[wldlyinaccurate]

I know that this type of functionality is useful, but please consider the performance implications before doing anything like this. Non-native implementations (i.e. not IntersectionObserver) _always_ use properties and methods that cause style and layout calculations which have a significant performance impact on low-powered devices.

A more comprehensive list is here[1], but the main culprits are HTMLElement.offset{Width,Height}, Element.client{Width,Height}, and Window.getComputedStyle(). Avoid these, _especially_ on events like scroll. You will ruin the experience for many of your users.

[1] https://gist.github.com/paulirish/5d52fb081b3570c81e3a

[gcb0]

and it's impossible to achieve it without probing those attributes.

some advertising libraries solve that, since the advertising industry deals in in-view ad impressions for a few years now, by being smart about when to do it. the best one is called safeFrames. others like moat detect if the user has a fast computer, abuse it to no end, and then extrapolate the results for the whole audience.

i expect all of them to move to the native APIs soon (but Android fragmentation will make it slow)