Add instant behavior to synced tab scrolling (#2747)

Co-authored-by: Chris Swithinbank <swithinbank@gmail.com>

Author: bbag

.changeset/chilly-paws-smell.md

@@ -0,0 +1,5 @@
+---
+"@astrojs/starlight": patch
+---
+
+Ensures `<Tab>` component toggling is stable when smooth scrolling is enabled via custom CSS

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',
 				});
 			}
 		}