Sorry sorry, I just checked it (was a couple of months ago I created the page) – it’s actually just a regular structure field and it’s made to look like a modal window on click by using a custom panel CSS.
So no plugin.
Sorry for the confusion.
Ahhh ok 
maybe you want to share the panel.css for this? Looks amazing and I personally like modals more then the inline solution currently included in Kirby! Of course, I could build it by my own but maybe you have already some experience with it 