window.onload not firing

I "think" the issue is related to the following change - https://github.com/jquery/jquery/issues/3194 in jQuery 3.x where onload is not guaranteed to fire inside a $(Document).ready state any more - which is what Drupal.behaviours is giving you.

I tested with drupal 8.9, php 7.4.

I've made a patch and it works for me.

ushma → changed the visibility of the branch 3418597-window.onload-not-firing to hidden.