Mouse Cursor not hidden in XAML based apps. 

In xaml, mouse must be actively turn off during initialization.

Setting Game.IsMouseVisible =false; has no effect because the code check
against current _isMouseVisible value, which is by false from the beginning.
Now, In order to hide the mouse cursor one must do:
IsMouseVisible = true;
IsMouseVisible = false;

Another thing: The execution path through
MetroGamePlatform.RunLoop()
MetroGameWindow.RunLoop()
which call SetCursor(false), doesn't seem to be active anymore.