Using PopupButton as Editor/Focus issues

I am using the PopupButton as a PropertyEditor. In the popup content I am using a ListBox. For the most part everything is working but...

At times when the popup is open and I use the mouse to select different elements (ListBoxItem) I lose Focus to the PropertyGridDataAccessor item or the property grid itself. Then if I use the keys to go up and down the listbox (popup is open), it doesn't work, focus will be going up and down the property grid.

It does depend upon how I arrive at the PopupButton. If the propertygrid is not in focus and I move directly to the PopupButton, click button, open the popup and select items everything works fine.

If I start with the property grid in focus, then get to the PopupButton, open the popup, then I have this problem.

Any help appreciated. Thanks.
jeff