I think it would be easier to make smooth looping animations if the timeline wrapped around.
For example if I have an animation that is 100 frames long (frames 0 to 99) and put one keyframe at 0 and another at 80. Since 80 is the last frame it would behave is if there was an identical frame to 0 at position 100, giving a smooth wrap around.
It would work the same for the start of the timeline, so if I only had keyframes at 15 and 80 (still assuming 100 frames total) it would make a virtual copy of 15 at 115 and a copy of 80 at frame -20 so that it can interpolate all values for all frames in the animation.
This has two IMO significant benefits for looping animations:
1. Not having to maintain a copy of the start frame at frame "animation length + 1".
2. It would be much easier to time-offset a single animated property. Unless I'm mistaken right now if you move an animation track you have to figure out the transform values at "animation length + 1" and copy those to a new keyframe at time=0.
It would of course only be enabled for looping animations
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