bug#69647: [PATCH 1/2] Delete trailing whitespace when formatting LSP documentation

[Jim Porter <jporterbugs@gmail.com>]

On 3/8/2024 1:02 PM, João Távora wrote:
> And that's probably the ideal solution too.  But as far as I remember,
> markdown-view-mode (or whatever it's called) is not used for viewing the
> documentation.  it is used somewhere in the pipeline to render the text in a
> buffer which is then yanked using `buffer-substring` (with properties).

Yep. 'eglot--format-markup' uses 'gfm-view-mode' in a temp buffer and 
returns it as a string after calling 'buffer-string'. That should be 
fine though. For example, last year I fixed a similar issue with Eglot's 
usage of Markdown mode where backslash escapes still showed up in the 
Eldoc buffer:

   https://github.com/joaotavora/eglot/issues/188
   https://github.com/jrblevin/markdown-mode/pull/756
   https://github.com/jrblevin/markdown-mode/issues/766

I expect that something similar could be done for the trailing spaces case.

> So I don't know how that'd work.  Maybe Philip's patch can be tweaked to leave
> those specific bits of trailing whitespace alone?

I think we should leave the trailing whitespace alone in Eglot entirely; 
the underline that Philip mentioned in the original report is from 
'markdown-line-break-face', which Markdown mode uses[1] to fontify only 
the trailing whitespace that means "insert a line break".

> And do we really want to preserve the LSP server's notion of a line
> break anyway?  It has no concept of how wide the  Emacs window is.
> I'd say paragraphs yes, line breaks no.
I think we do in this case, yes. (Although I guess this depends on how 
Markdown mode handles things.) These are "hard" line breaks 
corresponding to the "<br>" tag when you convert Markdown->HTML, so they 
should always get turned into a newline.

Normally if I have some text like "foo bar\nbaz" in Markdown, that 
should get rendered as "foo bar baz": the newline is just there to make 
the source text readable, but it's treated like a space. 
'markdown-view-mode' (and 'gfm-view-mode') don't do this though, so a 
soft newline stays a newline in the "view" output. This also means that 
the Markdown line break sequence ("  \n") isn't necessary in 
'markdown-view-mode' to get a line break. Just the "\n" is sufficient. 
(In this sense, we could remove the "  " and there'd be no issues with 
the *current* version of markdown-mode.)

If I were going to make a complete fix for this, I'd do most of it in 
markdown-mode and would do the following things:

1. Translate "\n" to " " in 'markdown-view-mode'
2. Translate "  \n" to "\n" in 'markdown-view-mode'
3. Turn on 'visual-line-mode' in the ElDoc buffer to get soft line wrapping

However, as a partial fix, you could probably just set 
'markdown-line-break-face' to be invisible when 
'markdown-hide-markup[-in-view-modes]' is non-nil somewhere in Markdown 
mode. I think that would be easy, although I tried to do this and the 
obvious method didn't work, so who knows?

[1] Or *should* use, anyway. I haven't tested all possible cases.