UserAgent: Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/54.0.2840.99 Safari/537.36

Steps to reproduce the problem:
1. create a div, we can call it "container".
2. give the container fixed width and height, and set the overflow to auto.
3. create a div inside the container, we can call it "content".
4. add some text inside content , but don't let the text exceed the bottom of the content(don't let the scroll bar appear)
5. write a event handler to mouse move, in order to add more text when you dragging to select text.
6. drag to select the text, when dragging, your js code added more text inside the content, so that the scroll bar appears, but when you drag your mouse beyond the bottom of the container, the container will not auto scroll.

7. release your mouse and repeat step 6, now the container will auto scroll.

To make things clear, here is the codepen link:
http://codepen.io/moonqqqq/pen/yVbVym

When you drag to select the text at first time, the container will not auto scroll.
In the second time, it will.

What is the expected behavior?
in step 6, the container should auto scroll to the bottom.

What went wrong?
The container will not auto scroll when there is no scroll bar at first.

Did this work before? N/A 

Does this work in other browsers? No
 the scrolling behavior is weird.

Chrome version: 54.0.2840.99  Channel: stable
OS Version: 10.0
Flash Version: Shockwave Flash 23.0 r0