Currently the gstreamer-rs backend has common media information change callback for any pipeline property change (including the stream collection change)
https://docs.rs/gstreamer-play/latest/src/gstreamer_play/auto/play_signal_adapter.rs.html#144
https://docs.rs/gstreamer-play/latest/src/gstreamer_play/auto/play_media_info.rs.html

And there are conversion from PlayMediaInfo to our Metadata (here) and if there are any change in compare with the latest metadata the media backend sends PlayerEvent::MetadataUpdated event (here) and a client must parsing this message and identify what was changed.

The proposal is to add new PlayerEvent::MediaTracksChanged event which will allow to inform a client if stream collection was changed and need to adjust list of the media tracks on its side.

Add new MediaTrack struct something like instead of Vec<String>

struct MediaTrack {
   type: Audio/Video/...,
   kind: "main", "", ...
   id: "0", "1", ....
   label: ...
   language: ...
}

See https://html.spec.whatwg.org/multipage/media.html#dom-videotrack-kind (as example)

And DO NOT use codec as a possible field (see https://github.com/servo/media/blob/main/backends/gstreamer/player.rs#L62) because it could be changed dynamically multiple times due to internal issue(?) of the media engine (outdated gstreamer version 1.20-2?)

The following test
./mach test-wpt -r /html/semantics/embedded-content/the-video-element/video_size_preserved_after_ended.html on
the next media file media/test-1s.webm is constantly switching between On2 VP9 and VP9 video codecs during playback with 2-100+ media update info messages.