refactor: stabilize live chat sync materialization (#1132)

Canonicalize session message/part materialization across load, prefetch, reconnect, and recovery paths so OpenChamber restores session snapshots through one consistent merge flow.

Preserve live assistant streaming text when stale or delayed snapshots arrive, while still replacing optimistic user parts with confirmed server snapshots to avoid duplicated user messages.

Narrow recovery triggers to explicit incomplete snapshot signals instead of broad session-event fallbacks, reducing unnecessary session refetches during active streaming.

Keep turn windowing aligned with parented assistant replies and add regression coverage for materialization gaps, stale snapshot protection, optimistic user replacement, reconnect recovery, and turn grouping.

packages/ui/src/sync/use-sync.ts

@@ -6,7 +6,6 @@ import { SESSION_CACHE_LIMIT } from "./types"
 import { pickSessionCacheEvictions } from "./session-cache"
 import {
   mergeOptimisticPage,
-  mergeMessages,
   type OptimisticItem,
 } from "./optimistic"
 import { useDirectoryStore, useSyncSDK, useSyncDirectory, useChildStoreManager } from "./sync-context"
@@ -18,6 +17,7 @@ import {
   setSessionPrefetch,
   clearSessionPrefetch,
 } from "./session-prefetch-cache"
+import { getSessionMaterializationStatus, materializeSessionSnapshots } from "./materialization"
 
 const SKIP_PARTS = new Set(["patch", "step-start", "step-finish"])
 const MESSAGE_PAGE_SIZE = 200
@@ -224,33 +224,19 @@ export function useSync() {
         }
 
         const current = store.getState()
-        const cached = current.message[sessionID] ?? []
-        const messages = options?.mode === "prepend"
-          ? mergeMessages(cached, merged.session)
-          : (cached.length > 0 ? mergeMessages(cached, merged.session) : merged.session)
+        const materialized = materializeSessionSnapshots(
+          current,
+          sessionID,
+          merged.session.map((info) => ({
+            info,
+            parts: merged.part.find((item) => item.id === info.id)?.part ?? [],
+          })),
+          { skipPartTypes: SKIP_PARTS, mode: options?.mode === "prepend" ? "prepend" : "merge" },
+        )
 
-        // Build part updates — preserve existing references on prepend to avoid flicker
-        const isPrepend = options?.mode === "prepend"
-        let partsChanged = false
-        const partUpdate: Record<string, Part[]> = { ...current.part }
-        for (const p of merged.part) {
-          if (isPrepend && partUpdate[p.id]) continue // already loaded
-          const filtered = p.part.filter((x: Part) => !SKIP_PARTS.has(x.type))
-          if (filtered.length) {
-            partUpdate[p.id] = filtered
-            partsChanged = true
-          }
-        }
-
-        const patch: Record<string, unknown> = {
-          message: messages !== cached ? { ...current.message, [sessionID]: messages } : current.message,
-        }
-        if (!isPrepend || partsChanged) {
-          patch.part = partUpdate
-        }
-        store.setState(patch)
+        store.setState({ message: materialized.message, part: materialized.part })
         setMetaFor(sessionID, {
-          limit: messages.length,
+          limit: materialized.messages.length,
           cursor: merged.cursor,
           complete: merged.complete,
           loading: false,
@@ -258,7 +244,7 @@ export function useSync() {
         setSessionPrefetch({
           directory,
           sessionID,
-          limit: messages.length,
+          limit: materialized.messages.length,
           cursor: merged.cursor,
           complete: merged.complete,
         })
@@ -281,7 +267,8 @@ export function useSync() {
 
       const current = store.getState()
       const m = getMetaFor(sessionID)
-      const cached = current.message[sessionID] !== undefined && m.limit > 0
+      const materialization = getSessionMaterializationStatus(current, sessionID)
+      const cached = materialization.hasMessages && materialization.renderable && m.limit > 0
       const hasSession = Binary.search(current.session, sessionID, (s) => s.id).found
       if (cached && hasSession && !force) return
 
@@ -401,6 +388,7 @@ export function useSync() {
 
   return useMemo(
     () => ({
+      ensureSessionRenderable: syncSession,
       syncSession,
       loadMore,
       hasMore,