Ask the recurrenceId, which is the part that survives expansion

Rules alone were still wrong, in the other direction. A live 0.16.19 was
asked to expand a real weekly series: the occurrences come back carrying
no rule at all — only the master has one — and Stalwart spells that
master's rule "recurrenceRule", singular, not the RFC 8984 array ihasmail
looks for. So a genuine occurrence would have read as a one-off, and the
delete dialog would have offered to delete "this event" while deleting
the series.

What an occurrence does carry is a recurrenceId, which a one-off never
has. Master by its rule under either name, occurrence by its
recurrenceId. The tests carry the shapes the live server returned.

web/src/store/__tests__/recurrence.test.ts

@@ -19,9 +19,15 @@ describe("isRecurring", () => {
     // and a base that is a different id. Neither makes it a series.
     expect(isRecurring(ev({ id: "eaaaaai", baseEventId: "i" }))).toBe(false);
   });
-  it("recognises a series by its recurrence rules", () => {
+  it("recognises a series by its rule, under either name", () => {
     expect(isRecurring(ev({ recurrenceRules: [{ "@type": "RecurrenceRule", frequency: "weekly" }] }))).toBe(true);
-    expect(isRecurring(ev({ baseEventId: "ev1", recurrenceRules: [{ "@type": "RecurrenceRule", frequency: "daily" }] }))).toBe(true);
     expect(isRecurring(ev({ excludedRecurrenceRules: [{ "@type": "RecurrenceRule", frequency: "monthly" }] }))).toBe(true);
+    // Stalwart 0.16 keeps a single rule under the singular name.
+    expect(isRecurring(ev({ recurrenceRule: { "@type": "RecurrenceRule", frequency: "weekly", count: 3 } }))).toBe(true);
+  });
+  it("recognises an occurrence, which arrives with no rule of its own", () => {
+    // A live 0.16.19 expands a weekly series into instances like this: an id
+    // per occurrence, a recurrenceId, and no rule attached.
+    expect(isRecurring(ev({ id: "iaaaaas", recurrenceId: "2030-03-11T10:00:00" }))).toBe(true);
   });
 });

web/src/store/calendar.ts

@@ -300,15 +300,22 @@ export const useCalendar = create<CalendarState>((set, get) => ({
 /**
  * Whether an event is part of a series.
  *
- * Not the same question as "does it have a baseEventId", nor "is that base some
- * other event". `CalendarEvent/query` runs with `expandRecurrences`, and Stalwart
- * hands back an instance id for everything it returns that way — a one-off event
- * included, whose own id (`eaaaaai`) differs from its base (`i`), verified
- * against a live 0.16.19. Recurrence rules are what make a series, so those are
- * what we ask about.
+ * Three things had to be checked against a live 0.16.19 to get this right, none
+ * of which the mock reproduces:
+ *
+ * - `baseEventId` says nothing. `CalendarEvent/query` runs with
+ *   `expandRecurrences`, and a one-off comes back as id `eaaaaai` over base
+ *   `i` — an instance id of its own, and a base that is a different id.
+ * - The rules say nothing on an instance. An occurrence of a weekly series
+ *   arrives with no rule attached at all; only the master carries one.
+ * - Stalwart names that rule `recurrenceRule`, singular, not the RFC 8984
+ *   `recurrenceRules` array.
+ *
+ * What an occurrence does carry is a `recurrenceId`, and a one-off never has
+ * one. Master or occurrence, that is what makes this a series.
  */
 export function isRecurring(ev: CalendarEvent): boolean {
-  return Boolean(ev.recurrenceRules?.length || ev.excludedRecurrenceRules?.length);
+  return Boolean(ev.recurrenceRule || ev.recurrenceRules?.length || ev.excludedRecurrenceRules?.length || ev.recurrenceId);
 }
 
 export function toInstance(e: CalendarEvent, calendars: Record<Id, Calendar>): EventInstance | null {