fix(client): stabilize syntax highlighting under load

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

@@ -19,7 +19,7 @@
  */
 
 import { createHighlighterCoreSync, createCssVariablesTheme } from 'shiki/core'
-import { createJavaScriptRegexEngine } from 'shiki/engine/javascript'
+import { createJavaScriptRegexEngine, defaultJavaScriptRegexConstructor } from 'shiki/engine/javascript'
 import langTs from '@shikijs/langs/typescript'
 import langBash from '@shikijs/langs/shellscript'
 import langJson from '@shikijs/langs/json'
@@ -140,6 +140,20 @@ const cssVariablesTheme = createCssVariablesTheme({
   fontStyle: true,
 })
 
+/**
+ * The client regex engine compiles each TextMate pattern when its scanner is
+ * created. Shiki otherwise defers patterns longer than 3,000 characters until
+ * their first match; that compilation counts against Shiki's 500 ms per-line
+ * budget and can return a partial token stream under host contention. Eager
+ * compilation leaves the same budget in place for scanning user content.
+ */
+const regexEngine = createJavaScriptRegexEngine({
+  forgiving: true,
+  regexConstructor: pattern => defaultJavaScriptRegexConstructor(pattern, {
+    lazyCompileLength: Number.POSITIVE_INFINITY,
+  }),
+})
+
 let singleton: HighlighterCore | undefined
 
 /** The synchronous highlighter (one instance per document); pre-warmed below, lazy as the fallback. */
@@ -147,7 +161,7 @@ function highlighter(): HighlighterCore {
   singleton ??= createHighlighterCoreSync({
     themes: [cssVariablesTheme],
     langs: LANGS,
-    engine: createJavaScriptRegexEngine({ forgiving: true }),
+    engine: regexEngine,
   })
   return singleton
 }