This is a really useful problem to solve. But, I would personally prefer to solve it with a polyfill for a standardized approach that will eventually receive native implementation.
The advantages of standardization and eventual native implementation outweigh the immediacy of a JS-based API designed in relative isolation. Why should I invest time learning this micro-library, when the API is certain to be different from the native implementation? The documentation doesn't seem to even acknowledge the existance of the standard (did the author do any research before implementing a one-off library? I have no way of knowing.), much less explain why it differs from the proposed standard.
Intersection Observer has been implemented in Chrome since 51, and Opera since 38. It's currently being implemented in Firefox ( https://bugzilla.mozilla.org/show_bug.cgi?id=1243846 ), and is "likely" from Edge (https://developer.microsoft.com/en-us/microsoft-edge/platfor... ).
in-view.js's API is certainly less likely to be the cross-platform API than Intersection Observer. Also, plenty of APIs are implemented by browsers at the editor's draft stage.