Thanks, the curve dragging is surprisingly easy. I think a very old discussion in a Google forum have me the idea how to implement it. It even works with keeping the handle directions (pressing Shift while dragging).

That tree on the left is difficult to get right on all devices. It has to support click, touch to click, drag and drop, swipe scrolling, touch scrolling, etc., and all that while acting as if it holds hundreds of elements. There are still a few issues, for example swipe scrolling and some jankiness on iOS, but I have them on my list.

Keyboard shortcuts are implemented for non-mobile devices. If you open a menu on the menu bar, you can see them at the right in the menu items.