bug#81661: 32.0.50; Offscreen context in show-paren-mode truncates multi-character keyword paren

> Date: Wed, 19 Aug 2026 23:58:37 -0400
> From: Troy Brown via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <[email protected]>
> 
> Some languages, such as Ruby, highlight keywords when
> `show-paren-mode` is enabled.  The following demonstrates an example
> with a "do loop". Open the following in `ruby-mode`.
> 
> ```ruby
> 5.times do |i|
> 
> 
> 
> 
>   print "Number: #{i}"
> end
> ```
> 
> Either resize the window so the first line is not showing or insert
> more empty lines to force it off the screen.  Execute "M-: (setq
> show-paren-context-when-offscreen t) RET" to enable offscreen context
> display.  Move point to the end of the "end" keyword.  The echo area
> will show "Matches 5.times d".  The "do" keyword is truncated to the
> first character rather than showing the entire keyword.  This same
> problem manifests using other forms of offscreen context display as
> well, such as the "overlay" or "child-frame" settings.
> 
> The underlying issue appears to be the use of the
> `blink-paren-open-paren-line-string` function to construct the line of
> context which will be displayed in the echo area (or elsewhere).  That
> function only takes a single parameter consisting of the position in
> the buffer where the "paren" begins.  The function assumes the keyword
> "paren" is only a single character rather than possibly being multiple
> characters long.  The function attempts to truncate the context line
> such that the "paren" is the last part of the context, but assumes
> it's only a single character in length resulting in the keyword being
> truncated.

Thanks.

It would be nice if you (or someone else) could explain how Ruby mode
sets up this keyword highlighting in show-paren-mode.  It seems that
it customizes show-paren-data-function in some way, but I cannot find
where that happens and what is the code of the customized function, I
can only see its bytecode.

I can see what you describe, but I see no such problem in c-ts-mode,
which highlights preprocessor directives using a similar technique.
In the c-ts-mode case I see "Matches #ifdef FOO_BAR_BAZ", which shows
the full match, not just the first character.  So I wonder what is
different in how ruby-mode does this, but I've hit a brick wall
because I cannot find the code which implements this for Ruby.

Regarding blink-paren-open-paren-line-string: I'm not sure your
analysis is correct (and the c-ts-mode case seems to confirm my
conclusion): it doesn't always assume "paren" is a single character,
not for showing the context.  See these comments there:

       ;; Show what precedes the open in its line, if anything.

       ;; Show what follows the open in its line, if anything.

       ;; Otherwise show the previous nonblank line,
       ;; if there is one.

       ;; There is nothing to show except the char itself.

It's only in the last 2 cases that there's the 1-character assumption,
right?

Maybe Dmitry could help.