fix(webui): a browser frame that never arrives now says so

packages/webui/src/client/features/browser/contracts/browser-port.ts

@@ -239,6 +239,15 @@ export interface BrowserPort {
       readonly viewport?: WebuiBrowserViewport;
     },
     onFrame: (frame: WebuiBrowserImageFrame) => void,
+    /**
+     * The service's refusal, when the stream gave up.
+     *
+     * The frames themselves say nothing about a stream that stopped, so the
+     * refusal is reported here rather than left to the panel to infer: a
+     * panel that is waiting and a panel whose stream failed look identical
+     * until one of them is told.
+     */
+    onFailure?: (error: Error) => void,
   ) => () => void;
   /**
    * The session's web tabs, as a snapshot, on a stream.

packages/webui/src/client/features/browser/ui/BrowserPanel.public.tsx

@@ -76,6 +76,10 @@ const PANEL_COPY = {
   absent: "当前运行时没有浏览器能力，浏览器面板不可用。",
   notReady: "浏览器能力尚未就绪（例如扩展未连接），面板暂时无法显示页面。",
   unknown: "当前运行时没有报告浏览器能力，面板按未授权处理。",
+  // The stream gave up. The reason is said once, beside this notice, as a
+  // toast; this sentence says the half that is about the panel — that the
+  // picture is not coming, and that opening the page again is the way back.
+  failed: "浏览器画面流已失败，画面不会再来。重新打开页面可以重试。",
 } as const;
 
 export interface BrowserPanelProps {
@@ -411,6 +415,19 @@ export function BrowserPanel({
         </FeedbackLiveRegion>
       ) : null}
 
+      {/*
+        A stream that gave up is not a page that is still loading. The empty
+        placeholder below looks the same in both cases, so the failure says
+        so — and it says it here, in the panel, rather than only in a toast
+        that a person can miss. It renders only when the panel failed, so the
+        default DOM is the one that was there before.
+      */}
+      {state?.status === "error" ? (
+        <FeedbackLiveRegion className="webui-browser-notice" testId="browser-panel-failed">
+          {PANEL_COPY.failed}
+        </FeedbackLiveRegion>
+      ) : null}
+
       <div className="webui-browser-stage" ref={stageRef}>
         {frameSrc ? (
           <img

packages/webui/src/client/features/browser/ui/BrowserPanelHost.public.tsx

@@ -372,6 +372,12 @@ export function BrowserPanelHost({
           dispatch({ type: "frame", frame });
         }
       },
+      // The service's refusal, not the panel's guess. A stream that gave up is
+      // reported as an error, so it stops looking like a page that is still
+      // loading — the two are the same picture on screen otherwise.
+      (error) => {
+        dispatch({ type: "failed", error: toBrowserError(error) });
+      },
     );
     return () => {
       stop();

packages/webui/src/client/platform/transport.ts

@@ -325,8 +325,8 @@ export function createWebuiTransport(options: WebuiTransportOptions): Required<W
         ...(body.runId ? { runId: body.runId } : {}),
       }, { reuseConnection: true }),
     sendBrowserCommand: (body) => request(SEND_BROWSER_COMMAND_OPERATION_NAME, body),
-    watchBrowserFrames: (body, onFrame) =>
-      watchBrowserFrames(WATCH_BROWSER_FRAMES_OPERATION_NAME, body, onFrame),
+    watchBrowserFrames: (body, onFrame, onFailure) =>
+      watchBrowserFrames(WATCH_BROWSER_FRAMES_OPERATION_NAME, body, onFrame, onFailure),
     watchBrowserWebTabs: (body, onSnapshot) =>
       watchBrowserWebTabs(WATCH_BROWSER_WEB_TABS_OPERATION_NAME, body, onSnapshot),
     openBrowserWebTab: (body) => request(OPEN_BROWSER_WEB_TAB_OPERATION_NAME, body),

packages/webui/src/client/platform/wire.ts

@@ -194,6 +194,7 @@ export interface WebuiWire {
     operation: string,
     requestBody: { readonly surface: WebuiBrowserSurface; readonly target: WebuiBrowserTargetId },
     onFrame: (frame: WebuiBrowserImageFrame) => void,
+    onFailure?: (error: Error) => void,
   ): () => void;
   /**
    * The web-tab snapshot stream.
@@ -614,13 +615,22 @@ export function createWebuiWire({
       readonly viewport?: WebuiBrowserViewport;
     },
     onFrame: (frame: WebuiBrowserImageFrame) => void,
+    onFailure?: (error: Error) => void,
   ): () => void {
     let stopped = false;
     const session = openWireSession(operation, requestBody, {
       frame: (_session, frame) => {
         // A stopped watch must not deliver, and `close()` is not a promise that
         // the socket stops emitting: the caller's flag is what guarantees it.
         if (stopped) return;
+        // An error frame is the service's answer: it says the stream gave up
+        // and why. Dropping it is what leaves a panel waiting for a picture
+        // that is never coming — the watcher has no way to tell "still
+        // loading" from "the service said it stopped", so it says neither.
+        if (frame.kind === "error") {
+          onFailure?.(new WebuiWireError(frame.code, frame.message, frame.requestId));
+          return;
+        }
         if (frame.kind !== "event" || !frame.body || typeof frame.body !== "object") return;
         onFrame(frame.body as WebuiBrowserImageFrame);
       },