bug#39390: 28.0.50; prettify-symbols-mode confuses display when start/end faces are different

[Raphael 'kena' Poss <knz@thaumogen.net>]

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On 02-02-2020 20:20, Eli Zaretskii wrote:
> The composition process needs that all of the
> characters to be composed have the same face.  That face is put on the
> characters by font-lock mode, not by prettify-symbols-mode. [...]
> Without all the characters to be replaced
> having the same font-lock face, you will not be able to "convince"
> prettify-symbols-mode to replace them

I think this wasn't clear in my original bug report: the replacement
*does* occur -- the display shows the result of the substitution upon
first display. (see first attached screenshot).

It even properly switches back and forth between the composition and
original form with unprettify-at-point. (see second attachment)

The problem is that the *redisplays* (e.g. as triggered by moving the
highlight around with hl-line-mode) mess up the lines around the
replacement. See third and fourth screenshots.

If you are telling me that (compose-region) should never have agreed to
operate on a range of symbols with different faces, then I have
counter-evidence that it does. If you are telling me that it shouldn't,
then maybe that's where the bug really lies.



-- 
Raphael 'kena' Poss

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