I have no idea why. It seems that the grip is drawn by taking bg_color,
then filling the rectangle with it, and drawing with the lighter version
on top of that. For some reason region clipping does not work, and it does
turn out to be a rectangle instead of a triangle. That said, triangle wouldn't
have looked much better, IMO.

Anyway, turns out, there's a style code for grip that was disabled.
Re-enabling it fixes everything for me.

Paned widget (which also has a grip) is not affected, since its grip is never drawn.