bug#32932: 27.0.50; render bugs on macOS Mojave

[Eli Zaretskii]

> From: Aaron Jensen <aaronjensen@gmail.com>
> Date: Wed, 31 Oct 2018 23:51:42 -0700
> Cc: Alan Third <alan@idiocy.org>, 32932@debbugs.gnu.org
> 
> getting set causes erase_phys_cursor to get called, which ultimately
> calls draw_phys_cursor_glyph, which calls draw_glyphs, which I believe
> is what is blanking the line. It appears to be more than just
> redrawing the glyph under the cursor.
> 
> Another clue is that it appear to only blank from where the cursor is
> to the end of the line. Anything before that isn’t cleared.

Can you find the reason for that?  In general, redrawing the cursor
should only redraw a single character, and sometimes the two adjacent
ones.  It shouldn't redraw more than that.

>From what you describe, it sounds like the problem is in the logic
that determines which parts to redraw, see update_text_area in
dispnew.c.