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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>