margin bottom seems to disappear on css grid when overflow appears.

no matter which way i approach the layout of my page (flexbox, css grid, fixed) i seem to loose margin/padding bottom when using css grid.

an example of the issue:

HTML

```
<div class="header">&nbsp;</div>
<div class="nav">&nbsp;</div>
<div class="article">
       <div class="settings__grid">
        <div class="settings__grid-item">&nbsp;</div>
        <div class="settings__grid-item">&nbsp;</div>
        <div class="settings__grid-item">&nbsp;</div>
        <div class="settings__grid-item">&nbsp;</div>
        <div class="settings__grid-item">&nbsp;</div>
        <div class="settings__grid-item">&nbsp;</div>
      </div>
</div>

```

CSS

```
html,
body {
  margin: 0;
  height: 100%;
}
.header {
  position: fixed;
  z-index: 2;
  top: 0;
  left: 0;
  width: 100%;
  height: 20px;
  background: white;
  box-shadow: 0px 2px 4px rgba(0, 0, 0, 0.24);
}
.nav {
  position: fixed;
  z-index: 1;
  top: 20px;
  left: 0;
  width: 200px;
  height: calc(100% - 20px);
  background: #FAF9F8;
  border-right: 1px solid #d9d9d9;
}

.article {
  background: #F3F3F5;
  position: relative;
  top: 20px;
  left: 200px;
  width: calc(100% - 200px);
  height: calc(100% - 20px);
  padding: 10px;
}

.settings__grid {
  display: grid;
  height: 100%;

  grid-template-columns: repeat(auto-fit, minmax(32rem, 1fr));
  grid-auto-rows: 14rem;
  grid-gap: 3rem;
}

.settings__grid-item {
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: space-evenly;
  text-align: center;
  background-color: rgba(0, 0, 0, 0.4);
  border-radius: 6px;
}

@media (max-width: 768px) {
  .nav {
    bottom: 0;
    left: 0;
    right: 0;
    top: auto;
    width: 100%;
    height: 60px;
  }
  .article {
    height: calc(100% - 80px);
    left: 0;
    width: 100%;
  }
}

```

<https://codepen.io/anon/pen/EzWmeg>

i expect the margin/padding to be consistent to what i define on parent but it doesn't and disappears / collapses