fix: session history not loading + new session showing stale messages

Bug 1 - Session opens with empty messages:
- loadChatHistory effect was re-running on every currentSessionId change,
  racing with openSessionById and overwriting loaded messages
- Removed currentSessionId from effect dependencies (only runs on mount/postId)
- Added recovery: if session has 0 messages but has post_id, try fetching
  from the post-based conversation endpoint as fallback

Bug 2 - Start New Session shows old messages:
- startNewConversation now sets isHydratingSessionRef=true before changing
  session state, preventing the persistence effect from saving stale data
- Fully resets: messages, plan, agentMode, keyword suggestions, providerInfo
- loadPostSessions called AFTER state reset to avoid stale renders

Also fixed:
- Legacy fallback now only fires when no session was resolved at all
  (prevents loading old post_meta data over session data)

assets/js/sidebar.js

@@ -1136,6 +1136,11 @@
 
 		React.useEffect(() => {
 			const loadChatHistory = async () => {
+				// Skip if we already have a session loaded (e.g., from openSessionById)
+				if (messages.length > 0 || isHydratingSessionRef.current) {
+					return;
+				}
+
 				try {
 					const headers = {
 						'X-WP-Nonce': wpAgenticWriter.nonce,
@@ -1148,7 +1153,7 @@
 					if (sessions.length > 0) {
 						if (sessions.length > 0) {
 							let selected = sessions[0];
-							const preferred = currentSessionId || (() => {
+							const preferred = (() => {
 								try {
 									return window.localStorage.getItem(`wpawSessionId_${postId}`) || '';
 								} catch (error) {
@@ -1185,8 +1190,8 @@
 						}
 					}
 
-					// Legacy endpoint fallback.
-					if (postId && historyMessages.length === 0) {
+					// Legacy endpoint fallback - only if no session found at all.
+					if (postId && historyMessages.length === 0 && !resolvedSessionId) {
 						const legacy = await fetch(`${wpAgenticWriter.apiUrl}/chat-history/${postId}`, {
 							method: 'GET',
 							headers,
@@ -1216,7 +1221,9 @@
 				}
 			};
 			loadChatHistory();
-		}, [postId, currentSessionId, sanitizeMessagesForStorage, hydrateSessionStateFromMessages]);
+		// Only run on mount / postId change — NOT on currentSessionId change
+		// Session switches are handled by openSessionById directly
+		}, [postId]);
 
 		const loadPostSessions = async () => {
 			const headers = {
@@ -1316,9 +1323,24 @@
 				setCurrentSessionId(sessionId);
 				const sessionMessages = Array.isArray(data?.messages) ? data.messages : [];
 
-				// If session has no messages but has a post_id, it may have been improperly persisted
-				if (sessionMessages.length === 0) {
-					wpawLog.warn('Session loaded with 0 messages:', sessionId);
+				// If session has no messages, try fetching from the conversations/post endpoint
+				// as a recovery mechanism (messages may be stored under post relationship)
+				if (sessionMessages.length === 0 && data?.post_id && Number(data.post_id) > 0) {
+					wpawLog.warn('Session has 0 messages, attempting post-based recovery:', sessionId);
+					try {
+						const postSessionRes = await fetch(`${wpAgenticWriter.apiUrl}/conversation/${data.post_id}`, {
+							method: 'GET',
+							headers,
+						});
+						if (postSessionRes.ok) {
+							const postSessionData = await postSessionRes.json();
+							if (Array.isArray(postSessionData?.messages) && postSessionData.messages.length > 0) {
+								sessionMessages.push(...postSessionData.messages);
+							}
+						}
+					} catch (e) {
+						// Non-fatal recovery attempt
+					}
 				}
 
 				lastPersistedMessagesRef.current = JSON.stringify(sanitizeMessagesForStorage(sessionMessages));
@@ -5627,13 +5649,27 @@
 					throw new Error('Failed to create a new conversation');
 				}
 				const data = await response.json();
+				// Fully reset state for clean slate
+				isHydratingSessionRef.current = true;
 				if (data?.session_id) {
 					setCurrentSessionId(data.session_id);
 				}
-				await loadPostSessions();
 				lastPersistedMessagesRef.current = JSON.stringify([]);
 				setMessages([]);
+				currentPlanRef.current = null;
+				setAgentMode('chat');
 				setShowWelcome(false);
+				setFocusKeywordSuggestions([]);
+				setSelectedFocusKeyword('');
+				setProviderInfo(null);
+				await loadPostSessions();
+				setTimeout(() => {
+					isHydratingSessionRef.current = false;
+					// Focus input
+					if (inputRef.current) {
+						inputRef.current.focus();
+					}
+				}, 50);
 			} catch (error) {
 				setMessages(prev => [...prev, {
 					role: 'system',