Re: [PATCH] User-defined fringe tooltips (a request for review)

[Eli Zaretskii <eliz@gnu.org>]

> From: Vladimir Kazanov <vekazanov@gmail.com>
> Date: Thu, 21 Dec 2023 16:51:15 +0000
> Cc: emacs-devel@gnu.org
> 
> > The code in note_mouse_highlight examines
> > the buffer text and overlays for text properties whose meaning is to
> > display a help-echo string, so why cannot you do the same to look for
> > a 'display' property of this specific structure, and extract the
> > help-echo directly from there at mouse-highlight time instead of at
> > glyph-generation time?
> 
> Yes, having Lispy stuff in the glyph_row struct doesn't make sense,
> and this is why I was in doubt.
> 
> Anyway, a new patch is attached with the approach you suggested (the
> way I understood it). I use "struct it" to iterate over the line for
> which the fringe indicator is defined, in note_mouse_highlight. The
> iterator then pulls out the display spec and saves the last left/right
> fringe captions it walked over.
> 
> Does that feel right?

No, there's no need to use the iterator.  Just look at the positions
recorded in the glyphs of the relevant glyph_row, and then look up
text properties at those positions using Fget_char_property etc., like
we already do to look for the 'help-echo' property in the same
function.  Here's a typical fragment:

	    Lisp_Object obj = glyph->object;
	    ptrdiff_t charpos = glyph->charpos;

	    /* Try text properties.  */
	    if (STRINGP (obj)
		&& charpos >= 0
		&& charpos < SCHARS (obj))
	      {
		help = Fget_text_property (make_fixnum (charpos),
					   Qhelp_echo, obj);

Since the 'display' property that shows a fringe bitmap can be on any
character of a screen line, you will need to loop over all the glyphs
of the glyph_row where the mouse pointer is, testing the 'display'
property as above.  Something like this:

  for (glyph = row->glyphs[TEXT_AREA];
       glyph < row->glyphs[TEXT_AREA] + row->used[TEXT_AREA];
       glyph++)

where 'row' is

   row = MATRIX_ROW (w->current_matrix, vpos);

and 'vpos' is returned by this call which we already have in
note_mouse_highlight:

      /* Find the glyph under X/Y.  */
      glyph = x_y_to_hpos_vpos (w, x, y, &hpos, &vpos, &dx, &dy, &area);

If Fget_text_property returns non-nil for the 'display' property, you
will then need to look at the value and see whether it specifies a
help-echo for the fringe; if it does, extract the string and assign it
to 'help_echo_string'.

Then you won't need to use 'struct it', and won't need any new caption
members in 'struct it'.

Thanks.