“In previous versions of Rhino, when the number of layers doesn’t exceed the vertical space in the layers panel, using the scroll wheel will scroll horizontally to show columns that are not visible”
I meant scrolling with the dedicated thumb wheel which is solely used for horizontal scrolling.
So in Rhino 6 it doesn’t matter how many layers there are, horizontal scrolling is always possible with the thumb wheel.
That does not take away what you tested, but possibly makes it into to similar issues.
Sound similar but not identical since I never mentioned scrolling through tabs.
Just scrolling horizontal with the wheel to see the different layer categories and names instead of clicking and moving the scroll bar manually.