Dropdown has no caret inside a parent with background

When a `Dropdown` component is inside of any element that is above its parent and has a `background-color`, the caret on the right doesn't show. It's very clearly because the `.hig__dropdown__input-caret` element is assigned `z-index: -1`, which places it under the parent element.

Here's a screenshot to demonstrate (the dropdowns on the right don't show carets):

Is there any reason that a `z-index` is assigned to the caret?

Start by inspecting the .hig__dropdown__input-caret style and reproduce the issue with a Dropdown inside an element that has a background-color. Check the stacking behavior around its z-index, then verify that carets remain visible in the reported parent-background scenario.

Written by the indexing model from the issue text.