The experimental stage-2 presenter (CAMetalLayer + display link) stuttered badly
in fullscreen but ran fine windowed. render() runs on the display-link / MAIN
thread and calls layer.nextDrawable(), which blocks that thread until a drawable
frees. With the layer's own displaySyncEnabled left on (default), present also
waits for the hardware vsync, so the block serializes the main thread to the
display — windowed, the WindowServer's looser compositing hides it; fullscreen's
tighter, more-direct path exposes it as judder. (Apple dev-forum guidance:
displaySync off measurably reduces nextDrawable() blocking.)
- displaySyncEnabled = false (macOS-only): the display link is already the per-
vsync pacing source, so the layer's redundant vsync wait only adds the stall.
- maximumDrawableCount = 3 (explicit): more in-flight headroom before
nextDrawable() has to block on the main thread.
Swift-only (no core/ABI change → no xcframework rebuild). Validated: swift build;
swift test (39 passed, 0 failures).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
enricobuehler
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