The Chromium Wheel Smooth Scroller extension breaks sidebar scrolling in app-layout.

[TrebuhD]

Description

We've been getting reports from users that when this extension is installed, the sidebar can't be scrolled. When the mouse cursor is above the sidebar, the main page content is being scrolled instead.

Expected outcome

The sidebar should be scrollable.

Actual outcome

The sidebar can't be scrolled when the Chromium Wheel Smooth Scroller extension is installed.

Live Demo

https://raw-dot-custom-elements.appspot.com/PolymerElements/app-layout/v2.1.0/app-layout/demo/demo5.html

Steps to reproduce

1. Install the extension and enable it.
2. Open a new tab and navigate to this app-layout demo.
3. Make the browser window shorter so that the scrollbar appears on the sidebar.
4. The sidebar is not scrollable.

Browsers Affected

• Chrome
• Firefox
• Safari 9
• Safari 8
• Safari 7
• Edge
• IE 11
• IE 10

[keanulee]

Seems to work when not using native Shadow DOM (https://raw-dot-custom-elements.appspot.com/PolymerElements/app-layout/v2.1.0/app-layout/demo/demo5.html?wc-shadydom=true), so I suspect extension incompatibility with Shadow DOM.