diff --git a/demo/smoke_device_position_history.mjs b/demo/smoke_device_position_history.mjs
index 4edf802e..4d939e63 100644
--- a/demo/smoke_device_position_history.mjs
+++ b/demo/smoke_device_position_history.mjs
@@ -97,12 +97,24 @@ const res = await page.evaluate(async () => {
   out.futureFieldsSurviveDrag = c._layout[deviceId].k === 0
     && c._layout[deviceId].future === 'kept';
   out.undoEnabled = !sr().querySelector('[data-device-position-history="undo"]').disabled;
+  // #397 AC1: after a write the local copy IS what went over the wire.
+  out.localCopyEqualsTheWire = writes.length > 0
+    && JSON.stringify(c._layout[deviceId]) === JSON.stringify({
+      ...c._layout[deviceId], ...writes[writes.length - 1].pos,
+    })
+    && JSON.stringify(serverLayout[deviceId]) === JSON.stringify(writes[writes.length - 1].pos);
 
   const afterDrag = structuredClone(c._layout[deviceId]);
   sr().querySelector('[data-device-position-history="undo"]').click();
   await idle();
-  out.undoRestoresExactStart = c._layout[deviceId].x === explicit.x
-    && c._layout[deviceId].y === explicit.y
+  // #397: the card now keeps what it sent — the canonical position — so a
+  // restored placement may differ from the raw `explicit` by the lattice snap
+  // (< 1e-9 of the plan, invisible). The equality is therefore stated to that
+  // precision, and the snap itself is pinned separately below: a real logical
+  // drift would exceed it by orders of magnitude.
+  const near = (a, b) => Math.abs(a - b) < 1e-9;
+  out.undoRestoresExactStart = near(c._layout[deviceId].x, explicit.x)
+    && near(c._layout[deviceId].y, explicit.y)
     && c._layout[deviceId].k === 0
     && c._devicePositionHistory.canRedo;
   sr().querySelector('[data-device-position-history="redo"]').click();
@@ -113,7 +125,7 @@ const res = await page.evaluate(async () => {
   }));
   await idle();
   out.keyboardUndoWorks = c._devicePositionHistory.canRedo
-    && c._layout[deviceId].x === explicit.x && c._layout[deviceId].y === explicit.y;
+    && near(c._layout[deviceId].x, explicit.x) && near(c._layout[deviceId].y, explicit.y);
   window.dispatchEvent(new KeyboardEvent('keydown', {
     key: 'z', code: 'KeyZ', ctrlKey: true, shiftKey: true,
     bubbles: true, composed: true, cancelable: true,
@@ -231,10 +243,22 @@ const res = await page.evaluate(async () => {
   out.nativeInputHistoryNotIntercepted = c._devicePositionHistory.size === sizeBeforeInput;
   input.remove();
 
-  // Same-content reload is an own/reconnect echo; different content is remote authority.
-  serverLayout = structuredClone(c._layout);
+  // Same-content reload is an own/reconnect echo; different content is remote
+  // authority. #397: the server snapshot is NOT copied from the card here — it
+  // already holds what actually went over the wire (the fake WS above stores
+  // `message.pos`). The former `serverLayout = structuredClone(c._layout)`
+  // erased by hand the very divergence this check exists to catch: the wire
+  // carries the canonical position while `_layout` kept the raw one, so the
+  // card mistook its own echo for a remote edit and cleared the stack. With
+  // the assignment gone the check reddens on the unfixed code.
+  const ownEchoServerPos = structuredClone(serverLayout[deviceId]);
+  const ownEchoLocalPos = structuredClone(c._layout[deviceId]);
   await c._reloadLayoutOnly();
   out.sameContentReloadKeepsHistory = c._devicePositionHistory.canUndo;
+  // The point of the check is only meaningful if the two sides were equal to
+  // begin with: pin that explicitly instead of assuming it.
+  out.ownEchoMatchesWhatWentOverTheWire =
+    JSON.stringify(ownEchoServerPos) === JSON.stringify(ownEchoLocalPos);
   serverLayout = {
     ...serverLayout,
     [deviceId]: { ...serverLayout[deviceId], x: serverLayout[deviceId].x + 0.01 },
@@ -243,6 +267,40 @@ const res = await page.evaluate(async () => {
   out.remoteContentClearsHistory = !c._devicePositionHistory.canUndo
     && !c._devicePositionHistory.canRedo;
 
+  // #397 AC5b: the echo of a DELETE must not clear the stack either — the
+  // delete branch removes a key instead of replacing a value, so proving the
+  // update branch says nothing about it.
+  const echoProbe = c._devices.find((candidate) => candidate.id !== deviceId
+    && candidate.bindingStatus?.kind !== 'ha_disabled');
+  if (echoProbe) {
+    c._layout = { ...c._layout, [echoProbe.id]: { s: echoProbe.space, x: 0.42, y: 0.42 } };
+    await c._persistDevicePlacement(echoProbe.id, { s: echoProbe.space, x: 0.42, y: 0.42 });
+    c._devicePositionHistory.push({
+      name: 'echo probe move',
+      before: { deviceId: echoProbe.id, spaceId: echoProbe.space, placement: null },
+      after: { deviceId: echoProbe.id, spaceId: echoProbe.space,
+        placement: { s: echoProbe.space, x: 0.42, y: 0.42 } },
+    });
+    await c._persistDevicePlacement(echoProbe.id, null);
+    await c._reloadLayoutOnly();
+    out.deleteEchoKeepsHistory = c._devicePositionHistory.canUndo
+      && c._layout[echoProbe.id] === undefined;
+
+    // #397 AC7: while a write is in flight the card is the authority — a
+    // server answer holding the OLD position must not win the merge.
+    const inFlightPos = { s: echoProbe.space, x: 0.63, y: 0.21 };
+    serverLayout = { ...serverLayout, [echoProbe.id]: { s: echoProbe.space, x: 0.1, y: 0.1 } };
+    c._sentPos.set(echoProbe.id, structuredClone(inFlightPos));
+    c._layout = { ...c._layout, [echoProbe.id]: structuredClone(inFlightPos) };
+    await c._reloadLayoutOnly();
+    out.inFlightPositionWinsTheMerge =
+      JSON.stringify(c._layout[echoProbe.id]) === JSON.stringify(inFlightPos);
+    c._sentPos.delete(echoProbe.id);
+  } else {
+    out.deleteEchoKeepsHistory = null;
+    out.inFlightPositionWinsTheMerge = null;
+  }
+
   // A valid command owns its original space and makes the result visible there.
   const otherDevice = c._devices.find((candidate) => candidate.space !== device.space
     && candidate.bindingStatus?.kind !== 'ha_disabled');
diff --git a/docs/CHANGELOG.md b/docs/CHANGELOG.md
index aaf5d06f..ba789f0b 100644
--- a/docs/CHANGELOG.md
+++ b/docs/CHANGELOG.md
@@ -2,6 +2,11 @@
 
 ## Unreleased
 
+- Undo/Redo of marker positions no longer goes dark on its own: the card now
+  keeps exactly what it sent to the server, so a reconnect or a second tab
+  stops looking like someone else's edit
+  ([#397](https://github.com/Matysh/houseplan-card/issues/397)).
+
 - Dangerous actions now use one accessible House Plan confirmation dialog
   instead of the browser prompt. Device, draft, plan and space deletion plus
   lock opening share clear consequences, safe Cancel/X/Escape behaviour and
diff --git a/docs/CHANGELOG.ru.md b/docs/CHANGELOG.ru.md
index 59485b0f..93eb5347 100755
--- a/docs/CHANGELOG.ru.md
+++ b/docs/CHANGELOG.ru.md
@@ -8,6 +8,11 @@
 
 ## Не выпущено
 
+- История Undo/Redo позиций маркеров больше не гаснет сама: карточка хранит
+  ровно то, что отправила серверу, поэтому переподключение или вторая вкладка
+  перестали выглядеть чужой правкой
+  ([#397](https://github.com/Matysh/houseplan-card/issues/397)).
+
 - Опасные действия теперь используют единый доступный диалог подтверждения
   House Plan вместо системного окна браузера. Удаление устройства, черновика,
   файла плана и пространства, а также открытие замка получили понятные
diff --git a/scripts/mutation-gate.mjs b/scripts/mutation-gate.mjs
index ade69499..a9629ef9 100644
--- a/scripts/mutation-gate.mjs
+++ b/scripts/mutation-gate.mjs
@@ -746,6 +746,18 @@ const MUTANT_DEFINITIONS = [
       replace: "      this._persistDecorStyle();\n    }, 0);",
     }],
   },
+  {
+    id: 'device-echo-keeps-local-noncanonical',
+    guard: 'node demo/smoke_device_position_history.mjs',
+    because: 'a card that keeps the raw position while sending the canonical '
+      + 'one mistakes its own echo for a remote edit and wipes the undo stack '
+      + '(#397 B3)',
+    patches: [{
+      file: 'src/houseplan-card.ts',
+      find: '            this._layout = { ...this._layout, [deviceId]: pos };',
+      replace: '            void pos;',
+    }],
+  },
   {
     id: 'camera-cancel-loses-zoom',
     guard: 'node demo/smoke_smooth_zoom.mjs',
diff --git a/src/houseplan-card.ts b/src/houseplan-card.ts
index 3ca1dae1..1f7a52a2 100755
--- a/src/houseplan-card.ts
+++ b/src/houseplan-card.ts
@@ -5241,7 +5241,12 @@ export class HouseplanCard extends LitElement {
             type: 'houseplan/layout/delete', device_id: deviceId,
           });
         } else {
+          // #397: keep locally exactly what goes on the wire, or the card's
+          // own echo reads as a remote edit and wipes the undo stack.
           const pos = canonicalizePosition(this._layout[deviceId]);
+          if (contentFingerprint(pos) !== contentFingerprint(this._layout[deviceId])) {
+            this._layout = { ...this._layout, [deviceId]: pos };
+          }
           pending = pos;
           this._sentPos.set(deviceId, pending);
           registered = true;
diff --git a/test/device-position-echo.test.mjs b/test/device-position-echo.test.mjs
new file mode 100644
index 00000000..a39cacac
--- /dev/null
+++ b/test/device-position-echo.test.mjs
@@ -0,0 +1,48 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import { readFileSync } from 'node:fs';
+import { canonicalizePosition } from '../test-build/coordinate-canonicalization.js';
+import { contentFingerprint } from '../test-build/visual-continuity.js';
+
+// #397: the card sends the canonical position to the server and must keep the
+// same value locally. While it kept the raw one, its own echo came back
+// looking foreign — the fingerprints differed — and the next reload wiped the
+// undo history the user had just filled (AC10 of #74).
+
+const CARD = readFileSync(new URL('../src/houseplan-card.ts', import.meta.url), 'utf8');
+
+test('#397: the premise holds — canonicalization is not identity', () => {
+  // If it were, the whole issue would be moot and the guards below decorative.
+  const raw = { s: 'ground', x: 0.024999999999999942, y: 0.63 / 3 };
+  const canonical = canonicalizePosition(raw);
+  assert.notEqual(contentFingerprint(canonical), contentFingerprint(raw),
+    'the lattice snap must actually change the value');
+  assert.ok(Math.abs(canonical.x - raw.x) < 1e-9,
+    'and it must be a snap, not a move — the difference is invisible on screen');
+});
+
+test('#397 AC1: the update branch stores what it sends, before sending it', () => {
+  const branch = CARD.slice(
+    CARD.indexOf('private async _persistDevicePlacement'),
+    CARD.indexOf('this._persistLocalLayout();', CARD.indexOf('private async _persistDevicePlacement')),
+  );
+  assert.ok(branch, 'the persist method must be found');
+  const write = branch.indexOf('this._layout = { ...this._layout, [deviceId]: pos }');
+  const send = branch.indexOf("type: 'houseplan/layout/update'");
+  const fingerprint = branch.indexOf('this._layoutContentFingerprint = contentFingerprint(this._layout)');
+  assert.ok(write > 0, 'the canonical position must be written back into _layout');
+  assert.ok(write < send,
+    'the local copy is updated BEFORE the wire, so a reload racing the answer '
+    + 'sees the value that was sent');
+  assert.ok(send < fingerprint,
+    'the fingerprint is taken after the write, over the canonical layout');
+});
+
+test('#397 AC5a: the delete branch removes the key before the fingerprint', () => {
+  const method = CARD.slice(CARD.indexOf('private async _persistDevicePlacement'));
+  const apply = method.indexOf('applyDevicePlacement(this._layout, deviceId, placement)');
+  const fingerprint = method.indexOf('this._layoutContentFingerprint = contentFingerprint(this._layout)');
+  assert.ok(apply > 0 && apply < fingerprint,
+    'both branches mutate _layout through applyDevicePlacement before the '
+    + 'fingerprint is recorded — deletion removes the key, not replaces a value');
+});