Update backdrop to support table loading state

[duncanuszkay-d2l]

Downstream ticket
Upstream ticket

We want to add a visual loading state to the table in the event that the data needs to be refreshed. This can occur after filters are applied, sort orders are changed, and possibly in future when we move to a new page.

In this PR, I'm updating the existing backdrop component in a few ways to make it compatible with this solution:

• I'm adding a contained option which limits the backdrop to the containing block instead of attaching it to the viewport
• When the backdrop is contained, it doesn't prevent body scrolling
• When aria-hiding elements, it now stops at the first positioned element rather than going all the way up to the document itself

Video Demo

Recording.2026-02-06.134345.mp4

[github-actions]

Thanks for the PR! 🎉

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[duncanuszkay-d2l]

One possible alternative is to either disable the aria-hiding stage entirely when the contained option is set, or possibly add another option to disable aria-hiding, but I felt that this option to hide within the container was the most intuitive for the downstream consumer.