Reveals an interactive popout menu on hover and focus.

<div class="reference" tabindex="0"><div class="popout-menu">Popout menu</div></div>

.reference {
  position: relative;
  background: tomato;
  width: 100px;
  height: 100px;
}
.popout-menu {
  position: absolute;
  visibility: hidden;
  left: 100%;
  background: #333;
  color: white;
  padding: 15px;
}
.reference:hover > .popout-menu,
.reference:focus > .popout-menu,
.reference:focus-within > .popout-menu {
  visibility: visible;
}

Explanation

1. position: relative on the reference parent establishes a Cartesian positioning context for its child.
2. position: absolute takes the popout menu out of the flow of the document and positions it in relation to the parent.
3. left: 100% moves the the popout menu 100% of its parent’s width from the left.
4. visibility: hidden hides the popout menu initially and allows for transitions (unlike display: none).
5. .reference:hover > .popout-menu means that when .reference is hovered over, select immediate children with a class of .popout-menu and change their visibility to visible, which shows the popout.
6. .reference:focus > .popout-menu means that when .reference is focused, the popout would be shown.
7. .reference:focus-within > .popout-menu ensures that the popout is shown when the focus is within the reference.

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