isAtBottom isn't always correct on High DPI screens

[pradyunsg]

Figured this out while diagnosing why my sidebar wasn't switching to the lowest item when scrolling to the bottom. According to MDN, pageYOffset is:

A floating-point number specifying the number of pixels the Document is scrolled vertically within its containing Window. This number is subpixel precise, so it may not be an integer.

Looks like my page ends on a subpixel? :)

(sorry -- I didn't make a JSFiddle -- it would've taken more time than contributing a fix!)

[pradyunsg]

This is a very WIP page, but hey, here's a GIF demoing the issue:

[pradyunsg]

Gentle nudge.