I'm afraid this isn't possible. However, that's my code for the SnackBar:

displaySnackBar(String message, BuildContext context) {
    final snackBar = SnackBar(
      duration: Duration(milliseconds: 2000),
      behavior: SnackBarBehavior.fixed,
      content: Text(message, style: TextStyle(color: Colors.black, fontWeight: FontWeight.normal)), 
      backgroundColor: Colors.white

It works very fine. The only problem: I want to add a simple shadow behind it. The behavior must be fixed.

Solution 1: Darshan

With default SnackBar widget and behavior.fixed, you may not be able to put a shadow effect. But you can achieve that using this plugin. It has an example of shadow at top of the bar:

enter image description here

Hope this helps.

Solution 2: Bassam

The answer: This is currently (without using a plugin) not possible.

Solution 3: Ataberk

If it is very important for someone you can use a plugin or similar effect can be achieved with BottomSheet. I know it is not actual SnackBar but you can implement needed SnackBar features by hand.

bottom sheet

BottomSheet idea is below. You can add needed features by hand.

    (context) => Stack(
          alignment: Alignment.bottomCenter,
          children: [
              onTap: () {
              child: Container(
                  height: double.infinity,
                  width: double.infinity,
                  color: Colors.black.withOpacity(.3)),
              height: MediaQuery.of(context).size.height * .6,
              width: double.infinity,
              margin: EdgeInsets.symmetric(horizontal: 8),
              padding: EdgeInsets.all(16),
              decoration: BoxDecoration(
                  color: backgroundAccentColor,
                  boxShadow: boxShadow,
                  borderRadius: BorderRadius.only(
                      topLeft: Radius.circular(12),
                      topRight: Radius.circular(12))),
              child: Text("Add new card"),
    backgroundColor: Colors.transparent);