[tk_popup] is Tk's method of posting a popup menu.

http://www.purl.org/tcl/home/man/tcl8.4/TkCmd/popup.html

Popups act funny on different platforms. Learn how to fix that in the [Pragmatic use of Pernicious Popups]. 

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[Category Widget] - [Tk syntax help] - [Arts and Crafts of Tcl-Tk Programming]