fix: disable hljs.highlightAuto for unknown languages, hide line numbers for plain text

- highlightAuto was misdetecting plain text (file trees, command output) as
  code and applying wrong syntax colors (red keywords, green tags, etc.)
- Now only highlights when language is explicitly specified and known to hljs
- Code blocks without a specified language render as clean plain text
- Line numbers hidden for plain text blocks (only shown for actual code)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

desktop/src/components/chat/CodeViewer.tsx

@@ -16,18 +16,23 @@ export function CodeViewer({ code, language, maxLines = 20, showLineNumbers = tr
   const isTruncated = !expanded && lines.length > maxLines
   const visibleCode = isTruncated ? lines.slice(0, maxLines).join('\n') : code
 
+  // Only highlight when language is explicitly known.
+  // Do NOT use highlightAuto — it misdetects plain text as code.
+  const hasLanguage = !!language && !!hljs.getLanguage(language)
+  const effectiveShowLineNumbers = showLineNumbers && hasLanguage
+
   const highlightedLines = useMemo(() => {
     return visibleCode.split('\n').map((line) => {
       try {
-        if (language && hljs.getLanguage(language)) {
-          return hljs.highlight(line || ' ', { language, ignoreIllegals: true }).value
+        if (hasLanguage) {
+          return hljs.highlight(line || ' ', { language: language!, ignoreIllegals: true }).value
         }
-        return hljs.highlightAuto(line || ' ').value
+        return escapeHtml(line || ' ')
       } catch {
         return escapeHtml(line)
       }
     })
-  }, [language, visibleCode])
+  }, [hasLanguage, language, visibleCode])
 
   const startLine = 1
 
@@ -55,22 +60,30 @@ export function CodeViewer({ code, language, maxLines = 20, showLineNumbers = tr
       <div className="max-h-[420px] overflow-auto">
         <div className="min-w-full font-[var(--font-mono)] text-[12px] leading-[1.3]">
           {visibleLines.map((line, index) => (
-            <div
-              key={`${startLine + index}-${line}`}
-              className="grid grid-cols-[3rem,minmax(0,1fr)] gap-0 hover:bg-[#f6f8fa]/50"
-            >
-              {showLineNumbers ? (
+            effectiveShowLineNumbers ? (
+              <div
+                key={`${startLine + index}-${line}`}
+                className="grid grid-cols-[3rem,minmax(0,1fr)] gap-0 hover:bg-[#f6f8fa]/50"
+              >
                 <span className="select-none border-r border-[#eaeef2] bg-[#fafbfc] px-2 py-px text-right text-[11px] text-[#8b949e]">
                   {startLine + index}
                 </span>
-              ) : (
-                <span className="w-0" />
-              )}
-              <span
-                className="overflow-hidden bg-white px-3 py-px whitespace-pre-wrap break-words text-[#24292f]"
-                dangerouslySetInnerHTML={{ __html: highlightedLines[index] ?? escapeHtml(line) }}
-              />
-            </div>
+                <span
+                  className="overflow-hidden bg-white px-3 py-px whitespace-pre-wrap break-words text-[#24292f]"
+                  dangerouslySetInnerHTML={{ __html: highlightedLines[index] ?? escapeHtml(line) }}
+                />
+              </div>
+            ) : (
+              <div
+                key={`${startLine + index}-${line}`}
+                className="hover:bg-[#f6f8fa]/50"
+              >
+                <span
+                  className="block bg-white px-3 py-px whitespace-pre-wrap break-words text-[#24292f]"
+                  dangerouslySetInnerHTML={{ __html: highlightedLines[index] ?? escapeHtml(line) }}
+                />
+              </div>
+            )
           ))}
         </div>
       </div>

desktop/src/components/chat/highlightCode.ts

@@ -12,14 +12,22 @@ export function highlightCodeLines(code: string, language?: string): string[] {
   return code.split('\n').map((line) => highlightCodeLine(line, language))
 }
 
+/** Returns true if the language is known to hljs and should be highlighted */
+export function isHighlightable(language?: string): boolean {
+  return !!language && !!hljs.getLanguage(language)
+}
+
 function highlightCodeLine(line: string, language?: string): string {
   const safeLine = line.length > 0 ? line : ' '
 
   try {
+    // Only highlight when language is explicitly specified and known.
+    // Do NOT use highlightAuto — it misdetects plain text (file trees,
+    // command output, etc.) as code and applies wrong colors.
     if (language && hljs.getLanguage(language)) {
-      return hljs.highlight(safeLine, { language }).value
+      return hljs.highlight(safeLine, { language, ignoreIllegals: true }).value
     }
-    return hljs.highlightAuto(safeLine).value
+    return escapeHtml(safeLine)
   } catch {
     return escapeHtml(safeLine)
   }

desktop/src/components/markdown/MarkdownRenderer.tsx

@@ -1,6 +1,6 @@
 import { useMemo } from 'react'
 import { marked, type Tokens } from 'marked'
-import { escapeHtml, highlightCodeLines } from '../chat/highlightCode'
+import { escapeHtml, highlightCodeLines, isHighlightable } from '../chat/highlightCode'
 
 type Props = {
   content: string
@@ -11,16 +11,27 @@ const renderer = new marked.Renderer()
 renderer.code = function ({ text, lang }: Tokens.Code) {
   const languageLabel = escapeHtml(lang || 'code')
   const lines = text.split('\n')
+  const hasLanguage = isHighlightable(lang)
   const highlightedLines = highlightCodeLines(text, lang)
 
-  const body = highlightedLines
-    .map((line, index) => `
-      <div class="grid grid-cols-[3rem,minmax(0,1fr)] gap-0 hover:bg-[#f6f8fa]/50">
-        <span class="select-none border-r border-[#eaeef2] bg-[#fafbfc] px-2 py-px text-right text-[11px] text-[#8b949e]">${index + 1}</span>
-        <span class="overflow-hidden bg-white px-3 py-px whitespace-pre-wrap break-words text-[#24292f]">${line || '&nbsp;'}</span>
-      </div>
-    `)
-    .join('')
+  // Show line numbers only when language is known (actual code).
+  // Plain text, file trees, command output etc. look better without them.
+  const body = hasLanguage
+    ? highlightedLines
+        .map((line, index) => `
+          <div class="grid grid-cols-[3rem,minmax(0,1fr)] gap-0 hover:bg-[#f6f8fa]/50">
+            <span class="select-none border-r border-[#eaeef2] bg-[#fafbfc] px-2 py-px text-right text-[11px] text-[#8b949e]">${index + 1}</span>
+            <span class="overflow-hidden bg-white px-3 py-px whitespace-pre-wrap break-words text-[#24292f]">${line || '&nbsp;'}</span>
+          </div>
+        `)
+        .join('')
+    : highlightedLines
+        .map((line) => `
+          <div class="hover:bg-[#f6f8fa]/50">
+            <span class="block bg-white px-3 py-px whitespace-pre-wrap break-words text-[#24292f]">${line || '&nbsp;'}</span>
+          </div>
+        `)
+        .join('')
 
   return `
     <div class="my-4 overflow-hidden rounded-lg border border-[#d0d7de] bg-[#f6f8fa] text-[#24292f]">