Fixed desktop text selection behavior.

src/App.test.tsx

@@ -0,0 +1,21 @@
+import { fireEvent, render, screen } from "@testing-library/react";
+import { describe, expect, it, vi } from "vitest";
+import App from "./App";
+
+vi.stubGlobal("Worker", class {
+  postMessage() {}
+  terminate() {}
+});
+
+describe("App canvas selection", () => {
+  it("does not expose a viewer canvas before a file is opened", () => {
+    render(<App />);
+    expect(screen.getByText("Open an Adobe XD file")).toBeInTheDocument();
+  });
+
+  it("keeps normal document text non-selectable by CSS", () => {
+    const { container } = render(<App />);
+    fireEvent.doubleClick(container);
+    expect(container.querySelector(".selectable-text")).not.toBeInTheDocument();
+  });
+});