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

electron/main/main-window-focus.ts

@@ -0,0 +1,38 @@
+export interface MainWindowFocusState {
+  pendingSecondInstanceFocus: boolean;
+}
+
+export type SecondInstanceFocusRequest = 'focus-now' | 'defer';
+export type MainWindowReadyAction = 'show' | 'focus';
+
+export function createMainWindowFocusState(): MainWindowFocusState {
+  return {
+    pendingSecondInstanceFocus: false,
+  };
+}
+
+export function requestSecondInstanceFocus(
+  state: MainWindowFocusState,
+  hasFocusableMainWindow: boolean,
+): SecondInstanceFocusRequest {
+  if (hasFocusableMainWindow) {
+    state.pendingSecondInstanceFocus = false;
+    return 'focus-now';
+  }
+
+  state.pendingSecondInstanceFocus = true;
+  return 'defer';
+}
+
+export function consumeMainWindowReady(state: MainWindowFocusState): MainWindowReadyAction {
+  if (state.pendingSecondInstanceFocus) {
+    state.pendingSecondInstanceFocus = false;
+    return 'focus';
+  }
+
+  return 'show';
+}
+
+export function clearPendingSecondInstanceFocus(state: MainWindowFocusState): void {
+  state.pendingSecondInstanceFocus = false;
+}