Nice job. Very strongly reminds me of Aseprite [1] (an offline pixel editor) which also takes a similar approach of layers of frames that can be animated.
The onion skin approach to toggling the number of +/- frames was a nice touch as well.
Recommendations:
- Quickly play/pause the animation with the spacebar
- A separately detachable thumbnail preview to get a high-level view of your current work.
- Overriding the delay on specific frames
[1] - https://www.aseprite.org
