deltaForKey method
Slide delta for key, or 0.0 if not currently sliding (or not
registered).
Implementation
double deltaForKey(TKey key) {
final nid = _nids[key];
if (nid == null || nid >= _slideByNid.length) return 0.0;
final slide = _slideByNid[nid];
return slide == null ? 0.0 : slide.currentDelta;
}