webtex generated image in Markdown does not convert to HTML due to newline

[cderv]

From https://groups.google.com/g/pandoc-discuss/c/rRAh4z3l_BY

Take this md document

$ echo -e '$$\n\\frac{x}{y}\n$$' > test.md
$ cat test.md
$$
\frac{x}{y}
$$

It can be converted using webtex in Markdown to use an image

$ pandoc -t gfm --webtex test.md
![
\\frac{x}{y}
](https://latex.codecogs.com/png.latex?%0A%5Cfrac%7Bx%7D%7By%7D%0A "
\frac{x}{y}
")

There is some new line inserted inside what should be an image link in markdown. I was expecting this ![\\frac{x}{y}](https://latex.codecogs.com/png.latex?%0A%5Cfrac%7Bx%7D%7By%7D%0A "\frac{x}{y}")

This creates issues when trying to convert to HTML

$ pandoc -t markdown --webtex -o test-webtex.md test.md

$ ​cat test-webtex.md
![
\\frac{x}{y}
](https://latex.codecogs.com/png.latex?%0A%5Cfrac%7Bx%7D%7By%7D%0A "
\frac{x}{y}
")

$pandoc -f markdown -t html test-webtex.md
<p>[
\frac{x}{y}](https://latex.codecogs.com/png.latex?%0A%5Cfrac%7Bx%7D%7By%7D%0A
” “)</p>```


$ pandoc -t native -f markdown test-webtex.md
[ Para
  ​​[ Str "["
  ​​, SoftBreak
  ​​, Str
      ​​"\\frac{x}{y}](https://latex.codecogs.com/png.latex?%0A%5Cfrac%7Bx%7D%7By%7D%0A"
  ​​, Space
  ​​, Str "\8221"
  ​​, SoftBreak
  ​​, RawInline (Format "tex") "\\frac{x}{y}"
  ​​, SoftBreak
  ​​, Str "\8220)"
  ​​]
]

---

From @jgm

Looks like the parser expects a non-space character after the
opening " in a link/image title. So we should avoid a break
there.

BAD:

% pandoc
[
a
](url "
title
")
<p>[ a](url ” title “)</p>

OK:

% pandoc
[
a
](url "title
title
")
<p><a href="url" title="title title">a</a></p>

---

With gfm reader it does not see the same thing

$ pandoc -t native -f gfm test-webtex.md
[ Para
   ​[ Image
       ​( "" , [] , [] )
       ​[ SoftBreak , Str "\\frac{x}{y}" , SoftBreak ]
       ​( "https://latex.codecogs.com/png.latex?%0A%5Cfrac%7Bx%7D%7By%7D%0A"
       ​, "\n\\frac{x}{y}\n"
       ​)
   ​]
]

From @jgm
gfm reader is commonmark and uses a different parser, which
doesn't mind the line break there.

So there are two possible solutions:

1. fix markdown reader so it's okay with the line break after "
   in title.

2. fix writer so it strips out the leading and trailing space of
   a title.

Probably worth doing both.