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 class name is tricky.
The pure name '''TScrollbar''' may be used to modify the original style:

======tcl
ttk::style configure TScrollbar -relief raised
======

Nevertheless, for any other purpose,the name '''Horizontal.TScrollbar''' or '''Vertical.TScrollbar''' is used.

To diplay the layout of the widget:

======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.

%| option | Description | Honored in styles | Not honored in styles |%
&| -orient | ? |  |  |&
&| -background | ? |  |  |&
&| -bordercolor | ? |  |  |&
&| -troughcolor | trough color |  |  |&
&| -lightcolor | ? |  |  |&
&| -darkcolor | color of the frames of each element | clam |  |&
&| -arrowcolor | arrow background color |  |  |&
&| -arrowsize | arrows width and height and thus width of the widget | alt, clam, default, native, classic | vista, winnative, xpnative |&
&| -gripcount | number of vertical lines on the thumb | clam | alt, classic, native |&
&| -sliderlength | No visual effect | clam, default |  |&


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