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/components/session/SessionSidebar.tsx

@@ -1087,7 +1087,7 @@ export const SessionSidebar: React.FC<SessionSidebarProps> = ({
     currentSessionId,
     sortedSessions,
     recentSessionIds: recentSessionIdsList,
-    loadMessages: sync.syncSession,
+    ensureSessionRenderable: sync.ensureSessionRenderable,
   });
 
   const sectionsForSidebarRender = React.useMemo(() => {

packages/ui/src/components/session/sidebar/SessionNodeItem.tsx

@@ -347,7 +347,7 @@ function SessionNodeItemComponent(props: Props): React.ReactNode {
     let skipped = 0;
     for (const child of children) {
       try {
-        await sync.syncSession(child.session.id);
+        await sync.ensureSessionRenderable(child.session.id);
         const childRecords = buildSessionMessageRecordsSnapshot(directoryStore.getState(), child.session.id).list;
         const childTitle = child.session.title || t('sessions.sidebar.session.export.untitledSubagent');
         const childAgent = (child.session as Session & { agent?: string }).agent;
@@ -379,7 +379,7 @@ function SessionNodeItemComponent(props: Props): React.ReactNode {
       return;
     }
 
-    await sync.syncSession(session.id);
+    await sync.ensureSessionRenderable(session.id);
 
     const records = buildSessionMessageRecordsSnapshot(directoryStore.getState(), session.id).list;
     if (records.length === 0) {

packages/ui/src/components/session/sidebar/hooks/useSessionPrefetch.ts

@@ -1,7 +1,7 @@
 import React from 'react';
 import type { Session } from '@opencode-ai/sdk/v2';
 import { useSessionUIStore } from '@/sync/session-ui-store';
-import { getSyncMessages } from '@/sync/sync-refs';
+import { getSyncSessionMaterializationStatus } from '@/sync/sync-refs';
 
 const SESSION_PREFETCH_HOVER_DELAY_MS = 180;
 const SESSION_PREFETCH_SETTLE_MS = 600;
@@ -12,10 +12,10 @@ type Args = {
   currentSessionId: string | null;
   sortedSessions: Session[];
   recentSessionIds?: string[];
-  loadMessages: (sessionId: string) => Promise<unknown>;
+  ensureSessionRenderable: (sessionId: string) => Promise<unknown>;
 };
 
-export const useSessionPrefetch = ({ currentSessionId, sortedSessions, recentSessionIds = [], loadMessages }: Args): void => {
+export const useSessionPrefetch = ({ currentSessionId, sortedSessions, recentSessionIds = [], ensureSessionRenderable }: Args): void => {
   const sessionPrefetchTimersRef = React.useRef<Map<string, number>>(new Map());
   const sessionPrefetchQueueRef = React.useRef<string[]>([]);
   const sessionPrefetchInFlightRef = React.useRef<Set<string>>(new Set());
@@ -36,30 +36,28 @@ export const useSessionPrefetch = ({ currentSessionId, sortedSessions, recentSes
         continue;
       }
 
-      // Check if messages already loaded in sync child store
-      const hasMessages = getSyncMessages(nextSessionId).length > 0;
-      if (hasMessages) {
+      // Check if the session is already renderable in the sync child store.
+      if (getSyncSessionMaterializationStatus(nextSessionId).renderable) {
         continue;
       }
 
       sessionPrefetchInFlightRef.current.add(nextSessionId);
-      void loadMessages(nextSessionId)
+      void ensureSessionRenderable(nextSessionId)
         .catch(() => undefined)
         .finally(() => {
           sessionPrefetchInFlightRef.current.delete(nextSessionId);
           pumpSessionPrefetchQueue();
         });
     }
-  }, [loadMessages]);
+  }, [ensureSessionRenderable]);
 
   const scheduleSessionPrefetch = React.useCallback((sessionId: string | null | undefined) => {
     if (!sessionId || sessionId === currentSessionId || typeof window === 'undefined') {
       return;
     }
 
-    // Already loaded in sync
-    const hasMessages = getSyncMessages(sessionId).length > 0;
-    if (hasMessages) {
+    // Already renderable in sync
+    if (getSyncSessionMaterializationStatus(sessionId).renderable) {
       return;
     }