Drag an event to move it, and its edge to resize it

The calendar could only be edited through the editor, so moving a meeting
half an hour meant opening a dialog, changing two fields and saving. Every
other surface a finger or a pointer drives already answers to a drag.

In the day and week grids an event moves by dragging it and changes length
by dragging its bottom edge, snapping to fifteen minutes. In the month
grid it moves to another day and keeps the time it had, because a month
cell is a day and nothing finer -- changing the hour as well would answer
a question nobody asked.

It goes through the same path a menu edit takes. A recurring event is
asked which dates it means, and the answer runs through runScoped, so a
date the server will only change as part of a whole series offers that
rather than failing.

Three things do not offer a drag, and the reasons are checked in one place
so no grid has to remember all three: a read-only calendar, an event with
no calendar, and a birthday -- which is derived from a contact and has
nothing on the server to move. The reserved classes the swipe gesture was
told to keep out of are exactly the ones that are draggable here, which is
what that reservation was for.

Invitations are not sent. A drag is a scheduling gesture, and mailing
every guest on each nudge of a block is not what the hand was asking for;
a change that should go out with notice goes through the editor.

The new time is computed in the event's own frame rather than through an
instant. Working it out from the reader's local hours and then
re-expressing it in the event's zone converts twice, and the two do not
cancel: caught in the browser, where an event moved two hours the first
time it was dragged in the month grid and then sat still, because after
that its stored time and the reader's agreed. Parsing the stored string
into its parts and adding minutes to those touches no zone at all, and a
resize sends only a duration, so the question does not arise there either.

web/src/lib/eventDrag.ts

@@ -0,0 +1,163 @@
+/**
+ * Moving and resizing an event by dragging it.
+ *
+ * The arithmetic lives here, away from the grids and under test, for the same
+ * reason the swipe thresholds do: the numbers are the whole thing, and a
+ * mistake in them moves somebody's meeting to the wrong hour rather than
+ * merely looking wrong.
+ *
+ * Nothing here talks to the server or knows what a scope is. It answers one
+ * question — given an event and a gesture, what are the new start and end —
+ * and the caller decides whether it is allowed to save that.
+ */
+import { addMinutes } from "./dates";
+import { isBirthdayEvent } from "./birthdays";
+import type { CalendarEvent } from "@/jmap/types";
+
+/**
+ * Fifteen minutes, which is the smallest slot anybody schedules against and
+ * the largest that still lands where the pointer looks like it is.
+ */
+export const SNAP_MINUTES = 15;
+
+/** An event has to keep some length; dragging its end past its start is not a request. */
+export const MIN_DURATION_MINUTES = 15;
+
+/** Round a count of minutes to the nearest slot, away from zero on a tie. */
+export function snap(minutes: number, slot: number = SNAP_MINUTES): number {
+  return Math.round(minutes / slot) * slot;
+}
+
+export interface Span {
+  start: Date;
+  end: Date;
+}
+
+/**
+ * Moved by a number of minutes, keeping its length.
+ *
+ * Both ends move together: dragging the middle of an event is asking for it to
+ * happen at another time, not to become a different length.
+ */
+export function movedBy(span: Span, deltaMinutes: number): Span {
+  const delta = snap(deltaMinutes);
+  return { start: addMinutes(span.start, delta), end: addMinutes(span.end, delta) };
+}
+
+/**
+ * Moved to another day, keeping its time of day and its length.
+ *
+ * This is the month grid, where a cell is a day and nothing finer. An event
+ * dragged from Tuesday to Friday should still be at two o'clock; changing the
+ * hour as well would be answering a question nobody asked.
+ */
+export function movedToDay(span: Span, day: Date): Span {
+  const length = span.end.getTime() - span.start.getTime();
+  const start = new Date(day.getFullYear(), day.getMonth(), day.getDate(), span.start.getHours(), span.start.getMinutes(), 0, 0);
+  return { start, end: new Date(start.getTime() + length) };
+}
+
+/**
+ * Resized from its end, never shorter than one slot.
+ *
+ * The floor is a clamp rather than a refusal: a drag that goes too far is
+ * still a drag, and stopping at fifteen minutes is what the reader sees
+ * happening while they do it.
+ */
+export function resizedBy(span: Span, deltaMinutes: number): Span {
+  const end = addMinutes(span.end, snap(deltaMinutes));
+  const minimum = addMinutes(span.start, MIN_DURATION_MINUTES);
+  return { start: span.start, end: end.getTime() < minimum.getTime() ? minimum : end };
+}
+
+/** Seconds, as an ISO 8601 duration — the shape `duration` takes on the wire. */
+export function formatDuration(seconds: number): string {
+  const total = Math.max(0, Math.round(seconds));
+  const days = Math.floor(total / 86400);
+  const hours = Math.floor((total % 86400) / 3600);
+  const minutes = Math.floor((total % 3600) / 60);
+  const secs = total % 60;
+  if (!total) return "PT0S";
+  const time = [hours && `${hours}H`, minutes && `${minutes}M`, secs && `${secs}S`].filter(Boolean).join("");
+  return `P${days ? `${days}D` : ""}${time ? `T${time}` : ""}`;
+}
+
+/**
+ * The patch a move or a resize sends.
+ *
+ * **Computed in the event's own frame, never through an instant.** An event
+ * carries a wall-clock `start` and a `timeZone`, and the grid draws it at the
+ * reader's local time. Working out a new time from those local hours and then
+ * re-expressing it in the event's zone converts twice, and the two conversions
+ * do not cancel: an event in a zone two hours from the reader's moved two
+ * hours the first time it was dragged, and then sat still, because after that
+ * its stored time and the reader's happened to agree.
+ *
+ * Parsing the stored string into its parts and adding minutes to those parts
+ * touches no zone at all, so there is nothing to get wrong. The zone itself is
+ * left exactly as it was: dragging an event is not a claim about where it
+ * happens.
+ */
+function parseStored(start: string): Date | null {
+  const m = /^(\d{4})-(\d{2})-(\d{2})T(\d{2}):(\d{2})(?::(\d{2}))?/.exec(start ?? "");
+  if (!m) return null;
+  return new Date(Number(m[1]), Number(m[2]) - 1, Number(m[3]), Number(m[4]), Number(m[5]), Number(m[6] ?? 0), 0);
+}
+
+function formatStored(d: Date): string {
+  const p = (n: number) => String(n).padStart(2, "0");
+  return `${d.getFullYear()}-${p(d.getMonth() + 1)}-${p(d.getDate())}T${p(d.getHours())}:${p(d.getMinutes())}:${p(d.getSeconds())}`;
+}
+
+export interface DragPatch {
+  start?: string;
+  duration?: string;
+}
+
+/** Moved by a number of minutes, in the event's own frame. */
+export function movePatch(storedStart: string, deltaMinutes: number): DragPatch {
+  const base = parseStored(storedStart);
+  if (!base) return {};
+  return { start: formatStored(addMinutes(base, snap(deltaMinutes))) };
+}
+
+/** Moved to another date, keeping the time of day it already had. */
+export function moveToDayPatch(storedStart: string, day: Date): DragPatch {
+  const base = parseStored(storedStart);
+  if (!base) return {};
+  const moved = new Date(day.getFullYear(), day.getMonth(), day.getDate(), base.getHours(), base.getMinutes(), base.getSeconds(), 0);
+  return { start: formatStored(moved) };
+}
+
+/**
+ * Resized from its end. Only the duration moves, so the start -- and with it
+ * the whole question of zones -- is not touched at all.
+ */
+export function resizePatch(currentSeconds: number, deltaMinutes: number): DragPatch {
+  const seconds = Math.max(MIN_DURATION_MINUTES * 60, currentSeconds + snap(deltaMinutes) * 60);
+  return { duration: formatDuration(seconds) };
+}
+
+/**
+ * Whether this event can be dragged at all.
+ *
+ * Three separate reasons it might not be, and they are checked here so no grid
+ * has to remember all three:
+ *
+ *  - **A birthday is derived**, not stored. There is nothing on the server to
+ *    move, and the date belongs to a contact rather than to a calendar.
+ *  - **The calendar may be read-only** — someone else's, shared without write
+ *    rights. This is the same question the popover asks before offering Edit.
+ *  - **An event with no calendar** has nowhere to be saved.
+ */
+export function canDragEvent(event: CalendarEvent | null | undefined, calendar: { myRights?: { mayWriteAll?: boolean; mayWriteOwn?: boolean } } | undefined): boolean {
+  if (!event || isBirthdayEvent(event.id)) return false;
+  if (!calendar) return false;
+  return Boolean(calendar.myRights?.mayWriteAll || calendar.myRights?.mayWriteOwn);
+}
+
+/** How far the pointer moved, in minutes, given a grid's pixels-per-hour. */
+export function pixelsToMinutes(deltaPixels: number, hourHeight: number): number {
+  if (!hourHeight) return 0;
+  return (deltaPixels / hourHeight) * 60;
+}