GUI Design problem: How can I remove space between widgets on a canvas?

[Jeff Epler]

The problem is probably in the portion of the widget called the "highlight
region".  Set the highlightthickness of the buttons to 0 to get rid of it.

This part of the widget is filled either with the highlightbackground
(typically grey) normally, or the highlightcolor (typically black) when the
widget has keyboard focus.

Jeff