WPF Popup hiding problem

Question

Suppose you have a ToggleButton for opening a Popup, same behaviour as all known elements as ComboBox etc.

... which is this c

Answer 1

Ok, here is some code that works for me (those are copy-pasted from working code with some of the not-interesting parts removed):

Here's the content of a ComboBox-like UserControl:

And from a custom template to an actual ComboBox: