docs: modernize protocol-handler docs (#29380)

* docs: modernize protocol-handler docs

* docs: iadd contextIsolation

* docs: add guide for launch-app-from-URL-in-other-app

* docs: address comments

* chore: fix brackets

* chore: add escaped brackets

docs/fiddles/system/protocol-handler/launch-app-from-URL-in-another-app/main.js

@@ -1,10 +1,39 @@
 // Modules to control application life and create native browser window
-const { app, BrowserWindow, dialog } = require('electron')
+const { app, BrowserWindow, ipcMain, shell } = require('electron')
 const path = require('path')
 
-// Keep a global reference of the window object, if you don't, the window will
-// be closed automatically when the JavaScript object is garbage collected.
-let mainWindow
+let mainWindow;
+
+if (process.defaultApp) {
+  if (process.argv.length >= 2) {
+    app.setAsDefaultProtocolClient('electron-fiddle', process.execPath, [path.resolve(process.argv[1])])
+  }
+} else {
+    app.setAsDefaultProtocolClient('electron-fiddle')
+}
+
+const gotTheLock = app.requestSingleInstanceLock()
+
+if (!gotTheLock) {
+  app.quit()
+} else {
+  app.on('second-instance', (event, commandLine, workingDirectory) => {
+    // Someone tried to run a second instance, we should focus our window.
+    if (mainWindow) {
+      if (mainWindow.isMinimized()) mainWindow.restore()
+      mainWindow.focus()
+    }
+  })
+
+  // Create mainWindow, load the rest of the app, etc...
+  app.whenReady().then(() => {
+    createWindow()
+  })
+  
+  app.on('open-url', (event, url) => {
+    dialog.showErrorBox('Welcome Back', `You arrived from: ${url}`)
+  })
+}
 
 function createWindow () {
   // Create the browser window.
@@ -12,58 +41,23 @@ function createWindow () {
     width: 800,
     height: 600,
     webPreferences: {
-      nodeIntegration: true
+      preload: path.join(__dirname, 'preload.js'),
     }
   })
 
-  // and load the index.html of the app.
   mainWindow.loadFile('index.html')
-
-  // Open the DevTools.
-  mainWindow.webContents.openDevTools()
-
-  // Emitted when the window is closed.
-  mainWindow.on('closed', function () {
-    // Dereference the window object, usually you would store windows
-    // in an array if your app supports multi windows, this is the time
-    // when you should delete the corresponding element.
-    mainWindow = null
-  })
 }
 
-// This method will be called when Electron has finished
-// initialization and is ready to create browser windows.
-// Some APIs can only be used after this event occurs.
-app.whenReady().then(createWindow)
-
-// Quit when all windows are closed.
+// Quit when all windows are closed, except on macOS. There, it's common
+// for applications and their menu bar to stay active until the user quits
+// explicitly with Cmd + Q.
 app.on('window-all-closed', function () {
-  // On macOS it is common for applications and their menu bar
-  // to stay active until the user quits explicitly with Cmd + Q
-  if (process.platform !== 'darwin') {
-    app.quit()
-  }
+  if (process.platform !== 'darwin') app.quit()
 })
 
-app.on('activate', function () {
-  // On macOS it is common to re-create a window in the app when the
-  // dock icon is clicked and there are no other windows open.
-  if (mainWindow === null) {
-    createWindow()
-  }
-})
-
-// In this file you can include the rest of your app's specific main process
-// code. You can also put them in separate files and require them here.
-
-if (process.defaultApp) {
-  if (process.argv.length >= 2) {
-    app.setAsDefaultProtocolClient('electron-api-demos', process.execPath, [path.resolve(process.argv[1])])
-  }
-} else {
-  app.setAsDefaultProtocolClient('electron-api-demos')
-}
-
-app.on('open-url', (event, url) => {
-  dialog.showErrorBox('Welcome Back', `You arrived from: ${url}`)
+// Handle window controls via IPC
+ipcMain.on('shell:open', () => {
+  const pageDirectory = __dirname.replace('app.asar', 'app.asar.unpacked')
+  const pagePath = path.join('file://', pageDirectory, 'index.html')
+  shell.openExternal(pagePath)
 })