Fix nested <details> tag

[timneutkens]

Description

Fix nested HTML block parsing in parseMarkdownIntoBlocks where nested elements of the same type (e.g. <details> inside <details>) would prematurely close the outer block, causing inner content to leak out as separate top-level blocks.

Type of Change

• Bug fix (non-breaking change which fixes an issue)

Related Issues

Changes Made

• Fixed parseMarkdownIntoBlocks in lib/parse-blocks.tsx to count both opening and closing tags of the tracked type when merging tokens inside an HTML block, instead of only checking for the first closing tag
• Added nested-details-tables.test.tsx with 5 focused tests covering the nested HTML block parsing bug

Testing

• All existing tests pass
• Added new tests for the changes
• Manually tested the changes

Test Coverage

• parseMarkdownIntoBlocks produces balanced open/close counts for 2-deep and 3-deep nesting
• Rendered DOM correctly nests inner <details> inside outer <details>
• Sibling content (headings, tables) after an inner </details> stays inside the outer <details>
• Only one top-level <details> is produced for a fully nested structure

Screenshots/Demos

Using the Next.js stats action markdown:

Before

After

Checklist

• My code follows the project's code style
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings or errors
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have created a changeset (pnpm changeset)

Changeset

• I have created a changeset for these changes

Additional Notes

Root cause: When merging tokens inside an HTML block, the code only checked for closing tags but never tracked new opening tags of the same type. A single nested </details> would pop the only entry on the HTML tracking stack, so the parser treated the outer block as closed and everything after became separate blocks.

[vercel]

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[vercel]

Additional Suggestion:

When the initial HTML block token contains multiple opening tags of the same type, only 1 entry is pushed to htmlStack, causing the first closing tag to prematurely terminate the HTML block merge.

[haydenbleasel]

awesome, plays nicely with our recent similar fix 🚢