What is the reason behind the child element's margin functioning properly when overflow:hidden is applied?

Every time I try to add an element such as h1 with margin: 30px 0;, the margin ends up extending beyond the container!

This issue has plagued me on multiple occasions, but I was able to resolve it by using overflow: hidden

I am curious to understand what causes this problem and why the solution of using overflow works?

You can view a JSFiddle example here https://jsfiddle.net/LeoAref/zv6c2c2d/

.container {
  background: #ccc;
}
.container.overflow {
  overflow: hidden;
}
.secTitle {
  margin: 30px 0;
}
code {
  color: blue;
}
<!-- secTitle margin goes outside the container -->
<div class="container">
  <h1 class="secTitle">Container without <code>overflow: hidden</code></h1>
</div>

<!-- works fine here -->
<div class="container overflow">
  <h1 class="secTitle">Container with <code>overflow: hidden</code></h1>
</div>

Answer ā„–1

What could be causing this issue?

The problem arises due to the collapsing of margins within the parent element in this scenario.

To provide more clarity, let's refer to the relevant information regarding margin collapsing:

Understanding Margin Collapse

In CSS, when the adjacent margins of multiple boxes combine, they can collapse into a single margin. This phenomenon is known as margin collapsing and results in a combined margin width that is determined by the maximum of the collapsed margins' widths. Negative margins are also taken into account, with certain calculations applied to arrive at the final margin width.

A key rule to prevent margin collapsing is explained as follows:

Margins of elements that create new block formatting contexts, such as floats and elements with non-'visible' overflow values, do not collapse with their inner children. [learn more]

In this particular case, the element's 'overflow' property is set to 'hidden', which prevents margin collapsing and confines the margins within the element itself.

If you require additional solutions, consider exploring further documentation.

Answer ā„–2

.container.overflow

It must now appear like this:

.container .overflow

.container {
    background: #ccc;
}

.container .overflow {
    overflow: hidden;
}

.secTitle {
    margin: 30px 0;
}

code {
    color: blue;
}
<!-- secTitle margin goes outside the container -->
<div class="container">
    <h1 class="secTitle">
        Container without 
        <code>overflow: hidden</code>
    </h1>
</div>

<!-- works fine here -->
<div class="container overflow">
    <h1 class="secTitle">
        Container with 
        <code>overflow: hidden</code>
    </h1>
</div>

Answer ā„–3

Why do margins function this way? Margins are designed to extend beyond the element, and in this case they are not taken into account for the h1 margins. The default setting of overflow is visible, causing it to render outside of the h1 container. It may be difficult to fully explain, but I have done my best to clarify. Hopefully this explanation was helpful.

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