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

[Raphael 'kena' Poss]

Hi Tassilo,

thank you for joining the conversation.

to summarize your standpoint is that:

1) configurations that specify a string are invalid.
2) multi-word matches are "asking for trouble".


Regarding point (1). First of all the function compose-region does
accept a string as per its documentation. Separately if there is
consensus that this type of configuration is invalid, the function
should be programmed to reject it.

Finally, let us not distract ourselves with this detail. The bug is
readily reproducible with a single character:

https://asciinema.org/a/IGDZhOMnmF7sAWJYEjRqPdboA


Regarding point (2). Multiple characters in the output actually works
fine most of the time. Please double check the title of this e-mail
thread and the reason why I reported the issue in the first place:

- replacing either a single character or multiple characters in the
buffer, even separated by blanks, is fine as long as they display with
just 1 face.

- replacing anything with either a single or multiple characters in the
result of the composition is fine as long as the input uses just 1 face.


There is an error in the display code when compose-region composes over
multiple faces, and I'd like us to focus on that. I'm pretty sure that
pretty-symbols-mode is just one of multiple ways one can trigger this bug.

-- 
Raphael 'kena' Poss