A [Ttk] (i.e. theme-able) [scrollbar] [widget].
    :   '''[ttk]::scrollbar''' ''pathName'' ?''options…''?
    :   http://www.tcl.tk/man/tcl/TkCmd/ttk_scrollbar.htm


**Layout names**

[HaO] 2019-04-29: The layouts are called: '''Horizontal.TScrollbar''' and '''Vertical.TScrollbar''':

======tcl
% 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 {}}}
======

A custom stlye may be defined as follows:

======tcl
ttk::style configure My.Vertical.TScrollbar -troughcolor yellow
pack [ttk::scrollbar .s -style My.Vertical.TScrollbar -orient vertical -command ".t yview"] -fill y -expand true -side right
pack [text .t -yscrollcommand ".s set"] -side left -fill both -expand true
.t insert 1.0 [string repeat 1\n 100]
======

The following styling options are supported:
The description is for a vertical slider. There are 5 regions named here: upper arrow, upper trough, thumb, lower trough, lower arrow.

+++
-orient  ?
-background  ?
-bordercolor  ?
-troughcolor  trough color
-lightcolor  ?
-darkcolor  color of the frames of each element
-arrowcolor  arrow background color
-arrowsize  arrows width and height and thus width of the widget
-gripcount number of vertical lines on the thumb
-sliderlength  Height of the thumb
+++

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