onScaleUpdate is being triggered for a GestureDetector even when the user is touching the screen with only a single finger. In this case, details.scale is 1.0. I don't know the behavior of iOS or Android, but it seems like scale update should not be triggered for a single finger.

This is a problem because when a user is placing or removing two fingers on the screen, often one finger will remain for a brief time, which will reset the scale factor and cause ugly jank.

The workaround would be for the developer to listen for onScaleStart and onScaleEnd and based on those listen for onScaleUpdate only during the time scale is active.

See also issue #13101