Scroll a div under another div - html

Question

I amd doing a small test about how the create a single web page with smooth scroll with html + css but I have a problem:
When I scroll down, the phone is supposed to go under the header, but instead it goes above it. How can I fix this?
the code css
section {
height: 100vh;
display: flex;
justify-content: center;
align-items: center;
}
section:nth-child(odd)
{
background: #ccc;
}
/*inicia smartphone*/
.smartphone {
position: relative;
width: 360px;
height: 640px;
margin: auto;
border: 16px blue solid;
border-top-width: 60px;
border-bottom-width: 60px;
border-radius: 36px;
}
/* The horizontal line on the top of the device */
.smartphone:before {
content: '';
display: block;
width: 60px;
height: 5px;
position: absolute;
top: -30px;
left: 50%;
transform: translate(-50%, -50%);
background: #333;
border-radius: 10px;
}
/* The circle on the bottom of the device */
.smartphone:after {
content: '';
display: block;
width: 35px;
height: 35px;
position: absolute;
left: 50%;
bottom: -65px;
transform: translate(-50%, -50%);
background: #333;
border-radius: 50%;
}
/* The screen (or content) of the device */
.smartphone .content {
position: inherit;
height: 100%;
background: red;
}
/*acaba smartphone*/
/*inicia navbar*/
ul{
width: 100%;
background: #000;
height: 50px;
margin: 0px;
padding: 0;
position: fixed;
top: 0;
letter-spacing: 0;
display: flex;
justify-content: space-around;
}
ul li{
list-style: none;
}
ul li a {
color: white;
line-height: 50px;
box-sizing: border-box;
font-weight: bold;
text-transform: uppercase;
text-decoration: none;
font-size: 100%;
}
#media (max-width: 576px) {
ul li a {
font-size:12px;
}
}
#media (max-width: 375px) {
ul li a {
font-size:10px;
}
}
/*acaba navbar*/
the html code
<ul>
<li>Inicio</li>
<li>Acerca de x</li>
<li>Quienes Somos</li>
<li>Contactanos</li>
</ul>
<!--Logotipo y Nombre-->
<section class="section1" id="Inicio">
<div class="smartphone">
<div class="content">
</div>
</div>
</section>
<!--Que es la app-->
<section class="section2" id="AcercaDeX">
<P>acerca de x </P>
</section>
<!--Quienes Somos-->
<section class="section1" id="QuienesSomos">
<P>quienes somos</P>
</section>
<!--Contactanos-->
<section class="section2" id="Contactanos">
<P>contactanos</P>
</section>
I tough it was aboutpositions, but honestly I do not know how to fix this

Answer 1

Both your header (ul) and the smartphone element have positioning, so easy fix is to add z-index property to both. Then make sure the header has higher value than the smartphone and you are there.
section {
height: 100vh;
display: flex;
justify-content: center;
align-items: center;
}
section:nth-child(odd) {
background: #ccc;
}
/*inicia smartphone*/
.smartphone {
position: relative;
width: 360px;
height: 640px;
margin: auto;
border: 16px blue solid;
border-top-width: 60px;
border-bottom-width: 60px;
border-radius: 36px;
z-index: 99;
}
/* The horizontal line on the top of the device */
.smartphone:before {
content: '';
display: block;
width: 60px;
height: 5px;
position: absolute;
top: -30px;
left: 50%;
transform: translate(-50%, -50%);
background: #333;
border-radius: 10px;
}
/* The circle on the bottom of the device */
.smartphone:after {
content: '';
display: block;
width: 35px;
height: 35px;
position: absolute;
left: 50%;
bottom: -65px;
transform: translate(-50%, -50%);
background: #333;
border-radius: 50%;
}
/* The screen (or content) of the device */
.smartphone .content {
position: inherit;
height: 100%;
background: red;
}
/*acaba smartphone*/
/*inicia navbar*/
ul {
width: 100%;
background: #000;
height: 50px;
margin: 0px;
padding: 0;
position: fixed;
top: 0;
letter-spacing: 0;
display: flex;
justify-content: space-around;
z-index: 100;
}
ul li {
list-style: none;
}
ul li a {
color: white;
line-height: 50px;
box-sizing: border-box;
font-weight: bold;
text-transform: uppercase;
text-decoration: none;
font-size: 100%;
}
#media (max-width: 576px) {
ul li a {
font-size: 12px;
}
}
#media (max-width: 375px) {
ul li a {
font-size: 10px;
}
}
/*acaba navbar*/
<ul>
<li>Inicio</li>
<li>Acerca de x</li>
<li>Quienes Somos</li>
<li>Contactanos</li>
</ul>
<!--Logotipo y Nombre-->
<section class="section1" id="Inicio">
<div class="smartphone">
<div class="content">
</div>
</div>
</section>
<!--Que es la app-->
<section class="section2" id="AcercaDeX">
<P>acerca de x </P>
</section>
<!--Quienes Somos-->
<section class="section1" id="QuienesSomos">
<P>quienes somos</P>
</section>
<!--Contactanos-->
<section class="section2" id="Contactanos">
<P>contactanos</P>
</section>