This appears to be a system settings option in Mac OS Sierra set under the
Dock, "prefer tabs when opening documents".  With that choice, when there
is the tab bar  in an Idle edit window (more than one window superimposed,
with tabs) the the whole bottom line with the horizontal separator line
segment disappears:  seems like no allowance is made for the vertical space
taken up by the tab bar at top.  There is a workaround:  turn off this OS
option, but unfortunate to need to do that, and the user has no warning
that it is necessary.  I only noticed this when I switched to a new machine
and was doing initial diddling with the system options, and did not realize
that i was not setting it up as I had it on my previous machine.

Dr. Andrew N. Harrington
  Computer Science Department
  Graduate Program Director gpd@cs.luc.edu
  Loyola University Chicago
  207 Doyle Center, 1052 W Loyola Ave.
http://www.cs.luc.edu/~anh
Phone: 773-508-3569
Dept. Fax:    773-508-3739
aharrin@luc.edu (as professor, not gpd role)

On Mon, Oct 1, 2018 at 9:30 PM Terry J. Reedy <report@bugs.python.org>
wrote:

>
> Terry J. Reedy <tjreedy@udel.edu> added the comment:
>
> I do not see this 64-bit 3.7.1rc1 (see #34863) on 10.13.6 with, I believe,
> default settings.
>
> Are you reporting multiple tabs as a bug, or a feature resulting from
> intentional settings?  If the latter, this may be something the tcl/tk does
> not support properly.  Ned and Tal, do either of you know anything about
> this?
>
> In any case, is the status bar missing, or is the cursor position missing
> from the status bar.
>
> ----------
> nosy: +ned.deily, taleinat
>
> _______________________________________
> Python tracker <report@bugs.python.org>
> <https://bugs.python.org/issue34864>
> _______________________________________
>