diff --git a/src/ui/editor.test.ts b/src/ui/editor.test.ts
new file mode 100644
index 0000000..0f5bcc2
--- /dev/null
+++ b/src/ui/editor.test.ts
@@ -0,0 +1,159 @@
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
+import { SumEditor } from './editor';
+
+vi.mock('@codemirror/view', () => {
+  class MockEditorView {
+    static updateListener = { of: vi.fn() };
+    static theme = vi.fn();
+    static lineWrapping = {};
+    static decorations = { from: vi.fn() };
+
+    state = {
+      doc: {
+        length: 10,
+        toString: vi.fn(() => "mock doc"),
+        lines: 2,
+        line: vi.fn((n) => ({ text: `line ${n}`, from: (n-1)*10, to: n*10 })),
+        lineAt: vi.fn((_pos) => ({ number: 1, from: 0, to: 10 })),
+        iterLines: vi.fn(function*() { yield "line 1"; yield "line 2"; }),
+      },
+      selection: { main: { empty: true, from: 0, to: 0, head: 0 } },
+    };
+    dom = { addEventListener: vi.fn() };
+    scrollDOM = { addEventListener: vi.fn() };
+    dispatch = vi.fn();
+    focus = vi.fn();
+    lineBlockAt = vi.fn(() => ({ top: 10 }));
+    documentTop = 0;
+
+    constructor(config: any) {
+      if (config.state?.doc) {
+        this.state.doc.toString = vi.fn(() => config.state.doc);
+      }
+    }
+  }
+
+  return {
+    EditorView: MockEditorView,
+    ViewPlugin: { fromClass: vi.fn() },
+    Decoration: { mark: vi.fn(), line: vi.fn(), none: {} },
+    keymap: { of: vi.fn() },
+    drawSelection: vi.fn(),
+  };
+});
+
+vi.mock('@codemirror/state', () => ({
+  EditorState: {
+    create: vi.fn((cfg) => cfg),
+  },
+  StateEffect: {
+    define: vi.fn(() => ({ of: vi.fn() })),
+  },
+  StateField: {
+    define: vi.fn(),
+  },
+  RangeSetBuilder: class {
+    add = vi.fn();
+    finish = vi.fn();
+  },
+}));
+
+vi.mock('@codemirror/commands', () => ({
+  defaultKeymap: [],
+  history: vi.fn(),
+  historyKeymap: [],
+}));
+
+vi.mock('@codemirror/autocomplete', () => ({
+  autocompletion: vi.fn(),
+  acceptCompletion: vi.fn(),
+}));
+
+describe('SumEditor', () => {
+  beforeEach(() => {
+    vi.stubGlobal('requestAnimationFrame', (cb: Function) => cb());
+    vi.stubGlobal('document', {
+      createDocumentFragment: vi.fn(() => ({
+        appendChild: vi.fn(),
+      })),
+      createElement: vi.fn((tag) => ({
+        tagName: tag,
+        className: '',
+        textContent: '',
+        dataset: {},
+        style: {},
+      })),
+    });
+  });
+  afterEach(() => {
+    vi.unstubAllGlobals();
+  });
+
+  const createMocks = () => {
+    const parent = { getBoundingClientRect: () => ({ top: 0 }) } as any;
+    const resultsEl = {
+      addEventListener: vi.fn(),
+      replaceChildren: vi.fn(),
+      getBoundingClientRect: () => ({ top: 0 }),
+      clientHeight: 500,
+    } as any;
+    const engine = { completions: vi.fn(() => []) } as any;
+    const cb = { onChange: vi.fn(), onCopy: vi.fn(), onResults: vi.fn(), onSelection: vi.fn() };
+    const evaluateDoc = vi.fn(() => []);
+    return { parent, resultsEl, engine, cb, evaluateDoc };
+  };
+
+  it('instantiates and triggers evaluation on init', () => {
+    const { parent, resultsEl, engine, cb, evaluateDoc } = createMocks();
+    const editor = new SumEditor(parent, resultsEl, engine, cb, evaluateDoc, 'init text');
+    expect(editor).toBeDefined();
+    expect(editor.getText()).toBe('init text');
+    expect(evaluateDoc).toHaveBeenCalledWith('init text');
+    expect(cb.onResults).toHaveBeenCalled();
+  });
+
+  it('setText dispatches changes to the view', () => {
+    const { parent, resultsEl, engine, cb, evaluateDoc } = createMocks();
+    const editor = new SumEditor(parent, resultsEl, engine, cb, evaluateDoc, '');
+    editor.setText('new text');
+    expect(editor.view.dispatch).toHaveBeenCalledWith({
+      changes: { from: 0, to: 10, insert: 'new text' }
+    });
+  });
+
+  it('refresh re-evaluates the current text and renders results', () => {
+    const { parent, resultsEl, engine, cb, evaluateDoc } = createMocks();
+    const editor = new SumEditor(parent, resultsEl, engine, cb, evaluateDoc, 'text');
+    editor.refresh();
+    expect(evaluateDoc).toHaveBeenCalledTimes(2); // init + refresh
+  });
+
+  it('focus focuses the view', () => {
+    const { parent, resultsEl, engine, cb, evaluateDoc } = createMocks();
+    const editor = new SumEditor(parent, resultsEl, engine, cb, evaluateDoc, '');
+    editor.focus();
+    expect(editor.view.focus).toHaveBeenCalled();
+  });
+
+  it('goToLine calculates correctly and dispatches selection', () => {
+    const { parent, resultsEl, engine, cb, evaluateDoc } = createMocks();
+    const editor = new SumEditor(parent, resultsEl, engine, cb, evaluateDoc, '');
+    editor.goToLine(2);
+    expect(editor.view.dispatch).toHaveBeenCalledWith({
+      selection: { anchor: 10, head: 20 },
+      scrollIntoView: true
+    });
+    expect(editor.view.focus).toHaveBeenCalled();
+  });
+
+  it('getSheetWithResults formats lines with their results', () => {
+    const { parent, resultsEl, engine, cb, evaluateDoc } = createMocks();
+    // evaluateDoc will return mock results
+    evaluateDoc.mockReturnValue([{ text: 'result 1', tokens: [] }, { text: 'result 2', tokens: [] }] as any);
+    const editor = new SumEditor(parent, resultsEl, engine, cb, evaluateDoc, '');
+
+    const sheet = editor.getSheetWithResults();
+    // doc mock returns line 1 and line 2
+    expect(sheet).toBe('line 1 = result 1\nline 2 = result 2');
+  });
+});