Suggested changes:
- change interval time from 200 to 50ms
- initialize `last*` vars together with the call to `setInterval()`
- clear interval when scroll positions stop changing, independent of
  history state

Additional changes:
- remove unused `throttle()` function
- move guarded block to inside `onScrollEnd()` since using history
  navigation will trigger our "popstate" callback and fire additional
  "scroll" and "scrollend" events, causing redundant expensive calls to
  `replaceState()`