++shmem, and thanks,

Your example illustrates the meaning of highlightthickness quite nicely.  Typing <Tab> several times makes it very clear why you might want to have a space there (I don't of course, because I'm only using the border Canvases to frame the inner one, and thus don't need to select them).

Similarly, just as I had never used highlightthickness before, highlightbackground was also new to me.  So that's 2 new things you've taught me in Tk -- thank you!