The MDI / Multiple Tabs Flat Navigation App Bar in Windows Store Apps sample was updated to version 126.96.36.199.
Now it also runs on Windows 8.1. Before the update, it crashes on Windows 8.1 when animation was disabled and the user right-clicks and added new content pages more than two times.
In case you cannot see any animation under Windows 8.1, please check the Performance Options setting “Animate controls and elements inside windows” (System / Advanced system settings / Performance / Settings). If this option is not set, transition is disabled for Windows Store apps too.