fix: lock sidebar width and add rendered markdown preview

Sidebar overflowed horizontally on long file/folder names, which broke
vertical scrolling too — locked width with min/max-width plus
overflow-x: hidden, and truncate folder names with ellipsis.

Documents now open in a rendered preview (marked + DOMPurify) by
default, with an 编辑/预览 toggle to switch to the CodeMirror source view.

frontend/package-lock.json

@@ -11,6 +11,8 @@
         "@codemirror/lang-html": "^6.4.10",
         "@codemirror/lang-markdown": "^6.4.0",
         "@uiw/react-codemirror": "^4.25.11",
+        "dompurify": "^3.4.12",
+        "marked": "^18.0.7",
         "react": "18.3.1",
         "react-dom": "18.3.1"
       },
@@ -1672,6 +1674,13 @@
         "@types/react": "*"
       }
     },
+    "node_modules/@types/trusted-types": {
+      "version": "2.0.7",
+      "resolved": "https://registry.npmjs.org/@types/trusted-types/-/trusted-types-2.0.7.tgz",
+      "integrity": "sha512-ScaPdn1dQczgbl0QFTeTOmVHFULt394XJgOQNoyVhZ6r2vLnMLJfBPd53SB52T/3G36VI1/g2MZaX0cwDuXsfw==",
+      "license": "MIT",
+      "optional": true
+    },
     "node_modules/@uiw/codemirror-extensions-basic-setup": {
       "version": "4.25.11",
       "resolved": "https://registry.npmjs.org/@uiw/codemirror-extensions-basic-setup/-/codemirror-extensions-basic-setup-4.25.11.tgz",
@@ -2243,6 +2252,15 @@
       "license": "MIT",
       "peer": true
     },
+    "node_modules/dompurify": {
+      "version": "3.4.12",
+      "resolved": "https://registry.npmjs.org/dompurify/-/dompurify-3.4.12.tgz",
+      "integrity": "sha512-zQvGet8Z2sWbQhCmfFz/T5QWH2oBmjnqK3qvOjaqaNLrLEF912WamU+ohnTp0TCep/MFVHpdJuCZEdFOdTnEFg==",
+      "license": "(MPL-2.0 OR Apache-2.0)",
+      "optionalDependencies": {
+        "@types/trusted-types": "^2.0.7"
+      }
+    },
     "node_modules/dunder-proto": {
       "version": "1.0.1",
       "resolved": "https://registry.npmjs.org/dunder-proto/-/dunder-proto-1.0.1.tgz",
@@ -2810,6 +2828,18 @@
         "@jridgewell/sourcemap-codec": "^1.5.5"
       }
     },
+    "node_modules/marked": {
+      "version": "18.0.7",
+      "resolved": "https://registry.npmjs.org/marked/-/marked-18.0.7.tgz",
+      "integrity": "sha512-iDVQ5ldaiKXn6b2JroX5kgRfmwgqolW7NpaEzTl1k/2Zh1njIEN9yniyLV/mOvWwtsE8OGgkjsCYvijuPk1dtA==",
+      "license": "MIT",
+      "bin": {
+        "marked": "bin/marked.js"
+      },
+      "engines": {
+        "node": ">= 20"
+      }
+    },
     "node_modules/math-intrinsics": {
       "version": "1.1.0",
       "resolved": "https://registry.npmjs.org/math-intrinsics/-/math-intrinsics-1.1.0.tgz",

frontend/package.json

@@ -13,6 +13,8 @@
     "@codemirror/lang-html": "^6.4.10",
     "@codemirror/lang-markdown": "^6.4.0",
     "@uiw/react-codemirror": "^4.25.11",
+    "dompurify": "^3.4.12",
+    "marked": "^18.0.7",
     "react": "18.3.1",
     "react-dom": "18.3.1"
   },

frontend/src/components/DocumentEditor.tsx

@@ -3,6 +3,7 @@ import CodeMirror from '@uiw/react-codemirror';
 import { markdown } from '@codemirror/lang-markdown';
 import { html } from '@codemirror/lang-html';
 import { deleteDocument, getDocument, saveDocument } from '../api/client';
+import MarkdownPreview from './MarkdownPreview';
 
 interface DocumentEditorProps {
   documentKey: string;
@@ -21,16 +22,17 @@ export default function DocumentEditor({ documentKey, onDirtyChange, onDeleted }
   const [saveError, setSaveError] = useState<string | null>(null);
   const [deleteError, setDeleteError] = useState<string | null>(null);
 
+  const [isPreview, setIsPreview] = useState(true);
+
   const isImage = contentType.startsWith('image/');
-  const extensions = useMemo(
+  const isMarkdown = contentType === 'text/markdown';
-    () => [contentType === 'text/markdown' ? markdown() : html()],
+  const extensions = useMemo(() => [isMarkdown ? markdown() : html()], [isMarkdown]);
-    [contentType],
-  );
 
   useEffect(() => {
     setLoading(true);
     setLoadError(null);
     setSaveError(null);
+    setIsPreview(true);
     getDocument(documentKey)
       .then((doc) => {
         setContent(doc.content);
@@ -124,9 +126,14 @@ export default function DocumentEditor({ documentKey, onDirtyChange, onDeleted }
             {deleting ? '删除中…' : '删除'}
           </button>
           {!isImage && (
+            <>
+              <button className="btn-secondary" onClick={() => setIsPreview((prev) => !prev)}>
+                {isPreview ? '编辑' : '预览'}
+              </button>
               <button className="btn-save" onClick={handleSave} disabled={!isDirty || saving}>
                 {saving ? '保存中…' : '保存'}
               </button>
+            </>
           )}
         </div>
       </div>
@@ -134,6 +141,10 @@ export default function DocumentEditor({ documentKey, onDirtyChange, onDeleted }
         <div className="image-preview-area" data-testid="image-preview">
           <img src={`data:${contentType};base64,${content}`} alt={documentKey.split('/').pop()} />
         </div>
+      ) : isPreview ? (
+        <div className="preview-area">
+          <MarkdownPreview content={content} isMarkdown={isMarkdown} />
+        </div>
       ) : (
         <div className="editor-area">
           <CodeMirror

frontend/src/components/FileExplorer.tsx

@@ -96,7 +96,7 @@ function FolderNode({
             <path d="M8 5v14l11-7z" />
           </svg>
         </span>
-        {name}
+        <span className="foldername">{name}</span>
       </div>
       {expanded && (
         <div className="tree-children">

frontend/src/components/MarkdownPreview.tsx

@@ -0,0 +1,23 @@
+import { useMemo } from 'react';
+import { marked } from 'marked';
+import DOMPurify from 'dompurify';
+
+interface MarkdownPreviewProps {
+  content: string;
+  isMarkdown: boolean;
+}
+
+export default function MarkdownPreview({ content, isMarkdown }: MarkdownPreviewProps) {
+  const html = useMemo(() => {
+    const rawHtml = isMarkdown ? (marked.parse(content, { async: false }) as string) : content;
+    return DOMPurify.sanitize(rawHtml);
+  }, [content, isMarkdown]);
+
+  return (
+    <div
+      className="markdown-preview"
+      data-testid="markdown-preview"
+      dangerouslySetInnerHTML={{ __html: html }}
+    />
+  );
+}

frontend/src/styles.css

@@ -192,9 +192,12 @@ body {
 /* ---------- Sidebar ---------- */
 .sidebar {
   width: 272px;
+  min-width: 272px;
+  max-width: 272px;
   background: var(--paper-alt);
   border-right: 1px solid var(--rule);
   overflow-y: auto;
+  overflow-x: hidden;
   padding: 16px 10px;
   flex-shrink: 0;
 }
@@ -228,6 +231,13 @@ body {
   white-space: nowrap;
   border-radius: 5px;
   user-select: none;
+  min-width: 0;
+}
+
+.tree-item .foldername {
+  flex: 1;
+  overflow: hidden;
+  text-overflow: ellipsis;
 }
 
 .tree-item:hover {
@@ -454,6 +464,103 @@ body {
   border-radius: var(--radius);
 }
 
+.preview-area {
+  flex: 1;
+  overflow-y: auto;
+  background: var(--paper);
+  padding: 32px 40px;
+}
+
+.markdown-preview {
+  max-width: 720px;
+  margin: 0 auto;
+  color: var(--ink);
+  line-height: 1.7;
+  font-size: 14px;
+}
+
+.markdown-preview h1,
+.markdown-preview h2,
+.markdown-preview h3,
+.markdown-preview h4 {
+  font-family: 'Fraunces', serif;
+  font-weight: 600;
+  color: var(--ink);
+  margin: 1.4em 0 0.6em;
+}
+
+.markdown-preview h1 {
+  font-size: 26px;
+}
+
+.markdown-preview h2 {
+  font-size: 21px;
+}
+
+.markdown-preview h3 {
+  font-size: 17px;
+}
+
+.markdown-preview p {
+  margin: 0.8em 0;
+}
+
+.markdown-preview a {
+  color: var(--pine);
+}
+
+.markdown-preview code {
+  font-family: 'IBM Plex Mono', monospace;
+  font-size: 12.5px;
+  background: var(--paper-alt);
+  padding: 2px 5px;
+  border-radius: 4px;
+}
+
+.markdown-preview pre {
+  background: var(--paper-alt);
+  border: 1px solid var(--rule);
+  border-radius: var(--radius);
+  padding: 14px 16px;
+  overflow-x: auto;
+}
+
+.markdown-preview pre code {
+  background: none;
+  padding: 0;
+}
+
+.markdown-preview ul,
+.markdown-preview ol {
+  padding-left: 1.6em;
+  margin: 0.8em 0;
+}
+
+.markdown-preview blockquote {
+  border-left: 3px solid var(--rule);
+  color: var(--ash);
+  margin: 0.8em 0;
+  padding: 2px 0 2px 16px;
+}
+
+.markdown-preview img {
+  max-width: 100%;
+  border-radius: var(--radius);
+}
+
+.markdown-preview table {
+  border-collapse: collapse;
+  width: 100%;
+  margin: 0.8em 0;
+}
+
+.markdown-preview th,
+.markdown-preview td {
+  border: 1px solid var(--rule);
+  padding: 6px 10px;
+  text-align: left;
+}
+
 /* ---------- Search overlay ---------- */
 .search-overlay {
   position: absolute;

frontend/tests/App.test.tsx

@@ -74,6 +74,8 @@ test('selecting a file from the tree loads it into the editor', async () => {
   const fileRow = await screen.findByText('架构设计.md');
   await user.click(fileRow);
 
+  await screen.findByTestId('markdown-preview');
+  await user.click(screen.getByRole('button', { name: '编辑' }));
   const textarea = await screen.findByTestId('codemirror-mock');
   expect(textarea).toHaveValue('# 架构设计');
 });
@@ -108,6 +110,8 @@ test('searching shows the results overlay, and clicking a result opens the docum
   await waitFor(() => {
     expect(screen.queryByTestId('search-results')).not.toBeInTheDocument();
   });
+  await screen.findByTestId('markdown-preview');
+  await user.click(screen.getByRole('button', { name: '编辑' }));
   const textarea = await screen.findByTestId('codemirror-mock');
   expect(textarea).toHaveValue('# 架构设计');
 });
@@ -180,6 +184,8 @@ test('switching files while the current document is dirty prompts for confirmati
   render(<App />);
 
   await user.click(await screen.findByText('文档A.md'));
+  await screen.findByTestId('markdown-preview');
+  await user.click(screen.getByRole('button', { name: '编辑' }));
   const textarea = await screen.findByTestId('codemirror-mock');
   await user.type(textarea, '修改内容');
 
@@ -206,13 +212,15 @@ test('confirming the discard prompt proceeds to switch files', async () => {
   render(<App />);
 
   await user.click(await screen.findByText('文档A.md'));
+  await screen.findByTestId('markdown-preview');
+  await user.click(screen.getByRole('button', { name: '编辑' }));
   const textarea = await screen.findByTestId('codemirror-mock');
   await user.type(textarea, '修改内容');
 
   await user.click(screen.getByText('文档B.md'));
 
   await waitFor(async () => {
-    expect(await screen.findByTestId('codemirror-mock')).toHaveValue('内容-文档B.md');
+    expect(screen.getByTestId('markdown-preview')).toHaveTextContent('内容-文档B.md');
   });
 });
 

frontend/tests/components/DocumentEditor.test.tsx

@@ -25,6 +25,45 @@ beforeEach(() => {
   vi.mocked(deleteDocument).mockReset();
 });
 
+async function switchToEditMode(user: ReturnType<typeof userEvent.setup>) {
+  await screen.findByTestId('markdown-preview');
+  await user.click(screen.getByRole('button', { name: '编辑' }));
+  return screen.findByTestId('codemirror-mock');
+}
+
+test('shows a rendered markdown preview by default', async () => {
+  vi.mocked(getDocument).mockResolvedValue({
+    key: '产品文档/架构设计.md',
+    content: '# 架构设计',
+    contentType: 'text/markdown',
+    encoding: 'utf-8',
+  });
+
+  render(<DocumentEditor documentKey="产品文档/架构设计.md" onDeleted={vi.fn()} />);
+
+  const preview = await screen.findByTestId('markdown-preview');
+  expect(preview.querySelector('h1')).toHaveTextContent('架构设计');
+  expect(screen.queryByTestId('codemirror-mock')).not.toBeInTheDocument();
+});
+
+test('clicking 编辑 switches to the code editor and back to 预览 restores the rendered view', async () => {
+  vi.mocked(getDocument).mockResolvedValue({
+    key: '产品文档/架构设计.md',
+    content: '# 架构设计',
+    contentType: 'text/markdown',
+    encoding: 'utf-8',
+  });
+  const user = userEvent.setup();
+
+  render(<DocumentEditor documentKey="产品文档/架构设计.md" onDeleted={vi.fn()} />);
+
+  const textarea = await switchToEditMode(user);
+  expect(textarea).toHaveValue('# 架构设计');
+
+  await user.click(screen.getByRole('button', { name: '预览' }));
+  expect(await screen.findByTestId('markdown-preview')).toBeInTheDocument();
+});
+
 test('shows an image preview instead of the code editor for image content types', async () => {
   vi.mocked(getDocument).mockResolvedValue({
     key: '产品文档/图片.png',
@@ -40,6 +79,7 @@ test('shows an image preview instead of the code editor for image content types'
   expect(img).toHaveAttribute('src', 'data:image/png;base64,iVBORw0KGgo=');
   expect(screen.queryByTestId('codemirror-mock')).not.toBeInTheDocument();
   expect(screen.queryByRole('button', { name: '保存' })).not.toBeInTheDocument();
+  expect(screen.queryByRole('button', { name: '编辑' })).not.toBeInTheDocument();
 });
 
 test('shows a loading state, then loads and displays document content', async () => {
@@ -54,8 +94,7 @@ test('shows a loading state, then loads and displays document content', async ()
 
   expect(screen.getByTestId('document-editor-loading')).toBeInTheDocument();
 
-  const textarea = await screen.findByTestId('codemirror-mock');
+  await screen.findByTestId('markdown-preview');
-  expect(textarea).toHaveValue('# 架构设计');
   expect(getDocument).toHaveBeenCalledWith('产品文档/架构设计.md');
   expect(screen.getByTestId('save-status')).toHaveTextContent('已归档');
 });
@@ -75,7 +114,7 @@ test('typing marks the document dirty and saving clears the dirty state', async
 
   render(<DocumentEditor documentKey="产品文档/架构设计.md" onDeleted={vi.fn()} />);
 
-  const textarea = await screen.findByTestId('codemirror-mock');
+  const textarea = await switchToEditMode(user);
   await user.clear(textarea);
   await user.type(textarea, '# 新内容');
 
@@ -108,7 +147,7 @@ test('save failure keeps the editor and unsaved content visible, shows an error,
 
   render(<DocumentEditor documentKey="产品文档/架构设计.md" onDeleted={vi.fn()} />);
 
-  const textarea = await screen.findByTestId('codemirror-mock');
+  const textarea = await switchToEditMode(user);
   await user.clear(textarea);
   await user.type(textarea, '# 未保存的修改');
 
@@ -142,7 +181,7 @@ test('calls onDirtyChange when dirty state changes', async () => {
 
   render(<DocumentEditor documentKey="产品文档/架构设计.md" onDirtyChange={onDirtyChange} onDeleted={vi.fn()} />);
 
-  const textarea = await screen.findByTestId('codemirror-mock');
+  const textarea = await switchToEditMode(user);
 
   await waitFor(() => {
     expect(onDirtyChange).toHaveBeenLastCalledWith(false);
@@ -169,7 +208,7 @@ test('clicking delete confirms, calls deleteDocument, and calls onDeleted on suc
 
   render(<DocumentEditor documentKey="产品文档/架构设计.md" onDeleted={onDeleted} />);
 
-  await screen.findByTestId('codemirror-mock');
+  await screen.findByTestId('markdown-preview');
   const deleteButton = screen.getByRole('button', { name: '删除' });
   await user.click(deleteButton);
 
@@ -195,7 +234,7 @@ test('clicking delete does nothing if the confirmation is declined', async () =>
 
   render(<DocumentEditor documentKey="产品文档/架构设计.md" onDeleted={onDeleted} />);
 
-  await screen.findByTestId('codemirror-mock');
+  await screen.findByTestId('markdown-preview');
   const deleteButton = screen.getByRole('button', { name: '删除' });
   await user.click(deleteButton);
 
@@ -217,7 +256,7 @@ test('shows an inline error and does not call onDeleted when delete fails', asyn
 
   render(<DocumentEditor documentKey="产品文档/架构设计.md" onDeleted={onDeleted} />);
 
-  await screen.findByTestId('codemirror-mock');
+  await screen.findByTestId('markdown-preview');
   const deleteButton = screen.getByRole('button', { name: '删除' });
   await user.click(deleteButton);