Bazen telefonlarda bazende eski bilgisayarlarda birbirine giriyor resmin yarısı ekranın yukarısına girmiş gibi gibi.. Bu durumu nasil cozebilirm


<body>
    <form action="login_authenticate.php" method="post">
        <div class="login-box">
            <img src="loginLogo.jpg"  />
            <h1>Yetkili Giriş</h1>
 
            <div class="textbox">
                <i class="fa fa-user" aria-hidden="true"></i>
                <input type="text" placeholder="Kullanıcı Adı"
                         name="adminname" value="">
            </div>
 
            <div class="textbox">
                <i class="fa fa-lock" aria-hidden="true"></i>
                <input type="password" placeholder="Şifre"
                         name="password" value="">
            </div>
 
            <input class="button" type="submit"
                     name="login" value="Giriş Yap">
        </div>
        
    </form>

</body>
body {
    margin: 0;
    padding: 0;
    font-family: sans-serif;
    background: url() no-repeat;
    background-size: cover;
}
 
.login-box {
    width: 480px;
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
    color: #191970;
}
 
.login-box h1 {
    text-align: center;
    font-size: 40px;
    border-bottom: 4px solid #191970;
    margin-bottom: 50px;
    padding: 13px;
}
 
.textbox {
    width: 100%;
    overflow: hidden;
    font-size: 20px;
    padding: 8px 0;
    margin: 8px 0;
    border-bottom: 1px solid #191970;
}
 
.fa {
    width: px;
    float: left;
    text-align: center;
}
 
.textbox input {
    border: none;
    outline: none;
    background: none;
    font-size: 18px;
    float: left;
    margin: 0 10px;
}
 
.button {
    width: 100%;
    padding: 8px;
    color: #ffffff;
    background: none #191970;
    border: none;
    border-radius: 6px;
    font-size: 18px;
    cursor: pointer;
    margin: 12px 0;
}