[Flutter] FlatButton 間の空白をなくす

結論

materialTapTargetSize プロパティで、shrinkWrap を指定すればいける

FlatButton(
      materialTapTargetSize: MaterialTapTargetSize.shrinkWrap, // これ
      child: Text("sample"),
      color: Colors.red,
      onPressed: () => print("sample"),
    );

padded → const MaterialTapTargetSize
Expands the minimum tap target size to 48px by 48px.
This is the default value of ThemeData.materialHitTestSize and the recommended size to conform to Android accessibility scanner recommendations.