CSS: Make element's z-index lower than a sibling's child - css

Question

The issue is the header: I need it to seem to be fixed on the fixed section and still be clickable.
When the first section scrolls over the header, it should cover the header.
I can move or change the header however I want:
<a href="#" class="header">
THIS IS THE HEADER
<span style="font-size: 12px;">(you can hover me)</span><br>
<span style="font-size: 14px; font-style: italic; letter-spacing: 0.05em;">which should be <br> OVER "invisible section" AND "fixed section" <br> BUT UNDER "first section", <br>WHILE maintaining fixed position</span>
</a>
and while I could add the header to the invisible section, I cannot change the overall structure of the sections:
<div class="section section-fixed">
fixed section underneath "invisible section"
</div>
<div class="wrapper">
<div class="section section-invisible">
<h2>invisible section</h2>
</div>
<div class="section section-first">
first section
</div>
</div>
I tried adding the header to the invisible section and while this works in keeping the header clickable, I cannot make it seem to be fixed the same as the fixed section.
I realise describing it might be confusing so here are some accompanying images and please check the FIDDLE
#import url("https://fonts.googleapis.com/css2?family=Baloo+Da+2&display=swap");
html {
box-sizing: border-box;
}
*, *:before, *:after {
box-sizing: inherit;
}
html, body {
margin: 0;
padding: 0;
}
body {
font-size: 16px;
font-family: 'Baloo Da 2', cursive;
}
.header {
background: floralwhite;
display: inline-block;
padding: 10px;
position: fixed;
z-index: 3;
left: 30px;
top: 30px;
color: inherit;
text-decoration: none;
line-height: 1.2;
box-shadow: 0 0 0 rgba(0, 0, 0, 0.5);
transition: all .5s ease;
}
.header:hover {
box-shadow: 2px 3px 10px rgba(0, 0, 0, 0.5);
background-color: pink;
}
.wrapper {
border: 2px solid red;
z-index: 2;
position: relative;
}
.section {
padding: 20px;
display: flex;
align-items: center;
justify-content: center;
height: 100vh;
}
.section-fixed {
background: gray;
position: fixed;
top: 0;
left: 0;
width: 100%;
z-index: 1;
}
.section-invisible {
align-items: flex-end;
padding: 0;
}
.section-invisible h2 {
background: goldenrod;
padding: 50px;
margin: 0;
width: 100%;
text-align: center;
}
.section-first {
background: darkslateblue;
color: white;
}
<a href="#" class="header">
THIS IS THE HEADER
<span style="font-size: 12px;">(you can hover me)</span><br>
<span style="font-size: 14px; font-style: italic; letter-spacing: 0.05em;">which should be <br> OVER "invisible section" AND "fixed section" <br> BUT UNDER "first section", <br>WHILE maintaining fixed position</span>
</a>
<div class="section section-fixed">
fixed section underneath "invisible section"
</div>
<div class="wrapper">
<div class="section section-invisible">
<h2>invisible section</h2>
</div>
<div class="section section-first">
first section
</div>
</div>
initial:
after scroll:

Answer 1

Update your z-index like below. The most important is to not set any z-index to wrapper:
#import url("https://fonts.googleapis.com/css2?family=Baloo+Da+2&display=swap");
html {
box-sizing: border-box;
}
*,
*:before,
*:after {
box-sizing: inherit;
}
html,
body {
margin: 0;
padding: 0;
}
body {
font-size: 16px;
font-family: 'Baloo Da 2', cursive;
}
.header {
background: floralwhite;
padding: 10px;
position: fixed;
z-index: 2; /* here */
left: 30px;
top: 30px;
color: inherit;
text-decoration: none;
line-height: 1.2;
box-shadow: 0 0 0 rgba(0, 0, 0, 0.5);
transition: all .5s ease;
}
.header:hover {
box-shadow: 2px 3px 10px rgba(0, 0, 0, 0.5);
background-color: pink;
}
.wrapper {
border: 2px solid red;
position: relative;
}
.section {
padding: 20px;
display: flex;
align-items: center;
justify-content: center;
height: 100vh;
}
.section-fixed {
background: gray;
position: fixed;
top: 0;
left: 0;
width: 100%;
z-index: 1; /* here */
}
.section-invisible {
align-items: flex-end;
padding: 0;
position: relative;
z-index: 1; /* here */
}
.section-invisible h2 {
background: goldenrod;
padding: 50px;
margin: 0;
width: 100%;
text-align: center;
}
.section-first {
background: darkslateblue;
color: white;
position: relative;
z-index: 2; /* here */
}
<a href="#" class="header">
THIS IS THE HEADER
<span style="font-size: 12px;">(you can hover me)</span><br>
<span style="font-size: 14px; font-style: italic; letter-spacing: 0.05em;">which should be <br> OVER "invisible section" AND "fixed section" <br> BUT UNDER "first section", <br>WHILE maintaining fixed position</span>
</a>
<div class="section section-fixed">
fixed section underneath "invisible section"
</div>
<div class="wrapper">
<div class="section section-invisible">
<h2>invisible section</h2>
</div>
<div class="section section-first">
first section
</div>
</div>
Related: Why can't an element with a z-index value cover its child?