I often have quite a large library in my tree panel, so i end up scrolling a lot to find the right element, which is quite tiring in the long run. Especially with a lot of subfolders, having to drag an image from the very bottom, to the top of the hierachy, is a bit annoying.

Instead of the scrolling option, why not use a panels with tabs? Like the panel setup Adobe uses in Illustrator, PS, etc.
See attachment for mockup of my suggestion.

It would help users to easiler get a overview, and the categories that users don't use, are easily accessible, but does not contribute to extra scrolling. ATM, i have no use for Constraints, Events and Skins.

Furthermore, in between the panels there could be these handles to make height adjustments. Perhaps even a collapse feature.

To sum it up, it would be neat to have some kind of "Adobe panel control" to increase the workflow speed
And of course, thanks for a brilliant animation tool!

/Bendsen

Sorry for the late respons, just returned from a long vacation.
But completely understandable, Shiu - but i hope that you one day can find some time to make a UI update
Anyhow, would love to see that "twin mockup"