Sometimes the glass is half empty of giggle water, sometimes half full of mud (well, that's more bipolar).

Or you know how they say there's a fine line between genius and madness? (If I had a nickel for every time I've been told, "... and you've crossed it." That line's the borderline!

It's become a noun dx in itself, but it's not, really, it's an adjective. Good catch.