snapshotRememberedKeys method
Returns the set of keys currently held in expansion memory, plus parents whose child list was emptied by a sync while they survived collapsed.
A key is present here only if it was previously removed by
syncRoots/syncChildren and its expansion state was recorded
for restoration on re-add, or if it is a retained parent whose
deliberate collapse must not be overridden when its children return.
Intended for callers (e.g. the auto-expand heuristic in
SyncedSliverTree) that need to distinguish a genuinely new key
from one that is being re-added after having been filtered out.
Implementation
Set<TKey> snapshotRememberedKeys() {
return {..._rememberedExpansion.keys, ..._emptiedWhileCollapsed};
}