UserAgent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/51.0.2704.79 Safari/537.36

Steps to reproduce the problem:
1. Get a mouse or a trackball (or at least imagine using one) 
2. Goto a website like Quora or Facebook, to a page where "endless scrolling" is enabled.
3. Now try to use the scroll bar to scroll downwards continuously. On a mouse or trackball, that means holding and dragging the scroll widget to the bottom.

What is the expected behavior?
The expected behavior is undefined because "continuous scrolling" is a non-standardized behavior, but I can imagine two expectations:

1. The scroll bar adjusts itself after the user has released their mouse button, unless the scrollable position no longer exists, in which case scrolling is stopped.
2. Scrolling is stopped when the content size changes.
3. Some other method is used to deal make scrolling continuous

What went wrong?
When the website adjusts its content (adds content), the scroll position changes from 85% to 50%, but the mouse is left at the 85% mark and the user still has drag control of the widget. Now, a small movement will cause scrolling at an incredible velocity (roughly as if you had dragged the scroller from the 50% mark to the 85% mark) and this in turn requests more content and if it weren't for the slowness of the rendering step, would create an infinite loop until the user stops scrolling and moves the mouse to the new widget location .

Did this work before? No 

Chrome version: 51.0.2704.79  Channel: n/a
OS Version: OS X 10.11.1
Flash Version: 

Laptops have become the standard for computers and everybody uses touch-pads now, but still a lot of people prefer desktops and mice/trackballs (hence the scrollers still exist). Such users shouldn't be limited to tapping the down keys on these strange new feed sites.

The simplest solution would be to delay update of the scroll bar until the user releases their mouse.