Get featured post url

gistfile1.php

<div class="case-study__screenshot"
  <?php if($latest_case_study->has_post_thumbnail()) {
    $thumb_id = get_post_thumbnail_id();
    $thumb_url_array = wp_get_attachment_image_src($thumb_id, 'medium', true);
    $thumb_url = $thumb_url_array[0];
  ?>
  style="background-image: url(<?php $thumb_url ?>);"
  <?php } ?>
  >
  </div>

try changing line 3 to...

 $thumb_id = get_post_thumbnail_id($latest_case_study->ID);