fix live tape scroll stability

apps/web/app/terminal.tsx

@@ -142,12 +142,12 @@ type TapeVirtualListConfig = {
 };
 
 const TAPE_VIRTUAL_CONFIG: Record<TapeVirtualPane, TapeVirtualListConfig> = {
-  options: { rowHeight: 36, overscan: 24, debugLabel: "options" },
-  equities: { rowHeight: 36, overscan: 20, debugLabel: "equities" },
-  flow: { rowHeight: 44, overscan: 16, debugLabel: "flow" },
-  alerts: { rowHeight: 44, overscan: 16, debugLabel: "alerts" },
-  classifier: { rowHeight: 44, overscan: 16, debugLabel: "classifier" },
-  dark: { rowHeight: 44, overscan: 16, debugLabel: "dark" }
+  options: { rowHeight: 36, overscan: 44, debugLabel: "options" },
+  equities: { rowHeight: 36, overscan: 36, debugLabel: "equities" },
+  flow: { rowHeight: 44, overscan: 24, debugLabel: "flow" },
+  alerts: { rowHeight: 44, overscan: 24, debugLabel: "alerts" },
+  classifier: { rowHeight: 44, overscan: 24, debugLabel: "classifier" },
+  dark: { rowHeight: 44, overscan: 24, debugLabel: "dark" }
 };
 
 export const getTapeVirtualConfig = (pane: TapeVirtualPane): TapeVirtualListConfig =>
@@ -844,6 +844,30 @@ export const appendHistoryTail = <T extends SortableItem>(
   return cap > 0 ? combined.slice(0, cap) : combined;
 };
 
+export const mergeHeldTapeHistory = <T extends SortableItem>(
+  displayedHistory: T[],
+  incomingHistory: T[],
+  frozenLiveHead: T[]
+): T[] => {
+  if (displayedHistory.length === 0) {
+    return appendHistoryTail([], incomingHistory, frozenLiveHead, 0);
+  }
+
+  const sortedDisplayed = appendHistoryTail([], displayedHistory, frozenLiveHead, 0);
+  const tail = sortedDisplayed.at(-1);
+  const tailTs = tail ? extractSortTs(tail) : Number.POSITIVE_INFINITY;
+  const tailSeq = tail ? extractSortSeq(tail) : Number.POSITIVE_INFINITY;
+  const olderIncoming = incomingHistory.filter((item) => {
+    const itemTs = extractSortTs(item);
+    if (itemTs < tailTs) {
+      return true;
+    }
+    return itemTs === tailTs && extractSortSeq(item) < tailSeq;
+  });
+
+  return appendHistoryTail(sortedDisplayed, olderIncoming, frozenLiveHead, 0);
+};
+
 export const getLiveHistoryRetentionCap = (subscription: LiveSubscription): number => {
   switch (subscription.channel) {
     case "options":
@@ -2491,6 +2515,7 @@ const usePausableTapeView = <T extends SortableItem & { seq: number }>(
   config: PausableTapeViewConfig<T>
 ): TapeState<T> => {
   const [data, setData] = useState<PausableTapeData<T>>(EMPTY_PAUSABLE_TAPE);
+  const displayedHistoryRef = useRef<T[]>([]);
   const holdForScroll = config.enabled ? (config.shouldHold ? config.shouldHold() : false) : false;
 
   useEffect(() => {
@@ -2557,13 +2582,31 @@ const usePausableTapeView = <T extends SortableItem & { seq: number }>(
   const status = config.enabled ? config.sourceStatus : "disconnected";
   const projected = projectPausableTapeState(data.visible, status, config.lastUpdate);
   const historyItems = config.historyTail ?? [];
-  const items = useMemo(() => composeTapeItems([], projected.items, historyItems), [projected.items, historyItems]);
+  const displayedHistoryItems = useMemo(() => {
+    if (!config.enabled) {
+      displayedHistoryRef.current = [];
+      return [];
+    }
+
+    if (!holdForScroll) {
+      displayedHistoryRef.current = historyItems;
+      return historyItems;
+    }
+
+    const next = mergeHeldTapeHistory(displayedHistoryRef.current, historyItems, projected.items);
+    displayedHistoryRef.current = next;
+    return next;
+  }, [config.enabled, historyItems, holdForScroll, projected.items]);
+  const items = useMemo(
+    () => composeTapeItems([], projected.items, displayedHistoryItems),
+    [projected.items, displayedHistoryItems]
+  );
 
   return {
     status,
     items,
     liveItems: projected.items,
-    historyItems,
+    historyItems: displayedHistoryItems,
     lastUpdate: projected.lastUpdate,
     replayTime: null,
     replayComplete: false,