通过使用ViewModel-first（绑定）方法进行奇怪的竞争条件

I am experimenting with a simple ViewModel-first based WPF application and some primitive navigation logic. The application consists of two views (screens). One screen contains a button "Go forward" and the other a button "Go backward". By pressing one of the buttons a delegate command is invoked, which in turn causes the shell view-model to switch the active screen. Screen 1 switches to Screen 2, whereas Screen 2 switches to Screen 1.I am experimenting with a simple ViewModel-firs