clips property
Blocks instead of keys — see MotionClip, kept sorted by start. A track has one or the other: a block's keys would be derived, so a key added to a block track replaces the blocks, and a block put on a key track replaces the keys. Two blocks that overlap crossfade over the overlap. The timeline draws bars for them; the file spells ClipTrack for one and ClipBlocks for several.
Implementation
final clips = <MotionClip>[];