1 files changed, 27 insertions, 0 deletions
diff --git a/src/client/scripts/scroll.ts b/src/client/scripts/scroll.ts
new file mode 100644
index 0000000000..76881bbde1
--- /dev/null
+++ b/src/client/scripts/scroll.ts
@@ -0,0 +1,27 @@
+export function getScrollContainer(el: Element | null): Element | null {
+	if (el == null || el.tagName === 'BODY') return null;
+	const style = window.getComputedStyle(el);
+	if (style.getPropertyValue('overflow') === 'auto') {
+		return el;
+	} else {
+		return getScrollContainer(el.parentElement);
+	}
+}
+
+export function getScrollPosition(el: Element | null): number {
+	const container = getScrollContainer(el);
+	return container == null ? window.scrollY : container.scrollTop;
+}
+
+export function onScrollTop(el: Element, cb) {
+	const container = getScrollContainer(el) || window;
+	const onScroll = ev => {
+		if (!document.body.contains(el)) return;
+		const pos = getScrollPosition(el);
+		if (pos === 0) {
+			cb();
+			container.removeEventListener('scroll', onscroll);
+		}
+	};
+	container.addEventListener('scroll', onScroll, { passive: true });
+}