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

["Clément Pit-Claudel" <cpitclaudel@gmail.com>]

On 26/04/2020 13.07, Eli Zaretskii wrote:
>> 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.

I typically want to align text relative to a prefix whose length depends on face properties, so if I use window-text-pixel-size, I need to redo the computation every time faces change.

>> 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.

What I meant to say is that if it's cheap enough to draw this:

⟝ bbbbbbbbbbb
⟝ bbbbbbbbbbb
⟝ bbbbbbbbbbb
⟝ bbbbbbbbbbb
⟝ bbbbbbbbbbb

(where "⟝" is a wrap-prefix on all lines but the first)

… then it should be cheap enough to draw this:

⟝ bbbbbbbbbbb
   bbbbbbbbbbb
   bbbbbbbbbbb
   bbbbbbbbbbb
   bbbbbbbbbbb

where the indentation is a specified space as wide as "⟝".

Clément.