Flutter Overlay Widget

I have a video call flutter app. Screen C represents the video call page and before entering C the user need to go from A to B. I need to create a feature just like how it is on WhatsApp video call when the user press back button the video call screen becomes smaller and can be dragged. The user is also able to interact with other pages without affecting the smaller video call screen. I need to do something similar in flutter. Right now the problem is that with the Navigator stack it is not possible to have two different screens visible at the same time. Are there any widgets or plugins or any other ideas I can use to implement this feature in flutter? Thanks in advance

So right now I tried resizing the Scaffold when the user presses back button in the video call screen so that it gives the effect of a smaller video call screen just like in WhatsApp but I am unable to see the previous screen underneath.

you can refer to this package,
https://pub.dev/packages/pip_flutter