Create a custom drop down menu?

[Chris M]

Well, the menu buttons work, the animation plays, it's even sticky: but now it slides down from over the header instead of under it.

Is this final dilemma something I will need to be content with?

You could either try a different animation, like Stretch Down, speed up the animation so it isn't as noticeable, or replicate the section and add it to the menu layer so it looks like it's refreshing itself.