A live library refresh or a sort change moved the highlight to a different
title, and Select then opened that one instead of the one the viewer chose.
Grid focus nodes are keyed by index while the cards are keyed by item, so a
merge that inserts an item before the focused slot leaves the highlight
pinned to the slot rather than the title: Flutter parks the node across the
rebuild and the replacement card re-attaches it. The library refresh already
compensated the scroll offset for that same index shift; focus is equally
index-pinned and was not compensated at all.
GridFocusNodeMixin.remapGridFocus carries the highlight with the item, and
the three grids that mutate content in place now use it: library browse, the
paginated card grid behind collections and playlists, and hub detail -- where
a user changing the sort was enough, no server push needed. Hub cards also
gain a key; they had none, so the element was silently updated with a
different item. The key is the global one, because aggregated hubs such as
Continue Watching can hold colliding per-server ids.
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