FirstFrameRendered

The first frame of this media item left the schedule. latency is measured from the open.

What "left the schedule" means is the strongest signal this build has: a renderer accepted the frame, or, with nothing attached, the schedule presented and released it. It is not a report that a pixel reached a display. A renderer that accepts a frame may still supersede it with a newer one or fail to draw it, and it counts those outcomes itself. Per-submission terminal feedback, which is what would make this event mean scanout, needs a per-submission renderer protocol that does not exist yet.