Calendar: edit and delete a single occurrence

Closes #132.

Stalwart 0.16.20 accepts a synthetic id on `CalendarEvent/set`, writing a
`recurrenceOverrides` entry rather than touching the series, so editing
one date of a recurring event is now something the server does and this
does too.

Editing asks the scope *before* the form opens, because it decides which
event the form is even about: a form populated from the master shows the
series' start date, so editing Wednesday's standup would have offered to
move Monday's. Deleting asks in place of the old confirm.

The patch is narrowed rather than posted hopefully. 0.16.20 sorts
per-occurrence properties into three groups and only one is honest: ten
are refused with `invalidProperties`, twelve more are dropped from the
patch while the response still reports success, and the rest are applied.
That silent middle group is how #26 reached a live server - a successful
response is not evidence anything was written - so `occurrencePatch`
throws on the first group, reports the second to the caller, and the
editor leaves out the five it always sends. A patch that would be
entirely dropped is not sent at all.

The refusal for an occurrence of a this-and-future change offers the
series instead of a bare error toast. Nothing here writes one of those,
but an event synced from another client can carry one.

Two things the scope prompt cost, both worth knowing. A dialog is queued
in a store the moment it is asked for, so it outlives the effect that
asked: without a ref guard a remount queues a second prompt the first
answer cannot retract. And gating the *answer* on the effect's cleanup
flag is worse - StrictMode runs mount, cleanup, mount, so the flag is
already set by the time anyone clicks and the editor never opens.

The mock expands recurrences for the first time, which is what makes any
of this developable. It hands out synthetic ids for everything including
one-offs, gives occurrences a `recurrenceId` and no rule, and reproduces
the refusals - including the silent drops, since a mock that applied them
would let a client that sends them look correct everywhere but a real
server.

web/src/store/__tests__/event-scope.test.ts

@@ -1,6 +1,6 @@
 import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
 import { CAP, client } from "@/jmap/client";
-import { eventIdForScope, isOccurrence, useCalendar } from "@/store/calendar";
+import { CalendarSetError, eventIdForScope, isOccurrence, isThisAndFutureRefusal, occurrencePatch, OccurrenceScopeError, useCalendar } from "@/store/calendar";
 import type { CalendarEvent, JmapSession } from "@/jmap/types";
 
 /**
@@ -161,3 +161,71 @@ describe("rsvp", () => {
     await expect(useCalendar.getState().rsvp(OCCURRENCE, "accepted")).rejects.toThrow(/not a participant/i);
   });
 });
+
+
+describe("occurrencePatch", () => {
+  it("lets through what one date will actually take", () => {
+    const { patch, dropped } = occurrencePatch({ title: "Just today", color: "#f00" });
+    expect(patch).toEqual({ title: "Just today", color: "#f00" });
+    expect(dropped).toEqual([]);
+  });
+
+  it("throws on a property the server refuses outright", () => {
+    // Loud is correct here: moving one occurrence to another calendar is not
+    // something the user can be quietly given a different answer to.
+    expect(() => occurrencePatch({ calendarIds: { c2: true } })).toThrow(OccurrenceScopeError);
+    expect(() => occurrencePatch({ useDefaultAlerts: false })).toThrow(/whole series/i);
+  });
+
+  it("removes an inherited property and reports it, rather than letting it vanish", () => {
+    // The server would take this patch, drop `privacy`, and answer "updated".
+    // Anything that believes the response believes the change landed.
+    const { patch, dropped } = occurrencePatch({ title: "x", privacy: "private", recurrenceRule: null });
+    expect(patch).toEqual({ title: "x" });
+    expect(dropped).toEqual(["privacy", "recurrenceRule"]);
+  });
+
+  it("judges a pointer patch on its first token, as the server does", () => {
+    expect(occurrencePatch({ "participants/me/participationStatus": "accepted" }).patch)
+      .toEqual({ "participants/me/participationStatus": "accepted" });
+    expect(occurrencePatch({ "participants/me/calendarAddress": "mailto:x@y" }).dropped)
+      .toEqual(["participants/me/calendarAddress"]);
+  });
+});
+
+describe("updateEvent, per occurrence", () => {
+  it("narrows the patch before sending it and reports what it kept back", async () => {
+    const calls = server();
+    const dropped = await useCalendar.getState().updateEvent(OCCURRENCE, { title: "Just today", privacy: "private" }, false, "occurrence");
+    expect(calls[0]!.update).toEqual({ iaaaaas: { title: "Just today" } });
+    expect(dropped).toEqual(["privacy"]);
+  });
+
+  it("sends nothing at all when a patch is entirely inherited", async () => {
+    // A request that could only be a no-op is worse than no request: the
+    // response would say "updated" and mean nothing by it.
+    const calls = server();
+    const dropped = await useCalendar.getState().updateEvent(OCCURRENCE, { privacy: "private" }, false, "occurrence");
+    expect(calls).toEqual([]);
+    expect(dropped).toEqual(["privacy"]);
+  });
+
+  it("leaves a series patch exactly as the caller wrote it", async () => {
+    const calls = server();
+    await useCalendar.getState().updateEvent(OCCURRENCE, { privacy: "private", useDefaultAlerts: false }, false, "series");
+    expect(calls[0]!.update!.i).toEqual({ privacy: "private", useDefaultAlerts: false });
+  });
+});
+
+describe("isThisAndFutureRefusal", () => {
+  it("recognises the refusal worth offering the series for", () => {
+    expect(isThisAndFutureRefusal(new CalendarSetError({
+      type: "invalidProperties",
+      description: "Occurrences of a this-and-future change cannot be modified individually.",
+    }))).toBe(true);
+  });
+  it("does not claim an unrelated refusal", () => {
+    expect(isThisAndFutureRefusal(new CalendarSetError({ type: "forbidden", description: "Nope." }))).toBe(false);
+    expect(isThisAndFutureRefusal(new Error("Occurrences of a this-and-future change"))).toBe(false);
+  });
+});

web/src/store/calendar.ts

@@ -83,6 +83,95 @@ export function isOccurrence(event: CalendarEvent): boolean {
   return event.baseEventId != null && event.baseEventId !== event.id;
 }
 
+/**
+ * What `CalendarEvent/set` will not take on a single occurrence, and why the
+ * client has to know rather than letting the server sort it out.
+ *
+ * 0.16.20's per-occurrence validator sorts properties into three groups, and
+ * only one of them is honest about itself:
+ *
+ * - **Rejected** — `invalidProperties`, *"This property cannot be modified on a
+ *   single occurrence."* Loud, and fine.
+ * - **Inherited** — dropped from the patch, and the response still says the
+ *   update succeeded. Nothing anywhere reports it.
+ * - Everything else, which is applied to the override.
+ *
+ * The middle group is the whole problem. It is the same failure as [#26], where
+ * a participant map addressed the RFC 8984 way was discarded without an error
+ * and the client showed the guests as saved: a successful response is not
+ * evidence that anything was written. So a per-occurrence patch is checked here
+ * before it is sent — rejected properties throw, inherited ones are reported to
+ * the caller — rather than being posted hopefully and believed.
+ *
+ * [#26]: https://github.com/Coffey-Labs/ihasmail/issues/26
+ */
+const OCCURRENCE_REJECTED = new Set([
+  "baseEventId", "calendarIds", "isDraft", "isOrigin", "utcStart", "utcEnd",
+  "useDefaultAlerts", "mayInviteSelf", "mayInviteOthers", "hideAttendees",
+]);
+
+/** Applied to the series and never to one date; dropped in silence if sent. */
+const OCCURRENCE_INHERITED = new Set([
+  "@type", "method", "organizerCalendarAddress", "privacy", "prodId",
+  "recurrenceId", "recurrenceIdTimeZone", "sentBy", "uid",
+  "recurrenceOverrides", "recurrenceRule", "relatedTo",
+]);
+
+/**
+ * A `notUpdated`/`notDestroyed` entry, kept whole rather than flattened.
+ *
+ * Some refusals are worth acting on rather than only showing: 0.16.20 will not
+ * edit an occurrence that belongs to a this-and-future change, and the useful
+ * response to that is to offer the series, which needs the reason and not just
+ * its text.
+ */
+export class CalendarSetError extends Error {
+  constructor(readonly setError: { type: string; description?: string; properties?: string[] }) {
+    super(setErrorMessage(setError));
+    this.name = "CalendarSetError";
+  }
+}
+
+/** Whether a refusal was "this occurrence belongs to a this-and-future change". */
+export function isThisAndFutureRefusal(err: unknown): boolean {
+  return err instanceof CalendarSetError && /this-and-future/i.test(err.setError.description ?? "");
+}
+
+export class OccurrenceScopeError extends Error {
+  constructor(readonly property: string) {
+    super(`"${property}" applies to the whole series and cannot be changed for one occurrence.`);
+    this.name = "OccurrenceScopeError";
+  }
+}
+
+/**
+ * A patch narrowed to what one occurrence will actually accept.
+ *
+ * Throws `OccurrenceScopeError` on a property the server would refuse, and
+ * returns the inherited ones it removed so a caller can say what it could not
+ * do for this date alone instead of claiming it did.
+ *
+ * Patch *pointers* are judged on their first token, the way the server does:
+ * `participants/{key}/participationStatus` is allowed, and
+ * `participants/{key}/calendarAddress` is one of the silent drops.
+ */
+export function occurrencePatch(patch: Record<string, unknown>): { patch: Record<string, unknown>; dropped: string[] } {
+  const out: Record<string, unknown> = {};
+  const dropped: string[] = [];
+  for (const [key, value] of Object.entries(patch)) {
+    const [head, , third] = key.split("/");
+    const root = head ?? key;
+    if (OCCURRENCE_REJECTED.has(root)) throw new OccurrenceScopeError(root);
+    if (OCCURRENCE_INHERITED.has(root)) { dropped.push(root); continue; }
+    if (root === "participants" && third === "calendarAddress") { dropped.push(key); continue; }
+    // `id` is immutable; the server errors on a value that is not the event's
+    // own, and ignores one that is. Neither is worth sending.
+    if (root === "id") { dropped.push(root); continue; }
+    out[key] = value;
+  }
+  return { patch: out, dropped };
+}
+
 /** A calendar somebody else shared, and the account it lives in. */
 export interface SharedCalendar {
   accountId: Id;
@@ -122,7 +211,8 @@ interface CalendarState {
   instancesIn(start: Date, end: Date): EventInstance[];
   getEvent(id: Id): Promise<CalendarEvent | null>;
   createEvent(event: Partial<CalendarEvent>, calendarId: Id, sendInvites: boolean): Promise<Id>;
-  updateEvent(event: CalendarEvent, patch: Record<string, unknown>, sendInvites: boolean, scope: EventScope): Promise<void>;
+  /** Returns the properties that had to be left to the series, if any. */
+  updateEvent(event: CalendarEvent, patch: Record<string, unknown>, sendInvites: boolean, scope: EventScope): Promise<string[]>;
   destroyEvent(event: CalendarEvent, sendInvites: boolean, scope: EventScope): Promise<void>;
   rsvp(event: CalendarEvent, status: "accepted" | "tentative" | "declined", comment?: string): Promise<void>;
   createCalendar(data: Partial<Calendar>): Promise<Id>;
@@ -399,10 +489,15 @@ export const useCalendar = create<CalendarState>((set, get) => ({
   async updateEvent(event, patch, sendInvites, scope) {
     const accountId = get().accountId!;
     const id = eventIdForScope(event, scope);
-    const res = await client.call<SetResponse>("CalendarEvent/set", { accountId, update: { [id]: patch }, sendSchedulingMessages: sendInvites });
+    // An occurrence takes less than the series does, and says so about only
+    // half of it. Narrow the patch here rather than posting it hopefully.
+    const { patch: body, dropped } = scope === "occurrence" ? occurrencePatch(patch) : { patch, dropped: [] as string[] };
+    if (!Object.keys(body).length) return dropped;
+    const res = await client.call<SetResponse>("CalendarEvent/set", { accountId, update: { [id]: body }, sendSchedulingMessages: sendInvites });
     const err = res.notUpdated?.[id];
-    if (err) throw new Error(setErrorMessage(err));
+    if (err) throw new CalendarSetError(err);
     get().invalidate();
+    return dropped;
   },
 
   async destroyEvent(event, sendInvites, scope) {
@@ -410,7 +505,7 @@ export const useCalendar = create<CalendarState>((set, get) => ({
     const id = eventIdForScope(event, scope);
     const res = await client.call<SetResponse>("CalendarEvent/set", { accountId, destroy: [id], sendSchedulingMessages: sendInvites });
     const err = res.notDestroyed?.[id];
-    if (err) throw new Error(setErrorMessage(err));
+    if (err) throw new CalendarSetError(err);
     set((s) => {
       const events = { ...s.events };
       // Drop both ids: the one that was sent, and the object as the caller

web/src/styles/app.css

@@ -310,6 +310,11 @@ a.menu-item:hover { color: var(--fg); }
 .dialog-body { padding: 8px 20px 16px; overflow: auto; }
 .dialog-foot { display: flex; align-items: center; justify-content: flex-end; gap: 8px; padding: 12px 20px 16px; border-top: 1px solid var(--border); }
 .dialog-foot .left { margin-right: auto; }
+/* "This occurrence or the whole series" — one button per answer, stacked, so
+   the destructive one is read rather than landed on by muscle memory. */
+.dialog-choices { display: flex; flex-direction: column; gap: 8px; }
+.dialog-choice { display: flex; flex-direction: column; align-items: flex-start; gap: 2px; width: 100%; text-align: left; padding: 10px 12px; height: auto; }
+.dialog-choice small { font-weight: 400; opacity: 0.75; }
 
 /* Toasts ----------------------------------------------------------------- */
 .toast-host { position: fixed; left: 50%; bottom: 24px; transform: translateX(-50%); z-index: 3000; display: flex; flex-direction: column; gap: 8px; align-items: center; pointer-events: none; padding: 0 12px; width: 100%; max-width: 520px; }

web/src/ui/dialog.tsx

@@ -86,9 +86,17 @@ export function Dialog({ open, onClose, title, children, footer, size = "md", cl
 
 /* ---------- Imperative confirm / prompt ---------- */
 
+export interface DialogChoice {
+  value: string;
+  label: string;
+  /** Shown under the label, for the choice that needs the caveat. */
+  hint?: string;
+  danger?: boolean;
+}
+
 interface ConfirmRequest {
   id: number;
-  kind: "confirm" | "prompt";
+  kind: "confirm" | "prompt" | "choice";
   title: string;
   message?: ReactNode;
   confirmLabel?: string;
@@ -96,6 +104,7 @@ interface ConfirmRequest {
   danger?: boolean;
   defaultValue?: string;
   placeholder?: string;
+  choices?: DialogChoice[];
   resolve: (v: boolean | string | null) => void;
 }
 
@@ -119,6 +128,18 @@ export function promptDialog(opts: { title: string; message?: ReactNode; default
   });
 }
 
+/**
+ * A question with more than two answers, which "this one or all of them" is.
+ *
+ * Resolves to the chosen `value`, or `null` if the dialog is dismissed —
+ * dismissing is not one of the choices, so a caller cannot mistake it for one.
+ */
+export function choiceDialog(opts: { title: string; message?: ReactNode; choices: DialogChoice[]; cancelLabel?: string }): Promise<string | null> {
+  return new Promise((resolve) => {
+    useConfirmStore.getState().push({ id: reqId++, kind: "choice", ...opts, resolve: (v) => resolve(typeof v === "string" ? v : null) });
+  });
+}
+
 export function ConfirmHost() {
   const req = useConfirmStore((s) => s.queue[0]);
   const pop = useConfirmStore((s) => s.pop);
@@ -132,21 +153,37 @@ export function ConfirmHost() {
   return (
     <Dialog
       open
-      onClose={() => done(req.kind === "prompt" ? null : false)}
+      onClose={() => done(req.kind === "confirm" ? false : null)}
       title={req.title}
       size="sm"
       footer={
-        <>
-          <button className="btn" onClick={() => done(req.kind === "prompt" ? null : false)}>
+        req.kind === "choice" ? (
+          <button className="btn" onClick={() => done(null)}>
             {req.cancelLabel ?? "Cancel"}
           </button>
-          <button className={`btn ${req.danger ? "btn-danger" : "btn-primary"}`} onClick={() => done(req.kind === "prompt" ? value : true)}>
-            {req.confirmLabel ?? (req.kind === "prompt" ? "OK" : "Confirm")}
-          </button>
-        </>
+        ) : (
+          <>
+            <button className="btn" onClick={() => done(req.kind === "prompt" ? null : false)}>
+              {req.cancelLabel ?? "Cancel"}
+            </button>
+            <button className={`btn ${req.danger ? "btn-danger" : "btn-primary"}`} onClick={() => done(req.kind === "prompt" ? value : true)}>
+              {req.confirmLabel ?? (req.kind === "prompt" ? "OK" : "Confirm")}
+            </button>
+          </>
+        )
       }
     >
       {req.message && <p style={{ marginTop: 0 }}>{req.message}</p>}
+      {req.kind === "choice" && (
+        <div className="dialog-choices">
+          {req.choices?.map((c) => (
+            <button key={c.value} className={`btn dialog-choice ${c.danger ? "btn-danger" : ""}`} onClick={() => done(c.value)}>
+              <span>{c.label}</span>
+              {c.hint && <small>{c.hint}</small>}
+            </button>
+          ))}
+        </div>
+      )}
       {req.kind === "prompt" && (
         <form
           onSubmit={(e) => {

web/src/views/calendar/CalendarContextMenu.tsx

@@ -1,13 +1,14 @@
 import { Calendar as CalIcon, CalendarDays, Copy, ExternalLink, Palette, Pencil, Plus, Tag, Trash2, X } from "lucide-react";
 import { useLocation } from "wouter";
 import type { CalendarEvent } from "@/jmap/types";
-import { useCalendar, isRecurring, type EventInstance } from "@/store/calendar";
+import { useCalendar, isRecurring, isOccurrence, type EventInstance, type EventScope } from "@/store/calendar";
 import { useSettings } from "@/store/settings";
 import { formatDayMonth } from "@/lib/datetime";
-import { MenuItem, MenuSep, MenuTitle, Popover, type Anchor } from "@/ui/popover";
 import { CALENDAR_COLORS } from "@/ui/misc";
+import { MenuItem, MenuSep, MenuTitle, Popover, type Anchor } from "@/ui/popover";
 import { confirmDialog } from "@/ui/dialog";
 import { toast } from "@/ui/toast";
+import { askDeleteScope, askEditScope, droppedMessage, runScoped } from "./scope";
 import { toLocalDateOnly } from "@/lib/dates";
 import { formatTime } from "@/lib/format";
 
@@ -65,9 +66,13 @@ export function CalendarContextMenu({ ctx, onClose, onOpen, onEdit, onCreate }:
   const participants = Object.keys(ev.participants ?? {}).length;
 
   const patch = async (p: Record<string, unknown>, msg: string) => {
+    const scope = await askEditScope(ev);
+    if (!scope) return;
     try {
-      await cal.updateEvent(ev, p, false, "series");
-      toast.success(msg);
+      const dropped = await runScoped(scope, (s) => cal.updateEvent(ev, p, false, s));
+      if (!dropped) return;
+      // A per-occurrence change can be accepted in part. Say which part.
+      toast.success(droppedMessage(dropped) ?? (scope === "occurrence" ? `${msg} for this date` : msg));
     } catch (err) {
       toast.error((err as Error).message);
     }
@@ -88,11 +93,16 @@ export function CalendarContextMenu({ ctx, onClose, onOpen, onEdit, onCreate }:
   };
   const del = async () => {
     onClose();
-    const recurring = isRecurring(ev);
-    if (!(await confirmDialog({ title: recurring ? "Delete all occurrences?" : "Delete this event?", confirmLabel: "Delete", danger: true }))) return;
+    let scope: EventScope | null = "series";
+    if (isRecurring(ev) && isOccurrence(ev)) {
+      scope = await askDeleteScope(ev);
+    } else if (!(await confirmDialog({ title: "Delete this event?", confirmLabel: "Delete", danger: true }))) {
+      scope = null;
+    }
+    if (!scope) return;
     try {
-      await cal.destroyEvent(ev, participants > 1, "series");
-      toast.success("Event deleted");
+      await runScoped(scope, (s) => cal.destroyEvent(ev, participants > 1, s));
+      toast.success(scope === "occurrence" ? "Occurrence deleted" : "Event deleted");
     } catch (err) {
       toast.error((err as Error).message);
     }

web/src/views/calendar/EventEditor.tsx

@@ -1,7 +1,7 @@
-import { useEffect, useMemo, useState } from "react";
+import { useEffect, useMemo, useRef, useState } from "react";
 import { Plus, Trash2, Users } from "lucide-react";
 import type { BusyPeriod, CalendarEvent, EmailAddress, JSCalendarAlert, JSCalendarParticipant, JSCalendarRecurrenceRule, JSCalendarNDay } from "@/jmap/types";
-import { useCalendar, myParticipantKeys, isRecurring, eventRule, makeParticipant, participantEmail } from "@/store/calendar";
+import { useCalendar, myParticipantKeys, isRecurring, isOccurrence, eventRule, makeParticipant, participantEmail, type EventScope } from "@/store/calendar";
 import { useSettings } from "@/store/settings";
 import { useSession } from "@/store/session";
 import { useContacts } from "@/store/contacts";
@@ -14,6 +14,7 @@ import { browserTimeZone, dateToZonedLocal, formatDuration, fromInputDateTime, l
 import { formatClock, formatNumericDate, formatWeekday } from "@/lib/datetime";
 import { WEEKDAYS, describeRule, presetFor, ruleFromPreset, type RecurrencePreset } from "@/lib/recurrence";
 import { newKey } from "@/lib/contacts";
+import { askEditScope, droppedMessage, runScoped } from "./scope";
 
 export interface EditorInit {
   event?: CalendarEvent;
@@ -24,25 +25,68 @@ export interface EditorInit {
 
 const ALERT_OPTIONS = [0, 5, 10, 15, 30, 60, 120, 1440, 2880, 10080];
 
+/**
+ * Fields this form always sends that a single occurrence will not take.
+ *
+ * `useDefaultAlerts` and `calendarIds` are refused with `invalidProperties`;
+ * the rest are dropped from the patch while the response still reports
+ * success. Both halves are reasons not to send them — the second more so,
+ * because nothing would say it had happened.
+ */
+const OCCURRENCE_OMIT = new Set(["useDefaultAlerts", "calendarIds", "recurrenceRule", "privacy", "organizerCalendarAddress"]);
+
 export function EventEditor({ init, onClose }: { init: EditorInit; onClose: () => void }) {
   const cal = useCalendar();
   const settings = useSettings((s) => s.settings);
   const session = useSession((s) => s.session);
   const [base, setBase] = useState<CalendarEvent | null | undefined>(init.event && !init.event.baseEventId ? init.event : undefined);
+  const [scope, setScope] = useState<EventScope | undefined>(init.event?.baseEventId ? undefined : "series");
   const editing = Boolean(init.event);
 
-  // Load base event for recurring instances
+  /*
+   * Which event this form is even about has to be settled before it opens.
+   *
+   * A form populated from the master shows the series' start date, so editing
+   * Wednesday's standup would offer to move Monday's — right for the series and
+   * wrong for one date. So the scope is asked first, and the occurrence itself
+   * is what the form loads when the answer is "this occurrence".
+   */
+  /*
+   * Asked once per event, and deliberately not tied to the effect's lifetime.
+   *
+   * Two things make the obvious version wrong. A dialog is queued in a store
+   * the moment it is requested, so it outlives the effect that asked for it: a
+   * re-run queues a second prompt the first answer cannot retract, and the
+   * reader is asked the same question twice. And gating the *answer* on a
+   * cleanup flag is worse — React's StrictMode runs mount, cleanup, mount, so
+   * the flag is already set by the time anyone clicks and the editor never
+   * opens at all. The ref is what makes this once; the answer is applied
+   * whenever it arrives.
+   */
+  const asked = useRef<string | null>(null);
   useEffect(() => {
-    if (init.event?.baseEventId) void cal.getEvent(init.event.baseEventId).then((e) => setBase(e));
-    else if (!init.event) setBase(null);
+    const ev = init.event;
+    if (!ev) { setBase(null); setScope("series"); return; }
+    if (!ev.baseEventId) { setBase(ev); setScope("series"); return; }
+    if (asked.current === ev.id) return;
+    asked.current = ev.id;
+    void (async () => {
+      const chosen = isRecurring(ev) && isOccurrence(ev) ? await askEditScope(ev) : "series";
+      if (!chosen) { onClose(); return; }
+      setScope(chosen);
+      if (chosen === "occurrence") setBase(ev);
+      else void cal.getEvent(ev.baseEventId!).then(setBase);
+    })();
     // eslint-disable-next-line react-hooks/exhaustive-deps
   }, [init.event?.id]);
 
-  if (base === undefined) return null;
-  return <EventForm key={base?.id ?? "new"} init={init} base={base} editing={editing} onClose={onClose} settingsTz={settings.timeZone ?? browserTimeZone} defaultAlert={settings.defaultAlertMinutes} myEmail={session?.username ?? ""} />;
+  if (base === undefined || scope === undefined) return null;
+  return <EventForm key={base?.id ?? "new"} init={init} base={base} scope={scope} editing={editing} onClose={onClose} settingsTz={settings.timeZone ?? browserTimeZone} defaultAlert={settings.defaultAlertMinutes} myEmail={session?.username ?? ""} />;
 }
 
-function EventForm({ init, base, editing, onClose, settingsTz, defaultAlert, myEmail }: { init: EditorInit; base: CalendarEvent | null; editing: boolean; onClose: () => void; settingsTz: string; defaultAlert: number; myEmail: string }) {
+function EventForm({ init, base, scope, editing, onClose, settingsTz, defaultAlert, myEmail }: { init: EditorInit; base: CalendarEvent | null; scope: EventScope; editing: boolean; onClose: () => void; settingsTz: string; defaultAlert: number; myEmail: string }) {
+  /** This form is editing one date rather than the series behind it. */
+  const oneDate = scope === "occurrence";
   const cal = useCalendar();
   const contacts = useContacts();
   const ev = base;
@@ -175,13 +219,23 @@ function EventForm({ init, base, editing, onClose, settingsTz, defaultAlert, myE
       };
       const invites = sendInvites && attendees.length > 0;
       if (ev) {
+        /*
+         * A single occurrence takes less than the series does. Four of the
+         * fields this form always sends are among them — `useDefaultAlerts`
+         * and `calendarIds` are refused outright, `recurrenceRule`,
+         * `privacy` and `organizerCalendarAddress` are dropped in silence —
+         * so they are left out here rather than sent and believed. The store
+         * still checks; this is what stops it having to complain.
+         */
+        const source = oneDate
+          ? Object.fromEntries(Object.entries(obj).filter(([k]) => !OCCURRENCE_OMIT.has(k)))
+          : obj;
         const patch: Record<string, unknown> = {};
-        for (const [k, v] of Object.entries(obj)) patch[k] = v === undefined ? null : v;
-        if (Object.keys(ev.calendarIds)[0] !== calendarId) patch.calendarIds = { [calendarId]: true };
-        // `ev` is the master: EventEditor resolves `baseEventId` when it opens
-        // on an occurrence, so the whole series is what this form edits.
-        await cal.updateEvent(ev, patch, invites, "series");
-        toast.success("Event updated");
+        for (const [k, v] of Object.entries(source)) patch[k] = v === undefined ? null : v;
+        if (!oneDate && Object.keys(ev.calendarIds)[0] !== calendarId) patch.calendarIds = { [calendarId]: true };
+        const dropped = await runScoped(scope, (s) => cal.updateEvent(ev, patch, invites, s));
+        if (!dropped) { setBusy(false); return; }
+        toast.success(droppedMessage(dropped) ?? (oneDate ? "This occurrence updated" : "Event updated"));
       } else {
         const clean: Record<string, unknown> = {};
         for (const [k, v] of Object.entries(obj)) if (v !== undefined) clean[k] = v;
@@ -206,7 +260,13 @@ function EventForm({ init, base, editing, onClose, settingsTz, defaultAlert, myE
   return (
     <Dialog open onClose={onClose} title={editing ? "Edit event" : "New event"} size="lg" footer={<><button className="btn" onClick={onClose}>Cancel</button><button className="btn btn-primary" disabled={busy} onClick={() => void save()}>{busy ? "Saving…" : editing ? "Save" : attendees.length && sendInvites ? "Send invites" : "Create"}</button></>}>
       <div className="event-form">
-        {ev && isRecurring(ev) && <div className="info-box mb-16">This is a recurring event — changes apply to the whole series.</div>}
+        {ev && isRecurring(ev) && (
+          <div className="info-box mb-16">
+            {oneDate
+              ? `Editing ${formatNumericDate(start)} only — the rest of the series is unchanged. Repeat, calendar and privacy belong to the series and are not shown.`
+              : "This is a recurring event — changes apply to the whole series."}
+          </div>
+        )}
         <div className="field"><input className="input" style={{ fontSize: "1.1em", height: 44 }} placeholder="Add title" autoFocus value={title} onChange={(e) => setTitle(e.target.value)} /></div>
         <div className="time-row mb-8">
           {allDay ? (
@@ -231,6 +291,7 @@ function EventForm({ init, base, editing, onClose, settingsTz, defaultAlert, myE
               {listTimeZones().map((t) => <option key={t} value={t}>{t}</option>)}
             </select>
           )}
+          {!oneDate && (
           <select className="select" style={{ width: "auto", height: 32 }} value={preset} onChange={(e) => { const p = e.target.value as RecurrencePreset; setPreset(p); if (p === "custom") setRule(rule ?? { "@type": "RecurrenceRule", frequency: "weekly", byDay: [{ "@type": "NDay", day: WEEKDAYS[(start.getDay() + 6) % 7]!.key }] }); else setRule(ruleFromPreset(p, start)); }}>
             <option value="none">Does not repeat</option>
             <option value="daily">Daily</option>
@@ -240,8 +301,9 @@ function EventForm({ init, base, editing, onClose, settingsTz, defaultAlert, myE
             <option value="yearly">Yearly</option>
             <option value="custom">Custom…</option>
           </select>
+          )}
         </div>
-        {preset === "custom" && (
+        {!oneDate && preset === "custom" && (
           <div className="card" style={{ marginBottom: 12 }}>
             <div className="row wrap" style={{ gap: 8 }}>
               <span>Repeat every</span>
@@ -271,7 +333,7 @@ function EventForm({ init, base, editing, onClose, settingsTz, defaultAlert, myE
         )}
         <div className="field-row">
           <div className="field"><label>Calendar</label>
-            <select className="select" value={calendarId} onChange={(e) => setCalendarId(e.target.value)}>
+            <select className="select" value={calendarId} disabled={oneDate} title={oneDate ? "An occurrence cannot be moved to another calendar on its own" : undefined} onChange={(e) => setCalendarId(e.target.value)}>
               {calendars.map((c) => <option key={c.id} value={c.id}>{c.name}</option>)}
             </select>
           </div>
@@ -329,7 +391,7 @@ function EventForm({ init, base, editing, onClose, settingsTz, defaultAlert, myE
             <div className="field-row">
               <div className="field"><label>Status</label><select className="select" value={status} onChange={(e) => setStatus(e.target.value as typeof status)}><option value="confirmed">Confirmed</option><option value="tentative">Tentative</option><option value="cancelled">Cancelled</option></select></div>
               <div className="field"><label>Show as</label><select className="select" value={freeBusy} onChange={(e) => setFreeBusy(e.target.value as typeof freeBusy)}><option value="busy">Busy</option><option value="free">Free</option></select></div>
-              <div className="field"><label>Visibility</label><select className="select" value={privacy} onChange={(e) => setPrivacy(e.target.value as typeof privacy)}><option value="public">Default</option><option value="private">Private</option><option value="secret">Secret</option></select></div>
+              {!oneDate && <div className="field"><label>Visibility</label><select className="select" value={privacy} onChange={(e) => setPrivacy(e.target.value as typeof privacy)}><option value="public">Default</option><option value="private">Private</option><option value="secret">Secret</option></select></div>}
             </div>
             <div className="field"><label>Category</label>
               <select className="select" value={category} onChange={(e) => setCategory(e.target.value)}>

web/src/views/calendar/EventPopover.tsx

@@ -1,9 +1,10 @@
 import { useState } from "react";
 import { AlignLeft, Bell, Calendar as CalIcon, Check, Clock, HelpCircle, Link2, MapPin, Pencil, Repeat, Trash2, Users, X, Mail } from "lucide-react";
-import { useCalendar, myParticipantKeys, isRecurring, eventRule, participantEmail, type EventInstance } from "@/store/calendar";
+import { useCalendar, myParticipantKeys, isRecurring, isOccurrence, eventRule, participantEmail, type EventInstance, type EventScope } from "@/store/calendar";
 import { Popover, type Anchor } from "@/ui/popover";
 import { confirmDialog } from "@/ui/dialog";
 import { toast } from "@/ui/toast";
+import { askDeleteScope, runScoped } from "./scope";
 import { formatTimeRange, humanDuration, parseDuration } from "@/lib/dates";
 import { describeRule } from "@/lib/recurrence";
 import { useCompose } from "@/store/compose";
@@ -28,13 +29,20 @@ export function EventPopover({ inst, anchor, onClose, onEdit }: { inst: EventIns
   const openCompose = useCompose((s) => s.open);
 
   const del = async () => {
-    const recurring = isRecurring(ev);
-    const ok = await confirmDialog({ title: recurring ? "Delete all occurrences?" : "Delete this event?", message: recurring ? "This will delete the entire series." : undefined, confirmLabel: "Delete", danger: true });
-    if (!ok) return;
+    // A series asks which; anything else is a plain confirm. `askDeleteScope`
+    // returns null for a dismissed dialog, which is a cancel and not a series.
+    let scope: EventScope | null = "series";
+    if (isRecurring(ev) && isOccurrence(ev)) {
+      scope = await askDeleteScope(ev);
+    } else {
+      const ok = await confirmDialog({ title: "Delete this event?", confirmLabel: "Delete", danger: true });
+      if (!ok) scope = null;
+    }
+    if (!scope) return;
     setBusy(true);
     try {
-      await cal.destroyEvent(ev, participants.length > 1, "series");
-      toast.success("Event deleted");
+      await runScoped(scope, (s) => cal.destroyEvent(ev, participants.length > 1, s));
+      toast.success(scope === "occurrence" ? "Occurrence deleted" : "Event deleted");
       onClose();
     } catch (err) {
       toast.error((err as Error).message);

web/src/views/calendar/scope.ts

@@ -0,0 +1,89 @@
+import { choiceDialog, confirmDialog } from "@/ui/dialog";
+import { isOccurrence, isRecurring, isThisAndFutureRefusal, type EventScope } from "@/store/calendar";
+import type { CalendarEvent } from "@/jmap/types";
+
+/**
+ * Ask which of a series a change is meant for, when there is a choice.
+ *
+ * There is only a choice when the object in hand is an occurrence of a real
+ * series: a one-off has a synthetic id too, but its only occurrence *is* the
+ * event, so asking would be a question with one true answer. `null` means the
+ * dialog was dismissed, which is not the same as "the whole series" — every
+ * caller has to treat it as a cancel.
+ *
+ * Until 0.16.20 there was nothing to ask: the server refused a write aimed at
+ * an occurrence, so "the whole series" was the only thing that could happen.
+ */
+export async function askScope(
+  event: CalendarEvent,
+  opts: { title: string; occurrenceLabel: string; seriesLabel: string; danger?: boolean; occurrenceHint?: string; seriesHint?: string },
+): Promise<EventScope | null> {
+  if (!isRecurring(event) || !isOccurrence(event)) return "series";
+  const answer = await choiceDialog({
+    title: opts.title,
+    choices: [
+      { value: "occurrence", label: opts.occurrenceLabel, hint: opts.occurrenceHint, danger: opts.danger },
+      { value: "series", label: opts.seriesLabel, hint: opts.seriesHint, danger: opts.danger },
+    ],
+  });
+  return answer === "occurrence" || answer === "series" ? answer : null;
+}
+
+/** The scope question for deleting. */
+export const askDeleteScope = (event: CalendarEvent): Promise<EventScope | null> =>
+  askScope(event, {
+    title: "Delete this event?",
+    occurrenceLabel: "This occurrence",
+    occurrenceHint: "Removes this date and leaves the rest of the series.",
+    seriesLabel: "All occurrences",
+    seriesHint: "Deletes the whole series. This cannot be undone.",
+    danger: true,
+  });
+
+/** The scope question for editing. */
+export const askEditScope = (event: CalendarEvent): Promise<EventScope | null> =>
+  askScope(event, {
+    title: "Change this event?",
+    occurrenceLabel: "This occurrence",
+    occurrenceHint: "Applies to this date only.",
+    seriesLabel: "All occurrences",
+    seriesHint: "Applies to every date in the series.",
+  });
+
+/**
+ * What to say when the server kept some of a per-occurrence change for the
+ * series. `dropped` comes back from `updateEvent`; an empty list says nothing.
+ */
+export function droppedMessage(dropped: string[]): string | null {
+  if (!dropped.length) return null;
+  const names = dropped.map((d) => d.replace(/^@/, "")).join(", ");
+  return `Saved for this date. ${names} ${dropped.length === 1 ? "applies" : "apply"} to the whole series and was left unchanged.`;
+}
+
+
+/**
+ * Run a scoped change, and offer the series if the server will not do one date.
+ *
+ * Stalwart refuses an occurrence that belongs to a this-and-future override —
+ * *"Occurrences of a this-and-future change cannot be modified individually."*
+ * Nothing ihasmail writes creates one, but an event synced from another client
+ * can carry one, so the refusal is reachable and a bare error toast would leave
+ * the reader with no way forward.
+ *
+ * The series is offered rather than silently substituted: they asked for one
+ * date, and doing the larger thing without saying so is the failure this whole
+ * area exists to avoid.
+ */
+export async function runScoped<T>(scope: EventScope, run: (scope: EventScope) => Promise<T>): Promise<T | null> {
+  try {
+    return await run(scope);
+  } catch (err) {
+    if (scope !== "occurrence" || !isThisAndFutureRefusal(err)) throw err;
+    const ok = await confirmDialog({
+      title: "This date cannot be changed on its own",
+      message: "It belongs to a change that was applied to this and all later occurrences, which the server will only edit as a whole. Apply to the entire series instead?",
+      confirmLabel: "Apply to series",
+    });
+    return ok ? await run("series") : null;
+  }
+}