On Thu, Dec 27, 2018 at 09:45:57AM -0600, Dale Sedivec wrote:
> You would probably know better than I would, but the only possible
> relevant differences I can think of are:
> 
> * I'm using a smaller font size
> * I'm using a Retina (i.e. scaled) display
> * I'm using macOS 10.13.6

These are all the same, but...

> I am attaching the Elisp script I used to reproduce this problem while
> bisecting, loaded like:
> 
>       nextstep/Emacs.app/Contents/MacOS/Emacs -Q --load repro.el

This reproduces instantly, like you say. Thank you! It looks to me
like the fringe background colour is being drawn right across the
line, which makes me think that the calculation to work out what to
blank in the fringe is going wrong.

I think that what’s happened is I’ve misunderstood one of the NS
rectangle function’s descriptions and assumed it did more error
checking than it does. It looks like the fix is simply to check that
the area that’s to be cleared is actually a legitimate rectangle.

Patch attached.
-- 
Alan Third