refactor: use overflow-y to determine scroll container

By using `overflow-y` instead of `overflow` using `endsWith` can be
avoided and represents the data we are actually interested in here
more accurately.

packages/client/src/scripts/scroll.ts

@@ -2,12 +2,8 @@ type ScrollBehavior = 'auto' | 'smooth' | 'instant';
 
 export function getScrollContainer(el: HTMLElement | null): HTMLElement | null {
 	if (el == null || el.tagName === 'HTML') return null;
-	const overflow = window.getComputedStyle(el).getPropertyValue('overflow');
-	if (
-		// xとyを個別に指定している場合、`hidden scroll`みたいな値になる
-		overflow.endsWith('scroll') ||
-		overflow.endsWith('auto')
-	) {
+	const overflow = window.getComputedStyle(el).getPropertyValue('overflow-y');
+	if (overflow === 'scroll' || overflow === 'auto') {
 		return el;
 	} else {
 		return getScrollContainer(el.parentElement);