fix: fix inconsistent behavior with threading email messages in the inbox/folders

lib/demo/demo-client.ts

@@ -388,6 +388,16 @@ export class DemoJMAPClient implements IJMAPClient {
     return { id: threadId, emailIds: emails.map(e => e.id) };
   }
 
+  async getThreads(threadIds: string[]): Promise<Thread[]> {
+    return threadIds
+      .map(tid => {
+        const emails = this.data.emails.filter(e => e.threadId === tid);
+        if (emails.length === 0) return null;
+        return { id: tid, emailIds: emails.map(e => e.id) };
+      })
+      .filter((t): t is Thread => t !== null);
+  }
+
   async getThreadEmails(threadId: string): Promise<Email[]> {
     return this.data.emails
       .filter(e => e.threadId === threadId)

lib/jmap/client-interface.ts

@@ -117,6 +117,7 @@ export interface IJMAPClient {
 
   // ── Threads ───────────────────────────────────────────────────
   getThread(threadId: string, accountId?: string): Promise<Thread | null>;
+  getThreads(threadIds: string[], accountId?: string): Promise<Thread[]>;
   getThreadEmails(threadId: string, accountId?: string): Promise<Email[]>;
 
   // ── Compose / Send ────────────────────────────────────────────

lib/jmap/client.ts

@@ -1904,6 +1904,24 @@ export class JMAPClient implements IJMAPClient {
     }
   }
 
+  async getThreads(threadIds: string[], accountId?: string): Promise<Thread[]> {
+    if (threadIds.length === 0) return [];
+    try {
+      const targetAccountId = accountId || this.accountId;
+      const response = await this.request([
+        ["Thread/get", { accountId: targetAccountId, ids: threadIds }, "0"],
+      ]);
+
+      if (response.methodResponses?.[0]?.[0] === "Thread/get") {
+        return (response.methodResponses[0][1].list || []) as Thread[];
+      }
+      return [];
+    } catch (error) {
+      console.error('Failed to get threads:', error);
+      return [];
+    }
+  }
+
   async getThreadEmails(threadId: string, accountId?: string): Promise<Email[]> {
     try {
       const targetAccountId = accountId || this.accountId;

lib/thread-utils.ts

@@ -5,8 +5,16 @@ import type { Email, ThreadGroup } from "./jmap/types";
  * Single-email threads are still returned as ThreadGroups with emailCount=1.
  * When disableThreading is true, each email is placed into its own group using
  * its message ID as the key, so the list shows individual messages.
+ *
+ * @param threadEmailCounts - Optional map of threadId → total email count across
+ *   all folders (from Thread/get). When provided, emailCount reflects the full
+ *   thread size rather than just the emails in the current folder.
  */
-export function groupEmailsByThread(emails: Email[], disableThreading = false): ThreadGroup[] {
+export function groupEmailsByThread(
+  emails: Email[],
+  disableThreading = false,
+  threadEmailCounts?: Map<string, number>,
+): ThreadGroup[] {
   if (!emails || emails.length === 0) {
     return [];
   }
@@ -53,7 +61,7 @@ export function groupEmailsByThread(emails: Email[], disableThreading = false):
       hasAttachment,
       hasAnswered,
       hasForwarded,
-      emailCount: sortedEmails.length,
+      emailCount: threadEmailCounts?.get(threadId) ?? sortedEmails.length,
     });
   }