The report will be sent to bug-gnu-emacs@gnu.org:

Please make sure that the bug is specific to the Mac port.
Other bugs should be sent to the place you are guided with
M-x report-emacs-bug on some official ports such as X11 or NS.

Please describe exactly what actions triggered the bug, and
the precise symptoms of the bug.  If you can, give a recipe
starting from `emacs -Q':

0. put the enclosed files in /tmp
1. emacs -Q
2. `M-: (ediff-merge-files-with-ancestor "/tmp/a.txt" "/tmp/b.txt" "/tmp/ancestor.txt") RET'
3. `SPC'
4. `= b y'

With the ancestor buffer containing this:

--8<---------------cut here---------------start------------->8---
This is the ancestor buffer
<<<<<<< variant A
quack!
>>>>>>> variant B
It has some old text in it
that has been lying around getting musty
for ages.  It's as old as the hills.  And now you want
####### Ancestor
It has some old text in it
that has been lying around getting musty
for aeons.  And now you want
======= end
me to compare it with your work?
Some nerve!
--8<---------------cut here---------------end--------------->8---

I expected it to do an ediff of the chunk headed by ">>>>>>> variant B"
with the chunk headed by "####### Ancestor" but instead I get the
following regions under comparison:

--8<---------------cut here---------------start------------->8---
It has some old text in it
--8<---------------cut here---------------end--------------->8---

                    - and -

--8<---------------cut here---------------start------------->8---
<<<<<<< variant A
quack!
>>>>>>> variant B
It has some old text in it
that has been lying around getting musty
for ages.  It's as old as the hills.  And now you want
--8<---------------cut here---------------end--------------->8---