fofos-admin/web/SitePreview.tsx

import { useCallback, useEffect, useRef, useState } from 'react';
import { useClient } from 'hooks/useClient';
import { readableError } from './errors';
import { DEVICES, usePreviewable, useViewMode } from './ViewMode';

/**
 * A page of the real site, embedded.
 *
 * ── Not a drawing of it ──
 *
 * The Layout section began with a React sketch of the header and footer. It looked plausible and it
 * was wrong, and worse it was a second implementation that would go on being wrong differently every
 * time either side changed. This renders the site's own page instead, so a change made in code shows
 * up here with no change here at all.
 *
 * ── Through the PLATFORM's dev proxy ──
 *
 * `/api/dev-proxy/:port/*` reaches any loopback port, rewrites the HTML, and injects a script that
 * patches fetch, XHR and WebSocket so URLs the page builds at RUNTIME are moved too. This plugin had
 * its own proxy for an afternoon and it was a worse copy of that.
 *
 * Same origin, which is what lets `theme` work at all — and what will let the page builder draw
 * handles on top of the real page when it gets there.
 */

/** Where the site's development server is. Asked once; it is a setting, not a live value. */
export const useDevPort = (): number | null => {
  const client = useClient();
  const clientRef = useRef(client);
  clientRef.current = client;
  const [port, setPort] = useState<number | null>(null);

  useEffect(() => {
    void clientRef.current
      .get<{ name: string; port: number | null }[]>('/fofos-admin/_config/servers')
      .then((servers) => setPort(servers.find((server) => server.name === 'development')?.port ?? null))
      .catch(() => setPort(null));
  }, []);

  return port;
};

export const SitePreview = ({
  port,
  path,
  theme,
  onDocument,
  reloadToken,
}: {
  port: number | null;
  /** The site path to show — `layout`, `about`, or `` for the front page. No leading slash. */
  path: string;
  /**
   * CSS custom properties to push into the frame, live.
   *
   * Optional because most previews have nothing to override. The Layout section is the exception: it
   * is editing the variables the page reads, so it can show a change without a save or a reload.
   */
  theme?: Record<string, string | null>;
  /**
   * Called with the frame's own document once it has loaded, and again after every reload.
   *
   * This is the door the page builder goes through. The frame is same-origin, so a caller can read the
   * rendered page's DOM and put its own elements inside it — chrome that belongs to the ADMIN, drawn on
   * the real page, without a line of it existing in the website's source.
   *
   * Return a cleanup and it runs before the next load and on unmount, so nothing injected outlives the
   * document it was injected into.
   *
   * ── Give it a stable identity, and change that identity to REPAINT ──
   *
   * It is re-run whenever it changes, so a `useCallback` whose dependencies are the data being drawn is
   * exactly right: new data, new function, redrawn. An inline arrow is a new function every render and
   * would reinject on every one.
   */
  onDocument?: (document: Document) => void | (() => void);
  /**
   * Change this to fetch the page again.
   *
   * The site renders from the database, so anything written there only appears when the document is
   * requested anew. The admin cannot draw the change itself — it does not know how to render a page,
   * and if it did it would be a second implementation of the one thing this preview exists to avoid.
   *
   * Remounts the frame rather than calling `contentWindow.location.reload()`. Both work while the frame
   * is same-origin, and the remount is one mechanism that would still work if it ever were not.
   */
  reloadToken?: number;
}) => {
  const client = useClient();
  /*
    Through a ref, and this is the whole reason the preview hung.

    `useClient()` rebuilds its verbs on EVERY render, so `get` has a new identity each time. With it in
    the dependency list the effect re-ran on every render, and its first act is `setReady(false)` —
    which renders again, which makes another `get`, which re-runs the effect. The frame never mounted
    and the panel said "Connecting…" forever.

    It is a known trap here: the same thing silently disabled every playback report in the Jellyfin
    player. The platform's own Preview panel avoids it by depending on neither the client nor the URL.
  */
  const clientRef = useRef(client);
  clientRef.current = client;

  const { device, mode } = useViewMode();
  const { width } = DEVICES[device];

  const [ready, setReady] = useState(false);
  const [failed, setFailed] = useState<string | null>(null);

  /*
    How much room there is, so the frame can be SCALED rather than squeezed.

    The iframe is always the real device width — 390, 834, 1440 — and shrunk to fit with a transform.
    Giving it the panel's width instead would mean the site's own media queries answered the panel's
    size, so "Desktop" in a narrow panel would render the tablet layout and label it desktop. That is
    the one thing a preview must not do.

    Measured rather than assumed because the panel is resizable and the workspace splits it.
  */
  const box = useRef<HTMLDivElement | null>(null);
  const [available, setAvailable] = useState({ width: 0, height: 0 });

  useEffect(() => {
    const element = box.current;
    if (!element) return;
    const observer = new ResizeObserver(([entry]) => {
      if (entry) setAvailable({ width: entry.contentRect.width, height: entry.contentRect.height });
    });
    observer.observe(element);
    return () => observer.disconnect();
  }, []);

  useEffect(() => {
    if (!port) return;
    let alive = true;
    setReady(false);
    setFailed(null);
    void clientRef.current
      .get<{ ok: boolean }>('/dev-proxy/session')
      .then(() => alive && setReady(true))
      .catch((failure: unknown) => alive && setFailed(readableError(failure)));
    return () => {
      alive = false;
    };
  }, [port]);

  /*
    The draft, pushed into the frame's own document.

    `setProperty` where there is a value and `removeProperty` where there is not — an empty string would
    set the variable to nothing rather than unsetting it, and the page's `var(--site-bg, #fff)` fallback
    would never come back. Choosing "default" has to actually restore the default.

    Keyed on the serialised theme rather than the object, which is a new identity every render. That is
    the same trap that hung this component on "Connecting…", one level down.
  */
  const frame = useRef<HTMLIFrameElement | null>(null);
  const serialised = JSON.stringify(theme ?? {});

  const applyTheme = useCallback(() => {
    const root = frame.current?.contentDocument?.documentElement;
    if (!root) return;
    for (const [property, value] of Object.entries(JSON.parse(serialised) as Record<string, string | null>)) {
      if (value) root.style.setProperty(property, value);
      else root.style.removeProperty(property);
    }
  }, [serialised]);

  // On every change, and again whenever the frame is (re)mounted — switching device remounts nothing,
  // but a reconnect or a section change does, and the draft has to survive that.
  useEffect(() => {
    applyTheme();
  }, [applyTheme, ready]);

  /*
    Hand the loaded document to whoever asked for it — on load, and again whenever they change.

    Two things have to happen and they race: the frame fetches a new document, and the caller fetches
    whatever it means to draw on it. Neither reliably finishes first. Running the injection ONLY on load
    meant painting with whatever data had arrived by then, which after creating a section was usually
    the data from before it existed.

    So `loaded` is state rather than a callback: the effect below depends on both it and `onDocument`,
    and runs again when either moves. Whichever arrives second triggers the paint that counts.
  */
  const [loaded, setLoaded] = useState(0);

  /*
    A remount is a NEW document, so `loaded` has to go back to nothing before it arrives.

    In an effect keyed on the token, and NOT in the iframe's ref. It was a ref callback, written inline
    — which React treats as a different ref every render, so it detaches and reattaches on each one,
    calling the callback with null and then the node. That reset `loaded` to 0 every render, which
    re-rendered, which reset it again: the paint never survived long enough to happen and the page came
    up with no controls at all.

    The lesson is the same one as `useClient` two files along: a value rebuilt every render must not be
    the thing something else keys on.
  */
  useEffect(() => {
    setLoaded(0);
  }, [reloadToken]);

  useEffect(() => {
    if (!loaded) return;
    const frameDocument = frame.current?.contentDocument;
    if (!frameDocument) return;

    const cleanup = onDocument?.(frameDocument);
    // Runs before the next paint and on unmount, so nothing injected outlives its document — and two
    // paints never leave two sets of controls.
    return () => cleanup?.();
  }, [loaded, onDocument]);

  const message =
    !port ? 'No development server is configured — set its port under Administration.'
    : failed ? `Could not authorise the preview: ${failed}`
    : !ready ? 'Connecting…'
    : null;

  /*
    Never scaled UP. A 390px phone blown up to fill a wide panel is a phone nobody has, and every
    judgement made against it — how big that headline is, how much fits above the fold — would be wrong
    in the direction of "looks fine".
  */
  const scale = available.width ? Math.min(1, available.width / width) : 1;

  /*
    All the room there is. WIDTH is the device; height is just how much of the page you can see.

    It was capped at the device's own height, on the reasoning that a 390x844 phone drawn 1200 pixels
    tall is not a phone. That was wrong, and the empty band under the preview was the tell: media
    queries answer WIDTH, so the layout at 390 is the phone's layout whatever the height. A taller frame
    shows more page, which is a scroll position rather than a different device.

    One real consequence, and it is the right one: the footer is `min-h-dvh`, so it sits at the bottom
    of the FRAME. A taller frame therefore puts it lower — exactly as a taller browser window does.
  */
  const shownHeight = Math.max(0, available.height);

  return (
    <section className="flex min-h-0 flex-1 flex-col gap-2">
      <div ref={box} className="min-h-0 flex-1">
        <div
          className="mx-auto overflow-hidden rounded-lg border border-border bg-white"
          // The scaled footprint, so the border hugs the frame and the page below does not jump when
          // the device changes.
          style={{ width: Math.min(width * scale, available.width || width), height: shownHeight }}
        >
          {message ? (
            <p className="p-6 text-center text-xs text-muted-foreground">{message}</p>
          ) : (
            /*
              No `sandbox`, for the reason the platform's own Preview gives: a sandbox WITHOUT
              allow-same-origin has an opaque origin and therefore no storage, and one WITH
              allow-same-origin and allow-scripts is worth nothing, because the frame can reach up and
              remove the attribute. This is the owner's own development server on this machine.
            */
            <iframe
              // The key is what makes `reloadToken` reload: a new key is a new element, so the browser
              // fetches the document again and `onLoad` runs, which re-injects the builder's chrome.
              key={reloadToken ?? 0}
              ref={frame}
              // Applied on load as well as on change: the document is replaced on a reload, and the
              // properties go with it.
              onLoad={() => {
                applyTheme();
                setLoaded((n) => n + 1);
              }}
              src={`/api/dev-proxy/${port}/${path}`}
              title={`The site at /${path}`}
              className="border-0"
              style={{
                width,
                height: shownHeight / scale,
                transform: `scale(${scale})`,
                transformOrigin: 'top left',
              }}
            />
          )}
        </div>
      </div>

      {/* Only while editing. In View the preview is the whole point, and a caption is a line of height
          taken from it to say something the device buttons already show. */}
      {mode === 'edit' && (
        <p className="shrink-0 text-[10px] text-muted-foreground">
          The development server, live at {width}px. Save to see a change here.
        </p>
      )}
    </section>
  );
};