Setting Window fullscreen mode to 'auto' does not work as expected

[ghost]

From the docs:
fullscreen: int or string, one of 0, 1, ‘fake’ or ‘auto’

Activate fullscreen. If set to 1, a resolution of width times height pixels will be used. If set to auto, your current display’s resolution will be used instead. This is most likely what you want. If you want to place the window in another display, use fake and adjust width, height, top and left.
http://kivy.org/docs/api-kivy.config.html#module-kivy.config

1. Config.set('graphics', 'fullscreen', 'auto')
   This works as expected. The display's resolution is used.
2. Window.fullscreen = 'auto'
   This behaves like Config.set('graphics', 'fullscreen', '1'), on a widescreen display it leads to a 4:3 ratio, letterboxed fullscreen.
   https://github.com/kivy/kivy/blob/master/kivy/core/window/__init__.py#L444