Q:

How to make part of flutter widget behave like a stack

What I am Implying to do is that if there is a Column and two containers in it.  I want another widget pinned to to the first container which overlaps parts of the 2nd container and move with the first column (if i apply some special scroll effects). 
Here is dummy code. 
          Column(
            children: <Widget>[
              Container(
                height: 300,
                child: <Some child widget which is pinned to this box and overlaps the second box>
              ),
              Container(
                height: 300,
              )
            ],
          )

A:

If I have understood your question properly one way is to use two stack's itself like as follows. 
Stack(
        children: <Widget>[
          Container(
            height: 300,
            margin: EdgeInsets.all(300),
          ),
          Stack(children: <Widget>[SizedBox(height: 300,),TheWidgetYouWantToOverlap()],)
        ],
      )

