fix: harden Windows single-instance startup (#498)

tests/unit/main-window-focus.test.ts

@@ -0,0 +1,32 @@
+import { describe, expect, it } from 'vitest';
+import {
+  consumeMainWindowReady,
+  createMainWindowFocusState,
+  requestSecondInstanceFocus,
+} from '@electron/main/main-window-focus';
+
+describe('main window focus coordination', () => {
+  it('defers second-instance focus until the main window is ready', () => {
+    const state = createMainWindowFocusState();
+
+    expect(requestSecondInstanceFocus(state, false)).toBe('defer');
+    expect(state.pendingSecondInstanceFocus).toBe(true);
+    expect(consumeMainWindowReady(state)).toBe('focus');
+    expect(state.pendingSecondInstanceFocus).toBe(false);
+  });
+
+  it('shows the main window normally when no second-instance focus is pending', () => {
+    const state = createMainWindowFocusState();
+
+    expect(consumeMainWindowReady(state)).toBe('show');
+    expect(state.pendingSecondInstanceFocus).toBe(false);
+  });
+
+  it('focuses immediately when the main window already exists', () => {
+    const state = createMainWindowFocusState();
+    requestSecondInstanceFocus(state, false);
+
+    expect(requestSecondInstanceFocus(state, true)).toBe('focus-now');
+    expect(state.pendingSecondInstanceFocus).toBe(false);
+  });
+});