@@ -0,0 +1,7 @@
+---
+---
+
+Test-only change to `@object-ui/plugin-kanban`: characterizes what a cross-column
+drag does with the landing position inside the target column (objectui#8826,
+question 3). No published behaviour changes — the pin records today's behaviour
+so a later change to it is announced rather than silent.
@@ -0,0 +1,322 @@
+/**
+ * ObjectUI
+ * Copyright (c) 2024-present ObjectStack Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+/**
+ * A CROSS-column drop loses the landing POSITION too — objectui#8826, question 3.
+ *
+ * ── What this file is ─────────────────────────────────────────────────────
+ * A CHARACTERIZATION pin, not a gate. objectui#8826 measured that a SAME-column
+ * reorder is persisted nowhere; its third question — is the landing position
+ * within the TARGET column also lost on a CROSS-column move? — was filed
+ * explicitly unmeasured, because the fix surface depends on the answer. This
+ * file is that measurement, frozen. It asserts today's behaviour so that a
+ * later change to it is announced rather than silent; it does NOT claim the
+ * behaviour is correct, and it rules nothing about questions 1 and 2 (whether
+ * in-column order should be persisted at all, and whether the optimistic update
+ * should be removed if not). Those are the maintainer's.
+ *
+ * ── The answer, and where the index dies ──────────────────────────────────
+ * `KanbanImpl.handleDragEnd`'s cross-column branch DOES compute a landing index
+ * (`overIndex`) and DOES splice the card in at it, then hands that index to
+ * `onCardMove` as its fourth argument. `ObjectKanban.handleCardMove` receives it
+ * as `_newIndex` and discards it outright (`void _newIndex`), and
+ * `persistCardMove` writes a body of exactly `{ [groupBy]: toColumnId }`. So the
+ * index reaches a parameter and stops there: no sink, no PATCH key, no storage.
+ * Column MEMBERSHIP flows back through the `columns` prop; ORDER never does —
+ * the board's order is whatever `bucketCardsIntoColumns` produces, which is the
+ * order of the underlying record array.
+ *
+ * ── ⚠️ The two data ownerships answer DIFFERENTLY, and that is the finding ──
+ * The card anticipated one answer; there are two, because the optimistic write
+ * that re-renders the board is itself gated on ownership.
+ *
+ *  - INTERNAL data (the board fetches): `persistCardMove`'s optimistic
+ *    `setFetchedData` re-renders `ObjectKanban`, `KanbanRenderer` re-buckets,
+ *    the `columns` identity changes and `KanbanImpl`'s mirror re-syncs from it.
+ *    The dropped card therefore leaves the slot the user dropped it in
+ *    IMMEDIATELY — inside the same `act()` — and lands wherever record order
+ *    puts it. This is WORSE than the same-column signature the card describes:
+ *    not a silent revert on some later reflow, but a visible snap on the spot.
+ *  - EXTERNAL data (a parent owns `data`, e.g. ListView): the optimistic write
+ *    is deliberately skipped, so nothing re-renders and the mirror is never
+ *    reset. The drop position SURVIVES on screen — until the parent's next data
+ *    reflow, which carries membership and not order. That is the card's
+ *    silent-revert signature exactly.
+ *
+ * ⇒ The position is lost on BOTH paths. They differ only in WHEN the user sees
+ *   it happen.
+ *
+ * ── The instrument ────────────────────────────────────────────────────────
+ * The card's recorder: the `columns` prop the board receives, captured on every
+ * render, via a module mock of `../KanbanImpl` that records and then renders the
+ * real board. (`columnsReachDomSameCommit-8534.test.tsx` has a recorder too, but
+ * it snapshots the DOM per commit at the `KanbanImpl` seam — a different
+ * instrument, and it never drives a cross-column drop.) The board is driven
+ * through `DndContext`'s real `onDragEnd`, captured by the second module mock:
+ * dnd-kit's pointer sensors need layout and pointer capture that happy-dom does
+ * not provide, so a synthesized drop on the production handler is the closest
+ * honest reproduction. Everything downstream of that call is the real path.
+ *
+ * Both controls the measurement needs are here as their own tests:
+ *  - LIT positive control — a cross-column move DOES produce a new `columns`
+ *    snapshot, with the membership change in it. The instrument is connected,
+ *    so "the drop order never appears" is a reading and not a dead wire.
+ *  - DEAD negative control — a drag that ends on nothing produces NO new
+ *    snapshot and no write. The instrument does not fire spuriously.
+ *
+ * ── The fixture is built so the two orders cannot coincide ────────────────
+ * The moved card is LAST in the record array and is dropped FIRST in the target
+ * column. A fixture where the dropped card also happens to sort first would
+ * read green whether the index survived or not.
+ */
+
+import React from 'react';
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
+import { render, screen, within, act, cleanup, waitFor } from '@testing-library/react';
+import '@testing-library/jest-dom';
+import { registerAllFields } from '@object-ui/fields';
+import { toast } from '@object-ui/components';
+import type { DataSource, ObjectKanbanSchema } from '@object-ui/types';
+import { ObjectKanban } from '../ObjectKanban';
+
+// Pay the board's lazy chunk at import time rather than racing it against a
+// `findBy` budget (AGENTS.md §测试纪律); specifier resolves to the same module
+// as `./KanbanImpl` in `../index`, so the `React.lazy` factory hits the cache.
+import '../KanbanImpl';
+
+// `vi.hoisted` so the mock factories — hoisted above every import — can reach
+// these boxes. A plain `const` would still be in its TDZ when the mocked
+// modules are first requested.
+const dnd = vi.hoisted(() => ({
+  onDragEnd: undefined as undefined | ((event: unknown) => void),
+}));
+const recorder = vi.hoisted(() => ({ snapshots: [] as string[] }));
+
+// Capture the board's real `onDragEnd` while still rendering the real provider,
+// so `@dnd-kit/sortable`'s hooks keep reading the same context instance.
+vi.mock('@dnd-kit/core', async (importOriginal) => {
+  const actual = await importOriginal<typeof import('@dnd-kit/core')>();
+  const ReactMod = await import('react');
+  const CapturingDndContext = (props: Record<string, unknown>) => {
+    dnd.onDragEnd = props.onDragEnd as (event: unknown) => void;
+    return ReactMod.createElement(actual.DndContext, props as never);
+  };
+  return { ...actual, DndContext: CapturingDndContext };
+});
+
+// THE INSTRUMENT: record the `columns` prop on every render of the board, then
+// render the real board with it. Recording in the component body (not an
+// effect) is deliberate — it is the prop AS DELIVERED that is under test.
+vi.mock('../KanbanImpl', async (importOriginal) => {
+  const actual = await importOriginal<typeof import('../KanbanImpl')>();
+  const ReactMod = await import('react');
+  const RecordingKanbanBoard = (props: Record<string, unknown>) => {
+    const cols = (props.columns as Array<Record<string, unknown>> | undefined) ?? [];
+    recorder.snapshots.push(
+      cols
+        .map((c) => {
+          const cards = (c.cards as Array<Record<string, unknown>> | undefined) ?? [];
+          return `${String(c.id)}:${cards.map((k) => String(k.title)).join(',')}`;
+        })
+        .join(' | '),
+    );
+    return ReactMod.createElement(actual.default as never, props as never);
+  };
+  return { ...actual, default: RecordingKanbanBoard };
+});
+
+registerAllFields();
+
+const objectDef = {
+  name: 'task',
+  fields: {
+    title: { type: 'text', label: 'Title' },
+    status: {
+      type: 'picklist',
+      label: 'Status',
+      options: [
+        { value: 'backlog', label: 'Backlog' },
+        { value: 'in_progress', label: 'In Progress' },
+      ],
+    },
+  },
+};
+
+/**
+ * Record order is the load-bearing part of this fixture. `Alpha` is LAST in the
+ * array and alone in `backlog`; the two cards already in `in_progress` come
+ * FIRST. Dropping `Alpha` onto `Xray` asks for slot 0 of the target column,
+ * while record order puts it in slot 2 — so drop order and record order are
+ * distinguishable, which is the whole point.
+ */
+const RECORDS = () => [
+  { id: 'x', title: 'Xray', status: 'in_progress' },
+  { id: 'y', title: 'Yankee', status: 'in_progress' },
+  { id: 'a', title: 'Alpha', status: 'backlog' },
+];
+
+/** What the recorder shows at mount, before any drag. */
+const AT_MOUNT = 'backlog:Alpha | in_progress:Xray,Yankee';
+/** What the drop asks for: `Alpha` in slot 0 of the target column. */
+const DROP_ORDER = 'backlog: | in_progress:Alpha,Xray,Yankee';
+/** What the prop actually carries back: record order, with `Alpha` last. */
+const RECORD_ORDER = 'backlog: | in_progress:Xray,Yankee,Alpha';
+
+const schema = {
+  type: 'object-kanban',
+  objectName: 'task',
+  groupBy: 'status',
+  cardTitle: 'title',
+  columns: [
+    { id: 'backlog', title: 'Backlog' },
+    { id: 'in_progress', title: 'In Progress' },
+  ],
+} satisfies ObjectKanbanSchema;
+
+function makeDataSource(update: DataSource['update']): DataSource {
+  return {
+    getObjectSchema: vi.fn(async () => objectDef),
+    find: vi.fn(async () => ({ value: RECORDS() })),
+    update,
+  } as unknown as DataSource;
+}
+
+/** The cards currently rendered inside a column, by the column's visible title. */
+function cardsIn(columnTitle: string): string[] {
+  const list = screen.getByRole('list', { name: `${columnTitle} cards` });
+  return within(list)
+    .queryAllByRole('listitem')
+    .map((el) => el.getAttribute('aria-label') ?? '');
+}
+
+/** An accepted write — this file measures the SUCCESS path, not a refusal. */
+const acceptingUpdate = () => vi.fn(async () => ({ id: 'a', status: 'in_progress' }));
+
+/**
+ * Mount on one of the two data ownerships and wait until it has settled: the
+ * object-schema fetch and (internal path) the record fetch both land as async
+ * state updates, and a stray one arriving AFTER the drag would re-render the
+ * board and add a snapshot that belongs to neither reading.
+ */
+async function mountBoard(mode: 'external' | 'internal', dataSource: DataSource) {
+  render(
+    <ObjectKanban
+      schema={schema}
+      dataSource={dataSource}
+      {...(mode === 'external' ? { data: RECORDS() } : {})}
+    />,
+  );
+  expect(await screen.findByText('Alpha')).toBeInTheDocument();
+  await act(async () => {
+    await Promise.resolve();
+    await Promise.resolve();
+  });
+  await waitFor(() => expect(cardsIn('Backlog')).toEqual(['Alpha']));
+  expect(recorder.snapshots[recorder.snapshots.length - 1]).toBe(AT_MOUNT);
+  return recorder.snapshots.length;
+}
+
+/** Drop `Alpha` onto `Xray` — slot 0 of "In Progress" — exactly as the board would. */
+async function dropAlphaOntoXray() {
+  expect(dnd.onDragEnd).toBeTypeOf('function');
+  await act(async () => {
+    dnd.onDragEnd!({ active: { id: 'a' }, over: { id: 'x' } });
+  });
+}
+
+beforeEach(() => {
+  dnd.onDragEnd = undefined;
+  recorder.snapshots.length = 0;
+  vi.spyOn(toast, 'error').mockImplementation(() => 'toast-id' as never);
+});
+
+afterEach(() => {
+  cleanup();
+  vi.restoreAllMocks();
+});
+
+describe('ObjectKanban — a cross-column drop loses the landing position too (#8826 Q3)', () => {
+  it('LIT POSITIVE CONTROL: the drop does produce a new `columns` snapshot, carrying the membership change', async () => {
+    // The card's own control. Without it, "the drop order never appears" would
+    // be indistinguishable from a recorder that was never wired up.
+    const before = await mountBoard('internal', makeDataSource(acceptingUpdate() as never));
+
+    await dropAlphaOntoXray();
+
+    const fresh = recorder.snapshots.slice(before);
+    expect(fresh.length).toBeGreaterThan(0);
+    const latest = fresh[fresh.length - 1];
+    // Membership DID flow back: `Alpha` left `backlog` and is in `in_progress`.
+    expect(latest).not.toBe(AT_MOUNT);
+    expect(latest).toMatch(/^backlog: \| in_progress:/);
+    expect(latest).toContain('Alpha');
+  });
+
+  it('DEAD NEGATIVE CONTROL: a drag that ends on nothing records no snapshot and writes nothing', async () => {
+    const update = acceptingUpdate();
+    const before = await mountBoard('internal', makeDataSource(update as never));
+
+    await act(async () => {
+      dnd.onDragEnd!({ active: { id: 'a' }, over: null });
+    });
+
+    expect(recorder.snapshots.slice(before)).toEqual([]);
+    expect(update).not.toHaveBeenCalled();
+  });
+
+  it('THE MEASUREMENT (internal data): the drop order is never recorded — the prop comes back in RECORD order', async () => {
+    const before = await mountBoard('internal', makeDataSource(acceptingUpdate() as never));
+
+    await dropAlphaOntoXray();
+
+    // The whole reading: across every snapshot this board has ever received,
+    // the order the user dropped in is not among them.
+    expect(recorder.snapshots).not.toContain(DROP_ORDER);
+    expect(recorder.snapshots.slice(before)).toEqual([RECORD_ORDER]);
+  });
+
+  it('THE MEASUREMENT (internal data): the card visibly leaves the dropped slot in the same act', async () => {
+    await mountBoard('internal', makeDataSource(acceptingUpdate() as never));
+
+    await dropAlphaOntoXray();
+
+    // Dropped at slot 0; on screen at slot 2. The optimistic write re-renders
+    // the board, which re-buckets and re-syncs the mirror from the prop.
+    expect(cardsIn('In Progress')).toEqual(['Xray', 'Yankee', 'Alpha']);
+    expect(cardsIn('Backlog')).toEqual([]);
+  });
+
+  it('THE ASYMMETRY (external data): no snapshot at all, so the dropped slot survives on screen', async () => {
+    const before = await mountBoard('external', makeDataSource(acceptingUpdate() as never));
+
+    await dropAlphaOntoXray();
+
+    // The optimistic write is skipped on this ownership, so nothing re-renders
+    // and `KanbanImpl`'s mirror is never reset — the local move stands.
+    expect(recorder.snapshots.slice(before)).toEqual([]);
+    expect(cardsIn('In Progress')).toEqual(['Alpha', 'Xray', 'Yankee']);
+    // ...and it is still not persisted anywhere: the next reflow from the
+    // parent will carry membership and not order.
+  });
+
+  it('THE WIRE: the landing index reaches no sink — the write body carries the column and nothing else', async () => {
+    for (const mode of ['internal', 'external'] as const) {
+      const update = acceptingUpdate();
+      await mountBoard(mode, makeDataSource(update as never));
+
+      await dropAlphaOntoXray();
+
+      expect(update).toHaveBeenCalledTimes(1);
+      // Exact body, not a subset match: an added positional key must fail here.
+      expect(update).toHaveBeenCalledWith('task', 'a', { status: 'in_progress' });
+      cleanup();
+      recorder.snapshots.length = 0;
+      dnd.onDragEnd = undefined;
+    }
+  });
+});