It seems that after a TkCanvas has been scrolled, there is a mismatch 
between coordinate values reported after a left-click on the canvas and 
where items have actually been drawn on the canvas.  

I'm trying to scroll a canvas and have the ability to draw a 'cursor' on 
the canvas at the location of a left mouse click, but what I find is that 
there is a mismatch between where the cursor is drawn and where the mouse 
is clicked if the canvas has been scrolled.  Can someone explain what 
happens to the coordinate system of a canvas after a scroll?

Phil