// In-app browser-history integration so Back/Forward step through a module's sub-views and
// overlays before leaving the page — not straight back to the previous module.
//
// Two primitives share the existing page-history (App stamps `sfPage` in history.state):
//   - useHistorySubview: a module's active sub-view rides in history.state._sub, so Back inside
//     a module (e.g. the sign-in card's "forgot password" → sign-in) returns to the parent view first.
//   - useBackToClose: while an overlay/modal is mounted it occupies one history entry; Back
//     closes the topmost one. A module-level stack keeps nesting correct — closing one modal by
//     its own button pops exactly its entry without tripping the modals underneath. An overlay
//     that is part of the view (a player) names itself in its entry's address.
//
// setPage() pushes a clean { sfPage } entry (dropping _sub/_ov), so each page starts at its root.
import { useCallback, useEffect, useEffectEvent, useMemo, useRef, useState } from "react";
import { navPosition, restoreNavPosition, type NavPosition } from "./listNav";
import { urlWithParams } from "./urlState";
import { useLatest } from "./useLatest";

// --- Per-history-entry scroll id + a nav bus (for scroll restoration) ------------------------
// Every navigation entry carries a stable numeric id in history.state.sfScroll. PageScroller
// remembers scroll offsets per (entry id + view key) so Back restores exactly where the user left
// an entry, while a forward navigation (a new id) starts at the top. We drive scroll ourselves, so
// the browser's own window-scroll restoration (which can't see our inner <main> scroller) is off.
if (typeof window !== "undefined" && "scrollRestoration" in window.history) {
  window.history.scrollRestoration = "manual";
}
let navCounter = 0;
// sfScroll ids are persisted into window.history.state and survive a full page reload, but this
// module-level counter resets to 0 on every load. Seed it past the current entry's id so freshly
// minted ids can't collide with ids already carried by pre-reload back-stack entries (ids are
// assigned in forward order, so the current entry's id is ≥ every ancestor's) — a collision would
// otherwise cross-contaminate PageScroller's per-entry scroll offsets after a reload + navigation.
if (typeof window !== "undefined") {
  const seed = (window.history.state as { sfScroll?: number } | null)?.sfScroll;
  if (typeof seed === "number") navCounter = seed;
}
/** A fresh entry id — stamp it into the state of every pushState that opens a new main-content view. */
export function nextNavId(): number {
  return ++navCounter;
}
/** The current entry's scroll id, assigning one (via replaceState) to entries that predate this — the
 *  initial load, or an entry pushed without one — so the id survives Back/Forward. */
export function ensureNavId(): number {
  const st = (window.history.state ?? {}) as { sfScroll?: number };
  if (typeof st.sfScroll === "number") {
    // Keep the counter ahead of any id we observe (e.g. a Back landed on a higher-id entry after a
    // reload seeded us low) so the next mint never duplicates a live entry's id.
    if (st.sfScroll > navCounter) navCounter = st.sfScroll;
    return st.sfScroll;
  }
  const id = nextNavId();
  window.history.replaceState({ ...st, sfScroll: id }, "");
  return id;
}
/** Read the current entry's scroll id without assigning one (0 if none). */
export function peekNavId(): number {
  const st = window.history.state as { sfScroll?: number } | null;
  return typeof st?.sfScroll === "number" ? st.sfScroll : 0;
}
const navListeners = new Set<() => void>();
/** Fire after a pushState that opens a new view but does NOT re-render PageScroller (a Plex sub-view
 *  open): lets the scroller run its restore/reset without waiting on a React render it won't get. */
export function notifyNav(): void {
  navListeners.forEach((cb) => cb());
}
export function subscribeNav(cb: () => void): () => void {
  navListeners.add(cb);
  return () => navListeners.delete(cb);
}

// --- The list item a view was left from (history.state.sfNav) -------------------------------------
// See lib/listNav "The item a view was left from". Stamped onto the entry being LEFT, right before a
// push opens a new view over it; read when Back/Forward lands on that entry, and dropped from it there,
// so a later landing on the same entry — an overlay closing over it — cannot pull focus to an old item.
// Installed at load, not by the first push: a stamp survives a reload in history.state, and the first
// Back after one must still find a listener.
let cancelNavRestore = () => {};
if (typeof window !== "undefined") {
  window.addEventListener("popstate", (e) => {
    cancelNavRestore();
    const pos = (e.state as { sfNav?: NavPosition } | null)?.sfNav;
    if (!pos) return;
    const { sfNav: _spent, ...st } = (window.history.state ?? {}) as Record<string, unknown>;
    window.history.replaceState(st, "");
    cancelNavRestore = restoreNavPosition(pos);
  });
}

/** Call right before a pushState that opens a new view over the current entry (a page, a sub-view, a
 *  channel, a tab). Returns the current state WITHOUT the stamp: a push that spreads the current state
 *  must spread this, so the new entry does not inherit a position it was never left from — nor an
 *  overlay's marks, which belong to the entry being left (an entry a reload under an open dialog
 *  started from carries them, and the first overlay over a view pushed from it took that view's entry
 *  over for its own). */
export function rememberNavPosition(): Record<string, unknown> {
  // A restore still waiting for the last landing's lists belongs to the page being left: left
  // running, it would focus "list N, item M" of whatever page this push opens.
  cancelNavRestore();
  const { sfNav: _stale, ...st } = (window.history.state ?? {}) as Record<string, unknown>;
  const pos = navPosition();
  window.history.replaceState(pos ? { ...st, sfNav: pos } : st, "");
  const { _ov: _overlay, _ovd: _depth, ...view } = st;
  return view;
}

// --- Is the entry a back() lands on still ours? --------------------------------------------
// Chrome keeps 50 entries a tab. When the list is full a new entry drops the OLDEST "skippable" one —
// an entry the page left by a pushState without a user gesture (an auto-skip countdown, a card that
// opened on its own) — not the oldest entry, so it can be this page's own entry under a sub-view.
// A programmatic back() then leaves the document for whatever older page sits below (measured
// 2026-10-07: an older entry whose restored Plex player started playing on its own). The Navigation
// API says which document the entry a back() lands on belongs to — counting the back()s of ours still
// in flight; a browser without it keeps the old assumption that our entries are all still there.
interface NavigationLike {
  currentEntry: { index: number } | null;
  entries: () => readonly { sameDocument: boolean }[];
}

/** Would `steps` more steps back (one, a history.back()) still land in this document? */
export function backStaysInDocument(steps = 1): boolean {
  const nav = (window as { navigation?: NavigationLike }).navigation;
  const at = nav?.currentEntry?.index;
  if (nav == null || at == null) return true;
  return nav.entries()[at - pendingSteps - steps]?.sameDocument === true;
}

// --- Sub-views (value carried in history.state._sub) ---------------------------------------

/** Optional address-bar mirroring for a sub-view, so a deep sub-view (e.g. a Plex show page) is
 * bookmarkable. history.state._sub stays the source of truth for Back/Forward; this only adds the URL. */
export interface SubviewUrl<T> {
  /** The query key this sub-view mirrors to (e.g. "v" for the Plex sub-view). */
  key: string;
  /** Serialize a sub-view to the param value, or null for views not mirrored (root / ephemeral ones). */
  encode: (view: T) => string | null;
  /** Parse a param value back to a sub-view, or null if it isn't a valid one. */
  decode: (raw: string) => T | null;
  /** What the history ENTRY keeps of a view, when that is less than the view opens with: a one-shot
   *  flag (the Plex player's `fromStart`) must not replay on a reload or a Forward onto the entry. */
  entry?: (view: T) => T;
}

export function useHistorySubview<T>(
  root: T,
  url?: SubviewUrl<T>,
): {
  view: T;
  /** `replace` rewrites the CURRENT entry instead of stacking a new one — for a sub-view that is the
   *  continuation of the navigation that just arrived here, not a step the user took inside the
   *  module. See the `jumpTo` hand-off in PlexBrowse. */
  open: (next: T, opts?: { replace?: boolean }) => void;
  /** The view moved on by itself (the Plex player stepped to the next episode): the entry it stands
   *  on says so from now on, without a navigation — the overlays over it stay open. */
  follow: (next: T) => void;
  back: () => void;
} {
  // Resolve the current entry's sub-view: history.state._sub for an in-app entry, else decode the
  // ?<key> URL param for a fresh bookmark (whose entry carries the param but no _sub), else root.
  // ⚠️ The seed AND the popstate handler must use THIS same fallback: a popstate can land on a
  // bookmark entry (URL-only, no _sub) — e.g. after an overlay's programmatic history.back() — and
  // reading history.state alone would wrongly collapse the bookmarked sub-view to root.
  const readView = (): T => {
    const fromState = window.history.state?._sub as T | undefined;
    if (fromState != null) return fromState;
    if (url) {
      const raw = new URLSearchParams(window.location.search).get(url.key);
      const decoded = raw != null ? url.decode(raw) : null;
      if (decoded != null) return decoded;
    }
    return root;
  };
  const [view, setView] = useState<T>(readView);
  // Where `follow` moved the view, and on which entry. An overlay over the view holds an entry of its
  // own (a copy of the view's state, same sfScroll id), and only the current entry can be rewritten:
  // the view's own entry under the overlay still says where it was until a step lands back on it.
  const followedRef = useRef<{ entry: number; view: T } | null>(null);
  const readLandedView = useEffectEvent((): T => {
    const followed = followedRef.current;
    if (followed == null || peekNavId() !== followed.entry) return readView();
    writeSubEntry(followed.view, url);
    return followed.view;
  });
  useEffect(() => {
    // Back/Forward restores the sub-view from the landed entry (state OR its restored ?<key> URL).
    const onPop = () => setView(readLandedView());
    window.addEventListener("popstate", onPop);
    return () => window.removeEventListener("popstate", onPop);
  }, []);
  // `open` and `back` are memoised so they are REFERENTIALLY stable, not merely behaviourally so.
  // They used to be plain inline arrows: correct, but a fresh identity every render, which forced
  // every consumer that listens for a sub-view change to disable the dependency check rather than
  // re-run its effect on each parent render (three such effects in PlexBrowse alone). `url` is a
  // module constant at every call site (PLEX_SUB_URL, MODE_URL) and the body otherwise touches only
  // module functions and the setter, so there is nothing left to churn on.
  // Both move the module to another view, so both are navigations: they close the overlays standing
  // over it first (`closeOverlaysThenNavigate`, synchronous when none is open). A step taken under an
  // open overlay landed as a USER Back — the overlay listener spent it closing the top overlay, so
  // leaving the Plex player with its stats readout up only closed the readout — and a push or a
  // replace on top of an overlay's entry is undone when that entry is reclaimed.
  const go = useCallback(
    (next: T, opts?: { replace?: boolean }) => {
      setView(next);
      // A navigation says where the view is now, on the entry it lands on or rewrites.
      followedRef.current = null;
      if (opts?.replace) {
        writeSubEntry(next, url);
        notifyNav();
        return;
      }
      const kept = url?.entry ? url.entry(next) : next;
      const href = url ? urlWithParams({ [url.key]: url.encode(next) }) : "";
      // A fresh sfScroll so the opened sub-view is its own scroll entry (starts at the top, and Back
      // restores the view we came from) — then notify PageScroller, which this push doesn't re-render.
      window.history.pushState(
        { ...rememberNavPosition(), _sub: kept, sfScroll: nextNavId() },
        "",
        href,
      );
      notifyNav();
    },
    [url],
  );
  const open = useCallback(
    (next: T, opts?: { replace?: boolean }) => closeOverlaysThenNavigate(() => go(next, opts)),
    [go],
  );
  // With no entry of this document below (see backStaysInDocument), the sub-view's own entry becomes
  // the root view in place — a step up the module, not out of the app.
  const rootRef = useLatest(root);
  const back = useCallback(
    () =>
      closeOverlaysThenNavigate(() => {
        if (backStaysInDocument()) window.history.back();
        else go(rootRef.current, { replace: true });
      }),
    [go, rootRef],
  );
  const follow = useCallback(
    (next: T) => {
      setView(next);
      followedRef.current = { entry: ensureNavId(), view: next };
      writeSubEntry(next, url);
    },
    [url],
  );
  // Memoised as a whole for the same reason: a consumer that wants "re-run when the sub-view moves"
  // can now write `[sub]` and mean it, instead of re-running on every parent render.
  return useMemo(() => ({ view, open, follow, back }), [view, open, follow, back]);
}

// Rewrite the CURRENT entry to hold `next`. Same entry, so it keeps its sfScroll id: PageScroller keys
// offsets per (entry, view), and the view key has changed, so the sub-view still opens at the top
// without minting an id that would orphan the one this entry already carries.
function writeSubEntry<T>(next: T, url: SubviewUrl<T> | undefined): void {
  const kept = url?.entry ? url.entry(next) : next;
  const href = url ? urlWithParams({ [url.key]: url.encode(next) }) : "";
  window.history.replaceState({ ...window.history.state, _sub: kept }, "", href);
}

// --- Overlays / modals (one history entry each, nesting-safe) -------------------------------
// A SINGLE module-level popstate handler owns the whole overlay stack, and history is kept in
// sync LAZILY via a coalesced microtask rather than eagerly per mount/unmount. This matters for
// modal→modal handoffs (e.g. About → Release notes): the leaving modal unmounts and the entering
// one mounts in the SAME React commit, so within one tick the desired overlay-entry count drops
// by one then rises by one — a net zero. Reconciling once, after the tick settles, sees that net
// zero and touches history NOT AT ALL: the new modal simply reuses the old one's entry.
//
// The previous eager design pushed on mount and called history.back() on unmount independently.
// Interleaved during a handoff that produced (a) a flash-and-vanish where the new modal's popstate
// listener mistook the leaving modal's back() for a genuine user Back and closed itself, and
// (b) a mangled history where the pointer ended up BEHIND the surviving entry, so a later browser
// Back walked off the app entirely instead of closing the modal. Coalescing removes both.
//
// A navigation never lands UNDER an open overlay: `setPage` and `openChannel` close every overlay
// first and push only once their entries are back (`closeOverlaysThenNavigate`). An earlier design
// let the push land above the overlays and "buried" their entries with a counter, and that
// counter was a whole family of bugs — a burial spent by a different overlay closing first, a
// navigation that could not report whether it pushed and was assumed to have, one browser Back that
// both undid the navigation AND closed the dialog above it, and the dead Back press after that (the
// last two measured in Chrome). With nothing ever buried, the stack and the history agree by
// construction.
// `url`: the keys an overlay names in the address of its own entry (and of the entries above it).
type Overlay = { token: number; cb: () => void; url?: OverlayUrl };
type OverlayUrl = Record<string, string | null>;
const overlayStack: Overlay[] = [];
// Every key an overlay has named in this document: an entry carries those of the overlays it stands
// for and none of the rest. The entry under an overlay is never written with its keys, so Back lands
// on an address without it; and one that still names a closed overlay (the entry a reload under an
// open player started from, a Forward onto an entry whose overlay is gone) is cleared once it is
// the current entry.
const overlayUrlKeys = new Set<string>();
let overlayCounter = 0;
// A plain function, not `++overlayCounter` inline: the React Compiler cannot lower an update of a
// module variable inside a hook, and skipped the whole hook for it (react-hooks/todo).
const nextOverlayToken = () => ++overlayCounter;
// Overlay history entries we've actually pushed. Reconciled toward overlayStack.length.
let historyDepth = 0;
// The entry this document was loaded on, when it was an overlay's (a reload under an open player):
// the first overlay to open takes it over rather than pushing another over it — pushed, Back would
// close the player onto that same page again, one dead step per reload. Until the first push or Back;
// a view pushed from it does not carry its mark (rememberNavPosition). `_ovd` on an overlay's entry is how many overlay
// entries it stood on, its own included: a reload under two (a dialog, or a maximised player, over
// the player) leaves the lower one's entry under the one taken over, and closing the taken-over
// overlay steps back over those too (`deadBelow`) — one Back closes it, as it opened.
let reloadedOverlayEntry: unknown = window.history.state?._ov ?? null;
const reloadedDepth: unknown = window.history.state?._ovd;
let deadBelow = 0;
// Programmatic history steps back still awaiting their popstate — each must be ignored — and how
// many entries they step over in all (a step over dead entries is one traversal of several).
let pendingProgrammatic = 0;
let pendingSteps = 0;
const pendingStepCounts: number[] = [];
function stepBack(steps: number): void {
  pendingProgrammatic++;
  pendingSteps += steps;
  pendingStepCounts.push(steps);
  if (steps === 1) window.history.back();
  else window.history.go(-steps);
}
function landedStepBack(): void {
  pendingProgrammatic--;
  pendingSteps -= pendingStepCounts.shift() ?? 1;
}
// The overlay at the bottom of the stack closing: back over its entry and any dead ones under it.
function stepsToClose(): number {
  const steps = historyDepth === 0 ? 1 + deadBelow : 1;
  if (historyDepth === 0) deadBelow = 0;
  return steps;
}
let reconcileScheduled = false;
let listenerInstalled = false;
// Re-checked after every reconcile — see `afterClosing`.
const syncWaiters = new Set<() => void>();

// Push/pop history entries until their count matches the live overlay count. Runs once per tick.
// A push waits while a back() of ours is still in flight: it would land on the entry just pushed and
// take it, leaving the books one entry above the history — and the next close then stepped back over
// the entry BELOW, the player's own (the Plex player's ⋯ → Settings, then a click outside the panel,
// closed the player). The popstate that lands the last back() reconciles again.
function reconcileHistory(): void {
  reconcileScheduled = false;
  const want = overlayStack.length;
  while (historyDepth < want && pendingProgrammatic === 0) {
    historyDepth++;
    const here = window.history.state?._ov;
    if (historyDepth === 1 && here != null && here === reloadedOverlayEntry) {
      reloadedOverlayEntry = null;
      if (typeof reloadedDepth === "number" && reloadedDepth > 1) deadBelow = reloadedDepth - 1;
      continue;
    }
    reloadedOverlayEntry = null;
    window.history.pushState(
      { ...window.history.state, _ov: nextOverlayToken(), _ovd: historyDepth + deadBelow },
      "",
    );
  }
  while (historyDepth > want) {
    historyDepth--;
    const steps = stepsToClose();
    if (backStaysInDocument(steps)) stepBack(steps);
    else if (steps > 1 && backStaysInDocument()) stepBack(1);
    else {
      // Nothing of ours below to step back to (see backStaysInDocument): the entry stays, unmarked.
      const {
        _ov: _drop,
        _ovd: _depth,
        ...st
      } = (window.history.state ?? {}) as Record<string, unknown>;
      window.history.replaceState(st, "");
    }
  }
  nameOverlaysHere();
  syncWaiters.forEach((cb) => cb());
}

// The address of the entry the open overlays stand on: their keys, every other overlay's cleared.
// Undefined (the address as it is) while no overlay names any. Written by `nameOverlaysHere` alone:
// a pushed entry starts as a copy of the one under it and is named once it is the current one.
function overlayHref(): string | undefined {
  if (overlayUrlKeys.size === 0) return undefined;
  const updates: OverlayUrl = {};
  for (const k of overlayUrlKeys) updates[k] = null;
  for (const o of overlayStack) Object.assign(updates, o.url);
  return urlWithParams(updates);
}

// Write the current entry's overlay keys, once the history has caught up with the stack (before
// that, the current entry may be one about to be pushed over or stepped back from).
function nameOverlaysHere(): void {
  if (historyDepth !== overlayStack.length || pendingProgrammatic > 0) return;
  const href = overlayHref();
  if (href != null && href !== hrefNow())
    window.history.replaceState(window.history.state, "", href);
}

const hrefNow = () => window.location.pathname + window.location.search + window.location.hash;

function scheduleReconcile(): void {
  if (reconcileScheduled) return;
  reconcileScheduled = true;
  queueMicrotask(reconcileHistory);
}

function ensureOverlayListener(): void {
  if (listenerInstalled) return;
  listenerInstalled = true;
  window.addEventListener("popstate", () => {
    reloadedOverlayEntry = null;
    if (pendingProgrammatic > 0) {
      landedStepBack(); // our own back(): consume and ignore
      // The last one landed: an overlay that opened while it was in flight still owes its entry.
      if (pendingProgrammatic === 0 && historyDepth < overlayStack.length) scheduleReconcile();
      else nameOverlaysHere();
      return;
    }
    // Genuine user Back: the browser already dropped one overlay entry, so close the topmost
    // overlay to match. Its unmount cleanup finds itself already off the stack and only re-syncs.
    // The last one closing over dead entries (see `deadBelow`) steps over them too.
    if (historyDepth > 0) historyDepth--;
    const dead = historyDepth === 0 ? deadBelow : 0;
    if (historyDepth === 0) deadBelow = 0;
    if (dead > 0 && backStaysInDocument(dead)) stepBack(dead);
    const top = overlayStack.pop();
    if (top) top.cb();
    scheduleReconcile();
  });
}

/** The history holds exactly one entry per live overlay and no programmatic back() is still in
 *  flight — so a pushState made now cannot be undone by one. */
function historyInSync(): boolean {
  return historyDepth === overlayStack.length && pendingProgrammatic === 0 && !reconcileScheduled;
}

// How long an overlay gets to actually close before its command is dropped. Closing is one React
// render (measured back()→popstate: 1–13 ms in Chrome); nothing in the app refuses a close today,
// but a future "discard your changes?" guard must not be navigated out from under.
const CLOSE_TIMEOUT_MS = 1500;
// An overlay's Back callback is a Back STEP: the contract lets one entry hold several, peeled one per
// call. Every overlay today closes in one call (the Plex player gives each layer its own entry), but a
// navigation must not be dropped by one that does not, so it asks again, a render apart (the callback
// reads what the last one closed), until the entry is gone or the timeout drops the command.
const RECLOSE_MS = 50;

/** Close `targets` (via `close`), then `run` once they have unmounted AND the history has caught up.
 *
 *  The trap: closing unmounts an overlay, whose cleanup reconciles by issuing a programmatic
 *  `history.back()`. A pushState made before that lands is undone by it, so a command that navigates
 *  silently does nothing but close. PlayerModal hit it first; the command palette the day it was
 *  written.
 *
 *  It waits for a STATE, not an event. An earlier helper waited for "a popstate" plus a 250 ms
 *  fallback — but one reconcile can issue several back()s, an overlay closed in the tick it opened
 *  issues none, and the fallback then ran with back()s still in flight. Here the reconcile and every
 *  popstate re-check one predicate.
 *
 *  `run` goes out on a fresh task, never inside the popstate dispatch that settled it: listeners
 *  registered before ours (NavigationProvider's) have read that event already, but ones after it
 *  have not, and a page applied from the OLD entry after our push would undo the navigation. */
function afterClosing(
  targets: readonly Overlay[],
  close: () => void,
  run: () => void,
  reclose = false,
): void {
  let done = false;
  let queued = false;
  const gone = () => targets.every((o) => !overlayStack.includes(o));
  const ready = () => !done && gone() && historyInSync();
  const stop = () => {
    done = true;
    syncWaiters.delete(check);
    window.removeEventListener("popstate", check);
    clearTimeout(timeout);
    clearInterval(again);
  };
  const again = reclose
    ? setInterval(() => {
        if (!done && !gone()) close();
      }, RECLOSE_MS)
    : undefined;
  function check(): void {
    if (queued || !ready()) return;
    queued = true;
    setTimeout(() => {
      queued = false;
      if (!ready()) return;
      stop();
      run();
    }, 0);
  }
  const timeout = setTimeout(() => {
    if (done) return;
    stop();
    // Still mounted: the overlay refused to close, so drop the command rather than navigate under
    // it. Gone but out of sync: a back() never reported back — run late rather than never.
    if (gone()) run();
  }, CLOSE_TIMEOUT_MS);
  syncWaiters.add(check);
  window.addEventListener("popstate", check);
  close();
  check();
}

/** Navigate over whatever overlays are open: close them all (topmost first, the order Back peels
 *  them in), then navigate once their entries are back. Synchronous when nothing is open — the
 *  ordinary rail click.
 *
 *  `setPage` and `openChannel` call it themselves, so no call site can forget it: a global shortcut,
 *  a toast's "View" clicked inside the player, a palette row, the player's channel link. Closing is
 *  what a page change already did to the module-owned overlays (it unmounts them); for the ones above
 *  the page column (About, the cheat sheet, the palette) it means the destination is what the user
 *  sees, not a dialog still standing over it. A navigation that turns out to go nowhere — Shift+W on
 *  Home with the player open — still closes, so the key always ends with the answer on screen (owner
 *  decision 2026-09-09). */
export function closeOverlaysThenNavigate(navigate: () => void): void {
  if (overlayStack.length === 0 && historyInSync()) {
    navigate();
    return;
  }
  const targets = [...overlayStack];
  const close = () =>
    [...targets]
      .reverse()
      .filter((o) => overlayStack.includes(o))
      .forEach((o) => o.cb());
  afterClosing(targets, close, navigate, true);
}

/** Close the overlay a control lives in, then run its command once that overlay's entry is back —
 *  the command palette's rows. Overlays underneath are left alone: a theme picked over the player
 *  must not close the player, and a row that navigates closes it through `setPage` anyway. */
export function closeTopOverlayThenRun(close: () => void, run: () => void): void {
  const top = overlayStack[overlayStack.length - 1];
  afterClosing(top ? [top] : [], close, run);
}

/** While the calling component is mounted, Back (or the browser/mouse back button) closes it via
 * `onClose` instead of navigating away. Mount the component only while the overlay is open.
 * `url`: what the overlay names in its entry's address (a null value: the key absent), followed as
 * it changes; the entry under it never carries those keys. */
export function useBackToClose(onClose: () => void, url?: OverlayUrl): void {
  const cb = useEffectEvent(onClose);
  const named = useEffectEvent(() => url);
  const entryRef = useRef<Overlay | null>(null);
  useEffect(() => {
    ensureOverlayListener();
    const entry: Overlay = { token: nextOverlayToken(), cb: () => cb() };
    entryRef.current = entry;
    overlayStack.push(entry);
    scheduleReconcile();
    return () => {
      entryRef.current = null;
      const idx = overlayStack.indexOf(entry);
      // idx === -1 means the shared handler already removed us on a real user Back; either way,
      // just re-sync history to the (possibly unchanged) overlay count on the next tick.
      if (idx !== -1) overlayStack.splice(idx, 1);
      scheduleReconcile();
    };
  }, []);
  // After the effect above (declaration order), so the first run names the entry before it is pushed.
  const urlDep = url ? JSON.stringify(url) : "";
  useEffect(() => {
    const entry = entryRef.current;
    const next = named();
    if (!entry || !next) return;
    entry.url = next;
    for (const k of Object.keys(next)) overlayUrlKeys.add(k);
    nameOverlaysHere();
  }, [urlDep]);
}

/** A Back-closable layer that exists only WHILE `active`. Stacks on top of `useBackToClose` and any
 * other active `useBackToExit` (LIFO) so nested views peel off one Back at a time — e.g. a player's
 * fullscreen sits above its maximise, which sits above the player itself: Back exits fullscreen,
 * then maximise, then closes the player. Leaving a layer by its own control (button/Esc) flips
 * `active` false, whose cleanup drops the entry and its history state on the next tick — exactly the
 * useBackToClose lifecycle, just gated on a flag. `onExit` runs on a genuine Back while active. */
export function useBackToExit(active: boolean, onExit: () => void): void {
  const cb = useEffectEvent(onExit);
  useEffect(() => {
    if (!active) return;
    ensureOverlayListener();
    const entry: Overlay = { token: nextOverlayToken(), cb: () => cb() };
    overlayStack.push(entry);
    scheduleReconcile();
    return () => {
      const idx = overlayStack.indexOf(entry);
      if (idx !== -1) overlayStack.splice(idx, 1);
      scheduleReconcile();
    };
  }, [active]);
}