feat(timeline): 'Back to where you were' after a jump (#127)

Clicking a reply quote (or arriving via a thread-panel / search / permalink
jump, i.e. an eventId route change) remembers the message that was in the
middle of the viewport and shows a Primary chip next to Jump to Latest —
'Back to where you were' — which scrolls/navigates back to it. Nothing is
rendered normally; the chip expires after 20 s or once used. Verified headless:
reply quote → original 120 messages up is shown + chip; chip → back at the
reply, chip gone.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/features/room/RoomTimeline.tsx

@@ -899,6 +899,45 @@ export function RoomTimeline({ room, eventId, roomInputRef, editor }: RoomTimeli
     ),
   );
 
+  // [Gitea #127] After a jump (reply quote, thread → main, search result) the
+  // reader is stranded far up the history; remember the message that was in
+  // the middle of the viewport and offer "Back to where you were".
+  const [returnPoint, setReturnPoint] = useState<{ eventId: string; at: number } | null>(null);
+  const captureReturnPoint = useCallback(() => {
+    const scroller = scrollRef.current;
+    if (!scroller) return;
+    const mid = scroller.getBoundingClientRect().top + scroller.clientHeight / 2;
+    const items = Array.from(scroller.querySelectorAll<HTMLElement>('[data-message-id]'));
+    const hit =
+      items.find((el) => {
+        const r = el.getBoundingClientRect();
+        return r.top <= mid && r.bottom >= mid;
+      }) ?? items[items.length - 1];
+    const id = hit?.getAttribute('data-message-id');
+    if (id) setReturnPoint({ eventId: id, at: Date.now() });
+  }, []);
+  // Route-driven jumps (thread panel / search / permalink): capture before the
+  // timeline is emptied by the effect below (effects run in declaration order).
+  const prevEventIdRef = useRef(eventId);
+  useEffect(() => {
+    const prev = prevEventIdRef.current;
+    prevEventIdRef.current = eventId;
+    if (eventId && eventId !== prev && prev === undefined) captureReturnPoint();
+  }, [eventId, captureReturnPoint]);
+  // Expire after 20 s.
+  useEffect(() => {
+    if (!returnPoint) return undefined;
+    const t = window.setTimeout(() => setReturnPoint(null), 20_000);
+    return () => window.clearTimeout(t);
+  }, [returnPoint]);
+  const handleReturn = useCallback(() => {
+    if (!returnPoint) return;
+    const target = returnPoint.eventId;
+    setReturnPoint(null);
+    if (eventId) navigateRoom(room.roomId, target);
+    else handleOpenEvent(target, false);
+  }, [returnPoint, eventId, navigateRoom, room.roomId, handleOpenEvent]);
+
   useEffect(() => {
     if (eventId) {
       setTimeline(getEmptyTimeline());
@@ -1010,9 +1049,10 @@ export function RoomTimeline({ room, eventId, roomInputRef, editor }: RoomTimeli
     async (evt) => {
       const targetId = evt.currentTarget.getAttribute('data-event-id');
       if (!targetId) return;
+      captureReturnPoint();
       handleOpenEvent(targetId);
     },
-    [handleOpenEvent],
+    [handleOpenEvent, captureReturnPoint],
   );
 
   const handleUserClick: MouseEventHandler<HTMLButtonElement> = useCallback(
@@ -2382,17 +2422,32 @@ export function RoomTimeline({ room, eventId, roomInputRef, editor }: RoomTimeli
             <span ref={atBottomAnchorRef} />
           </Box>
         </Scroll>
-        {!atBottom && (
+        {(!atBottom || returnPoint) && (
           <TimelineFloat position="Bottom">
-            <Chip
-              variant="SurfaceVariant"
-              radii="Pill"
-              outlined
-              before={<Icon size="50" src={Icons.ArrowBottom} />}
-              onClick={handleJumpToLatest}
-            >
-              <Text size="L400">Jump to Latest</Text>
-            </Chip>
+            <Box gap="200" alignItems="Center">
+              {returnPoint && (
+                <Chip
+                  variant="Primary"
+                  radii="Pill"
+                  outlined
+                  before={<Icon size="50" src={Icons.ArrowBottom} />}
+                  onClick={handleReturn}
+                >
+                  <Text size="L400">Back to where you were</Text>
+                </Chip>
+              )}
+              {!atBottom && (
+                <Chip
+                  variant="SurfaceVariant"
+                  radii="Pill"
+                  outlined
+                  before={<Icon size="50" src={Icons.ArrowBottom} />}
+                  onClick={handleJumpToLatest}
+                >
+                  <Text size="L400">Jump to Latest</Text>
+                </Chip>
+              )}
+            </Box>
           </TimelineFloat>
         )}
       </Box>