fix: use preventScroll on focus calls so autoFocus opt-in doesn't scroll page

When users explicitly set autoFocus={true}, the textarea now focuses
with { preventScroll: true } so the page doesn't jump. Also applies
to the modal-open focus path. Added tests verifying default no-focus
behavior and that opt-in focus uses preventScroll.

https://claude.ai/code/session_01LPiJ8GcUNtWdS2W6MYSE4b

Author: claude

packages/react-core/src/v2/components/chat/CopilotChatInput.tsx

@@ -252,7 +252,7 @@ export function CopilotChatInput({
     }
 
     if (config?.isModalOpen && !previousModalStateRef.current) {
-      inputRef.current?.focus();
+      inputRef.current?.focus({ preventScroll: true });
     }
 
     previousModalStateRef.current = config?.isModalOpen;
@@ -1224,7 +1224,7 @@ export namespace CopilotChatInput {
 
       useEffect(() => {
         if (autoFocus) {
-          internalTextareaRef.current?.focus();
+          internalTextareaRef.current?.focus({ preventScroll: true });
         }
       }, [autoFocus]);
 

packages/react-core/src/v2/components/chat/__tests__/CopilotChatInput.test.tsx

@@ -1006,5 +1006,27 @@ describe("CopilotChatInput", () => {
       // Clean up
       delete (HTMLElement.prototype as any).scrollIntoView;
     });
+
+    it("does not auto-focus the textarea by default", () => {
+      const focusSpy = vi.spyOn(HTMLElement.prototype, "focus");
+
+      renderWithProvider(
+        <CopilotChatInput onSubmitMessage={mockOnSubmitMessage} />,
+      );
+
+      expect(focusSpy).not.toHaveBeenCalled();
+      focusSpy.mockRestore();
+    });
+
+    it("auto-focuses with preventScroll when autoFocus is true", () => {
+      const focusSpy = vi.spyOn(HTMLElement.prototype, "focus");
+
+      renderWithProvider(
+        <CopilotChatInput autoFocus onSubmitMessage={mockOnSubmitMessage} />,
+      );
+
+      expect(focusSpy).toHaveBeenCalledWith({ preventScroll: true });
+      focusSpy.mockRestore();
+    });
   });
 });