编辑1：根据@YeasinSheikh的评论，请尝试以下操作，将填充添加到导航栏本身和FAB。
EDIT 2：一个新的解决方案来解决前一个的问题。

class Test2 extends StatelessWidget {
  const Test2({super.key});

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      floatingActionButtonLocation: FloatingActionButtonLocation.centerDocked,
      floatingActionButton: FloatingActionButton(
        backgroundColor: Colors.white,
        child: const Icon(
          Icons.ac_unit_sharp,
          color: Colors.black,
        ),
        onPressed: () {},
      ),
      bottomNavigationBar: BottomAppBar(
          notchMargin: 8,
          shape: const CircularNotchedRectangle(),
          color: Colors.red,
          child: Column(mainAxisSize: MainAxisSize.min, children: [
            Row(children: [
              Container(
                  height: 60,
                  width: 8,
                  color: Theme.of(context).scaffoldBackgroundColor),
              Expanded(child: Container(height: 60)),
              Container(
                  height: 60,
                  width: 8,
                  color: Theme.of(context).scaffoldBackgroundColor),
            ]),
            Container(height: 8, width: double.infinity, color: Theme.of(context).scaffoldBackgroundColor),
          ])),
      body: Column(
        children: [],
      ),
    );
  }
}