This Flutter app demos an easy way of adding an overlay container in a Stack that can be dragged up and down to resize and reveal it content.
The demo uses a stack with 2 widgets, the background one is purple with the text "Hello" on, the other one in the foreground is the VerticalDragContainer defined in the dragable.dart file.
To drag the container up (or down) the screen just click and hold the green bar while moving your finger(or mouse) up and down.
23 Jan 2019 - The widet has been updated to add the ability to click the titlebar to allow it to automatically expand and contract as well as the original drag feature
For help getting started with Flutter, view the online documentation.