bug#40856: Feature request: support arbitrary propertized strings in wrap-prefix specifications

[Eli Zaretskii]

> Cc: 40856@debbugs.gnu.org
> From: Clément Pit-Claudel <cpitclaudel@gmail.com>
> Date: Sun, 26 Apr 2020 12:09:31 -0400
> 
> > wrap-prefix can have value that uses :align-to, so I don't see why
> > you'd need to call window-text-pixel-size many times.  You could call> it 
> > once, and then use the result in the subsequent values of prefix.
> 
> True, if the prefix is the same on every line.
> But that calculation isn't very robust to changes in font sizes, other face 
> options, and windows.

I don't think I follow.  :align-to accepts pixel specification, so it
should be insensitive to font changes.

> > It should be possible to do that, but I'm not sure it will be cheaper
> > than calling window-text-pixel-size or something similar, because
> > (unlike with images) Emacs doesn't have the width already calculated
> > and stashed somewhere, it would need to lay out the string and
> > calculate the resulting pixel width each time it sees such a display
> > spec.
> 
> I think that would be great; the cost should be reasonable, since 1. it only 
> applies to the part of the buffer that is being displayed and 2. it won't be 
> costlier than using the stirng being measured as the wrap prefix.

I don't think your 2. is necessarily correct, because you want the
width computed each time the prefix spec is being used to produce
glyphs, by which time the string is old history.

> IOW, if we expect that it wouldn't be too costly to use a given string as the 
> wrap-prefix, then it shouldn't be too costly to use a specified space of that 
> width as the wrap prefix.

I don't think I follow the logic.  You intend to use :space in the
prefix spec, not the string.  So the display engine will have to
perform layout of the string and compute its pixel width each time it
produces a prefix for another line.  The string itself is either not
actually displayed, or is displayed on the first line of the
paragraph, before all the wrapped lines.  It may well be that
redisplay needs to produce the display of just some lines, which don't
even include the one with the string itself.