I look into the Caliburn.Micro source code and I think the PlatformProvider.Current is always set to DefaultPlatformProvider (which always returns false IsDesignMode). In other platforms, it should get replaced by XamlPlatformProvider, but WinRT doesn't
use Bootstrapper, so it never happens.
My work around right now is to add the following code at the beginning of Bind.AtDesignTimeChanged:
&& PlatformProvider.Current is DefaultPlatformProvider)
PlatformProvider.Current = new XamlPlatformProvider();
This way, XamlPlatformProvider is used when the Bind.AtDesignTime is set in XAML.
Please let me know if you have any comment.