Sidecar element visibility. Hidden element keeps its layout box, so it's not painted but keeps its size and becomes visible without re-layout.
Resolves source duration without loading media data. Element is left without source.
Source url, not set when source is held by track state itself
Loads source, completes when media data needed to show sidecar is loaded.
Source url, not set when source is held by track state itself, ie. frame images of frame image track
OptionalstartTime: numberSidecar time loading should start from, so the first fetched data is the one that will be played. Defaults to 0.
Releases media resources, so network, decoded data and buffers are freed. Element stays mounted with its layout box. Source is attached again with loadSource.
Owns sidecar media element and everything needed for its playback, mounted into host element.