A weird quirk of React in SSR ([[20221207104517-react-ssr]]) is that HTML events that fire before hydration won't re-fire once the code is hydrated. This ultimately leads to a strange race condition where certain things are sometimes fired.
An example is onLoad on an img element. This can be used to detect when an image loads. However if the image finishes loading on a page before hydration, the event isn't called, as the event handler isn't properly attached yet. Since the event has already happened, it will just never trigger.
e.g. a slow image will call onLoad, but a fast image (or a cached image) will not.
https://github.com/facebook/react/issues/12641 https://github.com/facebook/react/issues/15446 https://stackoverflow.com/questions/39777833/image-onload-event-in-isomorphic-universal-react-register-event-after-image-is