Yep, I've made yet another noob mistake.

I needed to compare two enum values, on a method that was accepting objects.

public string Whatever(object currentOption, object selectedOption)
{
    return
        currentOption == selectedOption
        ? "class='active' "
        : string.Empty;
}

Didn't work.

However,

public string Whatever(object currentOption, object selectedOption)
{
    return
        currentOption.Equals(selectedOption)
        ? "class='active' "
        : string.Empty;
}

Did work.

so, it appear that == isn't polymorphic so it did the object.Equals method which apparently looks for reference equality, rather than the enum Equals.

And that has been yet another future-reference-post ...