diff --git a/src/sidebar/flowchart-panel.js b/src/sidebar/flowchart-panel.js
new file mode 100644
index 0000000..c5ebd9b
--- /dev/null
+++ b/src/sidebar/flowchart-panel.js
@@ -0,0 +1,189 @@
+/**
+ * Sidebar panel: visual flow chart editor.
+ *
+ * Owns:
+ *   - flowchart-store (graph + undo/redo + subscribe)
+ *   - flowchart-canvas (SVG)
+ *   - flowchart-mermaid preview (text + injected renderer)
+ *   - debounced persistence to <userData>/flowchart-session.json
+ *   - panel-scoped keyboard shortcuts (Ctrl+Z / Ctrl+Shift+Z / Delete)
+ *   - "Insert at Cursor" button (reuses the existing `insert-content` IPC)
+ *
+ * @param {HTMLElement} container Mount point inside the sidebar panel
+ * @param {object} deps
+ * @param {() => string} deps.getUserDataPath   Absolute userData directory
+ * @param {(path:string) => Promise<string|null>} deps.readFile
+ * @param {(path:string, content:string) => Promise<void>} deps.writeFile
+ * @param {(text:string) => void} deps.insertAtCursor
+ * @param {(source:string, target:HTMLElement) => void} [deps.renderMermaid]
+ */
+
+'use strict';
+
+const { create: createStore } = require('../flowchart/flowchart-store');
+const { createCanvas } = require('../flowchart/flowchart-canvas');
+const { toMermaid } = require('../flowchart/flowchart-mermaid');
+
+const PREVIEW_DEBOUNCE_MS = 250;
+const PERSIST_DEBOUNCE_MS = 500;
+const PERSISTENCE_FILENAME = 'flowchart-session.json';
+
+function persistencePath(getUserDataPath) {
+  return `${getUserDataPath()}/${PERSISTENCE_FILENAME}`;
+}
+
+function debounce(fn, ms) {
+  let handle = null;
+  return (...args) => {
+    if (handle) clearTimeout(handle);
+    handle = setTimeout(() => {
+      handle = null;
+      fn(...args);
+    }, ms);
+  };
+}
+
+function renderFlowChartPanel(container, deps) {
+  const { getUserDataPath, readFile, writeFile, insertAtCursor, renderMermaid = () => {} } = deps;
+  if (typeof getUserDataPath !== 'function') {
+    throw new Error('flowchart-panel: getUserDataPath is required');
+  }
+  if (typeof readFile !== 'function' || typeof writeFile !== 'function') {
+    throw new Error('flowchart-panel: readFile and writeFile are required');
+  }
+  if (typeof insertAtCursor !== 'function') {
+    throw new Error('flowchart-panel: insertAtCursor is required');
+  }
+
+  container.innerHTML = `
+    <div class="flowchart-panel" tabindex="0">
+      <div class="flowchart-toolbar">
+        <button class="flowchart-insert-btn" title="Insert Mermaid block at cursor">
+          Insert at Cursor
+        </button>
+        <span class="flowchart-status" aria-live="polite"></span>
+      </div>
+      <div class="flowchart-split">
+        <div class="flowchart-canvas-host"></div>
+        <div class="flowchart-preview-host">
+          <pre class="flowchart-preview-source"></pre>
+          <div class="flowchart-preview-render"></div>
+        </div>
+      </div>
+    </div>
+  `;
+
+  const canvasHost = container.querySelector('.flowchart-canvas-host');
+  const previewSourceEl = container.querySelector('.flowchart-preview-source');
+  const previewRenderEl = container.querySelector('.flowchart-preview-render');
+  const insertBtn = container.querySelector('.flowchart-insert-btn');
+  const statusEl = container.querySelector('.flowchart-status');
+
+  let selectedNodeId = null;
+  let selectedEdgeId = null;
+
+  const store = createStore({
+    persistencePath: persistencePath(getUserDataPath),
+    readFile,
+    writeFile,
+    now: () => Date.now(),
+  });
+
+  // Hydrate from disk (defensively).
+  readFile(persistencePath(getUserDataPath))
+    .then((json) => {
+      if (json) store.deserialize(json);
+    })
+    .catch((err) => {
+      console.warn('flowchart-panel: failed to read session', err);
+    });
+
+  const canvas = createCanvas(canvasHost, store, {
+    onEdgeClick: (edgeId) => {
+      // Click an edge → prompt for kind and (optional) label. v2 can replace
+      // this with a real popover menu; the prompts are intentionally simple.
+      selectedEdgeId = edgeId;
+      selectedNodeId = null;
+      const edge = store.getGraph().edges.find((e) => e.id === edgeId);
+      if (!edge) return;
+      const nextKind = window.prompt('Edge kind (solid, dotted, thick):', edge.kind);
+      if (nextKind && ['solid', 'dotted', 'thick'].includes(nextKind)) {
+        store.setEdgeKind(edgeId, nextKind);
+      }
+      const nextLabel = window.prompt('Edge label (empty to clear):', edge.label || '');
+      if (nextLabel !== null) {
+        store.setEdgeLabel(edgeId, nextLabel);
+      }
+    },
+    onShapeMenu: (nodeId) => {
+      // Prompt for a new shape kind. v2: replace with a real context menu.
+      const next = window.prompt(
+        'New shape (process, decision, terminator, subroutine, document):'
+      );
+      if (next) store.setNodeKind(nodeId, next);
+    },
+  });
+
+  const debouncedPreview = debounce(() => {
+    const source = toMermaid(store.getGraph());
+    previewSourceEl.textContent = source;
+    try {
+      renderMermaid(source, previewRenderEl);
+    } catch (err) {
+      previewRenderEl.textContent = `Preview error: ${err && err.message ? err.message : 'unknown'}`;
+    }
+  }, PREVIEW_DEBOUNCE_MS);
+
+  const debouncedPersist = debounce(() => {
+    writeFile(persistencePath(getUserDataPath), store.serialize()).catch((err) => {
+      if (statusEl) statusEl.textContent = `Save failed: ${err.message || err}`;
+    });
+  }, PERSIST_DEBOUNCE_MS);
+
+  store.subscribe(() => {
+    debouncedPreview();
+    debouncedPersist();
+  });
+
+  insertBtn.addEventListener('click', () => {
+    const source = toMermaid(store.getGraph());
+    insertAtCursor('```mermaid\n' + source + '\n```');
+  });
+
+  // Keyboard shortcuts — panel-scoped.
+  container.addEventListener('keydown', (ev) => {
+    if (ev.ctrlKey && !ev.metaKey && ev.key.toLowerCase() === 'z') {
+      ev.preventDefault();
+      if (ev.shiftKey) store.redo();
+      else store.undo();
+      return;
+    }
+    if (ev.key === 'Delete' || ev.key === 'Backspace') {
+      if (selectedNodeId) {
+        ev.preventDefault();
+        store.removeNode(selectedNodeId);
+        selectedNodeId = null;
+      } else if (selectedEdgeId) {
+        ev.preventDefault();
+        store.disconnect(selectedEdgeId);
+        selectedEdgeId = null;
+      }
+    }
+  });
+
+  return {
+    getStore: () => store,
+    getSvg: () => canvas.getSvg(),
+    selectNode: (id) => {
+      selectedNodeId = id;
+    },
+    selectEdge: (id) => {
+      selectedEdgeId = id;
+    },
+    destroy: () => {
+      canvas.destroy();
+    },
+  };
+}
+
+module.exports = { renderFlowChartPanel };
diff --git a/tests/flowchart-panel.test.js b/tests/flowchart-panel.test.js
new file mode 100644
index 0000000..f0885f1
--- /dev/null
+++ b/tests/flowchart-panel.test.js
@@ -0,0 +1,142 @@
+/**
+ * @jest-environment jsdom
+ */
+jest.useFakeTimers();
+
+const { renderFlowChartPanel } = require('../src/sidebar/flowchart-panel');
+
+function mount(deps = {}) {
+  const container = document.createElement('div');
+  document.body.appendChild(container);
+  const api = renderFlowChartPanel(container, {
+    getUserDataPath: deps.getUserDataPath || (() => '/tmp/userdata'),
+    readFile: deps.readFile || jest.fn().mockResolvedValue(null),
+    writeFile: deps.writeFile || jest.fn().mockResolvedValue(undefined),
+    insertAtCursor: deps.insertAtCursor || jest.fn(),
+    renderMermaid: deps.renderMermaid || jest.fn(),
+    ...deps,
+  });
+  return { container, api };
+}
+
+describe('flowchart-panel: mounting', () => {
+  test('mounts canvas + preview panes', () => {
+    const { container } = mount();
+    expect(container.querySelector('.flowchart-panel')).not.toBeNull();
+    expect(container.querySelector('.flowchart-canvas-host')).not.toBeNull();
+    expect(container.querySelector('.flowchart-preview-host')).not.toBeNull();
+    expect(container.querySelector('svg.flowchart-canvas')).not.toBeNull();
+    expect(container.querySelector('.flowchart-insert-btn')).not.toBeNull();
+  });
+
+  test('exposes an <svg> on the returned api', () => {
+    const { api } = mount();
+    expect(api.getSvg().tagName.toLowerCase()).toBe('svg');
+  });
+});
+
+describe('flowchart-panel: persistence', () => {
+  test('reads from <userData>/flowchart-session.json on mount', async () => {
+    const readFile = jest.fn().mockResolvedValue(
+      JSON.stringify({
+        nodes: [{ id: 'n1', kind: 'process', x: 10, y: 10, label: 'Loaded' }],
+        edges: [],
+      })
+    );
+    mount({ readFile });
+    await Promise.resolve();
+    expect(readFile).toHaveBeenCalledWith('/tmp/userdata/flowchart-session.json');
+  });
+
+  test('writes debounced snapshot after a mutation (500ms)', async () => {
+    const writeFile = jest.fn().mockResolvedValue(undefined);
+    const { api } = mount({ writeFile });
+    const store = api.getStore();
+    store.addNode({ kind: 'process', x: 0, y: 0, label: 'New' });
+    expect(writeFile).not.toHaveBeenCalled();
+    jest.advanceTimersByTime(500);
+    // Allow the awaited writeFile microtask to resolve.
+    await Promise.resolve();
+    expect(writeFile).toHaveBeenCalledTimes(1);
+    const [path, content] = writeFile.mock.calls[0];
+    expect(path).toBe('/tmp/userdata/flowchart-session.json');
+    expect(JSON.parse(content).nodes).toHaveLength(1);
+  });
+
+  test('corrupt JSON does not crash mount', async () => {
+    const readFile = jest.fn().mockResolvedValue('{not-json');
+    expect(() => mount({ readFile })).not.toThrow();
+  });
+});
+
+describe('flowchart-panel: live preview', () => {
+  test('re-renders preview within 250ms of a store change', async () => {
+    const renderMermaid = jest.fn();
+    const { api } = mount({ renderMermaid });
+    api.getStore().addNode({ kind: 'process', x: 0, y: 0, label: 'Preview me' });
+    expect(renderMermaid).not.toHaveBeenCalled();
+    jest.advanceTimersByTime(250);
+    await Promise.resolve();
+    expect(renderMermaid).toHaveBeenCalledTimes(1);
+    const source = renderMermaid.mock.calls[0][0];
+    expect(source).toMatch(/^flowchart TD/);
+    expect(source).toMatch(/Preview me/);
+  });
+});
+
+describe('flowchart-panel: insert at cursor', () => {
+  test('clicking Insert wraps Mermaid source in a fenced code block', async () => {
+    const insertAtCursor = jest.fn();
+    const renderMermaid = jest.fn();
+    const { container, api } = mount({ insertAtCursor, renderMermaid });
+    api.getStore().addNode({ kind: 'process', x: 0, y: 0, label: 'Hi' });
+    jest.advanceTimersByTime(250);
+    await Promise.resolve();
+    container.querySelector('.flowchart-insert-btn').click();
+    expect(insertAtCursor).toHaveBeenCalledTimes(1);
+    const text = insertAtCursor.mock.calls[0][0];
+    expect(text.startsWith('```mermaid\n')).toBe(true);
+    expect(text.endsWith('\n```')).toBe(true);
+    expect(text).toMatch(/Hi/);
+  });
+});
+
+describe('flowchart-panel: keyboard shortcuts', () => {
+  test('Ctrl+Z triggers undo', () => {
+    const { api, container } = mount();
+    const store = api.getStore();
+    store.addNode({ kind: 'process', x: 0, y: 0, label: 'A' });
+    expect(store.canUndo()).toBe(true);
+    container.dispatchEvent(
+      new KeyboardEvent('keydown', { key: 'z', ctrlKey: true, bubbles: true })
+    );
+    expect(store.getGraph().nodes).toHaveLength(0);
+  });
+
+  test('Ctrl+Shift+Z triggers redo', () => {
+    const { api, container } = mount();
+    const store = api.getStore();
+    store.addNode({ kind: 'process', x: 0, y: 0, label: 'A' });
+    store.undo();
+    expect(store.canRedo()).toBe(true);
+    container.dispatchEvent(
+      new KeyboardEvent('keydown', {
+        key: 'z',
+        ctrlKey: true,
+        shiftKey: true,
+        bubbles: true,
+      })
+    );
+    expect(store.getGraph().nodes).toHaveLength(1);
+  });
+
+  test('Delete removes the selected node', () => {
+    const { api, container } = mount();
+    const store = api.getStore();
+    const n = store.addNode({ kind: 'process', x: 0, y: 0, label: 'A' });
+    // Select the node programmatically (canvas does this on pointerdown).
+    api.selectNode(n.id);
+    container.dispatchEvent(new KeyboardEvent('keydown', { key: 'Delete', bubbles: true }));
+    expect(store.getGraph().nodes).toHaveLength(0);
+  });
+});