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.