html - Last margin / padding collapsing in flexbox / grid layout

Question

I have a list of items that I'm trying to arrange into a scrollable horizontal layout with flexbox.

Each item in the container has a margin left and right, but the right margin of the last item is being collapsed.

Is there a way to stop this happening, or a good workaround?

ul {
  list-style-type: none;
  padding: 0;
  margin: 0;
  display: flex;
  height: 300px;
  overflow: auto;
  width: 600px;
  background: orange;
}
ul li {
  background: blue;
  color: #fff;
  padding: 90px;
  margin: 0 30px;
  white-space: nowrap;
  flex-basis: auto;
}

<div class="container">
  <ul>
    <li>Item 1</li>
    <li>Item 2</li>
    <li>Item 3</li>
    <li>Item 4</li>
  </ul>
</div>

Answer 1

Potential Problem #1

The last margin is not being collapsed. It's being ignored.

The overflow property applies only to content. It doesn't apply to padding or margins.

Here's what it says in the spec:

11.1.1 Overflow: the overflow property

This property specifies whether content of a block container element is clipped when it overflows the element's box.

Now let's take a look at the CSS Box Model:

^{source: W3C}

The overflow property is limited to the content box area. If the content overflows its container, then overflow applies. But overflow doesn't enter into the padding or margin areas (unless, of course, there is more content that follows).

---

Potential Problem #2

The problem with Potential Problem #1 is that it appears to fall apart outside of a flex or grid formatting context. For example, in a standard block layout, the last margin doesn't appear to collapse. So maybe overflow is permitted to cover margins / paddings, regardless of what it says in the spec.

div {
  height: 150px;
  overflow: auto;
  width: 600px;
  background: orange;
  white-space: nowrap;
}
span {
  background: blue;
  color: #fff;
  padding: 50px;
  margin: 0 30px;
  display: inline-block;
}

<div class="container">
    <span>Item 1</span>
    <span>Item 2</span>
    <span>Item 3</span>
    <span>Item 4</span>
</div>