Since a1b6a8971 only the selected sidecar attached at open, so mpv's
track-list carried one external subtitle and the track sheet could only
offer the rest as primary source switches - tap-and-hold on a
non-selected external track selected it as primary instead of secondary.
Real external files are cheap static fetches, so Jellyfin, Plex direct
play, and offline discovery now mark them preload and they ride along in
sub-files at open, keeping every external track selectable as a
secondary subtitle without a reopen. Embedded rows extracted on a
transcode stay lazy: extraction can stall behind the transcoder, which
is exactly what used to trip the sidecar open guard.
close#1860
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