Currently intersection observer not always initialized after hydration. For example - https://github.com/MailCheck-co/mailcheck.site/blob/master/src/components/Partners.svelte It's live on https://www.mailcheck.co/ - if you will start scrolling during load it will never trigger. That PR https://github.com/metonym/svelte-intersection-observer/pull/13 supposed to fix such a behavioure, but seems like problem does not fixed by moving subscription to onMount handler.