Plex sidecar subtitles are attached as MediaItem.SubtitleConfiguration and
tagged `external_<n>`, then recovered from the Format id the track selector
reports. Since media3 1.3.0, DefaultMediaSourceFactory always merges
side-loaded subtitles with the primary source and MergingMediaPeriod rewrites
every child format id to "<periodIndex>:<originalId>", so the tag arrives as
"1:external_0" - measured on device - or "0:1:external_0" behind the
container-sidecar merge. The prefix test therefore never matched and every
sidecar reached Dart as an embedded track with no URI.
A Plex sidecar's only identity is its stream key, which the app carries in
that URI, so both matchers failed on it: a server-selected sidecar could
never resolve and left subtitle selection pending, and a manually chosen one
could not be mapped back to a stream id to write to the server. The
already-attached branch of addSubtitleTrack compared the raw id too, so
re-selecting a loaded sidecar silently did nothing.
Route every write and readback of the tag through ExternalSubtitleIds, which
matches the final id segment, and cover it with an instrumentation test that
side-loads a subtitle through the real media3 media-source factory. Also stop
claiming a saved track selection when no server stream was identified - there
is no local store, so that path silently dropped the user's choice.
close#1713
edde746
d83d0790ba