fix: support fallback copy for webui replies

webui/src/components/MessageBubble.tsx

@@ -14,6 +14,7 @@ import { CliAppMentionText } from "@/components/CliAppMentionText";
 import { ImageLightbox } from "@/components/ImageLightbox";
 import { MarkdownText, preloadMarkdownText } from "@/components/MarkdownText";
 import { cn } from "@/lib/utils";
+import { copyTextToClipboard } from "@/lib/clipboard";
 import { formatTurnLatency } from "@/lib/format";
 import { toMediaAttachment } from "@/lib/media";
 import type {
@@ -71,8 +72,8 @@ export function MessageBubble({
   }, []);
 
   const onCopyAssistantReply = useCallback(() => {
-    if (!navigator.clipboard) return;
-    void navigator.clipboard.writeText(message.content).then(() => {
+    void copyTextToClipboard(message.content).then((ok) => {
+      if (!ok) return;
       setCopied(true);
       if (copyResetRef.current !== null) {
         window.clearTimeout(copyResetRef.current);

webui/src/lib/clipboard.ts

@@ -0,0 +1,40 @@
+export async function copyTextToClipboard(text: string): Promise<boolean> {
+  try {
+    if (navigator.clipboard?.writeText) {
+      await navigator.clipboard.writeText(text);
+      return true;
+    }
+  } catch {
+    // Fall through to the legacy path for browsers/WebViews where the
+    // Clipboard API exists but rejects outside a secure context.
+  }
+
+  return copyTextWithTextarea(text);
+}
+
+function copyTextWithTextarea(text: string): boolean {
+  if (typeof document.execCommand !== "function") {
+    return false;
+  }
+
+  const textarea = document.createElement("textarea");
+  textarea.value = text;
+  textarea.setAttribute("readonly", "");
+  textarea.style.position = "fixed";
+  textarea.style.top = "-9999px";
+  textarea.style.left = "-9999px";
+  textarea.style.opacity = "0";
+
+  document.body.appendChild(textarea);
+  textarea.focus();
+  textarea.select();
+  textarea.setSelectionRange(0, textarea.value.length);
+
+  try {
+    return document.execCommand("copy");
+  } catch {
+    return false;
+  } finally {
+    document.body.removeChild(textarea);
+  }
+}

webui/src/tests/message-bubble.test.tsx

@@ -167,6 +167,72 @@ describe("MessageBubble", () => {
     );
   });
 
+  it("copies completed assistant replies with the textarea fallback", async () => {
+    Object.defineProperty(navigator, "clipboard", {
+      configurable: true,
+      value: undefined,
+    });
+    const execCommand = vi.fn().mockReturnValue(true);
+    Object.defineProperty(document, "execCommand", {
+      configurable: true,
+      value: execCommand,
+    });
+    const message: UIMessage = {
+      id: "a-copy-fallback",
+      role: "assistant",
+      content: "Fallback copy reply.",
+      createdAt: Date.now(),
+    };
+
+    try {
+      render(<MessageBubble message={message} />);
+
+      fireEvent.click(screen.getByRole("button", { name: "Copy reply" }));
+
+      await waitFor(() => expect(execCommand).toHaveBeenCalledWith("copy"));
+      await waitFor(() =>
+        expect(screen.getByRole("button", { name: "Copied reply" })).toBeInTheDocument(),
+      );
+    } finally {
+      Reflect.deleteProperty(navigator, "clipboard");
+      Reflect.deleteProperty(document, "execCommand");
+    }
+  });
+
+  it("falls back when the Clipboard API rejects assistant reply copy", async () => {
+    const writeText = vi.fn().mockRejectedValue(new Error("not allowed"));
+    Object.defineProperty(navigator, "clipboard", {
+      configurable: true,
+      value: { writeText },
+    });
+    const execCommand = vi.fn().mockReturnValue(true);
+    Object.defineProperty(document, "execCommand", {
+      configurable: true,
+      value: execCommand,
+    });
+    const message: UIMessage = {
+      id: "a-copy-reject",
+      role: "assistant",
+      content: "Rejected clipboard copy.",
+      createdAt: Date.now(),
+    };
+
+    try {
+      render(<MessageBubble message={message} />);
+
+      fireEvent.click(screen.getByRole("button", { name: "Copy reply" }));
+
+      expect(writeText).toHaveBeenCalledWith("Rejected clipboard copy.");
+      await waitFor(() => expect(execCommand).toHaveBeenCalledWith("copy"));
+      await waitFor(() =>
+        expect(screen.getByRole("button", { name: "Copied reply" })).toBeInTheDocument(),
+      );
+    } finally {
+      Reflect.deleteProperty(navigator, "clipboard");
+      Reflect.deleteProperty(document, "execCommand");
+    }
+  });
+
   it("does not show copy actions for streaming placeholders", () => {
     const message: UIMessage = {
       id: "a-streaming",