bug#23574: 24.5; Overzealous underlining in emacs-nox

[Eli Zaretskii]

> Date: Fri, 10 Jun 2016 09:16:02 +0200
> From: martin rudalics <rudalics@gmx.at>
> CC: npostavs@users.sourceforge.net, 23574@debbugs.gnu.org, 
>  john.b.mastro@gmail.com, cwoodbury@azavea.com
> 
>  >> Suppose a user wants to use the same background for all spaces at the
>  >> ends of all lines of a buffer regardless of "the last face used on the
>  >> line".  How would she specify that?
>  >
>  > By putting the proper face property on the newline.
> 
> Which gets me back to my initial concern: If our user does that eagerly
> for the entire buffer, the overhead might be non-negligible.

I don't see why.  Redisplay only considers the visible portion of the
buffer.

> A loosely related question: Does for R2L text row->pixel_width for each
> glyph row indicate the width of that row occupied by text as it does for
> L2R text?

row->pixel_width doesn't count text glyphs, it counts all of the
glyphs in a glyph row, including the glyphs produced by the display
engine for its own purposes.  E.g., it always includes the space glyph
produced at the end of a line, which is needed for displaying the
cursor.  In the R2L case the width includes the stretch glyph
prepended on the left.

> Suppose we have a L2R line with dots indicating the empty space at
> the end of that line:
> 
> TTTTTTTTTTTTT....
> 
> R2L this line would appear as
> 
> ....TTTTTTTTTTTTT
> 
> Would these two lines have the same row->pixel_width?  Or, would the
> length of the stretch glyph added at the left of the R2L line be that of
> ‘window-text-width’ minus row->pixel_width?

The latter.  row->pixel_width is computed in compute_line_metrics,
after the stretch glyph (and any other glyphs needed for line display)
were already inserted.  compute_line_metrics doesn't care about what
glyphs are there, it counts them all.