photoswipe-previews.js

import PhotoSwipe from 'photoswipe/dist/photoswipe'
import PhotoSwipeUI_Default from 'photoswipe/dist/photoswipe-ui-default'

import { photoswipePreviews } from './previews'

const holder = document.querySelector('.pswp')

// initializing code

const openPhotoSwipe = ({ items }) => {
	
	// opening code
	
	const gallery = new PhotoSwipe(holder, PhotoSwipeUI_Default, items, { /* options */ })
	photoswipePreviews(gallery) // <- add this
	gallery.init()
}

photoswipe-template.pug

// Root element of PhotoSwipe. Must have class pswp.
.pswp(tabindex='-1' role='dialog' aria-hidden='true')
	//
		Background of PhotoSwipe.
		It's a separate element as animating opacity is faster than rgba().
	.pswp__bg
	// Slides wrapper with overflow:hidden.
	.pswp__scroll-wrap
		//
			Container that holds slides.
			PhotoSwipe keeps only 3 of them in the DOM to save memory.
			Don't modify these 3 pswp__item elements, data is added later on.
		.pswp__container
			.pswp__item
			.pswp__item
			.pswp__item
		// Default (PhotoSwipeUI_Default) interface on top of sliding area. Can be changed.
		.pswp__ui.pswp__ui--hidden
			.pswp__top-bar
				// Controls are self-explanatory. Order can be changed.
				.pswp__counter
				button.pswp__button.pswp__button--close(title='Close (Esc)')
				button.pswp__button.pswp__button--share(title='Share')
				button.pswp__button.pswp__button--fs(title='Toggle fullscreen')
				button.pswp__button.pswp__button--zoom(title='Zoom in/out')
				// Preloader demo https://codepen.io/dimsemenov/pen/yyBWoR
				// element will get class pswp__preloader--active when preloader is running
				.pswp__preloader
					.pswp__preloader__icn
						.pswp__preloader__cut
							.pswp__preloader__donut
			.pswp__share-modal.pswp__share-modal--hidden.pswp__single-tap
				.pswp__share-tooltip
			button.pswp__button.pswp__button--arrow--left(title='Previous (arrow left)')
			button.pswp__button.pswp__button--arrow--right(title='Next (arrow right)')
			.pswp__previews(data-previews)
			.pswp__caption
				.pswp__caption__center

posrcssrc.config.js

module.exports = {
	plugins: {
		'postcss-momentum-scrolling': [
			'scroll',
			'auto',
		],
		'postcss-position-alt': {},
		'postcss-short': {},
		'postcss-axis': {},
		'postcss-single-charset': {},
		'postcss-font-family-system-ui': {},
		'autoprefixer': {},
	},
}

previews.js

/**
 * @param {object} gallery
 */
export const photoswipePreviews = gallery => {
	let added = false
	const props = {
		gallery,
		selector: '[data-previews]',
	}
	gallery.listen('gettingData', () => {
		if (added) return
		added = true
		addPreviews(props)
	})
	gallery.listen('close', () => {
		removePreviews(props)
	})
	gallery.listen('afterChange', () => {
		setActivePreview(props)
	})
}

/**
 * @typedef photoSwipePreviewsMethodProps
 * @prop {object} gallery
 * @prop {string} selector
 */

/**
 * @param {photoSwipePreviewsMethodProps} props
 */
const addPreviews = ({ gallery, selector }) => {
	const { scrollWrap, items } = gallery
	const place = scrollWrap.querySelector(selector)
	for (const item of items) {
		const { msrc: preview } = item
		const element = document.createElement('img')
		element.setAttribute('src', preview)
		element.addEventListener('click', () => {
			gallery.goTo(items.indexOf(item))
		})
		place.appendChild(element)
	}
}

/**
 * @param {photoSwipePreviewsMethodProps} props
 */
const removePreviews = ({ gallery, selector }) => {
	const { scrollWrap } = gallery
	const place = scrollWrap.querySelector(selector)
	place.innerHTML = ''
}

/**
 * @param {photoSwipePreviewsMethodProps} props
 */
const setActivePreview = ({ gallery, selector }) => {
	const { scrollWrap, currItem } = gallery
	const { msrc: preview } = currItem
	const place = scrollWrap.querySelector(selector)
	const previewElements = place.querySelectorAll('img')
	for (const element of previewElements) {
		const src = element.getAttribute('src')
		const className = 'is-active'
		// Warning: collision possible if image not unique
		if (src === preview) {
			element.classList.add(className)
			element.scrollIntoView({ behavior: 'smooth' })
		}
		else element.classList.remove(className)
	}
}

previews.sass

.pswp
	&__caption
		bottom: 3em
	&__previews
		absolute: bottom left
		width: 100%
		display: flex
		flex-flow: row
		justify-content: center
		background-color: rgba(black, 0.3)
		overflow: auto
		// Hide scrollbar in Quantum (Firefox)
		scrollbar-width: none
		// Hide scrollbar in Blink (Chrome)
		&::-webkit-scrollbar
			display: none
		img
			size: 3em
			object-fit: cover
			opacity: 0.3
			transition: opacity 0.3s
			cursor: pointer
			&:hover
				opacity: 0.8
			&.is-active
				opacity: 1
				cursor: default

with justify-content:center it looks good on large screens and when you just have a few thumbnails but with many thumbnails you are unable to scroll to the beginning of the thumbnails. and if you try you will leave the lightbox because it responses on scroll events with an exit. i will try to fix this on my side with more html/css code for the thumbnails.

oh I see. I just added some images to my codepen and see this flexbox problem https://codepen.io/Grawl/pen/PoZBMPg

oh cool! can you update that codepen with your fixes? so anyone can use Photoswipe with good working previews.

Clicking on the thumbnails in https://codepen.io/Grawl/pen/ExPpXeW is not working

working for me