dialog should prevent scrolling when open

[keithamus]

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What are you trying to accomplish?

Another regression from #2496 is that, when open, the background is still scroll-able. (See whatwg/html#7732 for more). Our <modal-dialog> element hacks this by setting overflow:hidden when the dialog is open. This code fell out during #2496, this PR adds similar code back in.

(This is marked skip changeset as it fixes a regression in the pre-release #2492)

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• Fixes axe scan violation - This change fixes an existing axe scan violation.
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• Added/updated tests
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• Tested in Chrome
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⚠️ Visual differences found

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