queue_tab.dart was 7,567 lines with a single 7,300-line State class.
Continue the existing part-file split by moving six cohesive method
clusters into private extensions on _QueueTabState (same library, so
private state access is unchanged): selection mode, navigation/open
actions, collection item builders, filter UI, batch actions, and
queue/library item builders. setState calls in the parts go through a
_setState forwarder since @protected members cannot be called from
extensions. queue_tab.dart is now 2,462 lines.
# Conflicts:
# lib/screens/queue_tab.dart
zarzet
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