Trap 1: a clipping scroll container
overflow: hidden; height: 200px. A menu rendered as a child of this box
would be cut off at the fold. In the top layer it is not clipped at all.
Trap 2: a composited ancestor
transform: translateZ(0) makes this box the containing block for any
position: fixed descendant, so a fixed menu inside it would land at the
wrong coordinates. A popover in the top layer has no such ancestor.
Trap 3: the viewport corner
The target is pinned inside 50px of both the inline-end and the block-end edge, so the menu has room in neither axis and must flip on both.