diff --git a/frontend/src/components/FloatingAddButton.test.ts b/frontend/src/components/FloatingAddButton.test.ts
new file mode 100644
index 0000000..b6f2d82
--- /dev/null
+++ b/frontend/src/components/FloatingAddButton.test.ts
@@ -0,0 +1,175 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
+import { createApp, h, nextTick, ref } from 'vue'
+import FloatingAddButton from './FloatingAddButton.vue'
+
+const cleanups: Array<() => void> = []
+
+function pointerEvent(type: string, init: {
+  pointerId?: number
+  clientX?: number
+  clientY?: number
+  pointerType?: string
+  button?: number
+  isPrimary?: boolean
+} = {}) {
+  const event = new Event(type, { bubbles: true, cancelable: true })
+  Object.defineProperties(event, {
+    pointerId: { value: init.pointerId ?? 1 },
+    clientX: { value: init.clientX ?? 0 },
+    clientY: { value: init.clientY ?? 0 },
+    pointerType: { value: init.pointerType ?? 'touch' },
+    button: { value: init.button ?? 0 },
+    isPrimary: { value: init.isPrimary ?? true },
+  })
+  return event
+}
+
+async function mountButton(options: { show?: boolean; label?: string; reducedMotion?: boolean } = {}) {
+  const host = document.createElement('div')
+  document.body.append(host)
+  const show = ref(options.show ?? true)
+  const activations: Array<{ x: number; y: number }> = []
+  vi.stubGlobal('matchMedia', vi.fn().mockReturnValue({
+    matches: options.reducedMotion ?? false,
+    addEventListener: vi.fn(),
+    removeEventListener: vi.fn(),
+  }))
+  const app = createApp({
+    setup: () => () => h(FloatingAddButton, {
+      show: show.value,
+      label: options.label,
+      onActivate: (origin: { x: number; y: number }) => activations.push(origin),
+    }),
+  })
+  app.mount(host)
+  cleanups.push(() => { app.unmount(); host.remove() })
+  await nextTick()
+  return { host, show, activations, button: () => host.querySelector<HTMLButtonElement>('button') }
+}
+
+function setRect(button: HTMLButtonElement, left: number, top: number) {
+  Object.defineProperty(button, 'getBoundingClientRect', {
+    configurable: true,
+    value: () => ({ left, top, width: 56, height: 56, right: left + 56, bottom: top + 56, x: left, y: top, toJSON: () => ({}) }),
+  })
+}
+
+beforeEach(() => {
+  vi.useFakeTimers()
+  vi.stubGlobal('innerWidth', 390)
+  vi.stubGlobal('innerHeight', 844)
+  document.documentElement.style.setProperty('--safe-area-bottom', '20px')
+})
+
+afterEach(() => {
+  cleanups.splice(0).forEach((cleanup) => cleanup())
+  document.documentElement.style.removeProperty('--safe-area-bottom')
+  vi.useRealTimers()
+  vi.unstubAllGlobals()
+})
+
+describe('FloatingAddButton', () => {
+  it('renders an accessible native button and emits its visual center when clicked', async () => {
+    const { button, activations } = await mountButton({ label: '新建任务' })
+    const fab = button()!
+    setRect(fab, 100, 200)
+
+    expect(fab.tagName).toBe('BUTTON')
+    expect(fab.getAttribute('aria-label')).toBe('新建任务')
+    fab.focus()
+    expect(document.activeElement).toBe(fab)
+    fab.click()
+
+    expect(activations).toEqual([{ x: 128, y: 228 }])
+  })
+
+  it('reacts to show changes without emitting an activation', async () => {
+    const { button, show, activations } = await mountButton({ show: false })
+    expect(button()).toBeNull()
+
+    show.value = true
+    await nextTick()
+    expect(button()?.getAttribute('aria-label')).toBe('添加')
+    expect(activations).toEqual([])
+  })
+
+  it('drags, clamps, snaps to the nearest edge, and suppresses the trailing click', async () => {
+    const { button, activations } = await mountButton()
+    const fab = button()!
+    setRect(fab, 300, 700)
+    const capture = vi.fn()
+    Object.defineProperty(fab, 'setPointerCapture', { configurable: true, value: capture })
+
+    fab.dispatchEvent(pointerEvent('pointerdown', { pointerId: 7, clientX: 310, clientY: 710 }))
+    fab.dispatchEvent(pointerEvent('pointermove', { pointerId: 7, clientX: 50, clientY: 400 }))
+    await nextTick()
+
+    expect(capture).toHaveBeenCalledWith(7)
+    expect(fab.classList.contains('dragging')).toBe(true)
+    expect(fab.style.left).toBe('40px')
+    expect(fab.style.top).toBe('390px')
+
+    fab.dispatchEvent(pointerEvent('pointerup', { pointerId: 7, clientX: 50, clientY: 400 }))
+    await nextTick()
+    expect(fab.classList.contains('dragging')).toBe(false)
+    expect(fab.classList.contains('snapping')).toBe(true)
+    expect(fab.style.left).toBe('14px')
+    expect(fab.style.top).toBe('390px')
+
+    fab.click()
+    expect(activations).toEqual([])
+    vi.advanceTimersByTime(180)
+    fab.click()
+    expect(activations).toEqual([{ x: 328, y: 728 }])
+
+    vi.advanceTimersByTime(40)
+    await nextTick()
+    expect(fab.classList.contains('snapping')).toBe(false)
+  })
+
+  it('does not start dragging for secondary mouse or non-primary pointers', async () => {
+    const { button } = await mountButton()
+    const fab = button()!
+    setRect(fab, 100, 200)
+
+    fab.dispatchEvent(pointerEvent('pointerdown', { pointerType: 'mouse', button: 2 }))
+    fab.dispatchEvent(pointerEvent('pointermove', { clientX: 300, clientY: 500 }))
+    fab.dispatchEvent(pointerEvent('pointerdown', { pointerId: 2, isPrimary: false }))
+    fab.dispatchEvent(pointerEvent('pointermove', { pointerId: 2, clientX: 300, clientY: 500 }))
+    await nextTick()
+
+    expect(fab.classList.contains('dragging')).toBe(false)
+    expect(fab.getAttribute('style')).toBeNull()
+  })
+
+  it('snaps without animation when reduced motion is requested', async () => {
+    const { button } = await mountButton({ reducedMotion: true })
+    const fab = button()!
+    setRect(fab, 250, 300)
+
+    fab.dispatchEvent(pointerEvent('pointerdown', { clientX: 260, clientY: 310 }))
+    fab.dispatchEvent(pointerEvent('pointerup', { clientX: 280, clientY: 310 }))
+    await nextTick()
+
+    expect(fab.style.left).toBe('320px')
+    expect(fab.style.top).toBe('300px')
+    expect(fab.classList.contains('snapping')).toBe(false)
+  })
+
+  it('re-snaps a placed button after the viewport is resized', async () => {
+    const { button } = await mountButton()
+    const fab = button()!
+    setRect(fab, 250, 300)
+    fab.dispatchEvent(pointerEvent('pointerdown', { clientX: 260, clientY: 310 }))
+    fab.dispatchEvent(pointerEvent('pointerup', { clientX: 280, clientY: 310 }))
+    await nextTick()
+    expect(fab.style.left).toBe('320px')
+
+    vi.stubGlobal('innerWidth', 300)
+    window.dispatchEvent(new Event('resize'))
+    await nextTick()
+
+    expect(fab.style.left).toBe('230px')
+    expect(fab.style.top).toBe('300px')
+  })
+})
diff --git a/tests/test_recurrence_service_unit.py b/tests/test_recurrence_service_unit.py
new file mode 100644
index 0000000..cdc2bfe
--- /dev/null
+++ b/tests/test_recurrence_service_unit.py
@@ -0,0 +1,530 @@
+from datetime import UTC, datetime
+from types import SimpleNamespace
+from unittest.mock import AsyncMock
+from uuid import uuid4
+
+import pytest
+from fastapi import HTTPException
+from sqlalchemy import select, update
+from sqlalchemy.dialects import postgresql
+from sqlalchemy.ext.asyncio import async_sessionmaker, create_async_engine
+
+from backend import recurrence_service
+from backend.db import Base
+from backend.models import RecurrenceTemplate, Task, TaskList, User
+
+NOW = datetime(2026, 9, 19, 8, 0, tzinfo=UTC)
+DUE = datetime(2026, 9, 20, 9, 30, tzinfo=UTC)
+NEXT_DUE = datetime(2026, 9, 21, 9, 30, tzinfo=UTC)
+
+
+def make_user(*, timezone="Asia/Shanghai"):
+    return User(id=uuid4(), username="owner", password_hash="hash", timezone=timezone)
+
+
+def make_task(user, **overrides):
+    values = {
+        "id": uuid4(),
+        "user_id": user.id,
+        "list_id": uuid4(),
+        "title": "task",
+        "completed": False,
+        "due_at": DUE,
+        "due_has_time": True,
+        "version": 3,
+    }
+    values.update(overrides)
+    return Task(**values)
+
+
+def make_recurrence(user, task, **overrides):
+    values = {
+        "id": uuid4(),
+        "user_id": user.id,
+        "task_id": task.id,
+        "rrule": "FREQ=DAILY",
+        "starts_at": DUE,
+        "ends_at": None,
+        "trigger_mode": "scheduled",
+    }
+    values.update(overrides)
+    return RecurrenceTemplate(**values)
+
+
+def result_returning(task):
+    return SimpleNamespace(scalar_one_or_none=lambda: task)
+
+
+@pytest.fixture
+async def sqlite_db(tmp_path):
+    engine = create_async_engine(f"sqlite+aiosqlite:///{tmp_path / 'recurrence.db'}")
+    async with engine.begin() as connection:
+        await connection.run_sync(Base.metadata.create_all)
+    session_factory = async_sessionmaker(engine, expire_on_commit=False)
+    async with session_factory() as db:
+        yield db
+    await engine.dispose()
+
+
+async def persist_user_and_list(db, username):
+    user = User(username=username, password_hash="hash", timezone="Asia/Shanghai")
+    db.add(user)
+    await db.flush()
+    task_list = TaskList(user_id=user.id, name=f"{username} inbox", is_inbox=True)
+    db.add(task_list)
+    await db.flush()
+    return user, task_list
+
+
+@pytest.mark.asyncio
+async def test_lock_task_filters_owner_and_soft_deleted_tasks(sqlite_db):
+    owner, owner_list = await persist_user_and_list(sqlite_db, "owner")
+    foreign, foreign_list = await persist_user_and_list(sqlite_db, "foreign")
+    active = Task(user_id=owner.id, list_id=owner_list.id, title="active")
+    deleted = Task(
+        user_id=owner.id,
+        list_id=owner_list.id,
+        title="deleted",
+        deleted_at=NOW,
+    )
+    foreign_task = Task(user_id=foreign.id, list_id=foreign_list.id, title="foreign")
+    sqlite_db.add_all([active, deleted, foreign_task])
+    await sqlite_db.commit()
+
+    assert await recurrence_service.lock_task(sqlite_db, owner.id, active.id) is active
+    assert await recurrence_service.lock_task(sqlite_db, foreign.id, active.id) is None
+    assert await recurrence_service.lock_task(sqlite_db, owner.id, foreign_task.id) is None
+    assert await recurrence_service.lock_task(sqlite_db, owner.id, deleted.id) is None
+
+
+@pytest.mark.asyncio
+async def test_lock_task_actual_query_compiles_with_for_update():
+    class CapturingDb:
+        statement = None
+
+        async def scalar(self, statement):
+            self.statement = statement
+
+    db = CapturingDb()
+    await recurrence_service.lock_task(db, uuid4(), uuid4())
+
+    statement = db.statement
+    assert statement is not None
+    sql = str(statement.compile(dialect=postgresql.dialect()))
+
+    assert "tasks.user_id" in sql
+    assert "tasks.deleted_at IS NULL" in sql
+    assert sql.endswith(" FOR UPDATE")
+
+
+@pytest.mark.asyncio
+async def test_apply_task_changes_stale_version_cannot_update_database(sqlite_db):
+    owner, owner_list = await persist_user_and_list(sqlite_db, "owner")
+    task = Task(
+        user_id=owner.id,
+        list_id=owner_list.id,
+        title="original",
+        version=3,
+    )
+    sqlite_db.add(task)
+    await sqlite_db.commit()
+    task_id = task.id
+
+    with pytest.raises(HTTPException) as exc_info:
+        await recurrence_service.apply_task_changes(
+            sqlite_db,
+            user=owner,
+            task_id=task_id,
+            expected_version=2,
+            changes={"title": "stale write"},
+        )
+
+    await sqlite_db.rollback()
+    sqlite_db.expire_all()
+    persisted = await sqlite_db.get(Task, task_id)
+    assert exc_info.value.status_code == 409
+    assert persisted.title == "original"
+    assert persisted.version == 3
+
+
+@pytest.mark.asyncio
+async def test_optimistic_update_version_predicate_rejects_race(sqlite_db, monkeypatch):
+    owner, owner_list = await persist_user_and_list(sqlite_db, "owner")
+    task = Task(user_id=owner.id, list_id=owner_list.id, title="original", version=3)
+    sqlite_db.add(task)
+    await sqlite_db.commit()
+    task_id = task.id
+
+    original_execute = sqlite_db.execute
+    raced = False
+
+    async def execute_after_concurrent_version_change(statement, *args, **kwargs):
+        nonlocal raced
+        if not raced and getattr(statement, "is_update", False):
+            raced = True
+            await original_execute(
+                update(Task).where(Task.id == task_id).values(title="racer", version=4)
+            )
+            await sqlite_db.flush()
+        return await original_execute(statement, *args, **kwargs)
+
+    monkeypatch.setattr(sqlite_db, "execute", execute_after_concurrent_version_change)
+
+    with pytest.raises(HTTPException) as exc_info:
+        await recurrence_service.apply_task_changes(
+            sqlite_db,
+            user=owner,
+            task_id=task_id,
+            expected_version=3,
+            changes={"title": "loser"},
+        )
+
+    assert raced is True
+    assert exc_info.value.status_code == 409
+    sqlite_db.expire_all()
+    persisted = await sqlite_db.get(Task, task_id)
+    assert persisted.title == "racer"
+    assert persisted.version == 4
+
+
+@pytest.mark.asyncio
+async def test_recurring_completion_resets_only_completed_active_owned_subtasks(
+    sqlite_db, monkeypatch
+):
+    owner, owner_list = await persist_user_and_list(sqlite_db, "owner")
+    foreign, foreign_list = await persist_user_and_list(sqlite_db, "foreign")
+    parent = Task(
+        user_id=owner.id,
+        list_id=owner_list.id,
+        title="recurring parent",
+        due_at=DUE,
+        due_has_time=True,
+        version=3,
+    )
+    other_parent = Task(user_id=owner.id, list_id=owner_list.id, title="other parent")
+    sqlite_db.add_all([parent, other_parent])
+    await sqlite_db.flush()
+    recurrence = make_recurrence(owner, parent)
+    target = Task(
+        user_id=owner.id,
+        list_id=owner_list.id,
+        parent_id=parent.id,
+        title="target",
+        completed=True,
+        completed_at=NOW,
+        version=2,
+    )
+    incomplete = Task(
+        user_id=owner.id,
+        list_id=owner_list.id,
+        parent_id=parent.id,
+        title="already incomplete",
+        completed=False,
+        version=2,
+    )
+    deleted = Task(
+        user_id=owner.id,
+        list_id=owner_list.id,
+        parent_id=parent.id,
+        title="deleted child",
+        completed=True,
+        completed_at=NOW,
+        deleted_at=NOW,
+        version=2,
+    )
+    other_parent_child = Task(
+        user_id=owner.id,
+        list_id=owner_list.id,
+        parent_id=other_parent.id,
+        title="other parent child",
+        completed=True,
+        completed_at=NOW,
+        version=2,
+    )
+    foreign_child = Task(
+        user_id=foreign.id,
+        list_id=foreign_list.id,
+        parent_id=parent.id,
+        title="foreign child",
+        completed=True,
+        completed_at=NOW,
+        version=2,
+    )
+    sqlite_db.add_all(
+        [recurrence, target, incomplete, deleted, other_parent_child, foreign_child]
+    )
+    await sqlite_db.commit()
+    parent_id = parent.id
+    child_ids = [
+        target.id,
+        incomplete.id,
+        deleted.id,
+        other_parent_child.id,
+        foreign_child.id,
+    ]
+    monkeypatch.setattr(recurrence_service, "utcnow", lambda: NOW)
+    monkeypatch.setattr(recurrence_service, "occurrences", lambda *args: [NEXT_DUE])
+
+    _, changed_fields = await recurrence_service.apply_task_changes(
+        sqlite_db,
+        user=owner,
+        task_id=parent_id,
+        expected_version=3,
+        changes={"completed": True},
+    )
+    await sqlite_db.commit()
+    sqlite_db.expire_all()
+
+    rows = {
+        row.title: row
+        for row in (
+            await sqlite_db.scalars(select(Task).where(Task.id.in_(child_ids)))
+        ).all()
+    }
+    persisted_parent = await sqlite_db.get(Task, parent_id)
+    assert persisted_parent.completed is False
+    assert persisted_parent.due_at == NEXT_DUE
+    assert changed_fields == {"completed"}
+    assert (rows["target"].completed, rows["target"].completed_at, rows["target"].version) == (
+        False,
+        None,
+        3,
+    )
+    for title in ("deleted child", "other parent child", "foreign child"):
+        assert rows[title].completed is True
+        assert rows[title].completed_at == NOW
+        assert rows[title].version == 2
+    assert rows["already incomplete"].completed is False
+    assert rows["already incomplete"].version == 2
+
+
+@pytest.mark.parametrize("timezone", ["Not/A_Zone", ""])
+def test_user_zone_rejects_invalid_timezone(timezone):
+    with pytest.raises(HTTPException) as exc_info:
+        recurrence_service._user_zone(make_user(timezone=timezone))
+
+    assert exc_info.value.status_code == 422
+    assert exc_info.value.detail == "用户时区无效"
+
+
+@pytest.mark.asyncio
+async def test_apply_task_changes_returns_not_found_when_lock_finds_no_task():
+    db = AsyncMock()
+    db.scalar.return_value = None
+    user = make_user()
+
+    with pytest.raises(HTTPException) as exc_info:
+        await recurrence_service.apply_task_changes(
+            db, user=user, task_id=uuid4(), expected_version=1, changes={"title": "new"}
+        )
+
+    assert exc_info.value.status_code == 404
+    assert db.scalar.await_count == 1
+    db.execute.assert_not_awaited()
+
+
+@pytest.mark.asyncio
+async def test_apply_task_changes_rejects_stale_version_before_loading_recurrence():
+    user = make_user()
+    task = make_task(user)
+    db = AsyncMock()
+    db.scalar.return_value = task
+
+    with pytest.raises(HTTPException) as exc_info:
+        await recurrence_service.apply_task_changes(
+            db, user=user, task_id=task.id, expected_version=2, changes={"title": "new"}
+        )
+
+    assert exc_info.value.status_code == 409
+    assert db.scalar.await_count == 1
+    db.execute.assert_not_awaited()
+
+
+@pytest.mark.asyncio
+async def test_plain_completion_sets_timestamp_without_resetting_subtasks(monkeypatch):
+    user = make_user()
+    task = make_task(user)
+    updated = make_task(user, id=task.id, completed=True, version=4, completed_at=NOW)
+    db = AsyncMock()
+    db.scalar.side_effect = [task, None]
+    db.execute.return_value = result_returning(updated)
+    monkeypatch.setattr(recurrence_service, "utcnow", lambda: NOW)
+    changes = {"completed": True}
+
+    returned, changed_fields = await recurrence_service.apply_task_changes(
+        db, user=user, task_id=task.id, expected_version=3, changes=changes
+    )
+
+    assert returned is updated
+    assert changed_fields == {"completed"}
+    assert changes["completed_at"] == NOW
+    assert db.execute.await_count == 1
+
+
+@pytest.mark.asyncio
+async def test_uncompleting_task_clears_completed_timestamp(monkeypatch):
+    user = make_user()
+    task = make_task(user, completed=True, completed_at=NOW)
+    updated = make_task(user, id=task.id, completed=False, version=4, completed_at=None)
+    db = AsyncMock()
+    db.scalar.side_effect = [task, None]
+    db.execute.return_value = result_returning(updated)
+    monkeypatch.setattr(recurrence_service, "utcnow", lambda: NOW)
+    changes = {"completed": False}
+
+    await recurrence_service.apply_task_changes(
+        db, user=user, task_id=task.id, expected_version=3, changes=changes
+    )
+
+    assert changes["completed_at"] is None
+
+
+@pytest.mark.asyncio
+async def test_after_completion_reschedules_from_completion_and_records_it(monkeypatch):
+    user = make_user()
+    task = make_task(user)
+    recurrence = make_recurrence(
+        user,
+        task,
+        rrule=None,
+        trigger_mode="after_completion",
+        after_completion_days=2,
+    )
+    updated = make_task(user, id=task.id, due_at=NEXT_DUE, version=4)
+    db = AsyncMock()
+    db.scalar.side_effect = [task, recurrence]
+    db.execute.side_effect = [result_returning(updated), SimpleNamespace()]
+    monkeypatch.setattr(recurrence_service, "utcnow", lambda: NOW)
+    monkeypatch.setattr(recurrence_service, "_after_completion_due", lambda *args: NEXT_DUE)
+    changes = {"completed": True}
+
+    await recurrence_service.apply_task_changes(
+        db, user=user, task_id=task.id, expected_version=3, changes=changes
+    )
+
+    assert changes == {"completed": False, "due_at": NEXT_DUE, "completed_at": None}
+    assert recurrence.starts_at == NEXT_DUE
+    assert recurrence.last_completed_at == NOW
+    assert db.execute.await_count == 2
+
+
+@pytest.mark.asyncio
+async def test_scheduled_completion_advances_due_and_resets_completed_subtasks(monkeypatch):
+    user = make_user()
+    task = make_task(user)
+    recurrence = make_recurrence(user, task)
+    updated = make_task(user, id=task.id, due_at=NEXT_DUE, version=4)
+    db = AsyncMock()
+    db.scalar.side_effect = [task, recurrence]
+    db.execute.side_effect = [result_returning(updated), SimpleNamespace()]
+    monkeypatch.setattr(recurrence_service, "utcnow", lambda: NOW)
+    monkeypatch.setattr(recurrence_service, "occurrences", lambda *args: [NEXT_DUE])
+    changes = {"completed": True}
+
+    returned, changed_fields = await recurrence_service.apply_task_changes(
+        db, user=user, task_id=task.id, expected_version=3, changes=changes
+    )
+
+    assert returned is updated
+    assert changed_fields == {"completed"}
+    assert changes == {"completed": False, "due_at": NEXT_DUE, "completed_at": None}
+    assert recurrence.starts_at == NEXT_DUE
+    assert db.execute.await_count == 2
+
+
+@pytest.mark.asyncio
+async def test_scheduled_completion_stays_completed_when_rule_has_no_future_occurrence(monkeypatch):
+    user = make_user()
+    task = make_task(user)
+    recurrence = make_recurrence(user, task)
+    updated = make_task(user, id=task.id, completed=True, version=4, completed_at=NOW)
+    db = AsyncMock()
+    db.scalar.side_effect = [task, recurrence]
+    db.execute.return_value = result_returning(updated)
+    monkeypatch.setattr(recurrence_service, "utcnow", lambda: NOW)
+    monkeypatch.setattr(recurrence_service, "occurrences", lambda *args: [])
+    changes = {"completed": True}
+
+    await recurrence_service.apply_task_changes(
+        db, user=user, task_id=task.id, expected_version=3, changes=changes
+    )
+
+    assert changes == {"completed": True, "completed_at": NOW}
+    assert recurrence.starts_at == DUE
+    assert db.execute.await_count == 1
+
+
+@pytest.mark.asyncio
+async def test_removing_due_date_deletes_recurrence(monkeypatch):
+    user = make_user()
+    task = make_task(user)
+    recurrence = make_recurrence(user, task)
+    updated = make_task(user, id=task.id, due_at=None, version=4)
+    db = AsyncMock()
+    db.scalar.side_effect = [task, recurrence]
+    db.execute.return_value = result_returning(updated)
+    monkeypatch.setattr(recurrence_service, "utcnow", lambda: NOW)
+
+    await recurrence_service.apply_task_changes(
+        db, user=user, task_id=task.id, expected_version=3, changes={"due_at": None}
+    )
+
+    db.delete.assert_awaited_once_with(recurrence)
+
+
+@pytest.mark.asyncio
+async def test_moving_due_date_keeps_recurrence_anchor_in_sync(monkeypatch):
+    user = make_user()
+    task = make_task(user)
+    recurrence = make_recurrence(user, task)
+    updated = make_task(user, id=task.id, due_at=NEXT_DUE, version=4)
+    db = AsyncMock()
+    db.scalar.side_effect = [task, recurrence]
+    db.execute.return_value = result_returning(updated)
+    monkeypatch.setattr(recurrence_service, "utcnow", lambda: NOW)
+
+    await recurrence_service.apply_task_changes(
+        db, user=user, task_id=task.id, expected_version=3, changes={"due_at": NEXT_DUE}
+    )
+
+    assert recurrence.starts_at == NEXT_DUE
+    db.delete.assert_not_awaited()
+
+
+@pytest.mark.asyncio
+async def test_completion_request_with_explicit_due_does_not_reanchor_recurrence(monkeypatch):
+    user = make_user()
+    task = make_task(user, completed=True, completed_at=NOW)
+    recurrence = make_recurrence(user, task)
+    updated = make_task(user, id=task.id, completed=True, due_at=NEXT_DUE, version=4)
+    db = AsyncMock()
+    db.scalar.side_effect = [task, recurrence]
+    db.execute.return_value = result_returning(updated)
+    monkeypatch.setattr(recurrence_service, "utcnow", lambda: NOW)
+
+    await recurrence_service.apply_task_changes(
+        db,
+        user=user,
+        task_id=task.id,
+        expected_version=3,
+        changes={"completed": True, "due_at": NEXT_DUE},
+    )
+
+    assert recurrence.starts_at == DUE
+
+
+@pytest.mark.asyncio
+async def test_concurrent_update_loss_returns_conflict(monkeypatch):
+    user = make_user()
+    task = make_task(user)
+    db = AsyncMock()
+    db.scalar.side_effect = [task, None]
+    db.execute.return_value = result_returning(None)
+    monkeypatch.setattr(recurrence_service, "utcnow", lambda: NOW)
+
+    with pytest.raises(HTTPException) as exc_info:
+        await recurrence_service.apply_task_changes(
+            db, user=user, task_id=task.id, expected_version=3, changes={"title": "new"}
+        )
+
+    assert exc_info.value.status_code == 409