feat(workspace): add line numbers and horizontal scrolling to code block editor

frontend/src/features/workspace/CodeBlockEditor.tsx

@@ -347,6 +347,7 @@ export const CodeBlockEditor: React.FC<CodeBlockEditorProps> = ({
   const copyTimerRef = useRef<number | null>(null);
   const localTextareaRef = useRef<HTMLTextAreaElement | null>(null);
   const preRef = useRef<HTMLPreElement | null>(null);
+  const lineNumbersRef = useRef<HTMLDivElement | null>(null);
 
   useEffect(() => {
     return () => {
@@ -506,18 +507,42 @@ export const CodeBlockEditor: React.FC<CodeBlockEditorProps> = ({
   };
 
   const handleScroll = (e: React.UIEvent<HTMLTextAreaElement>) => {
+    const target = e.currentTarget;
     if (preRef.current) {
-      preRef.current.scrollTop = e.currentTarget.scrollTop;
-      preRef.current.scrollLeft = e.currentTarget.scrollLeft;
+      preRef.current.scrollTop = target.scrollTop;
+      preRef.current.scrollLeft = target.scrollLeft;
+    }
+    if (lineNumbersRef.current) {
+      lineNumbersRef.current.scrollTop = target.scrollTop;
     }
   };
 
+  const handleLineNumbersScroll = (e: React.UIEvent<HTMLDivElement>) => {
+    const target = e.currentTarget;
+    if (localTextareaRef.current) {
+      localTextareaRef.current.scrollTop = target.scrollTop;
+    }
+    if (preRef.current) {
+      preRef.current.scrollTop = target.scrollTop;
+    }
+  };
+
+  const handleLineNumbersWheel = (e: React.WheelEvent) => {
+    if (localTextareaRef.current) {
+      localTextareaRef.current.scrollTop += e.deltaY;
+    }
+  };
+
+  const lineCount = useMemo(() => {
+    return currentContent.split('\n').length;
+  }, [currentContent]);
+
   const rawLang = block.lang || '';
   const selectValue = rawLang ? resolveLanguageId(rawLang) || rawLang : '';
   const hasCustomUnknownLang = Boolean(selectValue && !KNOWN_OPTION_IDS.has(selectValue));
 
   return (
-    <div className="bg-code border border-border rounded-btn my-0.5 overflow-x-auto max-w-full">
+    <div className="bg-code border border-border rounded-btn my-0.5 max-w-full overflow-hidden">
       {/* Code Block Header */}
       <div className="flex items-center justify-between gap-2 px-2 py-1 border-b border-border text-[11px] font-mono text-faint">
         <div className="flex items-center gap-1.5 min-w-0">
@@ -584,41 +609,60 @@ export const CodeBlockEditor: React.FC<CodeBlockEditorProps> = ({
         </div>
       </div>
 
-      {/* Live Highlighted Overlay + Textarea */}
-      <div className="relative w-full overflow-x-auto">
-        <pre
-          ref={preRef}
+      {/* Live Highlighted Overlay + Textarea with Line Numbers */}
+      <div className="flex relative w-full overflow-hidden">
+        {/* Line Numbers Column */}
+        <div
+          ref={lineNumbersRef}
           aria-hidden="true"
-          className="absolute inset-0 m-0 p-2 font-mono text-xs text-main leading-relaxed whitespace-pre-wrap break-words [overflow-wrap:anywhere] pointer-events-none select-none overflow-hidden"
+          onScroll={handleLineNumbersScroll}
+          onWheel={handleLineNumbersWheel}
+          data-testid="code-line-numbers"
+          className="select-none py-2 pl-2.5 pr-2 font-mono text-xs text-faint leading-relaxed text-right border-r border-border/40 shrink-0 overflow-hidden"
         >
-          <code
-            className={prismLang ? `language-${prismLang}` : 'language-none'}
-            dangerouslySetInnerHTML={{
-              __html: highlightedHtml + (currentContent.endsWith('\n') ? '\n' : ''),
+          {Array.from({ length: lineCount }, (_, i) => (
+            <div key={i + 1} data-line-number={i + 1}>
+              {i + 1}
+            </div>
+          ))}
+        </div>
+
+        {/* Code Content (Overlay + Textarea) */}
+        <div className="relative flex-1 min-w-0 overflow-hidden">
+          <pre
+            ref={preRef}
+            aria-hidden="true"
+            className="absolute inset-0 m-0 p-2 font-mono text-xs text-main leading-relaxed whitespace-pre pointer-events-none select-none overflow-hidden"
+          >
+            <code
+              className={prismLang ? `language-${prismLang}` : 'language-none'}
+              dangerouslySetInnerHTML={{
+                __html: highlightedHtml + (currentContent.endsWith('\n') ? '\n' : ''),
+              }}
+            />
+          </pre>
+          <textarea
+            ref={(el) => {
+              localTextareaRef.current = el;
+              if (textareaRef) textareaRef.current = el;
+              registerTextareaRef?.(block.id, el);
             }}
+            rows={1}
+            value={currentContent}
+            readOnly={!canEdit}
+            spellCheck={false}
+            autoCapitalize="off"
+            autoComplete="off"
+            autoCorrect="off"
+            onFocus={handleFocus}
+            onBlur={handleBlur}
+            onChange={onTextareaChange}
+            onKeyDown={onTextareaKeyDown}
+            onScroll={handleScroll}
+            style={{ caretColor: 'var(--text-main)' }}
+            className="relative block w-full m-0 p-2 font-mono text-xs leading-relaxed whitespace-pre bg-transparent text-transparent caret-main selection:bg-accent/25 selection:text-transparent border-0 focus:outline-none focus:ring-0 resize-none overflow-x-auto"
           />
-        </pre>
-        <textarea
-          ref={(el) => {
-            localTextareaRef.current = el;
-            if (textareaRef) textareaRef.current = el;
-            registerTextareaRef?.(block.id, el);
-          }}
-          rows={1}
-          value={currentContent}
-          readOnly={!canEdit}
-          spellCheck={false}
-          autoCapitalize="off"
-          autoComplete="off"
-          autoCorrect="off"
-          onFocus={handleFocus}
-          onBlur={handleBlur}
-          onChange={onTextareaChange}
-          onKeyDown={onTextareaKeyDown}
-          onScroll={handleScroll}
-          style={{ caretColor: 'var(--text-main)' }}
-          className="relative block w-full m-0 p-2 font-mono text-xs leading-relaxed whitespace-pre-wrap break-words [overflow-wrap:anywhere] bg-transparent text-transparent caret-main selection:bg-accent/25 selection:text-transparent border-0 focus:outline-none focus:ring-0 resize-none overflow-hidden"
-        />
+        </div>
       </div>
     </div>
   );

frontend/src/features/workspace/__tests__/CodeBlockEditor.test.tsx

@@ -209,4 +209,55 @@ describe('CodeBlockEditor', () => {
     expect(res2.updates[res2.updates.length - 1]?.type).toBe('code');
     expect(res2.updates[res2.updates.length - 1]?.lang).toBe('json');
   });
+
+  it('renders line numbers column with aria-hidden, user-select: none, and correct line count including trailing empty lines and 100+ lines', () => {
+    // 1. Single empty line
+    const { container, rerender } = render(
+      <CodeBlockEditor block={makeCodeBlock({ id: 'code-empty', content: '' })} content="" canEdit />
+    );
+    const getLineContainer = () => container.querySelector('[data-testid="code-line-numbers"]') as HTMLElement;
+    expect(getLineContainer()).toBeTruthy();
+    expect(getLineContainer().getAttribute('aria-hidden')).toBe('true');
+    expect(getLineContainer().className).toContain('select-none');
+
+    let lineEls = container.querySelectorAll('[data-line-number]');
+    expect(lineEls).toHaveLength(1);
+    expect(lineEls[0].textContent?.trim()).toBe('1');
+
+    // 2. Trailing empty line gives an extra line number (e.g. line 1 + empty line 2)
+    rerender(
+      <CodeBlockEditor
+        block={makeCodeBlock({ id: 'code-trailing', content: `first line
+` })}
+        content={`first line
+`}
+        canEdit
+      />
+    );
+    lineEls = container.querySelectorAll('[data-line-number]');
+    expect(lineEls).toHaveLength(2);
+    expect(lineEls[0].textContent?.trim()).toBe('1');
+    expect(lineEls[1].textContent?.trim()).toBe('2');
+
+    // 3. 120 lines (100+ lines test)
+    const longCode = Array.from({ length: 120 }, (_, i) => `const x_${i + 1} = ${i + 1};`).join('\n');
+    rerender(
+      <CodeBlockEditor
+        block={makeCodeBlock({ id: 'code-long', content: longCode })}
+        content={longCode}
+        canEdit
+      />
+    );
+    lineEls = container.querySelectorAll('[data-line-number]');
+    expect(lineEls).toHaveLength(120);
+    expect(lineEls[0].textContent?.trim()).toBe('1');
+    expect(lineEls[99].textContent?.trim()).toBe('100');
+    expect(lineEls[119].textContent?.trim()).toBe('120');
+
+    // Textarea has whitespace-pre and overlay has whitespace-pre
+    const textarea = container.querySelector('textarea') as HTMLTextAreaElement;
+    const pre = container.querySelector('pre') as HTMLPreElement;
+    expect(textarea.className).toContain('whitespace-pre');
+    expect(pre.className).toContain('whitespace-pre');
+  });
 });