Jellyfin has no equivalent of Plex's bundled `?includeOnDeck=1`, so a show
detail open chained `/Shows/NextUp` behind the item fetch and the screen sat
on a spinner for both round trips. The second one is not needed to paint:
everything except the play button's episode label comes from the item.
`fetchItemWithOnDeck` now takes an `onItemReady` callback and invokes it as
soon as the item is known, when that is strictly before on-deck settles.
Plex returns both together and never invokes it.
Phone and desktop only. TV keeps its own reveal gate — `_isTvDetailReadyToReveal`
holds the foreground at opacity 0 until extras, related hubs, seasons and the
first episode page have all loaded, and those still run after the on-deck
lookup settles, so TV sees no change. Both halves are pinned by tests.
Measured on a remote Jellyfin server, 15 interleaved show-detail opens per
version: time to content 1264ms -> 1042ms (-18%), with the rest of the load
unchanged.
Seasons and extras deliberately still start after the whole lookup settles.
Starting them at the early paint measured worse (time to settled +21%)
because they contend with the on-deck request instead of overlapping it — the
same reason `/Shows/NextUp` is not fired in parallel with the item fetch.
That trade-off is also why TV was left alone rather than being unblocked by
moving those loads earlier.
Two ordering hazards the early paint introduces, both covered by
`media_detail_screen_test.dart`:
- The early call must not write on-deck. `_loadFullMetadata` runs again after
playback, and clearing there would blank the play button for the length of
the round trip. `onDeckSettled` marks the authoritative write, so a reload
that finds the series finished still clears it.
- A settled empty on-deck must not drop the episode-derived fallback that
`_ensureFallbackOnDeckEpisode` supplies.
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