handleSelectAll function
Implementation
bool handleSelectAll(FluentDocument document) {
final root = document.content;
final cursor = document.cursor;
if (root.nodes.isEmpty) return false;
// Use the document-cached stop rail instead of rebuilding O(n).
final stops = document.caretStops;
if (stops.isEmpty) return false;
// Select from first to last stop
final firstStop = stops.first;
final lastStop = stops.last;
cursor.moveTo(firstStop.fragmentId, firstStop.offset);
cursor.focusTo(lastStop.fragmentId, lastStop.offset);
// Sync SelectionManager to show visual selection
_syncSelectionManager(document);
// Select-all does NOT mutate content: cursor-only update.
document.cursorOnlyUpdate();
return true;
}