A Ttk (i.e. theme-able) scrollbar widget.
HaO 2019-04-29: The layouts are called: Horizontal.TScrollbar and Vertical.TScrollbar:
% ttk::style layout Vertical.TScrollbar
Vertical.Scrollbar.trough -sticky ns -children {Vertical.Scrollbar.uparrow -side top -sticky {} Vertical.Scrollbar.downarrow -side bottom -sticky {} Vertical.Scrollbar.thumb -sticky nswe -unit 1 -children {Vertical.Scrollbar.grip -sticky {}}}