Hide a div behind

Question

I try to do a simple think but I am stuck. I have a div "scrolling" with a fixed position. When I scroll the page, I want that the div "scrolling" goes behind all other div below.

I put an exemple in : exemple jsfiddle

.scrolling{
  top: 230px;
  background-color:lightblue;
  margin:auto;
  width:100%;
  text-align:center;
  position:fixed;
}

Thank you for your help

Answer 1

To answer your question, you need to understand concepts like z-index and/or how the stacking order works. There are great articles that will explain z-index, but one in particular I really enjoy is What no one told you about z-index.

A snippet from the referenced article above:

When you introduce the position property into the mix, any positioned elements (and their children) are displayed in front of any non-positioned elements. (To say an element is “positioned” means that it has a position value other than static, e.g., relative, absolute, etc.)

Interestingly enough, there's a CSS3 property that does something similar. You can address your issue, without the need of explicitly applying a z-index, by using transform.

For example, if we apply transform: translateX(0) to your JSFiddle, you'll see that the gray div now sits above your text. Here's an updated fiddle:

https://jsfiddle.net/gh94Lbad/

<div style="width:auto; height:50px; background-color:grey; transform: translateX(0)"></div>
<div style="width:auto; height:350px; background-color:grey;transform: translateX(0)">I want to be in displayed in front</div>

The reason transform works (by modifying the stacking context) is explained in the spec:

For elements whose layout is governed by the CSS box model, any value other than none for the transform property results in the creation of a stacking context. Implementations must paint the layer it creates, within its parent stacking context, at the same stacking order that would be used if it were a positioned element with z-index: 0. If an element with a transform is positioned, the z-index property applies as described in [CSS2], except that auto is treated as 0 since a new stacking context is always created.

Edit: As pointed out by @TemaniAfif, it's probably better to explain how this works by explaining the paint order of your elements:

For this you need to consider the painting order (w3.org/TR/css-position-3/#painting-order) where you find that transformed element are painted later thus they are above. (in the step (8) )

Answer 2

You need to use more z-index to element that you want to be seen and use less z-index to element "behind".

Answer 3

Pretty sure a (z index) is what you want to use here.

I updated your fiddle below. Z index allows you to position from front to back.

<div class="top">
  <div>
    Hello
  </div>
  <div>
    Hello
  </div>
</div>
<div class="scrolling">
  SCROLL
  <br>
  I want to stay behind the div grey
</div>


<div class="front" style="width:auto; height:350px; background-color:grey;">
  I want to be in displayed in front
</div>

.top{
  width:auto;
  background-attachment: fixed; 
  height:300px;  
  background-image: linear-gradient(pink, green);
  z-index: 0;
}
.scrolling{
  top: 230px;
  background-color:lightblue;
  margin:auto;
  width:100%;
  text-align:center;
  position:fixed;
  z-index: 0;
}
.front{
  z-index: 1;
  position: relative;
}

Answer 4

doable with z-index. But for the z-index to work, it must be position: absolute, position:fixed, or position:relative.

.top{
  width:auto;
  background-attachment: fixed; 
  height:300px;  
    background-image: linear-gradient(pink, green);
}
.scrolling{
  top: 230px;
  background-color:lightblue;
  margin:auto;
  width:100%;
  text-align:center;
  position:fixed;
  z-index:4;
}

<div class="top">
  <div>
    Hello
  </div>
  <div>
    Hello
  </div>
</div>
<div class="scrolling">
  SCROLL
  <br>
  I want to stay behind the div grey
</div>

<div style="width:auto; height:50px; background-color:grey;position:relative;z-index:5">

</div>
  
<div style="width:auto; height:350px; background-color:grey;position:relative;z-index:5">
  I want to be in displayed in front
</div>