This thread motivated me to improve the vArranger's FILL IN algorithm.

When you press the fill in button, the variation need to stop, and the arranger need to play the fill in.
But this does not sound so good, because notes actually playing of the variation are cut, and some notes of the fill in have been missed, as you did not played it since the beginning.

So the perfect solution does not exist.

Good news, the new vArranger update improve this a lot, so it is now very smooth. Maybe like Ketron? I don't know