The current GestureDetector does not allow the developer to have listeners for both drag gestures (horizontal or vertical) and scale gestures (two fingers). If you try to listen to both a scale gestures and drag gestures, you get an error message:

I/flutter (25289): Incorrect GestureDetector arguments.
I/flutter (25289): Simultaneously having a vertical drag gesture recognizer, a horizontal drag gesture recognizer, and
I/flutter (25289): a scale gesture recognizer will result in the scale gesture recognizer being ignored, since the
I/flutter (25289): other two will catch all drags.

I'm not sure why this happens, since scale is a two finger gesture, and drag is only one finger. Not being able to have both gestures recognized is problematic.