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

[Eli Zaretskii <eliz@gnu.org>]

> From: Vladimir Kazanov <vekazanov@gmail.com>
> Date: Sun, 7 Apr 2024 12:14:54 +0100
> Cc: emacs-devel@gnu.org
> 
> > Like I said earlier: I think it would be better to have the tooltip
> > text in a separate property, not as part of the 'display' property
> > spec.
> >
> > Also, I don't see a need for recording the positions in the glyph_row
> > structure, because finding the glyph that corresponds to buffer/string
> > text with that special property is easy enough.
> 
> Done.
> 
> The code in the patch goes through glyphs of the row under a mouse
> pointer, finds an object that generated the glyph (glyph->object,
> which can be a buffer or a string) and checks if
> left-fringe-help/right-fringe-help text properties are defined.
> Strings from the properties are then displayed as tooltips.
> 
> This only works for properties of visible text that has glyphs
> generated for it. Properties coming from invisible text and
> zero-length overlays are not represented as glyphs as much I can see
> from the code.

Since there can be only one fringe bitmap displayed on each screen
line, Lisp programs don't need the left/right-fringe-help to be put on
the same positions as the display property that generated the fringe
bitmaps.  They can place the left/right-fringe-help property on _any_
position within the same screen line.  (Of course, to make sure these
properties end up on the same screen line as the fringe bitmap, they
should be on positions as close as possible to the ones which cause
the bitmap to be displayed on the fringe, but that shouldn't be too
hard.)  So I think the danger of not showing the tooltip due to
invisible text or text that didn't produce any glyphs is not a real
one, provided that we lift the restriction of having the
left/right-fringe-help property on the same positions as the display
property which produces the fringe bitmap display.  Do you agree?

> ;;; Tooltips will NOT be shown
>   (insert (propertize "foo" 'display '(left-fringe left-arrow warning)
>                       'left-fringe-help "left tooltip"))
>   (insert (propertize "foo" 'display '(right-fringe left-arrow warning)
>                       'right-fringe-help "right tooltip"))
>   (insert (propertize "foo" 'left-fringe-help "left tooltip"))
> 
>   (insert (propertize "foo" 'right-fringe-help "right tooltip"))
> 
> (overlay-put
>    (make-overlay 1 2)
>    'before-string (propertize
>                    "x"
>                    'left-fringe-help "left tooltip"
>                    'display '(left-fringe right-arrow warning)))

It should be enough to put the left/right-fringe-help property on the
character immediately following the display property, to get the
tooltip to display in those cases.