I posted notice of this issue on #23551.

I did some research on simulating user interaction for testing.  The results are exemplified in the attached tk.generate_event.py with test setups and assertions.

Events are generated, to fire event handlers, with
  widget.generate_event(event, **kwds)
where keywords are used to set event attributes.  The uploaded file has examples.  The method is briefly described in
  http://infohost.nmt.edu/tcc/help/pubs/tkinter/web/universal.html
This reference has a more extensive coverage of events in a separate chapter.

The above does not mention the need to update the widget first.  I learned this from Stackoverflow answers to 'How to make generate_event work?" questions.  The tcl/tk doc, tcl.tk/man/tcl8.6/TkCmd/event.htm, adds the fact that text widgets require focus.

Even though button commands fire when releasing the 'main' button, event '<ButtonRelease-1>' does not work to fire commands.  Instead, use 'button.invoke()'.