remend miscounts intraword italic closers (`*foo*bar`) and appends a spurious trailing `*` to complete text

[JHSeo-git]

Bug Description

remend (applied whenever mode="streaming" and parseIncompleteMarkdown are at their defaults) skips intraword * delimiters when counting emphasis, treating them as unable to close an italic run. But per CommonMark, * can close emphasis intraword: *foo*bar parses as <em>foo</em>bar (micromark agrees — rendering the same string with mode="static" produces exactly that).

Because remend miscounts the run as unclosed, it appends a spurious * to the text. And since remend runs on every render while mode="streaming" — including renders after the stream has completed — the stray * ends up permanently in the final output, not just in a transient mid-stream state.

This is especially painful for Korean/Japanese content, where particles/suffixes attach directly to the emphasized word (e.g. *단어*로), so LLM output hits this pattern constantly. It corrupts the message whenever the number of intraword closers in a block is odd (even counts cancel out in remend's counter, which is why the bug looks intermittent).

Steps to Reproduce

1. Render <Streamdown>{'*foo*bar'}</Streamdown> (defaults: mode="streaming", parseIncompleteMarkdown on)
2. Compare with <Streamdown mode="static">{'*foo*bar'}</Streamdown>
3. The streaming-mode output has a literal trailing * that never goes away

Calling remend directly shows the corruption at the string level:

import remend from 'remend';

remend('*foo*bar');
// => '*foo*bar*'   (expected: unchanged — the input is complete, valid CommonMark)

Expected Behavior

<em>foo</em>bar — identical to mode="static" and to CommonMark reference implementations. Complete, valid markdown should pass through remend unchanged.

Actual Behavior

<em>foo</em>bar* — a spurious literal * is appended, and it persists after streaming finishes.

renderToStaticMarkup(<Streamdown>{'*foo*bar'}</Streamdown>);
// => <div><p><em>foo</em>bar*</p></div>

renderToStaticMarkup(<Streamdown mode="static">{'*foo*bar'}</Streamdown>);
// => <div><p><em>foo</em>bar</p></div>

More real-world examples (all complete texts, all corrupted the same way):

input	streaming-mode text	expected text
this is *real*ly good	this is really good*	this is really good
이것은 *기울임*으로 표시	이것은 기울임으로 표시*	이것은 기울임으로 표시

Code Sample

import { renderToStaticMarkup } from 'react-dom/server';
import { Streamdown } from 'streamdown';

// spurious trailing "*"
renderToStaticMarkup(<Streamdown>{'*foo*bar'}</Streamdown>);

Streamdown Version

2.5.0 (remend 1.3.0; also reproduces against current main)

React Version

18.2.0

Node.js Version

24.18.0

Browser(s)

Not applicable (Node.js)

Operating System

macOS

Additional Context

The skip happens in packages/remend/src/emphasis-handlers.ts — the "Skip if flanked by word chars on both sides" checks treat an intraword * as a non-delimiter. That matches CommonMark's rule for _, but not for *: an intraword * is both left- and right-flanking, so it can open and close emphasis (cf. the CommonMark spec example 5*6*78 → 5<em>6</em>78).

So when a run has a valid opener (e.g. after whitespace) and its closer happens to be intraword, remend's open-counter stays at 1 and the completion logic appends * at the end of the block. A fix could be to count intraword * as a potential closer whenever a run is currently open, mirroring micromark's delimiter matching.

Workaround we're using in the meantime: remend={{ italic: false }}.