On Sun, Jul 14, 2019 at 09:47:25AM -0700, Keith David Bershatsky wrote:
> Here are links to two (2) youtube videos showing the issue on Emacs
> 26.1 (built by https://emacsformacosx.com/ a few months ago) and
> also the master branch that I built using a commit from 07/10/2019
> (40cd6278ba965f3a9fcdddc568eb9ee7c69cf899). I don't have a 26.2
> build at the moment. The issue is less noticeable on the master
> branch, but can still be seen nevertheless. I have not studied the
> code as to what makes this feature work, but offhand would think
> that the header-line should not be overwritten by the row moving
> upwards (rather than updated after it has been overwritten).
> 
> Emacs 26.1:  https://youtu.be/Y8cZfURSwZc
> 
> Emacs Master Branch:  https://youtu.be/BO3r9_6OvYo

OK, that’s really not what I was expecting to see. 

It looks to me like the stretch glyph code in nsterm.m is just
nonsensical... I don’t really know what to make of it. I guess I’m
missing something?

Anyway, patch attached. I don’t see anything particularly wrong with
it applied, but it’s possible I’ve reintroduced some bug where the
modeline gets partially overwritten.

The patch is against master, but it’s probably possible to apply it to
26 manually.
-- 
Alan Third