Fixed extra spacing introduced by the scrollbar layout fix

The scrollbar fix we shipped earlier this month, to stop the layout shifting when a scrollbar appears or disappears, was being applied too broadly: it reserved scrollbar space on every individual element, not just the page itself, adding subtle extra spacing inside nested scrollable areas like tables, dropdowns, and modals. It’s now scoped correctly to just the page, so nested scrollable areas no longer have that extra gap.