Add instant behavior to synced tab scrolling

This forces the scroll behavior to be instant whenever clicking a synced tab triggers a window scroll, which fixes the odd "smooth scroll jump" on Starlight sites which use `html { scroll-behavior: smooth; }` in their global CSS. This provides a nicer UX on sites which use smooth scrolling, where e.g. clicking an item in the ToC smooth scrolls to that section on the page, but clicking a new synced tab doesn't cause an awkward jump as it scrolls to the new tab's position. It should be a non-breaking change to existing Starlight sites since the default scroll behavior of the window is already instant. 🙂

packages/starlight/user-components/Tabs.astro

@@ -224,6 +224,7 @@ if (isSynced) {
 				StarlightTabs.#syncTabs(this, newTab);
 				window.scrollTo({
 					top: window.scrollY + (this.getBoundingClientRect().top - previousTabsOffset),
+					behavior: 'instant',
 				});
 			}
 		}