A couple of suggestions for usability improvements:
1. Drag handling is buggy when I accidentally move the mouse outside of a box and then release. In that case it doesn't notice a mouse-up and the parameter gets stuck in the mouse-down state. Makes it difficult to draw in the leaf editor or to modify parameters by dragging them like sliders. Probably just need to register the mouse-up event handler on a common ancestor instead of the graph node editor boxes.
2. The little parameter "port" circles for dragging connections are a bit small, especially when zoomed out where you can see a significant portion of the whole scene. Usability would be improved by making them bigger (at least 25%?). It might just be the hit area that needs to be larger, not necessarily the graphical element.
Note: I'm using Firefox. Just tested in chromium and the mouse handler problem happens there as well but dragging to connect nodes seems a bit more usable in chromium.