The canvas coords are skewed by the highlight border and the relief.  By
default, they overlap onto the canvas coord space.  There are two ways
to deal with this:

## Method 1, zero out the highlightthickness and borderwidth
## (borderwidth defaults to zero) anyway
.canvas configure -borderwidth 0 -highlightthickness 0

## Method 2, adjust the canvas position to expose 0,0 properly
.canvas xview moveto 0
.canvas yview moveto 0

Method 2 is better, but you must do it anytime you adjust the canvas
borderwidth or highlightthickness.