bug#41454: 28.0.50; [".+" 0 font-shape-gstring] composition rule breaks paren highlighting

[Eli Zaretskii]

> From: Pip Cet <pipcet@gmail.com>
> Date: Fri, 22 May 2020 12:50:13 +0000
> 
> There seems to be a general problem using such regexps in the
> composition-function-table.

Could be, because it's a very unusual thing to do.

> If I evaluate this in emacs -Q (by placing point after it and hitting C-x C-e)
> 
> (custom-set-faces
>  '(default ((t (:family "Libertinus Serif" :height 330)))))
> (set-char-table-range composition-function-table t '([".+" 0
> font-shape-gstring]))
> 
> the font correctly changes to a very large Libertinus font. I then hit
> C-b C-d ) and the entire last line is highlighted, not just the
> opening parenthesis. After the blink delay is over, the opening
> parenthesis and the "s" following it are unhighlighted, but the rest
> of the line is not. It stays like that permanently (screenshot
> attached).

I think this is expected, since you told Emacs all those characters
are a single grapheme cluster.

The regexps in composition-function-table should match only characters
that are supposed to be composed.