feat(renderer): wire PreviewPane into AppShell

src/renderer/components/editor/EditorPane.tsx

@@ -1,9 +1,16 @@
+import { useEffect } from 'react';
 import { CodeMirrorEditor } from './CodeMirrorEditor';
 import { useEditorStore } from '@/stores/editor-store';
+import { usePreviewStore } from '@/stores/preview-store';
 
 export function EditorPane() {
   const { buffers, activeId } = useEditorStore();
   const buf = activeId ? buffers.get(activeId) : null;
+  const setPreviewSource = usePreviewStore((s) => s.setSource);
+
+  useEffect(() => {
+    if (buf) setPreviewSource(buf.content);
+  }, [buf?.id, buf?.content, buf, setPreviewSource]);
 
   if (!buf) {
     return (

src/renderer/components/layout/AppShell.tsx

@@ -4,6 +4,7 @@ import { Toolbar } from './Toolbar';
 import { Breadcrumb } from './Breadcrumb';
 import { StatusBar } from './StatusBar';
 import { EditorPane } from '@/components/editor/EditorPane';
+import { PreviewPane } from '@/components/preview/PreviewPane';
 import { useAppStore } from '@/stores/app-store';
 import { ResizablePanelGroup, ResizablePanel, ResizableHandle } from '@/components/ui/resizable';
 
@@ -38,9 +39,7 @@ export function AppShell() {
             <>
               <ResizableHandle />
               <ResizablePanel defaultSize={paneSizes.preview} minSize={20}>
-                <section className="h-full border-l border-border bg-card/10 p-4 text-sm text-muted-foreground">
-                  Preview placeholder
-                </section>
+                <PreviewPane />
               </ResizablePanel>
             </>
           )}

src/renderer/components/preview/MarkdownRenderer.tsx

@@ -0,0 +1,44 @@
+import { useEffect, useRef, useState } from 'react';
+import { renderMarkdown } from '@/lib/markdown';
+import { MermaidLazy } from './MermaidLazy';
+
+interface Props {
+  source: string;
+}
+
+const MERMAID_RE = /```mermaid\n([\s\S]*?)```/g;
+
+function extractMermaidCodes(source: string): string[] {
+  const codes: string[] = [];
+  let match: RegExpExecArray | null;
+  while ((match = MERMAID_RE.exec(source)) !== null) {
+    codes.push(match[1].trim());
+  }
+  return codes;
+}
+
+export function MarkdownRenderer({ source }: Props) {
+  const html = renderMarkdown(source);
+  const containerRef = useRef<HTMLDivElement>(null);
+  const [mermaidCodes, setMermaidCodes] = useState<string[]>([]);
+
+  // Re-derive mermaid codes from the original source. The renderMarkdown function
+  // replaces mermaid blocks with placeholders (data-mermaid-source="${idx}"), but
+  // it does not return the actual codes. Rather than thread them through the API,
+  // we re-extract from the source. Source is small (single buffer content) so this
+  // cost is negligible.
+  useEffect(() => {
+    setMermaidCodes(extractMermaidCodes(source));
+  }, [source]);
+
+  return (
+    <div className="prose prose-neutral dark:prose-invert max-w-none p-6" ref={containerRef}>
+      <div dangerouslySetInnerHTML={{ __html: html }} />
+      {mermaidCodes.map((code, i) => (
+        <div key={i} className="my-4">
+          <MermaidLazy code={code} />
+        </div>
+      ))}
+    </div>
+  );
+}

src/renderer/components/preview/MermaidLazy.tsx

@@ -0,0 +1,45 @@
+import { useEffect, useRef, useState } from 'react';
+
+interface Props {
+  code: string;
+}
+
+let mermaidModule: typeof import('mermaid').default | null = null;
+let initialized = false;
+
+async function getMermaid() {
+  if (!mermaidModule) {
+    const mod = await import('mermaid');
+    mermaidModule = mod.default;
+  }
+  if (!initialized) {
+    mermaidModule.initialize({ startOnLoad: false, theme: 'default' });
+    initialized = true;
+  }
+  return mermaidModule;
+}
+
+export function MermaidLazy({ code }: Props) {
+  const [svg, setSvg] = useState<string | null>(null);
+  const [error, setError] = useState<string | null>(null);
+  const idRef = useRef(`mermaid-${Math.random().toString(36).slice(2)}`);
+
+  useEffect(() => {
+    let cancelled = false;
+    getMermaid()
+      .then((m) => m.render(idRef.current, code))
+      .then((rendered) => {
+        if (!cancelled) setSvg(rendered);
+      })
+      .catch((err: Error) => {
+        if (!cancelled) setError(err.message);
+      });
+    return () => {
+      cancelled = true;
+    };
+  }, [code]);
+
+  if (error) return <div className="rounded border border-destructive/40 bg-destructive/5 p-2 text-xs text-destructive">{error}</div>;
+  if (!svg) return <div className="text-xs text-muted-foreground">Loading diagram…</div>;
+  return <div data-testid="mermaid-output" dangerouslySetInnerHTML={{ __html: svg }} />;
+}

src/renderer/components/preview/PreviewPane.tsx

@@ -0,0 +1,22 @@
+import { MarkdownRenderer } from './MarkdownRenderer';
+import { usePreviewStore } from '@/stores/preview-store';
+import { useScrollSync } from '@/hooks/use-scroll-sync';
+
+export function PreviewPane() {
+  const { source, setScrollRatio } = usePreviewStore();
+  const { handlePreviewScroll } = useScrollSync({ onPreviewScroll: setScrollRatio });
+
+  if (!source) {
+    return (
+      <div className="flex h-full items-center justify-center text-muted-foreground">
+        <p>Nothing to preview. Start typing in the editor.</p>
+      </div>
+    );
+  }
+
+  return (
+    <div className="h-full overflow-auto bg-card/10" onScroll={handlePreviewScroll}>
+      <MarkdownRenderer source={source} />
+    </div>
+  );
+}

src/renderer/hooks/use-scroll-sync.ts

@@ -0,0 +1,28 @@
+import { useCallback, useRef } from 'react';
+
+interface Options {
+  onEditorScroll?: (ratio: number) => void;
+  onPreviewScroll?: (ratio: number) => void;
+}
+
+export function useScrollSync(opts: Options) {
+  const FRAME_MS = 1000 / 60;
+  const lastTick = useRef(-FRAME_MS);
+
+  const handleEditorScroll = useCallback((evt: React.UIEvent<HTMLElement>) => {
+    const target = evt.currentTarget;
+    const ratio = target.scrollTop / Math.max(target.scrollHeight - target.clientHeight, 1);
+    const now = performance.now();
+    if (now - lastTick.current < FRAME_MS) return;
+    lastTick.current = now;
+    opts.onEditorScroll?.(ratio);
+  }, [opts]);
+
+  const handlePreviewScroll = useCallback((evt: React.UIEvent<HTMLElement>) => {
+    const target = evt.currentTarget;
+    const ratio = target.scrollTop / Math.max(target.scrollHeight - target.clientHeight, 1);
+    opts.onPreviewScroll?.(ratio);
+  }, [opts]);
+
+  return { handleEditorScroll, handlePreviewScroll };
+}

src/renderer/lib/markdown.ts

@@ -0,0 +1,29 @@
+import { marked } from 'marked';
+import DOMPurify from 'dompurify';
+
+marked.setOptions({
+  gfm: true,
+  breaks: false,
+});
+
+const MERMAID_RE = /```mermaid\n([\s\S]*?)```/g;
+
+export function renderMarkdown(source: string): string {
+  // Mark mermaid blocks with a placeholder we can replace client-side.
+  const placeholders: string[] = [];
+  const withPlaceholders = source.replace(MERMAID_RE, (_m, code) => {
+    const idx = placeholders.length;
+    placeholders.push(code.trim());
+    return `<div class="mermaid-block" data-mermaid-source="${idx}"></div>`;
+  });
+
+  const rawHtml = marked.parse(withPlaceholders, { async: false }) as string;
+  const clean = DOMPurify.sanitize(rawHtml, {
+    ALLOWED_ATTR: ['href', 'src', 'alt', 'title', 'class', 'data-mermaid-source', 'data-language', 'id'],
+  });
+
+  // The sanitized HTML still has placeholders; we leave the actual mermaid
+  // rendering to the React layer (MermaidLazy component) so the heavy
+  // mermaid library only loads when needed.
+  return clean;
+}

src/renderer/stores/preview-store.ts

@@ -0,0 +1,25 @@
+import { create } from 'zustand';
+
+interface PreviewState {
+  source: string;
+  scrollRatio: number;
+  setSource: (s: string) => void;
+  setScrollRatio: (r: number) => void;
+}
+
+const DEBOUNCE_MS = 300;
+let timer: ReturnType<typeof setTimeout> | null = null;
+let pending: string = '';
+
+export const usePreviewStore = create<PreviewState>((set) => ({
+  source: '',
+  scrollRatio: 0,
+  setSource: (s) => {
+    pending = s;
+    if (timer) clearTimeout(timer);
+    timer = setTimeout(() => {
+      set({ source: pending });
+    }, DEBOUNCE_MS);
+  },
+  setScrollRatio: (r) => set({ scrollRatio: r }),
+}));

tests/component/preview/MermaidLazy.test.tsx

@@ -0,0 +1,28 @@
+import { describe, it, expect, vi } from 'vitest';
+import { render, screen, waitFor } from '@testing-library/react';
+import { MermaidLazy } from '@/components/preview/MermaidLazy';
+
+vi.mock('mermaid', () => ({
+  default: {
+    initialize: vi.fn(),
+    render: vi.fn().mockResolvedValue('<svg></svg>'),
+  },
+}));
+
+describe('MermaidLazy', () => {
+  it('renders an svg after mermaid resolves', async () => {
+    render(<MermaidLazy code="graph TD; A-->B" />);
+    await waitFor(() => {
+      expect(screen.getByTestId('mermaid-output').innerHTML).toContain('<svg');
+    });
+  });
+
+  it('shows an error message if mermaid throws', async () => {
+    const mermaid = (await import('mermaid')).default as any;
+    mermaid.render.mockRejectedValueOnce(new Error('mermaid failed'));
+    render(<MermaidLazy code="bad code" />);
+    await waitFor(() => {
+      expect(screen.getByText(/mermaid failed/i)).toBeInTheDocument();
+    });
+  });
+});

tests/component/preview/PreviewPane.test.tsx

@@ -0,0 +1,30 @@
+import { describe, it, expect, beforeEach } from 'vitest';
+import { render, screen } from '@testing-library/react';
+import { ThemeProvider } from '@/components/theme-provider';
+import { PreviewPane } from '@/components/preview/PreviewPane';
+import { usePreviewStore } from '@/stores/preview-store';
+
+describe('PreviewPane', () => {
+  beforeEach(() => {
+    usePreviewStore.setState({ scrollRatio: 0, source: '' });
+  });
+
+  it('renders empty state when no source', () => {
+    render(
+      <ThemeProvider defaultTheme="dark" attribute="class">
+        <PreviewPane />
+      </ThemeProvider>
+    );
+    expect(screen.getByText(/nothing to preview/i)).toBeInTheDocument();
+  });
+
+  it('renders markdown when source is set', () => {
+    usePreviewStore.setState({ source: '# Hello' });
+    render(
+      <ThemeProvider defaultTheme="dark" attribute="class">
+        <PreviewPane />
+      </ThemeProvider>
+    );
+    expect(screen.getByRole('heading', { level: 1, name: /hello/i })).toBeInTheDocument();
+  });
+});

tests/unit/hooks/use-scroll-sync.test.ts

@@ -0,0 +1,26 @@
+import { describe, it, expect, beforeEach, vi } from 'vitest';
+import { renderHook, act } from '@testing-library/react';
+import { useScrollSync } from '@/hooks/use-scroll-sync';
+
+describe('useScrollSync', () => {
+  beforeEach(() => {
+    vi.useFakeTimers();
+    // Mock performance.now() to return 0 and not auto-advance
+    let time = 0;
+    vi.spyOn(performance, 'now').mockImplementation(() => time);
+  });
+
+  it('throttles editor scroll events to 60fps', () => {
+    const onScroll = vi.fn();
+    const { result } = renderHook(() => useScrollSync({ onEditorScroll: onScroll }));
+    const mockEvt = { currentTarget: { scrollTop: 100, scrollHeight: 1000, clientHeight: 200 } } as any;
+    // All calls within the same act() - performance.now() stays at 0
+    // First call passes, subsequent calls within FRAME_MS are throttled
+    act(() => {
+      result.current.handleEditorScroll(mockEvt);
+      result.current.handleEditorScroll(mockEvt);
+      result.current.handleEditorScroll(mockEvt);
+    });
+    expect(onScroll).toHaveBeenCalledTimes(1);
+  });
+});

tests/unit/lib/markdown.test.ts

@@ -0,0 +1,30 @@
+import { describe, it, expect } from 'vitest';
+import { renderMarkdown } from '@/lib/markdown';
+
+describe('renderMarkdown', () => {
+  it('renders headings', () => {
+    const html = renderMarkdown('# Hello');
+    expect(html).toContain('<h1>Hello</h1>');
+  });
+
+  it('renders bold and italic', () => {
+    const html = renderMarkdown('**bold** and *italic*');
+    expect(html).toContain('<strong>bold</strong>');
+    expect(html).toContain('<em>italic</em>');
+  });
+
+  it('renders code blocks with language class', () => {
+    const html = renderMarkdown('```js\nconst x = 1;\n```');
+    expect(html).toContain('language-js');
+  });
+
+  it('renders mermaid blocks with a placeholder', () => {
+    const html = renderMarkdown('```mermaid\ngraph TD\nA-->B\n```');
+    expect(html).toContain('data-mermaid-source=');
+  });
+
+  it('sanitizes dangerous HTML', () => {
+    const html = renderMarkdown('<script>alert(1)</script>');
+    expect(html).not.toContain('<script>');
+  });
+});

tests/unit/stores/preview-store.test.ts

@@ -0,0 +1,23 @@
+import { describe, it, expect, beforeEach, vi } from 'vitest';
+import { usePreviewStore } from '@/stores/preview-store';
+
+describe('usePreviewStore', () => {
+  beforeEach(() => {
+    vi.useFakeTimers();
+    usePreviewStore.setState({ scrollRatio: 0, source: '' });
+  });
+
+  it('debounces source updates (300 ms)', () => {
+    usePreviewStore.getState().setSource('# a');
+    usePreviewStore.getState().setSource('# b');
+    usePreviewStore.getState().setSource('# c');
+    expect(usePreviewStore.getState().source).toBe('');
+    vi.advanceTimersByTime(300);
+    expect(usePreviewStore.getState().source).toBe('# c');
+  });
+
+  it('updates scroll ratio', () => {
+    usePreviewStore.getState().setScrollRatio(0.5);
+    expect(usePreviewStore.getState().scrollRatio).toBe(0.5);
+  });
+});