diff --git a/internal/web/static/js/openfsd/airport-editor/history.js b/internal/web/static/js/openfsd/airport-editor/history.js
new file mode 100644
index 0000000..609a4d5
--- /dev/null
+++ b/internal/web/static/js/openfsd/airport-editor/history.js
@@ -0,0 +1,273 @@
+/**
+ * Pure undo/redo history stack for the airport editor.
+ * Snapshot-based; gesture coalescing for drags. No DOM.
+ *
+ * Dirty flags are not stored in snapshots — recompute via
+ * syncDirtyAfterHistoryApply after apply (K5 / K13).
+ */
+
+import { formatAptForDownload, formatAirForDownload } from './download.js';
+import { hashText, syncDirtyFromHash } from './model.js';
+
+/** @typedef {import('./model.js').EditorDocument} EditorDocument */
+/** @typedef {import('./model.js').Airport} Airport */
+/** @typedef {import('./model.js').Aircraft} Aircraft */
+
+/**
+ * @typedef {Object} EditorSnapshot
+ * @property {Airport|null} airport
+ * @property {Aircraft[]} aircraft
+ * @property {{ type: string, index: number }|null} selection
+ * @property {number} _nextSurfaceId
+ */
+
+/**
+ * @typedef {Object} HistoryStack
+ * @property {EditorSnapshot[]} undoStack
+ * @property {EditorSnapshot[]} redoStack
+ * @property {EditorSnapshot|null} gestureBaseline
+ * @property {number} maxDepth
+ */
+
+export const DEFAULT_HISTORY_MAX_DEPTH = 100;
+
+/**
+ * Deep-clone JSON-safe values. Prefer structuredClone; fallback JSON.
+ * @template T
+ * @param {T} value
+ * @returns {T}
+ */
+function deepClone(value) {
+  if (value === null || value === undefined) return value;
+  if (typeof structuredClone === 'function') {
+    try {
+      return structuredClone(value);
+    } catch {
+      /* fall through to JSON */
+    }
+  }
+  return JSON.parse(JSON.stringify(value));
+}
+
+/**
+ * @param {{ maxDepth?: number }} [opts]
+ * @returns {HistoryStack}
+ */
+export function createHistory(opts = {}) {
+  const maxDepth =
+    typeof opts.maxDepth === 'number' && opts.maxDepth > 0
+      ? Math.floor(opts.maxDepth)
+      : DEFAULT_HISTORY_MAX_DEPTH;
+  return {
+    undoStack: [],
+    redoStack: [],
+    gestureBaseline: null,
+    maxDepth,
+  };
+}
+
+/**
+ * Deep-clone document content fields into a snapshot.
+ * Mutating live `doc` after capture must not affect the returned snap.
+ * @param {EditorDocument} doc
+ * @returns {EditorSnapshot}
+ */
+export function captureSnapshot(doc) {
+  return {
+    airport: deepClone(doc.airport ?? null),
+    aircraft: deepClone(Array.isArray(doc.aircraft) ? doc.aircraft : []),
+    selection: deepClone(doc.selection ?? null),
+    _nextSurfaceId:
+      typeof doc._nextSurfaceId === 'number' && Number.isFinite(doc._nextSurfaceId)
+        ? doc._nextSurfaceId
+        : 1,
+  };
+}
+
+/**
+ * Apply snapshot into live doc (mutates doc). Does NOT touch dirty hashes,
+ * filenames, mode, or serverValidation.
+ * Deep-clones snap into doc so stack and live doc never share identity (K9).
+ * @param {EditorDocument} doc
+ * @param {EditorSnapshot} snap
+ */
+export function applySnapshot(doc, snap) {
+  doc.airport = deepClone(snap.airport ?? null);
+  doc.aircraft = deepClone(Array.isArray(snap.aircraft) ? snap.aircraft : []);
+  doc.selection = deepClone(snap.selection ?? null);
+  doc._nextSurfaceId =
+    typeof snap._nextSurfaceId === 'number' && Number.isFinite(snap._nextSurfaceId)
+      ? snap._nextSurfaceId
+      : 1;
+}
+
+/**
+ * Push a pre-mutation snapshot onto undo; clear redo; trim maxDepth.
+ * Always pushes — no equality skip on this path (v1).
+ * @param {HistoryStack} h
+ * @param {EditorSnapshot} snap
+ */
+export function pushUndo(h, snap) {
+  h.undoStack.push(snap);
+  h.redoStack.length = 0;
+  while (h.undoStack.length > h.maxDepth) {
+    h.undoStack.shift();
+  }
+}
+
+/**
+ * Record current doc as undo entry (convenience).
+ * Must be called BEFORE mutating doc for a discrete edit.
+ * @param {HistoryStack} h
+ * @param {EditorDocument} doc
+ */
+export function recordBeforeMutation(h, doc) {
+  pushUndo(h, captureSnapshot(doc));
+}
+
+/**
+ * Start a multi-event gesture (vertex/aircraft drag).
+ * Captures baseline once; ignores nested begins.
+ * @param {HistoryStack} h
+ * @param {EditorDocument} doc
+ */
+export function beginGesture(h, doc) {
+  if (h.gestureBaseline) return;
+  h.gestureBaseline = captureSnapshot(doc);
+}
+
+/**
+ * End gesture: if live doc content differs from baseline, push baseline to undo.
+ * Always clears gestureBaseline. Returns whether an entry was pushed.
+ * @param {HistoryStack} h
+ * @param {EditorDocument} doc
+ * @returns {boolean}
+ */
+export function commitGesture(h, doc) {
+  const base = h.gestureBaseline;
+  h.gestureBaseline = null;
+  if (!base) return false;
+  if (contentEquals(base, captureSnapshot(doc))) return false;
+  pushUndo(h, base);
+  return true;
+}
+
+/**
+ * Abort gesture without stacking. Does not restore doc.
+ * @param {HistoryStack} h
+ */
+export function discardGesture(h) {
+  h.gestureBaseline = null;
+}
+
+/**
+ * Undo: push current doc onto redo, apply top of undo onto doc.
+ * @param {HistoryStack} h
+ * @param {EditorDocument} doc
+ * @returns {{ ok: true, snap: EditorSnapshot } | { ok: false, reason: string }}
+ */
+export function undo(h, doc) {
+  if (h.gestureBaseline) {
+    return { ok: false, reason: 'gesture-active' };
+  }
+  if (!h.undoStack.length) return { ok: false, reason: 'empty' };
+  const current = captureSnapshot(doc);
+  const prev = h.undoStack.pop();
+  h.redoStack.push(current);
+  applySnapshot(doc, prev);
+  return { ok: true, snap: prev };
+}
+
+/**
+ * Redo: inverse of undo.
+ * @param {HistoryStack} h
+ * @param {EditorDocument} doc
+ * @returns {{ ok: true, snap: EditorSnapshot } | { ok: false, reason: string }}
+ */
+export function redo(h, doc) {
+  if (h.gestureBaseline) {
+    return { ok: false, reason: 'gesture-active' };
+  }
+  if (!h.redoStack.length) return { ok: false, reason: 'empty' };
+  const current = captureSnapshot(doc);
+  const next = h.redoStack.pop();
+  h.undoStack.push(current);
+  // Trim undo if redo branch grew past max (symmetric with pushUndo trim).
+  while (h.undoStack.length > h.maxDepth) {
+    h.undoStack.shift();
+  }
+  applySnapshot(doc, next);
+  return { ok: true, snap: next };
+}
+
+/**
+ * @param {HistoryStack} h
+ * @returns {boolean}
+ */
+export function canUndo(h) {
+  return h.undoStack.length > 0 && !h.gestureBaseline;
+}
+
+/**
+ * @param {HistoryStack} h
+ * @returns {boolean}
+ */
+export function canRedo(h) {
+  return h.redoStack.length > 0 && !h.gestureBaseline;
+}
+
+/**
+ * Empty both stacks and clear active gesture.
+ * @param {HistoryStack} h
+ */
+export function clearHistory(h) {
+  h.undoStack.length = 0;
+  h.redoStack.length = 0;
+  h.gestureBaseline = null;
+}
+
+/**
+ * Stable content key for airport + aircraft only (ignore selection).
+ * @param {EditorSnapshot} snap
+ * @returns {string}
+ */
+export function snapshotContentKey(snap) {
+  return JSON.stringify({ airport: snap.airport, aircraft: snap.aircraft });
+}
+
+/**
+ * Structural content equality for gesture no-op detection.
+ * Compares airport + aircraft only (selection ignored).
+ * @param {EditorSnapshot} a
+ * @param {EditorSnapshot} b
+ * @returns {boolean}
+ */
+export function contentEquals(a, b) {
+  return snapshotContentKey(a) === snapshotContentKey(b);
+}
+
+/**
+ * Seed last*DownloadHash from download-format text. Does not set dirty true.
+ * Leave dirty flags false when seeding a clean baseline (caller responsibility).
+ * Never sets hashes to null.
+ * @param {EditorDocument} doc
+ * @param {'apt'|'air'|'both'} [side='both']
+ */
+export function seedCleanContentHashes(doc, side = 'both') {
+  if (side === 'apt' || side === 'both') {
+    doc.lastAptDownloadHash = hashText(formatAptForDownload(doc));
+  }
+  if (side === 'air' || side === 'both') {
+    doc.lastAirDownloadHash = hashText(formatAirForDownload(doc));
+  }
+}
+
+/**
+ * Recompute aptDirty/airDirty from live last*DownloadHash vs download-format text.
+ * Does not mutate hashes. Safe for null airport / empty aircraft.
+ * @param {EditorDocument} doc
+ */
+export function syncDirtyAfterHistoryApply(doc) {
+  syncDirtyFromHash(doc, 'apt', formatAptForDownload(doc));
+  syncDirtyFromHash(doc, 'air', formatAirForDownload(doc));
+}
diff --git a/webjs/airport-editor/history.test.js b/webjs/airport-editor/history.test.js
new file mode 100644
index 0000000..86fedb5
--- /dev/null
+++ b/webjs/airport-editor/history.test.js
@@ -0,0 +1,587 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+
+import {
+  DEFAULT_HISTORY_MAX_DEPTH,
+  createHistory,
+  captureSnapshot,
+  applySnapshot,
+  pushUndo,
+  recordBeforeMutation,
+  beginGesture,
+  commitGesture,
+  discardGesture,
+  undo,
+  redo,
+  canUndo,
+  canRedo,
+  clearHistory,
+  contentEquals,
+  snapshotContentKey,
+  seedCleanContentHashes,
+  syncDirtyAfterHistoryApply,
+} from '../../internal/web/static/js/openfsd/airport-editor/history.js';
+import {
+  createEmptyDocument,
+  createEmptyAirport,
+  setAirport,
+  setAircraft,
+  addSurface,
+  setVertex,
+  deleteSurface,
+  addAircraft,
+  updateAircraft,
+  noteDownload,
+  markClean,
+  hashText,
+  SurfaceParking,
+  SurfaceTaxiway,
+  createDefaultAircraft,
+} from '../../internal/web/static/js/openfsd/airport-editor/model.js';
+import {
+  formatAptForDownload,
+  formatAirForDownload,
+} from '../../internal/web/static/js/openfsd/airport-editor/download.js';
+
+/** Build a small APT with one parking and one taxiway for mutations. */
+function seedDocWithGeometry() {
+  const doc = createEmptyDocument();
+  const apt = createEmptyAirport();
+  apt.icao = 'KBTV';
+  apt.fieldElev = 335;
+  setAirport(doc, apt, { markDirty: false });
+  addSurface(doc, {
+    kind: SurfaceParking,
+    name: 'G1',
+    points: [{ lat: 44.47, lon: -73.15 }],
+  });
+  addSurface(doc, {
+    kind: SurfaceTaxiway,
+    name: 'A',
+    points: [
+      { lat: 44.47, lon: -73.15 },
+      { lat: 44.48, lon: -73.14 },
+    ],
+  });
+  doc.aptDirty = false;
+  doc.selection = { type: 'surface', index: 1 };
+  return doc;
+}
+
+// ---------------------------------------------------------------------------
+// createHistory / constants
+// ---------------------------------------------------------------------------
+
+test('DEFAULT_HISTORY_MAX_DEPTH is 100', () => {
+  assert.equal(DEFAULT_HISTORY_MAX_DEPTH, 100);
+});
+
+test('createHistory defaults and custom maxDepth', () => {
+  const h = createHistory();
+  assert.deepEqual(h.undoStack, []);
+  assert.deepEqual(h.redoStack, []);
+  assert.equal(h.gestureBaseline, null);
+  assert.equal(h.maxDepth, DEFAULT_HISTORY_MAX_DEPTH);
+
+  const h2 = createHistory({ maxDepth: 3 });
+  assert.equal(h2.maxDepth, 3);
+
+  const h3 = createHistory({ maxDepth: 0 });
+  assert.equal(h3.maxDepth, DEFAULT_HISTORY_MAX_DEPTH);
+});
+
+// ---------------------------------------------------------------------------
+// capture / apply isolation (K9)
+// ---------------------------------------------------------------------------
+
+test('captureSnapshot deep-clones; live mutation does not affect snap', () => {
+  const doc = seedDocWithGeometry();
+  const snap = captureSnapshot(doc);
+
+  assert.equal(snap.airport.icao, 'KBTV');
+  assert.equal(snap.aircraft.length, 0);
+  assert.deepEqual(snap.selection, { type: 'surface', index: 1 });
+  assert.equal(snap._nextSurfaceId, doc._nextSurfaceId);
+
+  // Mutate live doc
+  doc.airport.icao = 'XXXX';
+  doc.airport.surfaces[1].points[0].lat = 99;
+  doc.selection = null;
+  doc._nextSurfaceId = 999;
+  doc.aircraft.push({ callsign: 'N1' });
+
+  assert.equal(snap.airport.icao, 'KBTV');
+  assert.equal(snap.airport.surfaces[1].points[0].lat, 44.47);
+  assert.deepEqual(snap.selection, { type: 'surface', index: 1 });
+  assert.notEqual(snap._nextSurfaceId, 999);
+  assert.equal(snap.aircraft.length, 0);
+});
+
+test('applySnapshot deep-clones into doc; stack snap remains isolated', () => {
+  const doc = seedDocWithGeometry();
+  const h = createHistory();
+  const before = captureSnapshot(doc);
+  pushUndo(h, before);
+
+  // Mutate to a different state
+  doc.airport.icao = 'KXXX';
+  setVertex(doc, 1, 0, { lat: 50, lon: -70 });
+  doc.selection = { type: 'surface', index: 0 };
+
+  const res = undo(h, doc);
+  assert.equal(res.ok, true);
+  assert.equal(doc.airport.icao, 'KBTV');
+  assert.equal(doc.airport.surfaces[1].points[0].lat, 44.47);
+
+  // Mutate live after apply — must not alter redo stack snaps (K9)
+  doc.airport.icao = 'MUTATED';
+  doc.airport.surfaces[0].points[0].lat = 1;
+  const stacked = h.redoStack[0];
+  assert.equal(stacked.airport.icao, 'KXXX');
+  assert.equal(stacked.airport.surfaces[1].points[0].lat, 50);
+  assert.equal(stacked.airport.surfaces[0].points[0].lat, 44.47);
+
+  // Mutating live doc must not alter the originally captured `before` snap either
+  // (before was pushed to undo then popped — still held by test via `before`)
+  assert.equal(before.airport.icao, 'KBTV');
+  assert.equal(before.airport.surfaces[1].points[0].lat, 44.47);
+
+  // Redo restores KXXX from stack clone; further live mutation does not corrupt it
+  const r2 = redo(h, doc);
+  assert.equal(r2.ok, true);
+  assert.equal(doc.airport.icao, 'KXXX');
+  // undo stack now has capture of pre-redo live state (MUTATED)
+  const u = h.undoStack[h.undoStack.length - 1];
+  assert.equal(u.airport.icao, 'MUTATED');
+  doc.airport.icao = 'AGAIN';
+  doc.airport.surfaces[0].name = 'ZZZ';
+  assert.equal(u.airport.icao, 'MUTATED');
+  assert.equal(u.airport.surfaces[0].name, 'G1');
+  // redo stack empty; original `before` still pristine
+  assert.equal(before.airport.icao, 'KBTV');
+  assert.equal(before.airport.surfaces[0].name, 'G1');
+});
+
+test('applySnapshot does not touch dirty hashes, filenames, mode', () => {
+  const doc = seedDocWithGeometry();
+  doc.aptDirty = true;
+  doc.airDirty = true;
+  doc.lastAptDownloadHash = 'deadbeef';
+  doc.lastAirDownloadHash = 'cafebabe';
+  doc.aptFilename = 'keep.apt';
+  doc.airFilename = 'keep.air';
+  doc.mode = 'taxi';
+  doc.serverValidation = { loading: false, stale: false };
+
+  const empty = captureSnapshot(createEmptyDocument());
+  applySnapshot(doc, empty);
+
+  assert.equal(doc.airport, null);
+  assert.equal(doc.aptDirty, true);
+  assert.equal(doc.airDirty, true);
+  assert.equal(doc.lastAptDownloadHash, 'deadbeef');
+  assert.equal(doc.lastAirDownloadHash, 'cafebabe');
+  assert.equal(doc.aptFilename, 'keep.apt');
+  assert.equal(doc.airFilename, 'keep.air');
+  assert.equal(doc.mode, 'taxi');
+  assert.deepEqual(doc.serverValidation, { loading: false, stale: false });
+});
+
+// ---------------------------------------------------------------------------
+// push / record / undo / redo
+// ---------------------------------------------------------------------------
+
+test('recordBefore + mutate + undo restores; redo restores post-state', () => {
+  const doc = seedDocWithGeometry();
+  const h = createHistory();
+  const icaoBefore = doc.airport.icao;
+
+  recordBeforeMutation(h, doc);
+  doc.airport.icao = 'KXXX';
+  doc.aptDirty = true;
+
+  assert.equal(canUndo(h), true);
+  assert.equal(canRedo(h), false);
+
+  const u = undo(h, doc);
+  assert.equal(u.ok, true);
+  assert.equal(doc.airport.icao, icaoBefore);
+  assert.equal(canRedo(h), true);
+
+  const r = redo(h, doc);
+  assert.equal(r.ok, true);
+  assert.equal(doc.airport.icao, 'KXXX');
+});
+
+test('pushUndo always grows stack (no identical-top elision)', () => {
+  const doc = seedDocWithGeometry();
+  const h = createHistory();
+  const a = captureSnapshot(doc);
+  const b = captureSnapshot(doc);
+  pushUndo(h, a);
+  pushUndo(h, b);
+  assert.equal(h.undoStack.length, 2);
+  assert.ok(contentEquals(h.undoStack[0], h.undoStack[1]));
+});
+
+test('redo cleared on new pushUndo after undo branch', () => {
+  const doc = seedDocWithGeometry();
+  const h = createHistory();
+
+  recordBeforeMutation(h, doc);
+  doc.airport.icao = 'A1';
+  undo(h, doc);
+  assert.equal(canRedo(h), true);
+
+  recordBeforeMutation(h, doc);
+  doc.airport.icao = 'A2';
+  assert.equal(canRedo(h), false);
+  assert.equal(h.redoStack.length, 0);
+});
+
+test('undo/redo empty and gesture-active reasons', () => {
+  const doc = createEmptyDocument();
+  const h = createHistory();
+
+  assert.deepEqual(undo(h, doc), { ok: false, reason: 'empty' });
+  assert.deepEqual(redo(h, doc), { ok: false, reason: 'empty' });
+
+  beginGesture(h, doc);
+  assert.deepEqual(undo(h, doc), { ok: false, reason: 'gesture-active' });
+  assert.deepEqual(redo(h, doc), { ok: false, reason: 'gesture-active' });
+});
+
+// ---------------------------------------------------------------------------
+// maxDepth
+// ---------------------------------------------------------------------------
+
+test('maxDepth drops oldest on overflow', () => {
+  const h = createHistory({ maxDepth: 3 });
+  const doc = createEmptyDocument();
+
+  for (let i = 0; i < 5; i++) {
+    doc._nextSurfaceId = i + 1;
+    pushUndo(h, captureSnapshot(doc));
+  }
+  assert.equal(h.undoStack.length, 3);
+  // Oldest remaining should be from when _nextSurfaceId was 3 (0-based loop: i=2 → id 3)
+  assert.equal(h.undoStack[0]._nextSurfaceId, 3);
+  assert.equal(h.undoStack[2]._nextSurfaceId, 5);
+});
+
+// ---------------------------------------------------------------------------
+// gestures
+// ---------------------------------------------------------------------------
+
+test('beginGesture / commitGesture coalesces multi-move into one undo entry', () => {
+  const doc = seedDocWithGeometry();
+  const h = createHistory();
+  const lat0 = doc.airport.surfaces[1].points[0].lat;
+
+  beginGesture(h, doc);
+  setVertex(doc, 1, 0, { lat: lat0 + 0.01, lon: -73.15 });
+  beginGesture(h, doc); // nested no-op
+  setVertex(doc, 1, 0, { lat: lat0 + 0.02, lon: -73.14 });
+  const pushed = commitGesture(h, doc);
+  assert.equal(pushed, true);
+  assert.equal(h.undoStack.length, 1);
+  assert.equal(h.gestureBaseline, null);
+
+  undo(h, doc);
+  assert.equal(doc.airport.surfaces[1].points[0].lat, lat0);
+});
+
+test('commitGesture no-op when content unchanged (selection-only ignored)', () => {
+  const doc = seedDocWithGeometry();
+  const h = createHistory();
+
+  beginGesture(h, doc);
+  doc.selection = { type: 'surface', index: 0 }; // selection change only
+  const pushed = commitGesture(h, doc);
+  assert.equal(pushed, false);
+  assert.equal(h.undoStack.length, 0);
+});
+
+test('commitGesture with no begin returns false', () => {
+  const doc = createEmptyDocument();
+  const h = createHistory();
+  assert.equal(commitGesture(h, doc), false);
+});
+
+test('discardGesture clears baseline without stacking', () => {
+  const doc = seedDocWithGeometry();
+  const h = createHistory();
+  beginGesture(h, doc);
+  setVertex(doc, 1, 0, { lat: 99, lon: 0 });
+  discardGesture(h);
+  assert.equal(h.gestureBaseline, null);
+  assert.equal(h.undoStack.length, 0);
+});
+
+test('canUndo / canRedo false during active gesture', () => {
+  const doc = seedDocWithGeometry();
+  const h = createHistory();
+  recordBeforeMutation(h, doc);
+  doc.airport.icao = 'X';
+  undo(h, doc); // leave something on redo
+  assert.equal(canUndo(h), false); // empty undo after single undo
+  // put one undo back
+  recordBeforeMutation(h, doc);
+  assert.equal(canUndo(h), true);
+  assert.equal(canRedo(h), false);
+
+  beginGesture(h, doc);
+  assert.equal(canUndo(h), false);
+  assert.equal(canRedo(h), false);
+  discardGesture(h);
+  assert.equal(canUndo(h), true);
+});
+
+// ---------------------------------------------------------------------------
+// clearHistory
+// ---------------------------------------------------------------------------
+
+test('clearHistory empties stacks and gesture', () => {
+  const doc = seedDocWithGeometry();
+  const h = createHistory();
+  recordBeforeMutation(h, doc);
+  beginGesture(h, doc);
+  // after begin, push another path: force redo empty, undo has 1, gesture set
+  clearHistory(h);
+  assert.equal(h.undoStack.length, 0);
+  assert.equal(h.redoStack.length, 0);
+  assert.equal(h.gestureBaseline, null);
+});
+
+// ---------------------------------------------------------------------------
+// contentEquals / selection + _nextSurfaceId
+// ---------------------------------------------------------------------------
+
+test('contentEquals true for deep-equal geometry, false when vertex moves', () => {
+  const doc = seedDocWithGeometry();
+  const a = captureSnapshot(doc);
+  const b = captureSnapshot(doc);
+  assert.equal(contentEquals(a, b), true);
+
+  b.selection = { type: 'aircraft', index: 0 };
+  assert.equal(contentEquals(a, b), true); // selection ignored
+
+  b.airport.surfaces[1].points[0].lat = 0;
+  assert.equal(contentEquals(a, b), false);
+
+  assert.equal(
+    snapshotContentKey(a),
+    JSON.stringify({ airport: a.airport, aircraft: a.aircraft }),
+  );
+});
+
+test('selection and _nextSurfaceId restore after undo of delete / add surface', () => {
+  const doc = seedDocWithGeometry();
+  const h = createHistory();
+  const nextIdBefore = doc._nextSurfaceId;
+  doc.selection = { type: 'surface', index: 0 };
+
+  recordBeforeMutation(h, doc);
+  deleteSurface(doc, 0);
+  assert.equal(doc.selection, null); // deleteSurface clears selection at index
+
+  undo(h, doc);
+  assert.deepEqual(doc.selection, { type: 'surface', index: 0 });
+  assert.equal(doc.airport.surfaces.length, 2);
+  assert.equal(doc.airport.surfaces[0].name, 'G1');
+  assert.equal(doc._nextSurfaceId, nextIdBefore);
+
+  // add surface path
+  recordBeforeMutation(h, doc);
+  const idBeforeAdd = doc._nextSurfaceId;
+  addSurface(doc, {
+    kind: SurfaceParking,
+    name: 'G2',
+    points: [{ lat: 1, lon: 2 }],
+  });
+  assert.ok(doc._nextSurfaceId > idBeforeAdd || doc.airport.surfaces.length === 3);
+
+  undo(h, doc);
+  assert.equal(doc.airport.surfaces.length, 2);
+  assert.equal(doc._nextSurfaceId, idBeforeAdd);
+});
+
+test('null airport baseline: place then undo restores airport null', () => {
+  const doc = createEmptyDocument();
+  const h = createHistory();
+  const before = captureSnapshot(doc);
+  assert.equal(before.airport, null);
+
+  pushUndo(h, before);
+  setAirport(doc, createEmptyAirport(), { markDirty: true });
+  addSurface(doc, {
+    kind: SurfaceParking,
+    name: 'G1',
+    points: [{ lat: 1, lon: 2 }],
+  });
+
+  undo(h, doc);
+  assert.equal(doc.airport, null);
+});
+
+// ---------------------------------------------------------------------------
+// Dirty / baseline helpers (K13)
+// ---------------------------------------------------------------------------
+
+test('bootstrap / empty seed: mutate then undo → clean via syncDirtyAfterHistoryApply', () => {
+  const doc = createEmptyDocument();
+  seedCleanContentHashes(doc, 'both');
+  assert.ok(doc.lastAptDownloadHash);
+  assert.ok(doc.lastAirDownloadHash);
+  assert.equal(doc.aptDirty, false);
+
+  const h = createHistory();
+  recordBeforeMutation(h, doc);
+  setAirport(doc, createEmptyAirport(), { markDirty: true });
+  addSurface(doc, {
+    kind: SurfaceParking,
+    name: 'G1',
+    points: [{ lat: 44.47, lon: -73.15 }],
+  });
+  assert.equal(doc.aptDirty, true);
+
+  undo(h, doc);
+  syncDirtyAfterHistoryApply(doc);
+  assert.equal(doc.airport, null);
+  assert.equal(doc.aptDirty, false);
+  assert.equal(doc.airDirty, false);
+});
+
+test('open-equivalent seed: set airport + seed apt → mutate → undo → clean', () => {
+  const doc = createEmptyDocument();
+  const apt = createEmptyAirport();
+  apt.icao = 'KBTV';
+  setAirport(doc, apt, { markDirty: false });
+  seedCleanContentHashes(doc, 'apt');
+  doc.aptDirty = false;
+
+  const h = createHistory();
+  recordBeforeMutation(h, doc);
+  doc.airport.icao = 'KXXX';
+  doc.aptDirty = true;
+
+  undo(h, doc);
+  syncDirtyAfterHistoryApply(doc);
+  assert.equal(doc.airport.icao, 'KBTV');
+  assert.equal(doc.aptDirty, false);
+});
+
+test('download then undo/redo dirty tracking', () => {
+  const doc = seedDocWithGeometry();
+  seedCleanContentHashes(doc, 'both');
+  doc.aptDirty = false;
+
+  const h = createHistory();
+  recordBeforeMutation(h, doc);
+  doc.airport.icao = 'KXXX';
+  doc.aptDirty = true;
+
+  // Download current (edited) content — marks clean with hash of KXXX
+  noteDownload(doc, 'apt', formatAptForDownload(doc));
+  assert.equal(doc.aptDirty, false);
+
+  undo(h, doc);
+  syncDirtyAfterHistoryApply(doc);
+  assert.equal(doc.airport.icao, 'KBTV');
+  assert.equal(doc.aptDirty, true); // hash(A) ≠ hash(B)
+
+  redo(h, doc);
+  syncDirtyAfterHistoryApply(doc);
+  assert.equal(doc.airport.icao, 'KXXX');
+  assert.equal(doc.aptDirty, false);
+});
+
+test('mark-clean then undo may re-dirty via recompute', () => {
+  const doc = seedDocWithGeometry();
+  seedCleanContentHashes(doc, 'both');
+  doc.aptDirty = false;
+
+  const h = createHistory();
+  recordBeforeMutation(h, doc);
+  doc.airport.icao = 'KXXX';
+  doc.aptDirty = true;
+
+  // Mark clean without changing hash (hash still seed of KBTV content)
+  markClean(doc, 'apt');
+  assert.equal(doc.aptDirty, false);
+
+  // Current content is KXXX; recompute would dirty — but we only recompute after history apply
+  // Undo to KBTV which matches seed → clean
+  undo(h, doc);
+  syncDirtyAfterHistoryApply(doc);
+  assert.equal(doc.airport.icao, 'KBTV');
+  assert.equal(doc.aptDirty, false);
+
+  // Redo to KXXX vs seed KBTV → dirty
+  redo(h, doc);
+  syncDirtyAfterHistoryApply(doc);
+  assert.equal(doc.airport.icao, 'KXXX');
+  assert.equal(doc.aptDirty, true);
+});
+
+test('null airport dirty symmetry when hash is non-null from prior download', () => {
+  const doc = seedDocWithGeometry();
+  const text = formatAptForDownload(doc);
+  noteDownload(doc, 'apt', text);
+  assert.equal(doc.aptDirty, false);
+  assert.ok(doc.lastAptDownloadHash);
+
+  const emptySnap = captureSnapshot(createEmptyDocument());
+  applySnapshot(doc, emptySnap);
+  syncDirtyAfterHistoryApply(doc);
+  assert.equal(doc.airport, null);
+  assert.equal(doc.aptDirty, true);
+  assert.notEqual(hashText(''), doc.lastAptDownloadHash);
+});
+
+test('seed recipe unity: seed and syncDirty use download-format helpers', () => {
+  const doc = createEmptyDocument();
+  seedCleanContentHashes(doc, 'both');
+  const expectedApt = hashText(formatAptForDownload(doc));
+  const expectedAir = hashText(formatAirForDownload(doc));
+  assert.equal(doc.lastAptDownloadHash, expectedApt);
+  assert.equal(doc.lastAirDownloadHash, expectedAir);
+  // empty download format is ''
+  assert.equal(formatAptForDownload(doc), '');
+  assert.equal(formatAirForDownload(doc), '');
+  assert.equal(expectedApt, hashText(''));
+  assert.equal(expectedAir, hashText(''));
+
+  syncDirtyAfterHistoryApply(doc);
+  assert.equal(doc.aptDirty, false);
+  assert.equal(doc.airDirty, false);
+});
+
+test('seedCleanContentHashes side=apt leaves air hash alone', () => {
+  const doc = createEmptyDocument();
+  doc.lastAirDownloadHash = 'keepme!!';
+  seedCleanContentHashes(doc, 'apt');
+  assert.equal(doc.lastAirDownloadHash, 'keepme!!');
+  assert.equal(doc.lastAptDownloadHash, hashText(formatAptForDownload(doc)));
+});
+
+test('air side: record / undo / seed clean for aircraft', () => {
+  const doc = createEmptyDocument();
+  const apt = createEmptyAirport();
+  apt.icao = 'KBTV';
+  setAirport(doc, apt, { markDirty: false });
+  setAircraft(doc, [createDefaultAircraft(apt, { lat: 44.47, lon: -73.15 })], {
+    markDirty: false,
+  });
+  seedCleanContentHashes(doc, 'both');
+  doc.airDirty = false;
+
+  const h = createHistory();
+  recordBeforeMutation(h, doc);
+  updateAircraft(doc, 0, { callsign: 'CHANGED' });
+
+  undo(h, doc);
+  syncDirtyAfterHistoryApply(doc);
+  assert.notEqual(doc.aircraft[0].callsign, 'CHANGED');
+  assert.equal(doc.airDirty, false);
+});