It's only like that in the preview itself, not in the actual shader. I did try to make it mask by alpha when that was checked, but for some reason it didn't seem to work - could be a Unity bug. I'll see if I can do more research on this at some point

If you're using a future alpha/beta of Unity, I won't really be able to debug the issue, as I don't have access to it yet

This is now fixed in Beta 0.23 :)

It's a bit of a mess. I'll look into this at some point