One thing I missed, you can close an item (pop-up) by clicking their own icon.
That’s really helpful to know, if you use it on mobile phones.
What do you think about an icon transformation:
If you open an item, the icon changes to a “close sign” or something similiar.
Here are some images I took from my phone:
This shows the normal close point for me
(btw, to the right of the icon is a polygon area 
)
And here would be an example of my idea:
+the icon of the activated item should always be on top of all other icons.
Also, it would be nice, if you can close an item by clicking the item in the side panel.
(But I think that was mentioned before already, if I remember right.)**
Best regards,
Michel
**edit:
I think I found the thread I remembered.