Fixed.  I added a light background as with Sphinx html. I am delighted with how well this is working so far.  I like the little always-there [TOC] button better than the browser sidebar.  Besides making it easy to verify the look of doc changes, I think it looks better. I like the appearance of the tk version, without the down arrow, a bit better than the ttk version with an arrow, but would switch, when possible, if there were functional reason (see below).

To improve the button: I want to add an activebackground (or foreground) so when moused over, the button is more obviously a live button to be pushed.  Any particular suggestion?  Is there a standard?  Or do I worry too much? Idle buttons don't change when moused over, but they have relief to convey 'push me'.

So that people can navigate without mouse, I tested adding takefocus=True to toc creation, takefocus=False to text, and toc.focus_set(). <space> (and <click>), <enter>, and <esc> work as expected. However the tk Menubutton (but not ttk version) seems to have a bug (at least here, ).  After <space>, it is left in a 'pressed' state, with a black outline, while the menu is displayed, and after.  (With <click> the outline and menu appear on button-press and the outline disappears on button-release, while the menu remains.) A click on the button is required to reset it and remove the outline.  Then either <space> or <click> work as before.  I tried an expanded command function to reset toc,
           def cmd(mark=tag):
                toc['state'] = 'disabled'
                toc['state'] = 'normal'
                text.see(mark)
but this does not work.  If this bug windows-only or general? Do you know of any other workaround to make the tk version work right?  Since click-to-reset is not obvious, and defeats keyboard-only use, I will not enable focus when click-reset is needed.