"TypeError: cyclic object value" when returning a list of shady DOM elements from a script in Selenium

[Artur-]

Description

When using Selenium and returning a list of elements from JavascriptExecutor.executeScript, it would seem that they are encoded more or less as JSON.stringify([element1, element2]). When any of the elements is inside a shadow root, this will fail with "TypeError: cyclic object value".

Live Demo

http://jsbin.com/gasepupuje/edit?html,console,output

Steps to Reproduce

1. Create div
2. Create span
3. Append span to div
4. Run JSON.stringify(div)
5. Append div to a shadow root
6. Run JSON.stringify(div)

Expected Results

Expected result would be {} both before and after the div has been attached. Acceptable result would be {"__shady":{}}

Actual Results

In Chrome and Safari TP, the result is {}, both before the div is attached and after
In Firefox and IE11, the result is {"__shady":{}} before the 'div' is attached to the shadow root and TypeError: cyclic object value in Firefox or Circular reference in value argument not supported in IE after it is attached

Browsers Affected

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

Versions

• webcomponents: v1.1.0 (master used in jsbin)

[Artur-]

Apparently this is also a problem with other properties of Polymer elements, such as __data

[TimvdLippe]

By forcing the polyfills, we can also observe this error in Chrome: http://jsbin.com/toguzelohu/edit?html,console,output