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

[Clément Pit-Claudel]

Hi all,

The wrap-prefix property is very convenient, but it's hard to use for alignment 
when the width of the item to align to is not easy to compute.  For example, to 
get the following effect, it's easy to use 'wrap-prefix "  "

- Test
  wrapped
  wrapped

But take a more complex case, like this:

xyz
⟝ abc
   wrapped
   wrapped

Depending on the font in use, the ⟝ character may be more or less wide.  So, 
the wrap prefix can't just be "  " here; it needs to be calculated using 
`window-text-pixel-size`, which is costly if it needs to be repeated for many 
lines.  Additionally, this calculation becomes obsolete and needs to be redone 
as soon as the user changes the font size.

Or consider the first example again, but this time with non-monospace text:

- Test
  wrapped
  wrapped

Depending on the width of "-" and of the space in the current font, the with of 
the wrapping prefix should be different.

Specified spaces can already measure the width of an image and many other 
elements.  Could they be extended to measure an arbitrary string?  Concretely, 
one would use (:space :width "- ") in the first case and (:space :width "⟝ ") 
in the second case, and get perfect alignment.  This is similar to what LaTeX 
calls "phantoms": spaces whose width is exactly that of their argument text.

Clément.