Enlarge to Fill Stand-Alone Image Viewer

Make stand-alone images fill the viewport regardless of whether they are too wide or too tall

You will need to install an extension such as Tampermonkey, Greasemonkey or Violentmonkey to install this script.

You will need to install an extension such as Tampermonkey to install this script.

You will need to install an extension such as Tampermonkey or Violentmonkey to install this script.

You will need to install an extension such as Tampermonkey or Userscripts to install this script.

You will need to install an extension such as Tampermonkey to install this script.

You will need to install a user script manager extension to install this script.

(I already have a user script manager, let me install it!)

You will need to install an extension such as Stylus to install this style.

You will need to install an extension such as Stylus to install this style.

You will need to install an extension such as Stylus to install this style.

You will need to install a user style manager extension to install this style.

You will need to install a user style manager extension to install this style.

You will need to install a user style manager extension to install this style.

(I already have a user style manager, let me install it!)

// ==UserScript==
// @name        Enlarge to Fill Stand-Alone Image Viewer
// @author      Jefferson "jscher2000" Scher
// @namespace   JeffersonScher
// @version     0.5
// @copyright   Copyright 2020 Jefferson Scher
// @license     BSD-3-Clause
// @description Make stand-alone images fill the viewport regardless of whether they are too wide or too tall
// @match       https://*/*
// @grant       none
// ==/UserScript==

var tgt;
function resizeImg(e){
  // don't resize when full size
  if (tgt.className.indexOf('overflowing') > -1) return;
  var whratio = tgt.width / tgt.height;
  var vpratio = window.innerWidth / window.innerHeight;
  if (whratio < vpratio){ // width needs more help
    tgt.width = parseInt(document.body.clientWidth);
    tgt.height = Math.round(tgt.width * (1 / whratio));
    // repeat to account for scroll bar
    tgt.width = parseInt(document.body.clientWidth);
    tgt.height = Math.round(tgt.width * (1 / whratio));
  } else {
    tgt.height = window.innerHeight;
    tgt.width = Math.round(tgt.height * whratio);
    // repeat to account for scroll bar
    tgt.height = window.innerHeight;
    tgt.width = Math.round(tgt.height * whratio);
  }
}
var resizeTimeout;
function resizeThrottler(){
  if(!resizeTimeout){
    resizeTimeout = window.setTimeout(function(){resizeTimeout = null; resizeImg();}, 200);
  }
}
if (document.querySelector('head').innerHTML.indexOf('TopLevelImageDocument.css') > -1) {
  tgt = document.querySelector('body img'); // first image in body
  tgt.addEventListener('click', resizeImg, false);
  window.addEventListener('resize', resizeThrottler, false); 
  window.setTimeout(resizeImg, 50);
}