extendStroke method
Implementation
void extendStroke(Offset point, double speed) {
final USignatureStroke? stroke = _active;
if (stroke == null) return;
final double factor = (speed / velocityScale).clamp(0.0, 1.0);
final double target = maxWidth - (maxWidth - minWidth) * factor;
_lastWidth += (target - _lastWidth) * 0.4;
stroke.points.add(USignaturePoint(point, _lastWidth));
notifyListeners();
}