I am not trying to re-invent the wheel.  I am trying to bring IDLE up to uniform standards.  In particular, make all dialogs usable from the keyboard.  The config dialog is the worst. See #27620, the master issue.

For a two button messagebox, I went back to 3.5.2, which still uses the commondialog box.  It has the default button marked as such.  <Return> always does the default action. <Escape> always cancels.  <Tab> moves the focus, but does not affect the above.  <Space> enters ' ' in text entry and 'clicks' buttons.  So this is at least 'a' Windows standard, as well as 'a' Mac standard.

The search dialogs act the same. The space behavior built-in to the TK widgets, at least on Windows. I presume this is a class binding.  The dialogs have no platform specific code, so they are also the defacto IDLE standard.

Serhiy, it is definitely a tk convention, and I presume more universal than that, that the 'default ring' always indicate what button/action is invoked by <Return>.  If the default moved with the focus, the default ring should move also.  I am not inclined to do that without a strong reason.

The button default ring is controlled by the 'default' option" 'active' = visible, 'normal' = possible, 'disabled' = not possible.  (From the tk docs.  The NMT reference is useless here.) I will add "default='active'" for the Ok button.

(Mark: "default=1", suggested on http://www.tkdocs.com/widgets/button.html, is a nasty bug.  "_tkinter.TclError: bad default "1": must be normal, active, or disabled".)