Re: Layered display API

[Eli Zaretskii <eliz@gnu.org>]

> From: Bo Lin <bo@dreamsphere.org>
> Cc: Dmitry Gutov <dgutov@yandex.ru>,  rudalics@gmx.at,  emacs-devel@gnu.org
> Date: Wed, 13 Aug 2014 22:06:26 -0400
> 
> Hi,
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >> > What I meant is this: if you need to display below the last line of
> >> > the buffer text, put the overlay at EOB, and include newlines in the
> >> > overlay string when you need to move to the next screen line.  To
> >> > align text horizontally you could use spaces or align-to display
> >> > properties in the string.
> >> 
> >> Yes, I might try this, as soon as there's some suggestion how to handle 
> >> the problem of `line-prefix' in this multi-overlay approach.
> >
> > Find the longest prefix and align everything so that the left edge
> > keeps clear of that?
> 
> Zero length overlays don't get displayed so this won't work when the
> buffer is empty.

When a buffer is empty, there are no line prefixes, right?  Or did I
misunderstand what you eman?

> >> >>      This is indeed a missing feature.  It should be easy enough to provide
> >> >>      some special kind of display property that would overlay any other
> >> >>      displayed content
> >> >>
> >> >>
> >> >> That would leave a question where will it have to be set on.
> >> >> Would that new kind of overlay be able to be displayed far
> >> >> from the position it's set on?
> >> >
> >> > No, I meant conceal the text produced by other display properties, and
> >> > display your overlay string instead.
> >> 
> >> It doesn't seem to be solving much: if I want to display something in 
> >> the middle of, say, large `display' text, there's no specific span of 
> >> text to set that new property on.
> >
> > You'd put it on the overlay string.
> 
> Ideally, for a popup tooltip, it should cover only the small
> rectangular area that is the tooltip, while leaving everything else
> intact to minimize visual disturbances. This is, AFAICT, currently
> impossible to achieve if you have to conceal anything with a
> 'display property, because: 1) in the simplest case the 'display is
> a string, but you don't know the proper display width of each
> character; and 2) the 'display can be not-strings.

I wasn't talking about the currently existing display properties, I
was talking about a new property with the capabilities I described.