fix(web): render CJK-adjacent strong emphasis

packages/client/ui-primitives/src/markdown/MarkdownText.tsx

@@ -5,13 +5,15 @@ import rehypeKatex from 'rehype-katex'
 import remarkGfm from 'remark-gfm'
 import remarkMath from 'remark-math'
 import { CodeBlock } from './CodeBlock.tsx'
+import { remarkCjkFriendlyStrong } from './remarkCjkFriendlyStrong.ts'
 import { remarkMathCompatibility } from './remarkMathCompatibility.ts'
 import 'katex/dist/katex.min.css'
 import css from './MarkdownText.module.css'
 
-const streamingRemarkPlugins = [remarkGfm]
+const streamingRemarkPlugins = [remarkGfm, remarkCjkFriendlyStrong]
 const settledRemarkPlugins = [
   remarkGfm,
+  remarkCjkFriendlyStrong,
   remarkMathCompatibility,
   remarkMath,
 ]

packages/client/ui-primitives/src/markdown/remarkCjkFriendlyStrong.ts

@@ -0,0 +1,88 @@
+/** Let asterisk strong emphasis close after punctuation when CJK prose continues without whitespace. */
+
+import { attention } from 'micromark-core-commonmark'
+import { unicodePunctuation } from 'micromark-util-character'
+import { classifyCharacter } from 'micromark-util-classify-character'
+import { codes, constants } from 'micromark-util-symbol'
+import type { Construct, Extension, State, Tokenizer } from 'micromark-util-types'
+
+interface RemarkProcessor {
+  data(): { micromarkExtensions?: Extension[] }
+}
+
+const cjkCharacter = new RegExp([
+  '\\p{Script_Extensions=Han}',
+  '\\p{Script_Extensions=Hiragana}',
+  '\\p{Script_Extensions=Katakana}',
+  '\\p{Script_Extensions=Hangul}',
+  '\\p{Script_Extensions=Bopomofo}',
+].join('|'), 'u')
+
+function isCjkCharacter(code: number | null): boolean {
+  return code !== null && code >= 0 && cjkCharacter.test(String.fromCodePoint(code))
+}
+
+const tokenizeCjkFriendlyAttention: Tokenizer = function (effects, ok, nok) {
+  const configuredAttentionMarkers = this.parser.constructs.attentionMarkers.null
+  if (configuredAttentionMarkers === undefined) {
+    throw new Error('micromark CommonMark attention markers are unavailable')
+  }
+  const attentionMarkers = configuredAttentionMarkers
+  const previous = this.previous
+  const before = classifyCharacter(previous)
+  let marker: number | null = codes.eof
+
+  return start
+
+  function start(code: number | null): State | undefined {
+    /* v8 ignore next -- this text construct is dispatched only for an asterisk. */
+    if (code !== codes.asterisk) return nok(code)
+    marker = code
+    effects.enter('attentionSequence')
+    return inside(code)
+  }
+
+  function inside(code: number | null): State | undefined {
+    if (code === marker) {
+      effects.consume(code)
+      return inside
+    }
+
+    const token = effects.exit('attentionSequence')
+    const after = classifyCharacter(code)
+    const open = !after || (after === constants.characterGroupPunctuation && Boolean(before))
+      || attentionMarkers.includes(code)
+    const commonMarkClose = !before
+      || (before === constants.characterGroupPunctuation && Boolean(after))
+      || attentionMarkers.includes(previous)
+    const markerCount = token.end.offset - token.start.offset
+    const cjkStrongClose = markerCount >= 2
+      && unicodePunctuation(previous)
+      && isCjkCharacter(code)
+    const close = commonMarkClose || cjkStrongClose
+
+    token._open = open
+    token._close = close
+    return ok(code)
+  }
+}
+
+const cjkFriendlyAttention: Construct = {
+  name: 'cjkFriendlyAttention',
+  resolveAll: attention.resolveAll,
+  tokenize: tokenizeCjkFriendlyAttention,
+}
+
+const cjkFriendlyStrong: Extension = {
+  text: { [codes.asterisk]: cjkFriendlyAttention },
+}
+
+/**
+ * Extend CommonMark asterisk strong emphasis for punctuation-delimited CJK prose.
+ * @returns Nothing.
+ */
+export function remarkCjkFriendlyStrong(this: RemarkProcessor): undefined {
+  const data = this.data()
+  const extensions = data.micromarkExtensions ?? (data.micromarkExtensions = [])
+  extensions.push(cjkFriendlyStrong)
+}