Trigger native fullscreen when lightbox opens

There is an unoficial fullscreen plugin by @arnowelzel that might be useful.

You may use asynchronous initialization to open gallery in fullscreen mode right away. Fullscreen API is supported in all major browsers, except mobile Safari. Fullscreen in mobile Chrome on Android works exceptionally well in landscape orientation as it removes toolbars.

To implement it, create a promise that is resolved after fullscreen mode is entered and pass it to lightbox as an openPromise option.

import PhotoSwipeLightbox from '/photoswipe/photoswipe-lightbox.esm.js';

// Simple fullscreen API
const fullscreenAPI = getFullscreenAPI();

// Create custom container
// which will be stretched to fullscreen.
//
// (we can not use PhotoSwipe root element (.pswp),
//  as it is created only after openPromise is resolved)
//
const pswpContainer = getContainer();

function getFullscreenPromise() {
  // Always resolve promise,
  // as wa want to open lightbox 
  // (no matter if fullscreen is supported or not)
  return new Promise((resolve) => {
    if (!fullscreenAPI || fullscreenAPI.isFullscreen()) {
      // fullscreen API not supported, or already fullscreen
      resolve();
      return;
    }

    document.addEventListener(fullscreenAPI.change, (event) => {
      pswpContainer.style.display = 'block';
      // delay to make sure that browser fullscreen animation is finished
      setTimeout(function() {
        resolve();
      }, 300);
    }, { once: true });

    fullscreenAPI.request(pswpContainer);
  });
}

const lightbox = new PhotoSwipeLightbox({
  gallery: '#gallery--native-fs',
  children: 'a',
  pswpModule: () => import('/photoswipe/photoswipe.esm.js'),

  // Add function that returns promise
  openPromise: getFullscreenPromise,
  
  // Append PhotoSwipe to our container
  appendToEl: fullscreenAPI ? pswpContainer : document.body,

  // disable opening/closing animations
  showAnimationDuration: 0,
  hideAnimationDuration: 0,

  // Add if you're using responsive images
  // since viewport size is unpredictable
  // at initialization
  preloadFirstSlide: false
});
lightbox.on('close', () => {
  pswpContainer.style.display = 'none';
  if (fullscreenAPI && fullscreenAPI.isFullscreen()) {
    fullscreenAPI.exit();
  }
});
lightbox.init();

// Simple fullscreen API helper,
// supports unprefixed and webkit-prefixed versions
function getFullscreenAPI() {
  let api;
  let enterFS;
  let exitFS;
  let elementFS;
  let changeEvent;
  let errorEvent;

  if (document.documentElement.requestFullscreen) {
    enterFS = 'requestFullscreen';
    exitFS = 'exitFullscreen';
    elementFS = 'fullscreenElement';
    changeEvent = 'fullscreenchange';
    errorEvent = 'fullscreenerror';
  } else if (document.documentElement.webkitRequestFullscreen) {
    enterFS = 'webkitRequestFullscreen';
    exitFS = 'webkitExitFullscreen';
    elementFS = 'webkitFullscreenElement';
    changeEvent = 'webkitfullscreenchange';
    errorEvent = 'webkitfullscreenerror';
  }

  if (enterFS) {
    api = {
      request: function (el) {
        if (enterFS === 'webkitRequestFullscreen') {
          el[enterFS](Element.ALLOW_KEYBOARD_INPUT);
        } else {
          el[enterFS]();
        }
      },

      exit: function () {
        return document[exitFS]();
      },

      isFullscreen: function () {
        return document[elementFS];
      },

      change: changeEvent,
      error: errorEvent
    };
  }

  return api;
};

function getContainer() {
  const pswpContainer = document.createElement('div');
  pswpContainer.style.background = '#000';
  pswpContainer.style.width = '100%';
  pswpContainer.style.height = '100%';
  pswpContainer.style.display = 'none';
  document.body.appendChild(pswpContainer);
  return pswpContainer;
}