This is a demo/example-implementation to a design solution that's used e.g. in the Soor.app. With Pull-To-Reach, the user can select toolbar items by swiping down rather than stretching his thumb up to the top:
Detailed instructions can also be found in this article: https://quickbirdstudios.com/blog/flutter-pull-to-refresh-was-yesterday-checkout-pull-to-reach/
And if you want to integrate this into your existing app, here is a great library which does that job for you: https://github.com/TiO-Design/pull_to_reach