bug#57728: 29.0.50; Emacs writes wrong glyph at the bottom-right corner of text terminals

[Eli Zaretskii]

> From: Gerd Möllmann <gerd.moellmann@gmail.com>
> Cc: gregory@heytings.org,  57728@debbugs.gnu.org,  akib@disroot.org
> Date: Mon, 12 Sep 2022 13:59:05 +0200
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >> From: Gerd Möllmann <gerd.moellmann@gmail.com>
> >> This is because 'write' deliberatly leaves the bottom-right corner alone
> >> if the terminal has "auto right margin" (autowrap).
> >
> > But AFAIU, the bug actually shows that we somehow _do_ write there,
> > although the display engine assumes we don't.  Right?
> 
> My theory is that no-one but tty_write_glyphs knows that it didn't write
> to the bottom-right corner.

You are saying that update_frame_line tells tty_write_glyphs to write
the '\', but tty_write_glyphs doesn't in this case?

But why would we need to write that character if we know it's already
there (because all lines end with it)?  That should only happen if we
scroll the region instead of rewriting lines one by one.  Is that what
happens in this case?