Currently, applying `box-shadow` to scrollbar pseudo-elements overflows in unexpected ways. I believe box shadows should behave like if the scrollbars pseudo-elements were organized this way:

```
<element>
	<::scrollbar>
		<::scrollbar-button></::scrollbar-button>
		<::scrollbar-track>
			<::scrollbar-track-piece :start></::scrollbar-track-piece>
			<::scrollbar-track-piece :end></::scrollbar-track-piece>
			<::scrollbar-thumb></::scrollbar-thumb>
		</::scrollbar-track>
		<::scrollbar-button></::scrollbar-button>
	</::scrollbar>
</element>
```

Allowing people to override the default overflow behavior with the `overflow` property, with `auto` and `scroll` being equivalent to `hidden`.