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F04 — Vsync / frame pacing ​

Every platform offers a different "draw now, in sync with the display" primitive. This demo drives redraw from the platform's native frame clock and measures how steady it actually is.

Requirements ​

  1. Redraw a trivially cheap frame (solid color flip) driven only by the platform frame clock — no sleep, no busy loop:
    • Wayland: wl_surface.frame callbacks.
    • X11: the Present extension (xcb_present_notify_msc / PresentCompleteNotify); if Present is unavailable, fall back to XSyncWaitCondition-free timed redraw and record the absence as the finding.
    • Win32: a DXGI waitable swapchain if feasible in plain D, else DwmFlush pacing — document which was chosen and why.
    • macOS: CADisplayLink (via NSView.displayLink(target:selector:)), noting CVDisplayLink's deprecation.
  2. Log frame_callback t=… for 600 consecutive frames; compute and print min/median/p99/max inter-frame delta and a coarse jitter histogram to stdout at exit.
  3. Record what happens when the window is occluded/minimized/hidden: does the clock stop, throttle, or free-run? (Wayland: frame callbacks simply stop — log the gap.)

Instrumentation ​

frame_callback t=<presentation-or-callback-time> per frame; vis_change state=… on occlusion events; final histogram … summary lines.

Findings to record ​

  • The jitter statistics per platform and what clock the timestamps come from.
  • Whether the primitive conveys presentation time or merely "draw now".
  • Throttling behavior when hidden — the contract a framework's requestRedraw must absorb.
  • Headless caveat: under headless weston/Xvfb the refresh is synthetic; record the configured rate and mark the numbers as headless-only.

Verification ​

Wayland/X11: Tier A (with the headless caveat). Win32: A[wine] numbers are Wine's DWM emulation — collect them, but queue a Tier C run for real numbers. macOS: A[ssh] if windows work over SSH; the display-link cadence on a real display is Tier C.