fix(gatsby-plugin-mdx): Do not leak frontmatter into page

[tyhopp]

Description

Fix for a recent change that introduced unexpected behavior where mdx frontmatter is rendered into pages.

Documentation

N/A

Related Issues

• [sc-51557]
• fix(gatsby-plugin-mdx): don't allow JS frontmatter by default #35830

[tyhopp]

When grayMatter was not passed options prior to this change, matter would always be undefined, resulting in no frontmatter rendered to the page. This changes removes matter entirely from the output node raw body.

Unsure if this is the correct fix since it's unclear to me why matter was included here in the first place (maybe it is stripped in later code paths?).

[pieh]

I tracked first introduction of it to ChristopherBiscardi/gatsby-mdx#329 - there is 0 context on it - to me it seem more like debugging code than anything else - the code is in the MDX format, so it would just output any content put there as content which doesn;t make much sense (to me anyway).

Especially with fact that matter in returned object means:

file.matter {String}: the raw, un-parsed front-matter string

[pieh]

I don't think we actually want matter there. As long as user continue to have access to frontmatter in the MDX file, we are not removing functionality.

For the behaviour change - gray-matter has some conditional paths based on wether any options were passed or not - so I would expect the difference in behaviour coming from some bugs in conditionals or maybe intentional, but confusing decisions based on that.

In any case, this looks good!

[pieh]

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