Say how often, and say what a reschedule costs

Occurrence ids carry their own start day, so moving the anchor or the
interval re-keys every occurrence and the marks under the old ids stop
being reachable. Nothing is rewritten — removeEvent makes the same trade,
and useMarkSet never removes because nothing else holds a copy — but the
reader is told the count first, the way removeGame reports blockedBy
instead of cascading.

Informs, never blocks. Renaming still costs nothing: the token is random
precisely so fixing a typo never moves an id, and movesOccurrences is what
keeps the warning off a rename and off a bare change of `until`.

cadenceLabel sits beside the form's own vocabulary so the sheet cannot
describe a rule differently from the control that set it.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

src/client/App.tsx

@@ -30,6 +30,7 @@ import {
 } from "./state/lens.ts";
 import { clockFor, formatRemaining } from "../shared/time.ts";
 import { dailySummary, isDaily, resolveDaily } from "../shared/daily.ts";
+import { nextOccurrences } from "../shared/recurrence.ts";
 import { orderGames } from "./state/gameOrder.ts";
 import { GameMetaProvider, type MetaResolver } from "./state/gameMeta.tsx";
 import { metaOnTheme, useTheme } from "./state/theme.ts";
@@ -705,6 +706,19 @@ export function App() {
               onSave: (_id: string, draft: EventDraft) =>
                 custom.editEvent(record.id, draft),
               onDelete: () => custom.removeEvent(record.id),
+              strandedBy: () => {
+                // What the reader has actually recorded against the occurrences
+                // this rule generates today, and would no longer reach once the
+                // ids move. Twelve is a season of a fortnightly rule — enough to
+                // make the number meaningful without walking a decade of a
+                // daily one.
+                if (record.repeat === null) return 0;
+                return nextOccurrences(record, now, 12).filter(
+                  (o) =>
+                    prog.progress[o.id] !== undefined ||
+                    (daily.logs[o.id]?.days.length ?? 0) > 0,
+                ).length;
+              },
             };
           })()}
         />

src/client/components/CustomForms.tsx

@@ -5,7 +5,12 @@ import {
   type CustomGames,
   type LaneId,
 } from "../../shared/custom.ts";
-import { comesRoundEarly, RepeatUnit } from "../../shared/recurrence.ts";
+import {
+  comesRoundEarly,
+  movesOccurrences,
+  RepeatUnit,
+  type Repeat,
+} from "../../shared/recurrence.ts";
 import { EventType } from "../../shared/schema.ts";
 import { useGameMeta } from "../state/gameMeta.tsx";
 import { readerInstant, type EventDraft } from "../state/useCustom.ts";
@@ -41,6 +46,32 @@ export const CUSTOM_HUES = [
 
 const TYPES = EventType.options;
 
+/**
+ * What a schedule change costs, or null when it costs nothing.
+ *
+ * Occurrence ids carry their own start day, so moving the anchor or the
+ * interval re-keys every occurrence and the marks stored under the old ids stop
+ * being reachable. Nothing is rewritten — `removeEvent` makes the same trade,
+ * and `useMarkSet.merge` never removes because nothing else holds a copy — but
+ * the reader is told the count first, the way `removeGame` reports `blockedBy`
+ * instead of cascading.
+ *
+ * Informs; never blocks.
+ */
+export function strandedNotice(count: number): string | null {
+  if (count <= 0) return null;
+  return `Changing the schedule will strand ${count} tick${
+    count === 1 ? "" : "s"
+  } you've already recorded.`;
+}
+
+/** How often a rule comes round, in the words the form offered. */
+export function cadenceLabel(repeat: Repeat | null): string | null {
+  if (repeat === null) return null;
+  if (repeat.interval === 1) return `every ${repeat.unit.replace(/s$/, "")}`;
+  return `every ${repeat.interval} ${repeat.unit}`;
+}
+
 function labelClass(): string {
   return "block text-xs font-medium text-muted";
 }
@@ -136,6 +167,7 @@ export function EventForm({
   initial,
   onSave,
   onCancel,
+  strandedBy,
 }: {
   /** Every lane an event can belong to — a source can miss an event too. */
   lanes: LaneId[];
@@ -143,6 +175,14 @@ export function EventForm({
   initial?: CustomEvent | undefined;
   onSave: (draft: EventDraft) => void;
   onCancel: () => void;
+  /**
+   * How many stored marks this draft's schedule would leave behind.
+   *
+   * Supplied by the caller because only it can see the mark stores. Absent —
+   * on the add form, where there is nothing to strand — the notice never
+   * renders.
+   */
+  strandedBy?: ((draft: EventDraft) => number) | undefined;
 }) {
   const gameMeta = useGameMeta();
   const start = fields(initial?.startsAt ?? null);
@@ -217,23 +257,32 @@ export function EventForm({
     !earlyReturn &&
     (repeatUnit === "never" || intervalValid);
 
+  const draft: EventDraft | null =
+    startsAt === null
+      ? null
+      : {
+          game, title, type,
+          summary: summary === "" ? null : summary,
+          startsAt, startHasTime: startTime !== "",
+          endsAt, endHasTime: endTime !== "",
+          repeat,
+        };
+  // Only a schedule change re-keys anything. Renaming does not — the token is
+  // random precisely so fixing a typo never costs the marks attached to it.
+  const stranded =
+    initial !== undefined && draft !== null && strandedBy !== undefined &&
+    movesOccurrences(initial, draft)
+      ? strandedBy(draft)
+      : 0;
+  const notice = strandedNotice(stranded);
+
   return (
     <form
       className="mt-3 rounded-xl border border-hairline p-3"
       onSubmit={(e) => {
         e.preventDefault();
-        if (!valid || startsAt === null) return;
-        onSave({
-          game,
-          title,
-          type,
-          summary,
-          startsAt,
-          startHasTime: startTime !== "",
-          endsAt,
-          endHasTime: endTime !== "",
-          repeat,
-        });
+        if (!valid || draft === null) return;
+        onSave(draft);
       }}
     >
       <label className={labelClass()}>
@@ -420,6 +469,10 @@ export function EventForm({
         </p>
       )}
 
+      {notice !== null && (
+        <p className="mt-2 text-xs leading-relaxed text-muted">{notice}</p>
+      )}
+
       <div className="mt-4 flex gap-2">
         <button
           type="submit"

src/client/components/EventDetail.tsx

@@ -7,7 +7,7 @@ import {
   type LaneId,
 } from "../../shared/custom.ts";
 import type { EventDraft } from "../state/useCustom.ts";
-import { EventForm } from "./CustomForms.tsx";
+import { cadenceLabel, EventForm } from "./CustomForms.tsx";
 import {
   formatAbsolute,
   formatRemaining,
@@ -76,6 +76,7 @@ export function EventDetail({
         games: CustomGames;
         onSave: (id: string, draft: EventDraft) => void;
         onDelete: (id: string) => void;
+        strandedBy: (draft: EventDraft) => number;
       }
     | undefined;
 }) {
@@ -159,6 +160,10 @@ export function EventDetail({
           <Field label="Type">{event.type}</Field>
         </dl>
 
+        {cadenceLabel(own?.record.repeat ?? null) !== null && (
+          <p className="text-xs text-faint">{cadenceLabel(own!.record.repeat)}</p>
+        )}
+
         {risk !== "fine" && effort !== undefined && clock.msRemaining !== null && (
           <p
             className="mt-4 rounded-lg border px-3 py-2 text-xs leading-relaxed"
@@ -241,6 +246,7 @@ export function EventDetail({
                   setEditing(false);
                 }}
                 onCancel={() => setEditing(false)}
+                strandedBy={own.strandedBy}
               />
             ) : (
               <div className="flex gap-2">

test/custom-ui.test.tsx

@@ -1,6 +1,10 @@
 import { describe, expect, test } from "bun:test";
 import { renderToStaticMarkup } from "react-dom/server";
-import { EventForm } from "../src/client/components/CustomForms.tsx";
+import {
+  cadenceLabel,
+  EventForm,
+  strandedNotice,
+} from "../src/client/components/CustomForms.tsx";
 import { YourOwn } from "../src/client/components/YourOwn.tsx";
 import { EventRow } from "../src/client/components/EventRow.tsx";
 import { AUTHOR, Colophon, REPO_URL } from "../src/client/components/Colophon.tsx";
@@ -415,3 +419,52 @@ describe("stating a repeat", () => {
     expect(html).toContain("no countdown");
   });
 });
+
+describe("what a reschedule costs", () => {
+  test("says nothing when nothing would be stranded", () => {
+    expect(strandedNotice(0)).toBe(null);
+  });
+
+  test("counts, and agrees with itself about plurals", () => {
+    expect(strandedNotice(1)).toContain("1 tick");
+    expect(strandedNotice(1)).not.toContain("ticks");
+    expect(strandedNotice(3)).toContain("3 ticks");
+  });
+
+  test("says what happens, not what is forbidden", () => {
+    // It informs; it never blocks. Their data is theirs to reorganise, and a
+    // form that refused the edit would be a worse answer than one that says
+    // what it costs — removeGame refuses because a cascade is unrecoverable,
+    // and an orphaned mark is not.
+    expect(strandedNotice(3)!.toLowerCase()).toContain("strand");
+  });
+});
+
+describe("the sheet says how often", () => {
+  test("a repeating event shows its cadence", () => {
+    const rule = CustomEvent.parse({
+      id: "myevent:k3f9qa2m01",
+      game: "mygame:limbus-company",
+      title: "Abyss",
+      type: "challenge",
+      summary: null,
+      startsAt: "2026-09-01T00:00:00.000Z",
+      startPrecision: "day",
+      endsAt: "2026-09-08T00:00:00.000Z",
+      endPrecision: "day",
+      repeat: { unit: "weeks", interval: 2, until: null },
+      at: AT,
+      updatedAt: AT,
+    });
+    expect(cadenceLabel(rule.repeat)).toBe("every 2 weeks");
+  });
+
+  test("an interval of one drops the number and the plural", () => {
+    expect(cadenceLabel({ unit: "weeks", interval: 1, until: null })).toBe("every week");
+    expect(cadenceLabel({ unit: "months", interval: 1, until: null })).toBe("every month");
+  });
+
+  test("a non-repeating event has no cadence to show", () => {
+    expect(cadenceLabel(null)).toBe(null);
+  });
+});