fix(ui): catalog continuation silently dropped staged attachments (#123581)

A continuable catalog session exposes the full attachment surface
(file inputs, plus-menu, paste), and the send button counts staged
attachments as composed content — but continueCatalogSession required a
non-empty text draft and always handed off attachments: []. Draft+images
sent text only with the images silently vanishing (payloads leaked,
never released); images-only made the enabled send button do nothing at
all: the worst bug class, an action with no visible outcome and no
recorded reason.

Carry independently-owned attachment clones through the continuation
handoff, mirroring the new-session sibling
(chat-pane-session-creation.ts), and treat attachments as composed
content in the guard so attachment-only continuations send. Empty
draft with no attachments still no-ops.

ui/src/pages/chat/chat-pane-history.test.ts

@@ -79,6 +79,7 @@ function createTestChatPane(params: { client: GatewayBrowserClient; sessions: Se
   const state = {
     agentsList: null,
     assistantAgentId: null,
+    chatAttachments: [],
     chatError: null,
     chatHistoryPagination: { hasMore: false },
     chatLoading: false,
@@ -717,6 +718,70 @@ describe("chat pane catalog continuation lifecycle", () => {
     expect(state.handleSendChat).not.toHaveBeenCalled();
   });
 
+  it("carries staged attachments through the continuation handoff", async () => {
+    const request = vi.fn().mockResolvedValue({ sessionKey: "agent:main:continued-attachments" });
+    const { key, pane, state } = createCatalogContinuationPane(request);
+    state.chatAttachments = [
+      {
+        id: "att-1",
+        mimeType: "image/png",
+        fileName: "shot.png",
+        dataUrl: "data:image/png;base64,AAAA",
+      },
+    ];
+
+    await pane.continueCatalogSession(key);
+
+    const handoff = consumePaneSessionHandoff(
+      pane.context,
+      pane.paneId,
+      "agent:main:continued-attachments",
+    );
+    expect(handoff?.send).toBe(true);
+    expect(handoff?.attachments).toHaveLength(1);
+    expect(handoff?.attachments[0]).toMatchObject({
+      mimeType: "image/png",
+      fileName: "shot.png",
+      dataUrl: "data:image/png;base64,AAAA",
+    });
+  });
+
+  it("continues an attachment-only draft instead of silently ignoring the send", async () => {
+    const request = vi.fn().mockResolvedValue({ sessionKey: "agent:main:attachment-only" });
+    const { key, pane, state } = createCatalogContinuationPane(request);
+    state.chatMessage = "";
+    state.chatAttachments = [
+      {
+        id: "att-2",
+        mimeType: "image/png",
+        fileName: "only.png",
+        dataUrl: "data:image/png;base64,BBBB",
+      },
+    ];
+
+    await pane.continueCatalogSession(key);
+
+    expect(request).toHaveBeenCalledWith("sessions.catalog.continue", key);
+    const handoff = consumePaneSessionHandoff(
+      pane.context,
+      pane.paneId,
+      "agent:main:attachment-only",
+    );
+    expect(handoff?.send).toBe(true);
+    expect(handoff?.attachments).toHaveLength(1);
+  });
+
+  it("still ignores a continuation with no draft and no attachments", async () => {
+    const request = vi.fn();
+    const { key, pane, state } = createCatalogContinuationPane(request);
+    state.chatMessage = "   ";
+    state.chatAttachments = [];
+
+    await pane.continueCatalogSession(key);
+
+    expect(request).not.toHaveBeenCalled();
+  });
+
   it("does not stage or send a continuation rejected by its logical pane", async () => {
     const request = vi.fn().mockResolvedValue({ sessionKey: "agent:main:rejected-continuation" });
     const { key, pane, state } = createCatalogContinuationPane(request);

ui/src/pages/chat/chat-pane-history.ts

@@ -17,7 +17,10 @@ import {
   areUiSessionKeysEquivalent,
   parseAgentSessionKey,
 } from "../../lib/sessions/session-key.ts";
-import { replaceChatAttachmentsFromEditor } from "./attachment-payload-store.ts";
+import {
+  cloneChatAttachmentsForIndependentOwner,
+  replaceChatAttachmentsFromEditor,
+} from "./attachment-payload-store.ts";
 import type { ChatHistoryPagination } from "./chat-history-pagination.ts";
 import {
   loadChatHistory,
@@ -535,8 +538,14 @@ export abstract class ChatPaneHistory extends ChatPaneSession {
     const scope = this.captureConnectionScope();
     const state = scope?.state;
     const client = scope?.client;
-    const draft = state?.chatMessage.trim();
-    if (!scope || !state || !client || !draft || !this.catalogSession?.canContinue) {
+    const draft = state?.chatMessage.trim() ?? "";
+    // Attachments count as composed content: an image-only continuation must
+    // send, not silently no-op while the send button looks live.
+    if (!scope || !state || !client || !this.catalogSession?.canContinue) {
+      return;
+    }
+    const attachments = cloneChatAttachmentsForIndependentOwner(state.chatAttachments);
+    if (!draft && attachments.length === 0) {
       return;
     }
     const sourceSessionKey = state.sessionKey;
@@ -573,7 +582,7 @@ export abstract class ChatPaneHistory extends ChatPaneSession {
         return;
       }
       preparePaneSessionHandoff(this.context, this.paneId, result.sessionKey, {
-        attachments: [],
+        attachments,
         draft,
         send: true,
       });