Field verdict on the depth-2 gate (M4 iPad Pro, 2752x2064@120): display stage
22-28 ms vs depth-1's 14 — a REGRESSION, not the predicted 5-8. Post-mortem:
any bounded-FIFO pacing keeps a STANDING queue on the always-vsync-latch
platforms. One burst fills every admitted slot and, with arrivals and latches
then running at the same rate, occupancy never returns to zero — each gate
slot costs one full refresh, permanently (the ladder fits exactly: arrival ~3
slots -> 30+ ms, depth 2 -> 22-28, depth 1 -> 14). A bounded FIFO caps the
queue; nothing ever drains it. And depth 1 serializes presents on the on-glass
callback's delivery lag, so neither rung can approach the sub-refresh floor.
Stage-4 (PresentPacing.deadline) inverts drawable ownership instead:
- A CAMetalDisplayLink on its own runloop thread vends ONE deadline-timed
drawable per refresh (preferredFrameLatency 1) into a newest-wins LatestBox;
an unpresented vend is replaced by the next (back to the pool).
- The render thread pairs it with the newest decoded frame the moment either
half arrives — the common case presents a frame INSTANTLY into an already-
vended drawable, latching the upcoming refresh. No image queue can form and
nothing waits on on-glass callbacks (they only feed the meters now).
- A stashed drawable can lag a mid-session HDR reconfigure by one vend:
encodePresent skips the mismatched-format vend (frame re-rings) instead of
tripping Metal validation.
- iOS/iPadOS defaults to stage-4; tvOS keeps stage-3 until its own A/B
(PUNKTFUNK_PRESENTER=stage4); macOS resolves "stage4" back to its default
(the sync-off/DCP-panic saga — deadline pacing lands there deliberately or
not at all). Settings picker gains Stage 4 with the derived default marker.
- Gate depth defaults to 1 everywhere again; PUNKTFUNK_GATE_DEPTH stays only
to reproduce the standing-queue ladder on-device.
- PUNKTFUNK_PRESENT_DEBUG gains latchMs p50/max (present-issue -> on-glass:
standing queue reads ~n x period, healthy reads < 1 period) + noDrawable=.
swift test (14/14) + full Punktfunk-iOS device build green.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
enricobuehler
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