Почему при добавлении к header свойства position:fixed пропадает вертикальный скролл?

Question

[losos_lapolosos]

Вот html:

<header>
		<div class="bg">
			<div class="container">
				<div class="top-contact">
					<div class="RusNum">8 800 231-54-93 Бесплатно по России</div>
					<div class="PitNum">8 981 822-25-30 Санкт-Петербург</div>
					<div class="mail">[e-mail]</div>
				</div>
			</div>
		</div>
		<div class="container">				
			<div class="menu">
				<div class="logo"><img src="img/logo.png" alt="Логотип"></div>
				<nav class="desktopNav">
					<a href="#" class="ArticleOfMenu">Главная</a>
					<a href="#" class="ArticleOfMenu">О продукте</a>
					<a href="#" class="ArticleOfMenu">Как это работает</a>
					<a href="#" class="ArticleOfMenu">Применение</a>
					<a href="#" class="ArticleOfMenu">Контакты</a>
				</nav>
				<div class="call"><a class="callButton" id="btn_modal_window">Звонок</a></div>
			</div>
		</div>
</header>

Вот css:

header{
	width: 100%;
	position: fixed;

}

.bg{
	background: #E1E1E1;
}

.top-contact{
	display: -webkit-flex;
	display: -moz-flex;
	display: -ms-flex;
	display: -o-flex;
	display: flex;
	justify-content: space-between;
	font-weight: 400;
	padding: 10px 0;
}

.menu{
	background: #fff;
	display: -webkit-flex;
	display: -moz-flex;
	display: -ms-flex;
	display: -o-flex;
	display: flex;
	justify-content: space-between;
	align-items: center;
	padding: 30px 0;
}

.logo{
	width: 25%;
}

.logo img{
	width: 100%;
	max-width: 182px;
	height: auto;
}

.desktopNav{
	font-weight: 500;
	display: -webkit-flex;
	display: -moz-flex;
	display: -ms-flex;
	display: -o-flex;
	display: flex;
	justify-content: center;
	width: 100%;
}

.desktopNav a{
	padding: 10px;
	border-bottom: 2px solid #30A261;
	text-decoration: none;
	color: #000;
	text-align: center;
	margin-left: 1%;
}

.desktopNav a:hover{
	background: #30A261;
	transform: translateY(-10px);
	color: #fff;
	-webkit-transition: all 0.5s ease;
	-moz-transition: all 0.5s ease;
	-ms-transition: all 0.5s ease;
	-o-transition: all 0.5s ease;
	transition: 0.5s ease all;
}

.desktopNav a:not(:hover){
	background: #fff;
	transform: translateY(0);
	color: #000;
	-webkit-transition: all 0.5s ease;
	-moz-transition: all 0.5s ease;
	-ms-transition: all 0.5s ease;
	-o-transition: all 0.5s ease;
	transition: all 0.5s ease
}


@keyframes moving{
	from{
		transform: translateX(3px);
	}
	20%{
		transform: translateX(-6px);
	}
	40%{
		transform: translateX(3px);
	}
	60%{
		transform: translateX(-6px);
	}
	80%{
		transform: translateX(3px);
	}
	to{
		transform: translateX(0px);
	}
}


.call a{
	display: block;
	padding: 12px 30px;
	border: 2px solid #30A261;
	text-decoration: none;
	color: #30A261;
	cursor: pointer;
}

.call a:hover{
	animation-name: moving;
	animation-duration: 0.5s;
	background: #30A261;
	color: #fff;
	-webkit-transition: all 0.25s ease;
	-moz-transition: all 0.25s ease;
	-ms-transition: all 0.25s ease;
	-o-transition: all 0.25s ease;
	transition: all 0.25s ease
}

.call a:not(:hover){
	background: #fff;
	color: #30A261;
	-webkit-transition: all 0.3s ease;
	-moz-transition: all 0.3s ease;
	-ms-transition: all 0.3s ease;
	-o-transition: all 0.3s ease;
	transition: all 0.3s ease
}