From 6870262f8d50407115160979b483a43503ee854f Mon Sep 17 00:00:00 2001
From: syuilo <syuilotan@yahoo.co.jp>
Date: Wed, 19 Aug 2020 17:52:11 +0900
Subject: [PATCH] enhance(client): Better element visible detection

---
 src/client/scripts/paging.ts |  5 ++---
 src/client/scripts/scroll.ts | 10 ++++++++--
 2 files changed, 10 insertions(+), 5 deletions(-)

diff --git a/src/client/scripts/paging.ts b/src/client/scripts/paging.ts
index 006d23875c..538615afa1 100644
--- a/src/client/scripts/paging.ts
+++ b/src/client/scripts/paging.ts
@@ -1,5 +1,4 @@
-import Vue from 'vue';
-import { getScrollPosition, onScrollTop } from './scroll';
+import { onScrollTop, isTopVisible } from './scroll';
 
 const SECOND_FETCH_LIMIT = 30;
 
@@ -148,7 +147,7 @@ export default (opts) => ({
 		},
 
 		prepend(item) {
-			const isTop = this.isBackTop || (document.body.contains(this.$el) && (getScrollPosition(this.$el) === 0));
+			const isTop = this.isBackTop || (document.body.contains(this.$el) && isTopVisible(this.$el));
 
 			if (isTop) {
 				// Prepend the item
diff --git a/src/client/scripts/scroll.ts b/src/client/scripts/scroll.ts
index bad31a80f5..18c3366891 100644
--- a/src/client/scripts/scroll.ts
+++ b/src/client/scripts/scroll.ts
@@ -13,12 +13,18 @@ export function getScrollPosition(el: Element | null): number {
 	return container == null ? window.scrollY : container.scrollTop;
 }
 
+export function isTopVisible(el: Element | null): boolean {
+	const scrollTop = getScrollPosition(el);
+	const topPosition = el.offsetTop; // TODO: container内でのelの相対位置を取得できればより正確になる
+
+	return scrollTop <= topPosition;
+}
+
 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) {
+		if (isTopVisible(el)) {
 			cb();
 			container.removeEventListener('scroll', onScroll);
 		}