Hi Boyan,
Indeed, in your sample it is not reproductible but with a few modifications it is :
* First, resize your MyWindow and the button into your xaml :
* Then, Add a little sleep before opening window to simulate a little normal treatment we can have on a real application (myWindow.xaml.cs)
* Finally, to reproduce you have to click fast (as a double click) one time into the button and the second time near of the button into the background of the window : the second window opens behind the first (picture is the result)
Raphael.