assets.previewStatus

Read the preview work queue — the serial drain that renders every queued preview.png (from generatePreviews backfills, from staging an asset whose preview is missing or stale, and from views that ask for a picture). Returns how many renders are still pending, which asset is rendering right now, the session's completed/failed totals, and the most recent failure records. Poll it after generatePreviews until pending reaches 0 and active is empty; a non-empty failures list names each asset that could not render and why.

While pending is above 0 and nothing renders, held names what the queue is waiting for: the engine in play mode (the queue renders in edit) or a render device being rebuilt after a loss. A render that cannot finish does not wait: when the render device is abandoned, a subject draws nothing, or the drain rendering it ends part way, the entry moves to failures with the reason, and the zm push refusal for an asset that needs its preview.png says where the queue stands on it. activeFrames counts the engine frames the batch under way has run, which tells a slow batch from one that is not moving, and drainer is the handle of the task draining the queue (task.status(drainer) reads it). Reading the status starts a drain when work is queued and none is running.