Hmm, in the new ones I was derailed by the icons of background tabs being not lined up with the OSX color pills. Yet the old mockups show a curve similar to the tab shape that creates a very welcome break in the design, explaining that "here are the tabs", and "here are the window controls", while the curve slant explained the height position difference between the pills and the icons/text (notice how the line linking the green pill center to the home icon center looks perpendicular to the centermost part of the separator curve, which cuts this segment in half). Without that curve, the new design simply feels broken. What's worse, since it's right there on the top-left, this is the absolute first experience of the software from the user's eye. It does not feel robust at all.

Ah, I must have been confusing these with Stephen Horlander's early mockups. Thanks for the detective work.