Version: 51.0.2704.103
OS: Ubuntu 14.04 (probably also on other OS)

What steps will reproduce the problem?
(1) Open https://jsfiddle.net/f8hrzwvw/7/
(2) The scrollbar in the result area is a custom scrollbar
(3) Click default scrollbar button so that the scrollbar becomes a default one because the class attribute is changed;
(4) Click custom scrollbar button to have the class be set back to "custom_scrollbar".

What is the expected output?
The scrollbar should be updated back to a custom scrollbar.

What do you see instead?
There is no repaint of the scrollable area.

Moreover, if uncomment the css style for custom_scrollbar, the buttons would behave as expected. Suspect there is an invalidation bug where changing the class isn't updating scrollbar style.