Wow. Just wow. I figured it out while building the smaller example for you.
We’re running GoJS v2.1.49 – as soon as I used v.2.3.12, the mouse events started bubbling and the panning works. So that GoJS + LeafletJS sample requires a certain level of GoJS to work.
If you run it on GoJS 2.1.49 the panning will not work.