Issue

backgroundColor for AnimatableStackView not set by FillDesignable protocol and remains nil

Apparently UIStackView is a non-drawing view so drawRect() is never called and the backgroundColor is not applied.

EDIT: This obviously affects setting the opacity as well

Possible Solutions

We can possibly solve this by:

1. Adding a backing CAShapeLayer with the background color in layoutSubviews()
2. Adding a backing UIView with the background color in layoutSubviews()

This is not my domain of expertise so I thought I would bounce some ideas off the rest of you.