fix: SeText heading markdown rendering differently from web version

[divyanshu-patil]

Summary

This PR fixes a Markdown rendering inconsistency where setext headings (headings created by underlining a line with --- or ===) were being rendered differently in the mobile app compared to the web app.

Issue

Given input like:

text 1
text 2
text 3
---
text 4

The mobile parser (based on commonmark) follows the strict CommonMark spec, which states that a setext underline converts the entire preceding paragraph into a heading — not just the line directly above the underline. Since text 1, text 2, and text 3 have no blank lines between them, the parser treats them as a single paragraph node, and that whole block becomes the heading. text 4 is left as a separate paragraph.

This does not match the web app's behavior, where only text 3 (the line immediately preceding the ---) becomes the heading, and text 1 / text 2 remain a normal paragraph.

Root cause

In blocks.js, the setext heading block-start handler assigns the entire accumulated paragraph string (container._string_content, which includes all prior lines) directly to the new heading node:

heading._string_content = container._string_content;

Because container._string_content contains every line collected in the paragraph so far ("text 1\ntext 2\ntext 3\n"), all of it gets wrapped in the heading, rather than isolating just the final line.

Fix

• Split container._string_content into individual lines and drop the trailing empty entry produced by the final line break.
• Pop off only the last line and use it as the heading's content, so the heading now reflects only the line directly above the setext underline.
• If any lines remain before the last one, rebuild them into a new paragraph node (with recalculated sourcepos) and insert it into the tree immediately before the heading, preserving them as normal paragraph text instead of discarding them.
• If the original paragraph only had one line, behavior is unchanged (heading is created directly from it, no extra paragraph node needed).

Follow-up fix during testing

An initial version of the patch attempted to reassign the heading's sourcepos directly:

heading.sourcepos = [...]; // TypeError: Cannot assign to property 'sourcepos' which has only a getter

This threw a runtime error because sourcepos on Node is a getter-only property (backed internally by _sourcepos). The fix was updated to assign to the internal field instead:

heading._sourcepos = [...];

This resolves the crash while keeping the source position metadata accurate for the newly split heading and paragraph nodes.

Ticket Link

Checklist

• Added or updated unit tests (required for all new features)
• Has UI changes
• Includes text changes and localization file updates
• Have tested against the 5 core themes to ensure consistency between them.
• Have run E2E tests by adding label E2E/Run (or E2E/Run-iOS / E2E/Run-Android for platform-specific runs).

Device Information

This PR was tested on:

Screenshots

Web

Before

After

Release Note

Fixed setext-style Markdown headings (underlined with --- or ===) incorrectly converting all preceding paragraph lines into the heading instead of only the line directly above the underline, to match web app behavior.

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• patches/commonmark+0.31.2-0.patch

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📝 Walkthrough

Walkthrough

This PR updates the commonmark patch file (patches/commonmark+0.31.2-0.patch) to fix a comment typo and change setext heading resolution logic so that any preceding lines in an accumulated paragraph are preserved as a separate paragraph node before the heading, rather than being discarded.

Changes

Commonmark Patch Update

Layer / File(s)	Summary
Comment fix and formatting patches/commonmark+0.31.2-0.patch	Corrects a comment typo ("definitiosn" → "definitions") and reformats a refmap trimming call to use slice(pos).
Setext heading paragraph preservation patches/commonmark+0.31.2-0.patch	Splits accumulated paragraph text by line so the last line becomes the setext heading's content, while preceding lines are inserted as a separate paragraph node with adjusted source positions before the heading.

Estimated code review effort: 2 (Simple) | ~10 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: fixing setext heading rendering to match the web version.
Description check	✅ Passed	The description accurately explains the setext heading bug, the fix, and the source position adjustment.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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[divyanshu-patil]

hello @enahum can you take a look at this PR, it overrides the default behaviour of the CommonMark but brings UI consistency across web and app version

Thank you for you time !

[enahum]

@divyanshu-patil Thanks for the PR, but I think this fits better in https://github.com/mattermost/commonmark.js instead of a patch. @hmhealey thoughts?

[divyanshu-patil]

@enahum Fair point, but the CommonMark setext spec actually says:

A setext heading consists of one or more lines of text, not interrupted by a blank line

So this behavior is spec-compliant, raising it in commonmark.js won't change anything.

The real issue is that web doesn't match this behavior. Web uses marked (also CommonMark-compliant, just faster), but it handles this differently. E.g.:

hello
hello
---

renders as:

hello
hello

Even GitHub follows the spec-correct behavior here, so web looks like the odd one out.

I think the fix is on the web side, probably something overriding marked's default behavior. Happy to dig in and fix it if that works for you!