bug#33891: 26.1.90; Occasional redrawing problems on macOS

["John Wiegley" <johnw@gnu.org>]

I have no way to reproduce this, I just want the developers to know since Eli
tells me there have been recent changes in the redisplay code.

Emacs 26.1 was pretty rock solid on macOS, using --with-ns. In 26.1.90, I
notice semi-frequent redisplay problems that don't reproduce when I try the
same command again.

For example, sometimes when I enter Gnus, and everything looks fine, suddenly
the majority of the screen will go black (except for the current summary
line). Moving the cursor brings back the article I'm reading, but even C-l
doesn't redraw the summary lines. I have to page down and page back up to get
them to redraw again.

This happens in all types of buffers: code, eshell, Gnus, Org, etc. The basic
behaviors I've seen are that either the current line goes black, and
everything else is fine, or the whole frame goes black.

I'd never seen this behavior before 26.1.90.

-- 
John Wiegley                  GPG fingerprint = 4710 CF98 AF9B 327B B80F
http://newartisans.com                          60E1 46C4 BD1A 7AC1 4BA2