bug#61361: cursor cannot be at the start of overlay that starts with a newline

[Eli Zaretskii <eliz@gnu.org>]

> Date: Wed, 8 Feb 2023 17:16:40 +0200
> Cc: 61361@debbugs.gnu.org
> From: Dmitry Gutov <dgutov@yandex.ru>
> 
> On 08/02/2023 17:09, Eli Zaretskii wrote:
> >> From: Xinyang Chen<chenxy@mit.edu>
> >> Date: Wed, 8 Feb 2023 09:25:07 -0500
> >>
> > But the 'cursor' property just supplies the index in the display
> > string where your Lisp program wants to place the cursor.  Emacs
> > cannot place point inside a display string.  So the display engine
> > needs to find where that index is on display.  And it cannot find that
> > place because there's no glyph that corresponds to the newline.
> 
> So, no chance of the display engine detecting that the same display 
> string with the 'cursor' property has a newline at that position?

"No chance" is too strong: this is software, after all.

But it's not easy to do that.  It involves several places with tricky
code, that currently all work in unison, and make the same
assumptions:

  . the decision where to place the cursor works on a screen-line
    basis: as we perform layout of each screen line, we decide whether
    the cursor should be in that line, and if so, on what glyph of
    that line
  . the decision whether a screen line is a candidate for showing the
    cursor when that screen line ends in a newline from a display or
    an overlay string, and/or when there's a gap in buffer positions
    shown on the screen due to buffer text hidden by some feature
  . the tricky (to say the least) code which finds the glyph where to
    put the cursor on a given screen line when buffer positions change
    non-monotonically with screen positions (due to bidirectional
    text) and/or have gaps due to overlay and display strings (it is
    here that the 'cursor' property is implemented)

Historically, the 'cursor' property is a relatively late addition to
the display engine, it was added in Emacs 22.1.  It complicated the
display code a little, but then along came bidirectional display and
complicated that _a_lot_.  So we are now at a place where the original
design of the Emacs 21 display never meant to be.

The entire design of overlay string display is problematic for adding
significant features, because when an overlay string is rendered, we
lose too much information about the original overlay (e.g., we don't
even record where in the buffer the overlay was positioned, nor the
overlay from which the string came).  So any feature that needs to
support properties of overlay strings must actually go back to buffer
text and look up the overlays there to find the one we want!

> That's unfortunate: that means we'll need to create some wonky 
> workaround for displaying a completion preview (in Company) when a 
> completion starts with a newline.

Yes, I know.  If someone wants to work on lifting this limitation, I
can offer help, but I don't think I'll have time (nor motivation, to
say the truth) to work on this myself.