add lightbox extra-zoom on click

app/assets/javascripts/discourse/lib/lightbox.js

@@ -8,14 +8,32 @@
 Discourse.Lightbox = {
   apply: function($elem) {
     $LAB.script("/javascripts/jquery.magnific-popup-min.js").wait(function() {
-      $('a.lightbox', $elem).each(function(i, e) {
+      $("a.lightbox", $elem).each(function(i, e) {
         $(e).magnificPopup({
-          type: 'image',
-          closeOnContentClick: true,
+          type: "image",
+          closeOnContentClick: false,
+
+          callbacks: {
+            open: function() {
+              var self = this,
+                  wrap = this.wrap,
+                  img = this.currItem.img,
+                  maxHeight = img.css("max-height");
+
+              wrap.on("click.pinhandler", "img", function() {
+                wrap.toggleClass("mfp-force-scrollbars");
+                img.css("max-height", wrap.hasClass("mfp-force-scrollbars") ? "none" : maxHeight);
+              });
+            },
+            beforeClose: function() {
+              this.wrap.off("click.pinhandler");
+              this.wrap.removeClass("mfp-force-scrollbars");
+            }
+          },
 
           image: {
             titleSrc: function(item) {
-              return item.el.attr('title') + ' &middot; <a class="image-source-link" href="' + item.src + '" target="_blank">' + I18n.t("lightbox.download") + '</a>';
+              return item.el.attr("title") + ' &middot; <a class="image-source-link" href="' + item.src + '" target="_blank">' + I18n.t("lightbox.download") + '</a>';
             }
           }
 

app/assets/stylesheets/desktop/magnific-popup.scss

@@ -639,3 +639,14 @@ button {
     }
   }
 }
+
+
+.mfp-force-scrollbars {
+  &.mfp-wrap {
+    overflow-y: auto !important;
+    overflow-x: auto !important;
+  }
+  .mfp-img {
+    max-width: none;
+  }
+}