its rare that one goes to a page that can load enough 'continuous single page' content to make the scroll itself shrink tiny but..
to replicate this: just go to archive.org and in the second search field type in 'macaddict' then go, and keep scrolling the results down till it finally stops loading any further content. now try to manually scroll back up and you seem to be grabbing the lower-right corner of the browser window instead
I know that it is still possible to just about carefully select the very 1/4 top of the tiny scroll itself instead of grabbing the window border otherwise but still for these less dexterous mouse users this might be a 'bad bug' to them tho
(sorry I couldn't upload the screenshots myself right now as the built-in screenshoter doesn't capture cursor icons themselves)