some fixes

Sources/Shared/Models/OlderHistoryScrollPolicy.swift

@@ -0,0 +1,30 @@
+import Foundation
+
+/// One interactive backward-history page handed to the UI: whether older
+/// history remains and the cursor the next request must use.
+struct OlderHistoryScrollPage: Equatable, Sendable {
+    let hasMore: Bool
+    let nextBefore: String?
+}
+
+/// Interactive scroll-back policy. The cursor for the next page must come
+/// from the server's RSM <first> of the page that was just returned, not from
+/// the oldest local message: a page can consist entirely of reactions,
+/// retractions or already-known stanzas, which insert no new message rows, so
+/// a locally computed anchor would never advance and every subsequent scroll
+/// would re-request the same page forever (spinner spins, nothing loads).
+enum OlderHistoryScrollPolicy {
+    /// - Parameter complete: the server's <fin complete="true"> flag.
+    /// - Parameter pageFirstID: RSM <first> UID of the returned page — the
+    ///   oldest item in it, i.e. the anchor for the next, older page.
+    static func page(
+        complete: Bool,
+        pageFirstID: String?
+    ) -> OlderHistoryScrollPage {
+        let nextBefore = ArchiveSyncCheckpoint.normalizedCursor(pageFirstID)
+        // An empty page (or one without RSM) cannot advance the cursor, so it
+        // must never report "more" — otherwise the UI would loop on it.
+        let hasMore = !complete && nextBefore != nil
+        return OlderHistoryScrollPage(hasMore: hasMore, nextBefore: nextBefore)
+    }
+}