Re: Getting column position accounting for overlays

[Eli Zaretskii <eliz@gnu.org>]

> From: Yuri D'Elia <wavexx@thregr.org>
> Date: Sat, 8 Aug 2015 16:31:28 +0200
> 
> On 07/08/15 20:19, Eli Zaretskii wrote:
> >> https://github.com/wavexx/rigid-tabs.el
> > 
> > If you need to align text on display, you should use the the
> > (space :align-to POS) display spec, see the node "Specified Space"
> > in the ELisp manual.  This lets the display engine do the job for
> > you, so you don't need to figure out the dimensions of the text
> > on display.
> 
> Ah yes, but that's precisely the problem: I need to calculate POS
> beforehand.

Yes, of course.  But you will have the same problem if you do get the
text dimensions: you need to compare that with something to do useful
things with the value.  Right?

> >> If you're trying to display anything related to the /visual/ appearance,
> >> you'll have to consider the possibility of overlays and thus run into
> >> the same issue.
> > 
> > Can you envision other use cases, besides alignment?
> 
> Looking for solutions

I actually asked about the problems, not the solutions.

> I came across a few snippets in emacswiki that
> dump the content of the overlays into another buffer (sort of "save what
> you see"). But all other cases were about calculating the current visual
> column for alignment purposes.
> 
> It's not so trivial, considering that you basically have to handle all
> 'display cases manually to replicate what emacs does, you might have
> overlapping overlays, and so on. I also think that it's bound to break
> if 'display gets extended to support new properties.

It's simply impossible, except in a few special cases.  If we need
such a feature that should work everywhere, we will need to ask the
display engine to supply this information.

But I still am unsure when would this be needed.