Grease Pencil - Time Offset Modifier, interactive mode (for making seamless loops)

A proposal for making the Time Offset Modifier a fantastic tool for creating seamlessly looping 2d animations with overlapping motion (e.g. fire, water, wind visual effects).

Proposal: The Time Offset Modifier gets a new toggle for 'Interactive Mode'. With interactive mode turned on, it behaves as follows:

1. The onion skin system displays the playhead's neighbouring frames after the Time Offset Modifier adjustment is calculated . In this mockup we see the keyframes at the start of the looping sequence show up as onion skin guides (although the playhead isn't adjacent to them)

2. The keyframe that the user is editing in the 3d view isn't necessarily a keyframe at the current playhead position. Instead the user is editing the source keyframe that the Time Offset Modifier is displaying. In this mockup the playhead at frame 9, and the keyframe at frame 1 is being drawn into.

With interactive mode you'd create a seamless looping sequence as follows.

1. Set your loop duration first, turn on Interactive Mode on the Time Offset Modifier
2. Keep advancing the playhead one step at a time, drawing new frames as you go, using onion skins as a guide for the motion.
3. Keep going until you're satisfied

Note: other animation software uses the concept of keyframe instances. You can use keyframe instances to create a similar workflow to the above. See this proposal . I believe keyframe instances would be a more powerful/flexible approach to this problem (wider range of use cases, including this one), but I'm adding this Time Modifier proposal here in case it's significantly easier to implement.