flowchart LR
  subgraph V["window visible (expected, not measured)"]
    V1["nextDrawable"] --> V2["compositor shows frame<br/>at a vertical blank"] --> V3["drawable returns<br/>every N refreshes"]
  end
  subgraph O["window occluded / screen locked"]
    O1["nextDrawable"] --> O2["frame never shown<br/>(presentedTime 0)"] --> O3["drawable returns<br/>every 8-10 ms, any N"]
  end