Make Menu.popup accept no parameter

atom/browser/api/lib/menu.coffee

@@ -79,7 +79,11 @@ Menu::_init = ->
         v8Util.setHiddenValue group[0], 'checked', true unless checked
 
 Menu::popup = (window, x, y) ->
-  throw new TypeError('Invalid window') unless window?.constructor is BrowserWindow
+  unless window?.constructor is BrowserWindow
+    # Shift.
+    y = x
+    x = window
+    window = BrowserWindow.getFocusedWindow()
   if x? and y?
     @_popupAt(window, x, y)
   else

docs/api/menu.md

@@ -237,9 +237,9 @@ Generally, the `template` is just an array of `options` for constructing a
 You can also attach other fields to the element of the `template` and they
 will become properties of the constructed menu items.
 
-### `Menu.popup(browserWindow[, x, y])`
+### `Menu.popup([browserWindow, x, y])`
 
-* `browserWindow` BrowserWindow
+* `browserWindow` BrowserWindow (optional)
 * `x` Number (optional)
 * `y` Number (**required** if `x` is used)