Window Size when SizeToContent is not specified

Question

When the following XAML used, the window size is not 5000x5000, but some small window where the button is cropped.

Answer 1

If the width and height are not specified WPF uses 60% of the width of you primary screen (where the windows is actually displayed) and 60% of its height. This is done by the Application singleton where all Windows ultimately register themselfes.