SwiftUI: Semi-transparent box appears below textfield when text entered?

I'm just learning, so possibly missing something obvious. I have a basic login form with two textfields. As soon as I enter some text in either field, a semi-transparent box appears below the textfield, overlapping other content. It will disappear if escape is pressed, something like an autocomplete drop-down might.

Any idea what is creating this, and how to not do that? Screenshot and code below.

Also... if anyone knows how to make the button taller (i.e. padding around the text but inside the button), that would be nice. It's awfully narrow. I tried adding padding on the Text element, but that doesn't expand the button frame.

struct SignInView: View {
    @State var email: String = ""
    @State var password: String = ""    

    var body: some View {
        VStack {
            VStack(alignment: .leading) {
                Text("Email").font(.callout).bold()
                TextField("bobby@dontworry.com", text: $email)
                    .disableAutocorrection(true)
                    .textFieldStyle(RoundedBorderTextFieldStyle())

                Text("Password").font(.callout).bold().padding(.top)
                SecureField("******************", text: $password)
                    .disableAutocorrection(true)
                    .textFieldStyle(RoundedBorderTextFieldStyle())

                Button(action: {
                    print("Sign In")
                }) {
                    HStack {
                        Spacer()
                        Text("Sign In")
                        Spacer()
                    }
                }
                    .disabled(email.isEmpty || password.isEmpty)
                    .padding(.top)
            }.padding()
        }.padding()
    }
}