedEntry = rundown.entries[nestedEntryId];
@@ -239,7 +251,7 @@ function Operator({ rundown, rundownMetadata, customFields, settings }: Operator
                       />
                     );
                   })}
-                </Fragment>
+                </div>
               );
             }
             return null;