If you define both onPanUpdate and onScaleUpdate you get this error:

Incorrect GestureDetector arguments.
I/flutter ( 3574): Having both a pan gesture recognizer and a scale gesture recognizer is redundant; scale is a superset of pan. Just use the scale gesture recognizer.

But how? onPanUpdate tells you dx and dy from the last position. While onScaleUpdate tells you focalPoint and localFocalPoint with no initial position. There's no easy way to convert between them.

Or am I missing something?

Frankly, if onScaleUpdate is a superset of onPanUpdate it should contain at least the exact same fields as onPanUpdate.