gasket, such as it is, very thin, 1/4 inch rubber, flimsy, and insubstantial, along the very top of the windshield glass

Come on!! I can't believe no one went out to check for this on their own cars after reading about this...!

None of the moderators...nobody?!

Mine looks like, ChiefPontiac.
But I suspect your's has lifted (as you mentioned).

First thing first, was the windscreen ever replaced, or pressure applied to the inside towards the top area (significate pressure)?
Secondly I would not apply any cement. I would go to a auto windscreen shop, let them have a look, may just need to be heated and reset. Failing which, automotive silicone would work, and can be removed if ever needed. But remember to apply it properly and neatly, for the cosmetic of it.

Thanks Zech, yours looks great, the way it should. I have found I can get an entire replacement gasket for the windshield for around $26.00.
Also found a gasket for the 2018 Cx3 on Amazon and have ordered it.
Mine looks like yours along the bottom and up the sides. Perhaps something was done to it in a previous life.
Anyway the ordered gasket material should work.