Here's the answer:
"Use your mouse wheel / magic mouse swipe" while hovering over the button. No need to open the menu with a mouse wheel or trackpad swipe. Still, personally I think it's silly. There's more than enough room to make the big "Algorithm select" button have edges that are actually arrows that select next and previous.. same for the saturation algorithms. There is plenty of space for some subtle nice arrows.
Like I said, my influence on GUI / UI is virtually nil.. and yes, it's annoying me quite a bit.

