If you put Blazy within a scrolling container, provide valid comma separated CSS selectors, except <code>#drupal-modal, .is-b-scroll</code>, e.g.: <code>#my-scrolling-container, .another-scrolling-container</code>. Known scrolling containers are <code>#drupal-modal</code> like seen at Media library, parallax containers with fixed height replacing default browser scrollbar. A scrolling modal with an iframe like Entity Browser has no issue since the scrolling container is the entire DOM. Must know <code>.blazy</code> parent container, or itself, which has CSS rules containing <code>overflow</code> with values anything but <code>hidden</code> such as <code>auto or scroll</code>. Press <code>F12</code> at any browser to inspect elements. IO does not need it, old bLazy does. Default to known <code>#drupal-modal, .is-b-scroll</code>. The <code>.is-b-scroll</code> is for modules which cannot reach this UI without extra legs. Symptons: eternal blue loader while should be loaded.