You are on point, the extension starts to slightly malfunction if the user starts to resize the browser window vertically. This is because of the assumption that `(window.pageYOffset + window.innerHeight) / document.body.scrollHeight` will approximately equal 1 when the user has scrolled to the bottom of the page (which apparently, is not necessarily the case).

Will address this issue ASAP. [1]

Thanks for finding an edge case which I did not notice :)

1. https://github.com/kugurerdem/mark-scroll-positions/issues/1