It kinda basically was just drool (see this thread: https://forums.macrumors.com/threads/1508549/)
Turns out it was just the rounded corners of Preview at fullscreen. The telltale sign was that it was a symmetrical pattern on both bottom corners of the screen. Hopefully this'll diffuse any potential anxiety about the quality of the new iMacs... at least it did for me.. mine's arriving tomorrow![]()
Good to know!