MahApps Metro Expander Does Not Close All The Way

[MemeMachineSO]

I had originally posted on StackOverflow, but no one seemed to know a solution so I will post here instead.

Here is a reproducible example that shows the behavior of MahApps vs normal WPF.

I have a Window which just contains an expander. I want it to be an 'overlay' of sorts, where all you can see is the expander. This works fine in normal WPF, but when I go to use MahApps Metro, the expander will not close all the way.

Take a look at the behavior of normal WPF vs MahApps: