I was on twitter and this came up;

Try GlideOverlay instead. I can send you a DM with some sample code. Or ping on SNDevs.

--- Jonathan Jacob (@TheBurgerlution) July 24, 2019

So I had to make a page for it.

It appears to be a undocumented type of modal.

It's used throughout ServiceNow on VTB, Change, and Surveys.