ISCG.user.js

// ==UserScript==
// @name         I see cute girls
// @namespace    ISCG
// @version      0.00000001
// @description  ...
// @author       made by fredgy-kun
// @match        https://*.donmai.us/posts*
// @match        https://*.donmai.us/
// @exclude      https://*.donmai.us/posts/*
// @grant        none
// @run-at      document-end
// ==/UserScript==

let tsize = localStorage.getItem(`tsize`)|0;
tsize = tsize > 0 ? tsize : 483;
let tcheck = localStorage.getItem(`tcheck`) !== `false`;

const extWhitelist = new Set([`.png`, `.jpg`, `.gif`]);

let thumbsParent =document.getElementsByClassName('post-gallery-grid')[0];

let iscgControls;
if (!document.getElementById(`iscgControls`)) {
	document.getElementById(`subnav-menu`).insertAdjacentHTML(`beforeend`,
		`<li id='iscgControls' style='--iscg-tsize-str:"";'>
			<input type='range' min='150' max='700' value='${tsize|0}' id='iscgSlider' style='width: 200px;'>
			<input type='checkbox' ${tcheck ? `checked` : ``} id='iscgCheckbox'>
			<span id='iscgOutput'></span>
		</li>`);

	document.head.insertAdjacentHTML(`beforeend`,
		`<style id='iscgStylesheet'>
			.iscgEnabled article.post-preview img{
				height: auto !important;
				width: auto !important;

			}
			.iscgEnabled article.post-preview img {
				max-width: var(--iscg-tsize) !important;
				max-height: var(--iscg-tsize) !important;
                display: inline-table;
			}

            .post-gallery-grid.post-gallery-150 .posts-container {
                grid-template-columns: repeat(auto-fill, minmax(var(--iscg-tsize), 1fr));
            }

            .post-preview img {
                border: 3px !important;
            }

			#iscgOutput::before {
				content: var(--iscg-tsize-str);
			}
		</style>`);

	iscgControls = document.getElementById(`iscgControls`);
}

document.getElementById(`iscgSlider`).oninput = function () {
	tsize = this.value;
    if (tcheck){
	applyTsize();
    }
	localStorage.setItem(`tsize`, tsize);
}

document.getElementById(`iscgCheckbox`).onclick = function () {
	tsize = localStorage.getItem(`tsize`);
    tcheck = this.checked;

	if (!tcheck) {
		tsize = 150;}

	alterThumbs();
	applyTsize();

	localStorage.setItem(`tcheck`, tcheck);
};

function applyTsize() {
	thumbsParent.classList.toggle(`iscgEnabled`, tcheck);
	thumbsParent.style.setProperty(`--iscg-tsize`, `${tsize|0}px`);
	iscgControls.style.setProperty(`--iscg-tsize-str`, `'${tsize|0}px'`);
}

function alterThumbs() {
	for (let thumb of thumbsParent.getElementsByClassName(`post-preview`)) {
		let url = null;
		if (tcheck) {/* use samples: */
			let sampUrl = tryParseHref(thumb.dataset.largeFileUrl);
			if (sampUrl && extWhitelist.has(sampUrl.pathname.slice(-4).toLowerCase())) {
				url = sampUrl;}
		} else {/* revert back to thumbnails: */
			url = tryParseHref(thumb.dataset.previewFileUrl);
		}

		if (url) {
            let el = thumb.querySelectorAll("img")[0];

            if(el){
             el.src = url;
            }

			/*for (let s of thumb.querySelectorAll("img")[0]) {
				s.srcset = url.href;}
                */
			/* load-balancing idea:
			= sampUrl.href.replace(/(:\/\/\w+\.)/, [ "://shima.", "://saitou.", "://kagamihara."][Math.floor(Math.random() * 3)]);}) */
		}
	}
}

function initialise() {
	if (tcheck) {alterThumbs();}
	applyTsize();
}

initialise();
$(document).on(`danbooru:post-preview-updated`, initialise); //breakes if not inside  unrelated https://stackoverflow.com/a/524721

function tryParseHref(href) {
	try {
		return new URL(href);
	} catch (x) {
		return null;};
};

The script is no longer working. 😭

Can you please update it?