buttons class spills over into internal child of custom element (shadyDom)

[elonmallin]

Description

If you have any element inside the paper-dialog that has the class "buttons" it will get some css applied from the paper-dialog. This happens with the shady dom at least.

Expected outcome

If a internal child element of a custom element has an internal class named buttons, that class should not be targeted by the paper-dialog and apply css to it.

Actual outcome

paper-dialog applies css to any child with the class buttons, even if that child is inside a custom element and you expect things to be scoped. Tested in shady dom only.

Steps to reproduce

<paper-dialog>
  <h3>Title</h3>
  <div> // with or without class="buttons"
    <my-custom-element></my-custom-element> // My internal has an element with a class=buttons
  </div>
</paper-dialog>

Browsers Affected

• [x ] Chrome