Reapply "fix(app): more startup efficiency (#18985)"

This reverts commit cbe1337f24.

packages/app/src/pages/session/use-session-hash-scroll.ts

@@ -8,6 +8,9 @@ export const useSessionHashScroll = (input: {
   sessionID: () => string | undefined
   messagesReady: () => boolean
   visibleUserMessages: () => UserMessage[]
+  historyMore: () => boolean
+  historyLoading: () => boolean
+  loadMore: (sessionID: string) => Promise<void>
   turnStart: () => number
   currentMessageId: () => string | undefined
   pendingMessage: () => string | undefined
@@ -181,6 +184,21 @@ export const useSessionHashScroll = (input: {
     queue(() => scrollToMessage(msg, "auto"))
   })
 
+  createEffect(() => {
+    const sessionID = input.sessionID()
+    if (!sessionID || !input.messagesReady()) return
+
+    visibleUserMessages()
+
+    let targetId = input.pendingMessage()
+    if (!targetId && !clearing) targetId = messageIdFromHash(location.hash)
+    if (!targetId) return
+    if (messageById().has(targetId)) return
+    if (!input.historyMore() || input.historyLoading()) return
+
+    void input.loadMore(sessionID)
+  })
+
   onMount(() => {
     if (typeof window !== "undefined" && "scrollRestoration" in window.history) {
       window.history.scrollRestoration = "manual"