function createOpportunityCalendar({ environment, loadLibrary, getState, getItems, getVersion,
  formatRevenue, formatDate, getOwnerName, createDetailLink, createCopyButton, createCloseIcon,
  createStageBadge, createStatusBadge, opportunityUrl, modalId, closeSrPopover, createViewIcon }) {
  const { document } = environment;
  let elements = null;
  let calendar = null;
  let library = null;
  let selectedView = "overview";
  let anchoredVersion = null;
  let details = null;
  let lastEvents = "";
  let dayCounts = new Map();
  const dayBadges = new Map();
  let resizeObserver = null;
  let layoutFrame = null;
  let eventLimit = 2;
  function visibleEventLimit(height, weeks, headerHeight) {
    return Math.max(0, Math.min(2, Math.floor((height / Math.max(1, weeks) - headerHeight - 26) / 48)));
  }
  function scheduleLayout() {
    for (const [name, icon] of [["today", "today"], ["dayGridMonth", "calendar"], ["dayGridWeek", "week"]]) {
      const button = elements?.calendarHost.querySelector?.(`.fc-${name}-button`);
      if (button && !button.querySelector("svg")) button.prepend(createViewIcon(icon));
    }
    const win = environment.window;
    if (!win?.requestAnimationFrame || layoutFrame !== null) return;
    layoutFrame = win.requestAnimationFrame(() => {
      layoutFrame = null;
      if (!calendar || selectedView !== "calendar" || !elements.calendarHost.clientHeight) return;
      calendar.updateSize();
      const frame = elements.calendarHost.querySelector(".fc-daygrid-day-frame");
      const events = frame?.querySelector(".fc-daygrid-day-events");
      if (!frame || !events) return;
      const scroller = frame.closest(".fc-scroller");
      const weeks = elements.calendarHost.querySelectorAll(".fc-daygrid-body tr").length;
      if (!scroller || !weeks) return;
      elements.calendarHost.style.setProperty("--opportunities-calendar-weeks", String(weeks));
      const headerHeight = events.getBoundingClientRect().top - frame.getBoundingClientRect().top;
      const limit = visibleEventLimit(scroller.clientHeight, weeks, headerHeight);
      if (limit !== eventLimit) { eventLimit = limit; calendar.setOption("dayMaxEvents", limit); }
    });
  }
  function dateKey(date) {
    return `${date.getFullYear()}-${String(date.getMonth() + 1).padStart(2, "0")}-${String(date.getDate()).padStart(2, "0")}`;
  }
  function fiscalWeekLabel(date) {
    // FullCalendar's week hook supplies a UTC date marker, not a local instant.
    const start = new Date(Date.UTC(date.getUTCFullYear(), date.getUTCMonth(), date.getUTCDate()));
    start.setUTCDate(start.getUTCDate() - start.getUTCDay());
    const end = new Date(start.getTime() + 6 * 86400000);
    const year = end.getUTCMonth() >= 5 ? end.getUTCFullYear() : end.getUTCFullYear() - 1;
    const fiscalStart = new Date(Date.UTC(year, 5, 1));
    const firstSunday = fiscalStart.getTime() - fiscalStart.getUTCDay() * 86400000;
    const representative = start < fiscalStart ? fiscalStart : start;
    const quarter = Math.floor(((representative.getUTCMonth() + 7) % 12) / 3) + 1;
    const week = Math.round((start.getTime() - firstSunday) / (7 * 86400000)) + 1;
    return `FY${String(year + 1).slice(-2)}-Q${quarter}-W${week}`;
  }
  function updateDayBadge(badge, key) {
    const count = dayCounts.get(key) || 0;
    badge.hidden = count === 0;
    badge.textContent = String(count);
    badge.title = `${count} ${count === 1 ? "opportunity" : "opportunities"}`;
    badge.setAttribute("aria-label", badge.title);
    badge.setAttribute("data-empty", String(count === 0));
  }
  function node(tag, className, text) {
    const el = document.createElement(tag);
    el.className = className;
    if (text !== undefined) el.textContent = text;
    return el;
  }
  function closeDate(value) {
    if (typeof value !== "string" || !/^\d{4}-\d{2}-\d{2}(?:T|$)/.test(value)) return null;
    const day = value.slice(0, 10);
    const date = new Date(`${day}T00:00:00Z`);
    return Number.isFinite(date.getTime()) && date.toISOString().slice(0, 10) === day ? day : null;
  }
  function eventsFor(items) {
    return items.flatMap((item, index) => {
      const start = closeDate(item.EffectiveDate);
      if (!start) return [];
      return [{ id: String(item.OptyNumber || item.OptyId || index), title: String(item.Name || item.OptyNumber || "-"),
        start, allDay: true, extendedProps: { item }, classNames: [`opportunities-calendar-stage-${stage(item)}`] }];
    });
  }
  function stage(item) { return ["SQL", "PIPELINE", "UPSIDE", "FORECAST", "WON"].includes(item.ForecastGroup_c) ? item.ForecastGroup_c : "OTHER"; }
  function initialDate(items, now = new Date()) {
    const today = `${now.getFullYear()}-${String(now.getMonth() + 1).padStart(2, "0")}-${String(now.getDate()).padStart(2, "0")}`;
    const dates = items.map(item => closeDate(item.EffectiveDate)).filter(Boolean).sort();
    return dates.find(date => date >= today) || dates.at(-1) || today;
  }
  function closeDetails(restoreFocus = true) {
    if (!details) return false;
    const previous = details;
    details = null;
    previous.overlay.remove();
    if (restoreFocus) (previous.trigger?.isConnected ? previous.trigger : elements?.tabs.calendar)?.focus();
    return true;
  }
  function showDialog(heading, content, contentControls, trigger) {
    closeDetails(false);
    const host = document.getElementById(modalId);
    if (!host || host.hidden) return;
    const overlay = node("section", "opportunities-extension-action-form-modal opportunities-calendar-detail");
    overlay.setAttribute("role", "dialog"); overlay.setAttribute("aria-modal", "true");
    overlay.setAttribute("aria-labelledby", "opportunities-calendar-detail-title");
    const panel = node("div", "opportunities-extension-action-form-dialog");
    const header = node("header", "opportunities-extension-account-plans-header");
    const title = node("h2", "", heading);
    title.id = "opportunities-calendar-detail-title";
    const dismiss = node("button", "opportunities-extension-icon-button");
    dismiss.type = "button"; dismiss.title = "Close"; dismiss.setAttribute("aria-label", "Close opportunity details");
    dismiss.append(createCloseIcon()); dismiss.addEventListener("click", () => closeDetails());
    header.append(title, dismiss);
    panel.append(header, content); overlay.append(panel); host.append(overlay);
    details = { overlay, trigger };
    overlay.addEventListener("click", event => { if (event.target === overlay) closeDetails(); });
    overlay.addEventListener("keydown", event => {
      if (event.key === "Escape") { event.preventDefault(); event.stopPropagation(); closeDetails(); }
      if (event.key === "Tab") {
        const controls = [dismiss, ...contentControls];
        const index = controls.indexOf(document.activeElement);
        if (index < 0 || (event.shiftKey ? index === 0 : index === controls.length - 1)) {
          event.preventDefault(); controls[event.shiftKey ? controls.length - 1 : 0].focus();
        }
      }
    });
    dismiss.focus();
  }
  function showDetails(item, trigger) {
    const content = node("div", "opportunities-calendar-detail-content");
    const link = createDetailLink(item.OptyNumber || "-", item.OptyNumber ? opportunityUrl + encodeURIComponent(item.OptyNumber) : "");
    const identity = node("div", "opportunities-extension-opty-number-content");
    identity.append(link);
    const copy = item.OptyNumber ? createCopyButton(item.OptyNumber) : null;
    if (copy) identity.append(copy);
    const summary = node("dl", "opportunities-extension-action-form-context");
    for (const [label, value] of [["Customer", item.CustomerAccount?.PartyUniqueName], ["Owner", getOwnerName(item)],
      ["Close Date", closeDate(item.EffectiveDate) ? formatDate(item.EffectiveDate) : "N/A"], ["Amount", formatRevenue(item)]]) {
      summary.append(node("dt", "", label), node("dd", "", String(value || "-")));
    }
    const badges = node("div", "opportunities-calendar-detail-badges");
    badges.append(createStatusBadge(item.StatusCode), createStageBadge(item.ForecastGroup_c));
    content.append(identity, summary, badges);
    showDialog(item.Name || item.OptyNumber || "Opportunity", content, item.OptyNumber ? [link, copy] : [], trigger);
  }
  function showDayOpportunities({ date, allSegs, jsEvent }) {
    jsEvent.preventDefault();
    const trigger = jsEvent.currentTarget || elements?.tabs.calendar;
    const content = node("div", "opportunities-calendar-day-list");
    const controls = [];
    const events = new Map(allSegs.map(({ event }) => [event.id, event]));
    for (const event of events.values()) {
      const item = event.extendedProps.item;
      const button = node("button", "opportunities-calendar-day-item"); button.type = "button";
      const summary = `${item.OptyNumber || "-"} | ${item.ForecastGroup_c || "N/A"} | ${formatRevenue(item)}`;
      button.append(node("strong", "", event.title), node("span", "", summary));
      button.title = `${event.title}\n${summary}`;
      button.addEventListener("click", () => showDetails(item, trigger));
      controls.push(button); content.append(button);
    }
    showDialog(`${formatDate(date.toISOString().slice(0, 10))} | ${events.size} ${events.size === 1 ? "opportunity" : "opportunities"}`, content, controls, trigger);
    // A truthy non-string result prevents FullCalendar from opening its measured popover.
    return true;
  }
  function mount(body, tableSurface) {
    resizeObserver?.disconnect();
    if (layoutFrame !== null) environment.window?.cancelAnimationFrame(layoutFrame);
    layoutFrame = null; eventLimit = 2;
    calendar?.destroy(); calendar = null; lastEvents = ""; anchoredVersion = null; selectedView = "overview";
    dayBadges.clear(); dayCounts.clear();
    closeDetails(false);
    const workspace = node("section", "opportunities-extension-views");
    const tablist = node("div", "opportunities-extension-view-tabs");
    tablist.setAttribute("role", "tablist"); tablist.setAttribute("aria-label", "Opportunity views");
    const tabs = {};
    const surface = node("section", "opportunities-extension-calendar");
    const message = node("p", "opportunities-calendar-message"); message.setAttribute("role", "status");
    const calendarHost = node("div", "opportunities-calendar-grid");
    const undated = node("details", "opportunities-calendar-undated");
    surface.append(message, calendarHost, undated);
    for (const [key, label, panel] of [["overview", "Overview", tableSurface], ["calendar", "Calendar", surface]]) {
      const tab = node("button", ""); tab.type = "button"; tab.id = `opportunities-view-${key}-tab`;
      tab.append(createViewIcon(key), node("span", "", label));
      panel.id = `opportunities-view-${key}`; panel.setAttribute("role", "tabpanel"); panel.setAttribute("aria-labelledby", tab.id);
      tab.setAttribute("role", "tab"); tab.setAttribute("aria-controls", panel.id);
      tab.addEventListener("click", () => selectView(key));
      tab.addEventListener("keydown", event => {
        if (!["ArrowLeft", "ArrowRight", "Home", "End"].includes(event.key)) return;
        event.preventDefault();
        const next = event.key === "Home" ? "overview" : event.key === "End" ? "calendar" : key === "overview" ? "calendar" : "overview";
        selectView(next); tabs[next].focus();
      });
      tabs[key] = tab; tablist.append(tab);
    }
    workspace.append(tablist, tableSurface, surface); body.append(workspace);
    elements = { workspace, tableSurface, surface, message, calendarHost, undated, tabs };
    if (environment.window?.ResizeObserver) {
      resizeObserver = new environment.window.ResizeObserver(scheduleLayout);
      resizeObserver.observe(calendarHost);
    }
    selectView("overview");
  }
  function selectView(view) {
    if (!elements || !["overview", "calendar"].includes(view)) return;
    closeDetails(false); closeSrPopover(); selectedView = view;
    elements.tableSurface.hidden = view !== "overview"; elements.surface.hidden = view !== "calendar";
    for (const [key, tab] of Object.entries(elements.tabs)) {
      tab.setAttribute("aria-selected", String(view === key)); tab.tabIndex = view === key ? 0 : -1;
    }
    render(getItems(), getState());
    if (view === "calendar") calendar?.updateSize();
    scheduleLayout();
  }
  function render(items, state) {
    if (!elements || selectedView !== "calendar") return;
    const { surface, message, calendarHost, undated } = elements;
    surface.setAttribute("aria-busy", String(state.status === "loading"));
    if (["loading", "error", "empty", "idle"].includes(state.status)) {
      message.textContent = state.message || (state.status === "loading" ? "Loading opportunities..." : "No opportunities found.");
      message.hidden = false; calendarHost.hidden = true; undated.hidden = true; closeDetails(false); return;
    }
    message.textContent = items.length ? "" : "No opportunities found."; message.hidden = !!items.length;
    calendarHost.hidden = false;
    const events = eventsFor(items);
    dayCounts = new Map();
    for (const event of events) dayCounts.set(event.start, (dayCounts.get(event.start) || 0) + 1);
    for (const { badge, key } of dayBadges.values()) updateDayBadge(badge, key);
    const anchor = initialDate(state.items);
    if (!calendar) {
      try {
        library ||= loadLibrary();
        calendar = new library.Calendar(calendarHost, {
          initialView: "dayGridMonth", initialDate: anchor, height: "100%", expandRows: true, fixedWeekCount: false,
          datesSet: scheduleLayout,
          locale: "en", firstDay: 0, editable: false, selectable: false, eventInteractive: true,
          headerToolbar: { left: "prev,next today", center: "title", right: "dayGridMonth,dayGridWeek" },
          buttonText: { today: "Today", month: "Month", week: "Week" },
          buttonHints: { prev: "Previous", next: "Next", today: "Today", dayGridMonth: "Month", dayGridWeek: "Week" },
          dayMaxEvents: 2, moreLinkText: count => `... +${count}`, displayEventTime: false,
          moreLinkHint: count => `View ${count} more ${count === 1 ? "opportunity" : "opportunities"}`,
          moreLinkClick: showDayOpportunities,
          weekNumbers: true,
          weekNumberContent: ({ date }) => ({ domNodes: [node("span", "opportunities-calendar-fiscal-week", fiscalWeekLabel(date))] }),
          dayCellDidMount: ({ el, date }) => {
            const badge = node("span", "opportunities-calendar-day-count");
            const key = dateKey(date);
            updateDayBadge(badge, key);
            el.querySelector(".fc-daygrid-day-frame").append(badge);
            dayBadges.set(el, { badge, key });
          },
          dayCellWillUnmount: ({ el }) => { dayBadges.get(el)?.badge.remove(); dayBadges.delete(el); },
          eventOrder: "title", eventDisplay: "block", events,
          eventContent: ({ event }) => {
            const item = event.extendedProps.item;
            const card = node("div", "opportunities-calendar-event-content");
            const summary = node("span", "", `${item.OptyNumber || "-"} | ${item.ForecastGroup_c || "N/A"} | ${formatRevenue(item)}`);
            summary.title = summary.textContent;
            card.append(node("strong", "", event.title), summary);
            return { domNodes: [card] };
          },
          eventDidMount: ({ el, event }) => {
            el.title = `${event.title}\n${event.extendedProps.item.CustomerAccount?.PartyUniqueName || ""}\n${formatDate(event.startStr)} | ${event.extendedProps.item.ForecastGroup_c || "-"}`;
            el.setAttribute("role", "button"); el.setAttribute("aria-label", el.title);
          },
          eventClick: ({ event, el, jsEvent }) => { jsEvent.preventDefault(); showDetails(event.extendedProps.item, el); }
        });
        calendar.render(); lastEvents = JSON.stringify(events); anchoredVersion = getVersion();
      } catch {
        calendar?.destroy(); calendar = null; message.textContent = "Unable to display the calendar. Use the overview or try again.";
        message.hidden = false; calendarHost.hidden = true; undated.hidden = true; return;
      }
    }
    const signature = JSON.stringify(events);
    calendar.batchRendering(() => {
      if (anchoredVersion !== getVersion()) { calendar.gotoDate(anchor); anchoredVersion = getVersion(); closeDetails(false); }
      if (signature !== lastEvents) { calendar.removeAllEvents(); calendar.addEventSource(events); lastEvents = signature; closeDetails(false); }
    });
    const missing = items.filter(item => !closeDate(item.EffectiveDate));
    undated.hidden = !missing.length; undated.textContent = "";
    if (missing.length) {
      undated.append(node("summary", "", `No valid Close Date (${missing.length})`));
      for (const item of missing) {
        const button = node("button", "", `${item.OptyNumber || "-"} | ${item.Name || "-"}`); button.type = "button";
        button.addEventListener("click", () => showDetails(item, button)); undated.append(button);
      }
    }
    scheduleLayout();
  }
  return { mount, render, selectView, closeDetails, closeDate, eventsFor, initialDate, fiscalWeekLabel, visibleEventLimit };
}