Function computeUnseenDivider

src/components/FullscreenLayout.tsx:239–256

(messages: readonly Message[], dividerIndex: number | null)

 * until Claude's text response lands.
 */
export function computeUnseenDivider(messages: readonly Message[], dividerIndex: number | null): UnseenDivider | undefined {
  if (dividerIndex === null) return undefined;
  // Skip progress and null-rendering attachments when picking the divider
  // anchor — Messages.tsx filters these out of renderableMessages before the
  // dividerBeforeIndex search, so their UUID wouldn't be found (CC-724).
  // Hook attachments use randomUUID() so nothing shares their 24-char prefix.
  let anchorIdx = dividerIndex;
  while (anchorIdx < messages.length && (messages[anchorIdx]?.type === 'progress' || isNullRenderingAttachment(messages[anchorIdx]!))) {
    anchorIdx++;
  }
  const uuid = messages[anchorIdx]?.uuid;
  if (!uuid) return undefined;
  const count = countUnseenAssistantTurns(messages, dividerIndex);
  return {
    firstUnseenUuid: uuid,
    count: Math.max(1, count)
  };
}

/**
 * Layout wrapper for the REPL. In fullscreen mode, puts scrollable