21.0.96 and 21.0.97: bizarre next-line behavior

[E. Jay Berkenbilt]

Of course, just as I finally give up and send the bug report, I figure
out how to reproduce it....

This happens in a buffer with no newline.

Start emacs -q.  In *scratch*:

M-x fundamental-mode
M-x set-variable next-line-add-newlines t

Then go to the end of the buffer and type

one two three

and without hitting return or doing anything that would cause a
newline to appear M-b and C-n.  Notice that the "three" gets moved to
the next line.