There are many paned window implementations, including a core one that was released in 8.4.

http://www.purl.org/tcl/home/man/tcl8.4/TkCmd/panedwindow.html
See also [paned window].
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Here is information on Iwidgets implementation:

[http://incrtcl.sourceforge.net/iwidgets/iwidgets/panedwindow.gif]

Docs can be found at http://incrtcl.sourceforge.net/iwidgets/iwidgets/panedwindow.html ('''the example provided may be outdated and fail to work with the latest Tcl/Tk''') and http://purl.org/tcl/home/man/iwidgets3.0/panedwindow.n.html

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[Category Widget] - [Category Command], a part of [Tk] and [incr Widgets]