`jQuery( window ).is( "*" )` should return `false`. Returning `true` should be considered a bug. Having non-elements fail all selectors is fine, since that's correct behavior.

I feel like we've had this discussion a few times, but the general concept is that `window` and `document` need to return sane values for all methods so that plugin authors don't have to constantly add checks and write exceptions around the various types of input.