fix: Avoid need to document Select "gotcha"

[devongovett]

Noticed in audit that #8419 had added docs for a "gotcha" in regards to the hidden select causing scrolling. I'd rather we fix this by default instead. In this case, we can use position: fixed on the visually hidden element so it is positioned within the viewport and doesn't cause scrolling. This works because the visually hidden element is also aria-hidden, so positioning it at the top of the page doesn't affect the screen reader cursor placement.

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[snowystinger]

This causes a problem if it's inside an element that has drag enabled. The default drag preview will include the entire bounding box union of the target + the hidden select.
In this screenshot, there is a gridlist, and the item with a picker is being dragged. As you can see, it's including the Hello text in the top left as well as the first grid item in the drag preview.