gallery.html

<link rel="stylesheet" href="/theme/3rd/photoswipe/dist/photoswipe.css"> 
<link rel="stylesheet" href="/theme/3rd/photoswipe/dist/default-skin/default-skin.css"> 
<script src="/theme/3rd/photoswipe/dist/photoswipe.min.js"></script> 
<script src="/theme/3rd/photoswipe/dist/photoswipe-ui-default.min.js"></script> 

    <style>
        .gallery
        {
            display: flex;
            flex-wrap: wrap;
        }

        .galleryitem {
            background-size: cover;
            background-repeat: no-repeat;
            background-position: 50% 50%;
            width: 240px;
            height: 240px;
            margin: 2px;
            position: relative;
        }
        @media (max-width: 767px) and (orientation: portrait) {
            .galleryitem {
                width: 140px;
                height: 140px;
            }
        }

        .itemcap {
            text-align: center;
            position: absolute;
            overflow: hidden;
            white-space: nowrap;
            text-overflow: ellipsis;
            bottom: 0px;
            width: 100%;
            left: 0px;
            color: rgb(204, 204, 204);
            background-color: rgba(0, 0, 0, 0.5);
        }
    </style>

    <div class="gallery">
        {% for name, photo, thumb, exif, caption, size in article.photo_gallery %}
        
        <a href="javascript:openGallery({{ loop.index-1 }})" title="{{ name }}" exif="{{ exif }}" caption="{{ caption }}">
            <div class="galleryitem" data-index="{{ loop.index }}" style="background-image: url('{{ SITEURL }}/{{ thumb }}')">
                {%  if caption %}
                    <span class="itemcap">
                        {{ caption }}</span>
                {%  endif %}
            </div>
        </a>
        {% endfor %}
    </div>

    <div class="pswp" tabindex="-1" role="dialog" aria-hidden="true">
        <div class="pswp__bg"></div>
        <div class="pswp__scroll-wrap">
            <div class="pswp__container">
                <div class="pswp__item"></div>
                <div class="pswp__item"></div>
                <div class="pswp__item"></div>
            </div>
            <div class="pswp__ui pswp__ui--hidden">

                <div class="pswp__top-bar">
                    <div class="pswp__counter"></div>

                    <button class="pswp__button pswp__button--close" title="Close (Esc)"></button>

                    <!-- button class="pswp__button pswp__button--share" title="Share"></button -->

                    <button class="pswp__button pswp__button--fs" title="Toggle fullscreen"></button>

                    <button class="pswp__button pswp__button--zoom" title="Zoom in/out"></button>
                    <div class="pswp__preloader">
                        <div class="pswp__preloader__icn">
                          <div class="pswp__preloader__cut">
                            <div class="pswp__preloader__donut"></div>
                          </div>
                        </div>
                    </div>
                </div>

                <div class="pswp__share-modal pswp__share-modal--hidden pswp__single-tap">
                    <div class="pswp__share-tooltip"></div> 
                </div>

                <button class="pswp__button pswp__button--arrow--left" title="Previous (arrow left)">
                </button>

                <button class="pswp__button pswp__button--arrow--right" title="Next (arrow right)">
                </button>

                <div class="pswp__caption">
                    <div class="pswp__caption__center"></div>
                </div>

            </div>

        </div>

    </div>

    <script>
            var slides = [];
            {% for name, photo, thumb, exif, caption, size in article.photo_gallery %}
                slides.push({ src: '{{photo}}', msrc: '{{thumb}}', w: {{size[0]}}, h: {{size[1]}}, title: '{{ caption }}' });
            {% endfor %}

            var pswpElement = document.querySelectorAll('.pswp')[0];
            function openGallery(clickedindex) {
                var options = {
                    closeOnScroll: false,
                    closeOnVerticalDrag: false,
                    showHideOpacity: true,
                    getThumbBoundsFn: function(index) {
                        // find thumbnail element
                            var thumbnail = document.querySelectorAll('.galleryitem')[index];

                            // get window scroll Y
                            var pageYScroll = window.pageYOffset || document.documentElement.scrollTop; 
                            // optionally get horizontal scroll

                            // get position of element relative to viewport
                            var rect = thumbnail.getBoundingClientRect(); 

                            // w = width
                            return {x:rect.left, y:rect.top + pageYScroll, w:rect.width};

                    },
                    index: clickedindex
                };
                var gallery = new PhotoSwipe( pswpElement, PhotoSwipeUI_Default, slides, options);
                gallery.init();
            }
        
    </script>