flattenContainer method
Returns the flattened fragment list for container with global
offsets, using the document cache when available.
Implementation
List<(Fragment, int, int)> flattenContainer(FNode container) {
_flattenedCache ??= {};
final cached = _flattenedCache![container.id];
if (cached != null) return cached;
final result = flattenFragmentsSimple(container);
_flattenedCache![container.id] = result;
return result;
}