Re: "Final" version of tty child frames

[Eli Zaretskii]

> From: Gerd Möllmann <gerd.moellmann@gmail.com>
> Cc: Emacs Devel <emacs-devel@gnu.org>
> Date: Wed, 23 Oct 2024 06:40:11 +0200
> 
> Gerd Möllmann <gerd.moellmann@gmail.com> writes:
> 
> > Feng Shu <tumashu@163.com> writes:
> >
> >> Gerd Möllmann <gerd.moellmann@gmail.com> writes:
> >>
> >>> Feng Shu <tumashu@163.com> writes:
> >>>
> >>>> border do not work well when current buffer is CJK, maybe because CJK
> >>>> Char width is not 1
> >>>
> >>> Could you please send me a bit of CJK text so that I have something I
> >>> can test this with?
> >
> > Thanks!
> 
> Hm, okay, but not sure how to fix that.
> 
> The problem comes indeed from the root frame displaying characters that
> are more then 1 column wide, say N. When we output such a character to
> the terminal, the cursor moves by N columns. Emacs represents that in
> the glyph matrix by producing N glyphs for the character, all bu tthe
> first have a padding flag set. So we have
> 
>   ... (C, 0) (C, 1) ...
> 
> in the glpyh matrix, where C is the character, and the number is the
> padding flag.
> 
> Now assume, for example, that the child frame is posititoned so that its
> left border where we want to display a '|' is in the column for the (C,
> 1).
> 
> That can't work, because writing C to the terminal moves 2 columns and 
> will skip over the column where we want the '|' to appear.

I think the solution should be the same as what we do when a line
needs to wrap (i.e. be continued) and we don't have enough columns to
put all the N glyphs before the continuation glyph: we don't show the
entire N-glyph group.  IOW, the border glyph should be shown instead
of multi-column character, padded with enough spaces before it to make
up for the M < N glyphs of C that could be shown before the border.
In your example above, C will not be shown, and instead we will show
one space glyph followed by the '|' glyph.