Dropdowns do not overlay other rows

[plequang]

Description

When using a dropdown like <paper-dropdown-menu> in an item template, the opened dropdown does not overlay the item rows below.

Can be demonstrated by adding a dropdown menu in the item template of the full demo:

Cause

The problem comes from iron-list, which adds translate3d styles to the template instances created for the rows. Translate3d create a stacking context, so all elements inside a row won't be able to overflow outside of the row.

See also PolymerElements/iron-list#242