fix: keep occurrence changes on their days when a series moves or ends

Three faults in how a series' changed and cancelled occurrences follow a
change of the whole series (found in review, live on 1.0 and 0.16.23 for
the web's version of the same code):

- A rule that names its days - a series on Mondays and Tuesdays, one on
  the 5th and the 6th of each month - keeps its occurrences on those days
  when its start moves. Moving every occurrence by a day moved each
  override along anyway, so the Monday's landed on the Tuesday's key and
  one silently overwrote the other: the cancelled Tuesday came back with
  the Monday's title, and the Monday lost its change. Under such a rule
  every override but the first occurrence's now stays on its day, at
  the series' new time; the same holds for the new series of "this and
  following". Two overrides that still meet on one key (an hourly series
  turned all-day) no longer overwrite each other: the one that stays
  where it is, else the first, is kept, and the others are left out and
  logged.
- "Does not repeat" for every occurrence kept the first occurrence's
  override on the single event, because a series' start counts as an
  occurrence of any rule: cancelled, 1.0 left the event out of every
  range query (still stored); retitled, 0.16.23 listed it twice. A series
  made single now keeps no overrides.
- The SEQUENCE a change of every occurrence raises (0.16.6 to 0.16.23
  refuse a lower one) came from the series as the app held it. A series
  opened from a reminder or a link was that copy, possibly older than the
  server's. The store now reads a series from the server every time a
  scope edit asks for it, so the raise, the overrides a split or a move
  rewrites and the rule all start from what the server holds.

src/lib/__tests__/recurrence-scope-edit.test.ts

@@ -1239,3 +1239,144 @@ describe.each<[string, Version]>([['1.0', '1.0'], ['0.16', '0.16']])('the series
     expect(deps.deleteEvent).not.toHaveBeenCalled();
   });
 });
+
+// A rule that names its days keeps its occurrences on them: a Monday and
+// Tuesday series moved by a day for every occurrence still meets on Mondays
+// and Tuesdays. Moved along, the Monday's override landed on the Tuesday's
+// and one overwrote the other: the cancelled Tuesday came back with the
+// Monday's title, and the Monday lost its change (live on 1.0 and 0.16.23).
+describe.each<[string, Version]>([['1.0', '1.0'], ['0.16', '0.16']])('overrides of a series whose rule names its days on Stalwart %s', (_label, version) => {
+  const participants = { org: participant(version, ORGANIZER, true, 'accepted'), b: participant(version, ATTENDEE, false, 'accepted') };
+  const special = (start: string, title: string) => (version === '1.0'
+    ? { title }
+    : { updated: '2026-11-20T09:00:00Z', start, duration: 'PT1H', participants, title });
+  function withDays(rule: Record<string, unknown>, overrides: Record<string, unknown>, start = '2026-12-07T10:00:00'): CalendarEvent {
+    return {
+      ...series(version), start, utcStart: berlinUtc(start), utcEnd: berlinUtc(start, 1),
+      participants, recurrenceRules: [{ '@type': 'RecurrenceRule', ...rule }], recurrenceOverrides: overrides,
+    } as unknown as CalendarEvent;
+  }
+  const mondayAndTuesday = () => withDays(
+    { frequency: 'weekly', byDay: [{ day: 'mo' }, { day: 'tu' }], count: 8 },
+    { '2026-12-14T10:00:00': special('2026-12-14T10:00:00', 'Planning, Monday special'), '2026-12-15T10:00:00': { excluded: true } },
+  );
+  const allFrom = (master: CalendarEvent, recurrenceId: string, hours: number) => {
+    const occurrence = {
+      ...master, id: 'maaaaaf', baseEventId: 'b', start: recurrenceId, recurrenceId, recurrenceIdTimeZone: 'Europe/Berlin',
+      utcStart: berlinUtc(recurrenceId), utcEnd: berlinUtc(recurrenceId, 1),
+    } as CalendarEvent;
+    const updates = seriesUpdatesFromOccurrence(occurrence, master, editorUpdates(occurrence), editorOn(occurrence, moveBy(occurrence, hours)), context);
+    // Unchanged overrides are not written again.
+    return { updates, overrides: (updates.recurrenceOverrides ?? master.recurrenceOverrides) as Record<string, Record<string, unknown>> };
+  };
+
+  it('keeps each override on its day when every occurrence moves by a day', () => {
+    const master = mondayAndTuesday();
+    const { updates, overrides } = allFrom(master, '2026-12-21T10:00:00', 24);
+
+    expect(updates.start).toBe('2026-12-08T10:00:00');
+    expect(Object.keys(overrides).sort()).toEqual(['2026-12-14T10:00:00', '2026-12-15T10:00:00']);
+    expect(overrides['2026-12-14T10:00:00']).toMatchObject({ title: 'Planning, Monday special' });
+    if (version === '0.16') expect(overrides['2026-12-14T10:00:00'].start).toBe('2026-12-14T10:00:00');
+    expect(overrides['2026-12-15T10:00:00']).toEqual({ excluded: true });
+  });
+
+  it('keeps each override on its day, at the new time, when the series also moves by an hour', () => {
+    const master = mondayAndTuesday();
+    const { overrides } = allFrom(master, '2026-12-21T10:00:00', 25);
+
+    expect(Object.keys(overrides).sort()).toEqual(['2026-12-14T11:00:00', '2026-12-15T11:00:00']);
+    expect(overrides['2026-12-14T11:00:00']).toMatchObject({ title: 'Planning, Monday special' });
+    if (version === '0.16') expect(overrides['2026-12-14T11:00:00'].start).toBe('2026-12-14T11:00:00');
+    expect(overrides['2026-12-15T11:00:00']).toEqual({ excluded: true });
+  });
+
+  it('keeps month days as well', () => {
+    // The 5th and the 6th of each month; 5 January retitled, 6 January cancelled.
+    const master = withDays(
+      { frequency: 'monthly', byMonthDay: [5, 6], count: 6 },
+      { '2027-01-05T10:00:00': special('2027-01-05T10:00:00', 'Planning, January'), '2027-01-06T10:00:00': { excluded: true } },
+      '2026-12-05T10:00:00',
+    );
+    const { overrides } = allFrom(master, '2027-02-05T10:00:00', 24);
+
+    expect(Object.keys(overrides).sort()).toEqual(['2027-01-05T10:00:00', '2027-01-06T10:00:00']);
+    expect(overrides['2027-01-05T10:00:00']).toMatchObject({ title: 'Planning, January' });
+    expect(overrides['2027-01-06T10:00:00']).toEqual({ excluded: true });
+  });
+
+  it('keeps them on their days for this and the following occurrences too', async () => {
+    const master = mondayAndTuesday();
+    const occurrence = {
+      ...master, id: 'maaaaaf', baseEventId: 'b', start: '2026-12-08T10:00:00', recurrenceId: '2026-12-08T10:00:00',
+      recurrenceIdTimeZone: 'Europe/Berlin', utcStart: berlinUtc('2026-12-08T10:00:00'), utcEnd: berlinUtc('2026-12-08T10:00:00', 1),
+    } as CalendarEvent;
+    const deps = actions(master);
+
+    // From Tuesday 8 December on, moved by a day to the Wednesday.
+    await saveOccurrenceEdit('this_and_future', {
+      event: occurrence, updates: editorUpdates(occurrence), editor: editorOn(occurrence, moveBy(occurrence, 24)),
+      sendScheduling: true, userEmails: [ORGANIZER],
+    }, deps);
+
+    const created = deps.createEvent.mock.calls[0][0];
+    expect(created.start).toBe('2026-12-09T10:00:00');
+    expect(Object.keys(created.recurrenceOverrides ?? {}).sort()).toEqual(['2026-12-14T10:00:00', '2026-12-15T10:00:00']);
+    expect(created.recurrenceOverrides!['2026-12-14T10:00:00']).toMatchObject({ title: 'Planning, Monday special' });
+    expect(created.recurrenceOverrides!['2026-12-15T10:00:00']).toEqual({ excluded: true });
+  });
+});
+
+// Two occurrences can still meet on one key - an hourly series turned
+// all-day puts every hour of a day on that day. One override is kept, the
+// first; none overwrites another.
+describe('overrides that would meet on one occurrence', () => {
+  it('keeps the first and leaves out the others instead of overwriting it', () => {
+    const master = {
+      ...series('1.0'), start: '2026-12-07T10:00:00', utcStart: '2026-12-07T09:00:00Z', utcEnd: '2026-12-07T10:00:00Z',
+      recurrenceRules: [{ '@type': 'RecurrenceRule', frequency: 'hourly', count: 3 }],
+      recurrenceOverrides: {
+        '2026-12-07T10:00:00': { title: 'First hour' },
+        '2026-12-07T11:00:00': { title: 'Second hour' },
+        '2026-12-07T12:00:00': { excluded: true },
+      },
+    } as unknown as CalendarEvent;
+    const occurrence = {
+      ...master, id: 'maaaaag', baseEventId: 'b', start: '2026-12-07T11:00:00', recurrenceId: '2026-12-07T11:00:00',
+      utcStart: '2026-12-07T10:00:00Z', utcEnd: '2026-12-07T11:00:00Z',
+    } as CalendarEvent;
+    const day = startOfDay(getEventStartDate(occurrence));
+
+    const updates = seriesUpdatesFromOccurrence(
+      occurrence, master,
+      editorUpdates(occurrence, { start: '2026-12-07T00:00:00', duration: 'P1D', timeZone: null, showWithoutTime: true }),
+      editorOn(occurrence, { when: { allDay: true, start: day, end: day } }), context,
+    );
+
+    expect(updates.recurrenceOverrides).toEqual({ '2026-12-07T00:00:00': { title: 'First hour' } });
+  });
+});
+
+// "Repeat: none" for every occurrence makes the series one event. Its start
+// is an occurrence of any rule, so the first occurrence's override stayed on
+// the single event: cancelled, 1.0 left the event out of every range query
+// (still stored); retitled, 0.16.23 listed it twice (live).
+describe.each<[string, Version]>([['1.0', '1.0'], ['0.16', '0.16']])('a series made one event on Stalwart %s', (_label, version) => {
+  it('keeps none of its overrides', () => {
+    const first = version === '1.0'
+      ? { excluded: true }
+      : { start: '2026-10-05T10:00:00', duration: 'PT1H', title: 'Kick-off', participants: series(version).participants };
+    const master = { ...series(version), recurrenceOverrides: { ...series(version).recurrenceOverrides, '2026-10-05T10:00:00': first } } as CalendarEvent;
+    const occurrence = serverOccurrence(version);
+
+    const updates = seriesUpdatesFromOccurrence(
+      occurrence, master,
+      // What the editor writes for "Does not repeat" on a series.
+      editorUpdates(occurrence, { recurrenceRules: null, recurrenceOverrides: null, excludedRecurrenceRules: null }),
+      editorOn(occurrence, { ruleChanged: true }), context,
+    );
+
+    expect(updates.recurrenceRules).toBeNull();
+    expect(updates.recurrenceOverrides).toBeNull();
+  });
+});

src/lib/recurrence-scope-edit.ts

@@ -43,6 +43,7 @@ import {
   isValidTimeZone,
   localDateTimeToInstant,
 } from './calendar-timezone';
+import { debug } from './debug';
 
 /** What the event editor had when it saved, besides the event data it wrote. */
 export interface SeriesEdit {
@@ -103,6 +104,15 @@ interface SeriesFrame {
   recurrenceRules: CalendarEvent['recurrenceRules'];
 }
 
+/**
+ * Whether `rules` name the days their occurrences fall on (BYDAY,
+ * BYMONTHDAY, BYYEARDAY): a start moved by days does not take those
+ * occurrences along.
+ */
+function namesItsDays(rules: CalendarEvent['recurrenceRules']): boolean {
+  return (rules ?? []).some((rule) => !!rule.byDay?.length || !!rule.byMonthDay?.length || !!rule.byYearDay?.length);
+}
+
 /**
  * The key an override of the series `from` keeps in the series `to`, or
  * null when `to` has no such occurrence any more. It moves with its
@@ -112,16 +122,52 @@ interface SeriesFrame {
  * where it was - a weekday rule the start moved away from - keeps the key,
  * and an occurrence the new rule does not give is gone. A date the override
  * adds to the series, which no rule gives, moves along.
+ *
+ * A rule that names its days keeps every occurrence but the anchor on its
+ * day, at the series' new time: moved by a day, the Monday of a Monday and
+ * Tuesday series is still that Monday. Moved along, its override landed on
+ * the Tuesday's and took its place: a cancelled Tuesday came back with the
+ * Monday's title, and the Monday lost its change (live on 1.0 and 0.16.23).
  */
 function overrideKeyIn(key: string, from: SeriesFrame, to: SeriesFrame, anchor = from.start): string | null {
   const shifted = movedLike(key, anchor, to.start);
   const moved = to.showWithoutTime ? `${shifted.slice(0, 10)}T00:00:00` : shifted;
   if (!seriesOccursAt(from.start, from.recurrenceRules, key)) return moved;
+  if (namesItsDays(to.recurrenceRules) && wallClockMs(key) !== wallClockMs(anchor)) {
+    const sameDay = `${key.slice(0, 10)}${moved.slice(10)}`;
+    return seriesOccursAt(to.start, to.recurrenceRules, sameDay) ? sameDay : null;
+  }
   if (seriesOccursAt(to.start, to.recurrenceRules, moved)) return moved;
   if (moved !== key && seriesOccursAt(to.start, to.recurrenceRules, key)) return key;
   return null;
 }
 
+/**
+ * `overrides` with the key each takes (keyOf), those without one left out,
+ * and never two on one key: an override that stays where it is comes first,
+ * and one that would land on a key already taken is left out and logged
+ * rather than overwrite it. Two occurrences of one series never share a
+ * recurrence id.
+ */
+function rekeyed(
+  overrides: OverrideMap,
+  keyOf: (key: string) => string | null,
+): Array<{ key: string; target: string; override: OverrideMap[string] }> {
+  const moves = Object.entries(overrides)
+    .map(([key, override]) => ({ key, target: keyOf(key), override }))
+    .filter((move): move is { key: string; target: string; override: OverrideMap[string] } => move.target !== null);
+  moves.sort((a, b) => Number(b.target === b.key) - Number(a.target === a.key));
+  const taken = new Set<string>();
+  return moves.filter((move) => {
+    if (!taken.has(move.target)) {
+      taken.add(move.target);
+      return true;
+    }
+    debug.warn('calendar', 'Two recurrence overrides map to one occurrence; keeping the first', move.target, move.key);
+    return false;
+  });
+}
+
 /**
  * `entry` with its attendees asked again (participantsAskedAgain): a whole
  * participants map (0.16) with their answers back at needs-action, a
@@ -362,13 +408,17 @@ function seriesOverrides(
     showWithoutTime: series.showWithoutTime ?? !!master.showWithoutTime,
     recurrenceRules: options.ruleChanged ? series.recurrenceRules ?? null : master.recurrenceRules,
   };
+  // A series made single ("Repeat: none") keeps no overrides. Its start is
+  // an occurrence of any rule (seriesOccursAt), so the first occurrence's
+  // override stayed on the single event: cancelled, it hid the event from
+  // every range query on 1.0 (still stored); retitled, 0.16.23 listed it
+  // twice (live).
+  if (options.ruleChanged && !to.recurrenceRules?.length) return null;
   const turned = from.showWithoutTime !== to.showWithoutTime;
   const opened = resolveOverrideKey({ start: occurrence.start, recurrenceId: occurrence.recurrenceId ?? master.start }, overrides);
 
   const result: OverrideMap = {};
-  for (const [key, override] of Object.entries(overrides)) {
-    const target = overrideKeyIn(key, from, to);
-    if (!target) continue;
+  for (const { key, target, override } of rekeyed(overrides, (key) => overrideKeyIn(key, from, to))) {
     const { updated: _updated, ...rest } = override as OverrideEntry;
     let entry: OverrideEntry = rest;
     if (!entry.excluded) {
@@ -380,7 +430,10 @@ function seriesOverrides(
           const pinned = wallClockMs(entry.start) === wallClockMs(key)
             && (entry.timeZone === undefined || entry.timeZone === master.timeZone);
           follows = pinned || key === opened;
-          if (follows) entry.start = movedLike(entry.start, from.start, to.start);
+          // The time its occurrence had is the time it has now; one of its
+          // own takes the editor's move when the editor was opened on it.
+          if (pinned) entry.start = target;
+          else if (follows) entry.start = movedLike(entry.start, from.start, to.start);
         }
         if (resized && typeof entry.duration === 'string'
           && (key === opened || parseDurationSeconds(entry.duration) === parseDurationSeconds(master.duration))) {
@@ -730,9 +783,7 @@ function followingOverrides(
   const turned = from.showWithoutTime !== to.showWithoutTime;
   const resized = parseDurationSeconds(to.duration) !== parseDurationSeconds(series.duration);
   const result: OverrideMap = {};
-  for (const [key, override] of Object.entries(overrides)) {
-    const target = overrideKeyIn(key, from, to, splitKey);
-    if (!target) continue;
+  for (const { key, target, override } of rekeyed(overrides, (key) => overrideKeyIn(key, from, to, splitKey))) {
     const opened = key === splitKey;
     const { updated: _updated, ...rest } = override as OverrideEntry;
     let entry = withParticipantsOf(rest, to.participants);

src/stores/__tests__/calendar-store-series-edits.test.ts

@@ -113,6 +113,8 @@ describe('the series itself opened from a reminder on the calendar store', () =>
 
   it.each(['0.16.23', '1.0'])('overrides its first occurrence in the owner\'s account on Stalwart %s', async () => {
     useCalendarStore.setState({ events: [] });
+    // As the owner's account holds it: no overrides yet.
+    mockGetEvents.mockImplementation(async (ids: string[]) => (ids[0] === 'b' ? [{ ...master, recurrenceOverrides: null }] : []));
     const store = useCalendarStore.getState();
 
     await saveOccurrenceEdit('this', {
@@ -192,3 +194,34 @@ describe('a change of an occurrence whose override has no organizer (Stalwart 0.
     });
   });
 });
+
+// The SEQUENCE a change of every occurrence raises is the server's: a series
+// opened from a reminder a while ago - or shown as itself in the grid - is
+// an older copy, and a raise from its number goes out lower than what the
+// attendees hold, which 0.16 (0.16.6 to 0.16.23) refuses.
+describe.each(['0.16.23', '1.0'])('a change of every occurrence from a series opened a while ago on Stalwart %s', (version) => {
+  it('raises the SEQUENCE the server holds, not the stale copy\'s', async () => {
+    const overrides = version === '1.0'
+      ? { '2026-10-19T10:00:00': { title: 'Planning, special' } }
+      : { '2026-10-19T10:00:00': { title: 'Planning, special', start: '2026-10-19T10:00:00', duration: 'PT1H', sequence: 2 } };
+    const stale = { ...master, sequence: 2, recurrenceOverrides: overrides } as unknown as CalendarEvent;
+    const current = {
+      ...master, sequence: 5,
+      recurrenceOverrides: version === '1.0' ? overrides : { '2026-10-19T10:00:00': { ...overrides['2026-10-19T10:00:00'], sequence: 5 } },
+    } as unknown as CalendarEvent;
+    mockGetEvents.mockImplementation(async (ids: string[]) => (ids[0] === 'b' ? [current] : []));
+    useCalendarStore.setState({ events: [] });
+    const store = useCalendarStore.getState();
+
+    await saveOccurrenceEdit('all', {
+      event: stale,
+      updates: { title: 'Planning, room 2', start: stale.start, duration: 'PT1H', timeZone: 'Europe/Berlin', participants },
+      editor: { attendees: [{ name: '', email: 'userb@example.org' }], when: editorTimesFromEvent(stale) },
+      sendScheduling: true, userEmails: [ORGANIZER],
+    }, { updateEvent: store.updateEvent, createEvent: store.createEvent, getMasterEvent: store.getMasterEvent });
+
+    expect(mockUpdate.mock.calls.map(([id, patch, schedule]) => [id, patch, schedule])).toEqual([
+      ['b', { title: 'Planning, room 2', sequence: 6, 'recurrenceOverrides/2026-10-19T10:00:00/sequence': 6 }, true],
+    ]);
+  });
+});

src/stores/__tests__/calendar-store.test.ts

@@ -1034,10 +1034,18 @@ describe('calendar-store', () => {
       expect(master?.start).toBe('2026-03-02T09:00:00');
     });
 
-    it('getMasterEvent returns a master as-is', async () => {
-      const ev = { id: 'ev1', recurrenceRules: [{ frequency: 'daily' }] } as any;
-      expect(await useCalendarStore.getState().getMasterEvent(ev)).toBe(ev);
-      expect(mockGetEvents).not.toHaveBeenCalled();
+    // A copy the app holds - opened from a reminder a while ago, or shown as
+    // itself in the grid - may be older than the server's: the scope edits
+    // build on its overrides and its SEQUENCE.
+    it('getMasterEvent reads a master again from the server', async () => {
+      useCalendarStore.setState({ calendars: [], events: [] });
+      const ev = { id: 'ev1', recurrenceRules: [{ frequency: 'daily' }], sequence: 1 } as any;
+      mockGetEvents.mockResolvedValue([{ ...ev, sequence: 4 }]);
+
+      const master = await useCalendarStore.getState().getMasterEvent(ev);
+
+      expect(mockGetEvents).toHaveBeenCalledWith(['ev1'], undefined);
+      expect(master?.sequence).toBe(4);
     });
   });
 

src/stores/calendar-store.ts

@@ -254,8 +254,8 @@ export interface CalendarState {
     options?: { sendSchedulingMessages?: boolean },
   ) => Promise<void>;
   deleteEvent: (id: string) => Promise<void>;
-  // Resolve a client-side expanded occurrence (or a master) to its master
-  // event, fetching it from the server when the expansion replaced it.
+  // Resolve an expanded occurrence (or a master) to its master event, read
+  // from the server: a copy the app holds may be older.
   getMasterEvent: (event: CalendarEvent) => Promise<CalendarEvent | null>;
   // Invitations / scheduling
   rsvpEvent: (
@@ -1044,25 +1044,20 @@ export const useCalendarStore = create<CalendarState>()(
   },
 
   getMasterEvent: async (event) => {
-    if (event.recurrenceRules?.length && !event.recurrenceId && !isServerRecurrenceInstance(event)) {
-      // The series itself, opened from a reminder or a link rather than the
-      // grid: known from here on, so a change to it is sent to its own
-      // account under its raw id (resolveMutationTarget).
-      if (!get().events.some((e) => e.id === event.id)) knownMasters.set(event.id, event);
-      return event;
-    }
     // Expansion replaces the master with its occurrences, each pointing back
     // at the stored event: through baseEventId when the server expanded it,
-    // through originalId when the device did.
+    // through originalId when the device did. The stored event is read
+    // again every time, also for the series itself (opened from a reminder
+    // or a link) and one the grid shows as itself: a copy held here can be
+    // older than the server's, and the scope edits build on its overrides
+    // and its SEQUENCE - a change that raises it from a stale one goes out
+    // lower than the attendees' copies, which 0.16 refuses.
     const realId = seriesIdOf(event);
-    const inStore = get().events.find(
-      (e) => !e.recurrenceId && !!e.recurrenceRules?.length && !isServerRecurrenceInstance(e)
-        && seriesIdOf(e) === realId && (e.accountId ?? undefined) === (event.accountId ?? undefined),
-    );
-    if (inStore) return inStore;
     const fetched = (await fetchEvents([realId], event.accountId)) ?? [];
     const master = fetched[0];
     if (!master) return null;
+    // Known from here on, so a change to it is sent to its own account under
+    // its raw id (resolveMutationTarget), whether or not the grid shows it.
     const mapped = mapServerEventToStoreEvent(master, get().calendars, event.accountId);
     knownMasters.set(mapped.id, mapped);
     return mapped;