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Re: use of `mouse-face' to delimit text zones [was: bidi-display-reorder

From: Eli Zaretskii
Subject: Re: use of `mouse-face' to delimit text zones [was: bidi-display-reordering is now non-nil by default]
Date: Tue, 23 Aug 2011 19:15:56 +0300

> From: "Drew Adams" <address@hidden>
> Date: Tue, 23 Aug 2011 08:29:22 -0700
> Cc: 'Eli Zaretskii' <address@hidden>, address@hidden
> Grep the Lisp sources for `mouse-face', then check occurrences where we 
> retrieve
> that property to determine the boundaries and position of various text
> "candidates" (e.g. look for `next(-single)-char-property'). We do this for
> comint history, Dired, tmm, gnus articles, quail completions,... 
> [...]
> FWIW, in Icicles, just as in the vanilla Emacs completion code, I make heavy 
> use
> of the fact that the `mouse-face' property indicates the boundaries of
> completion candidates.

Heavy use of this doesn't yet make it right.  Good (read:
"non-kludgey") programming practices should not use for some feature
symbols whose name or documentation give no clue of such a use.
Should I count the number of bugs submitted by you in the name of this
same principle?

The documentation of mouse-face says:

       This property is used instead of `face' when the mouse is on or
       near the character.  For this purpose, "near" means that all text
       between the character and where the mouse is have the same
       `mouse-face' property value.

That's it.  Not a word about potential use of that for doing anything
else besides displaying the text.

How is Joe R. Hacker supposed to know that adding invisible characters
to a portion of text covered by mouse-face will break completion??

Btw, mouse-face can also be put on overlay strings and display
strings; how can you get the underlying text then?


> things might be a lot more complex if two properties were used or if
> displayed-text limits did not correspond to the other (new) limits.

Like what?

> And tomorrow you or someone else might make the same argument about any new
> property we introduced to delimit candidates.

If the face is called `completion-candidate', who would "make the same

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