after hiding the lightbox, focus the original image's magnify button

Otherwise, the focus was reset, which would mean you'd have to navigate
all the way back to the image from the start.

themes/default/files/scripts/display-base.js

@@ -137,10 +137,13 @@ var display = Numbas.display = /** @lends Numbas.display */ {
     style_options_localstorage_key: 'numbas-style-options',
 
     /** Show the lightbox.
+     *
+     * @param {Element} original - The original image element which is going to be copied into the lightbox.
      */
-    show_lightbox: function() {
+    show_lightbox: function(original) {
         lightbox.classList.add('shown');
         lightbox.focus();
+        display.lightbox_original_element = original;
     },
 
     /** Hide the lightbox.
@@ -150,6 +153,9 @@ var display = Numbas.display = /** @lends Numbas.display */ {
         lightbox.classList.remove('shown');
         lightbox.innerHTML = '';
         display.lightbox_pressing_state = 'none';
+        if(display.lightbox_original_element) {
+            display.lightbox_original_element.querySelector('button').focus();
+        }
     },
 
     setExam: function(exam) {
@@ -253,7 +259,7 @@ var display = Numbas.display = /** @lends Numbas.display */ {
             function activate() {
                 lightbox.innerHTML = '';
                 lightbox.appendChild(elem);
-                Numbas.display.show_lightbox();
+                Numbas.display.show_lightbox(wrapper);
             }
 
             button.addEventListener('click', function(e) {