From e499ea0ed66b31964f79261b41f58a288b0cdb6f Mon Sep 17 00:00:00 2001
From: Hailey <me@haileyok.com>
Date: Thu, 4 Jul 2024 09:20:33 -0700
Subject: [PATCH] handle off screen visibility observer.

---
 src/view/com/notifications/Feed.tsx |  11 +--
 src/view/com/util/List.web.tsx      | 119 +++++++++++++++++-----------
 2 files changed, 73 insertions(+), 57 deletions(-)

diff --git a/src/view/com/notifications/Feed.tsx b/src/view/com/notifications/Feed.tsx
index 90f2785b21..e2f12e84f1 100644
--- a/src/view/com/notifications/Feed.tsx
+++ b/src/view/com/notifications/Feed.tsx
@@ -25,7 +25,6 @@ import {LoadMoreRetryBtn} from '#/view/com/util/LoadMoreRetryBtn'
 import {CenteredView} from '#/view/com/util/Views'
 import {FeedItem} from './FeedItem'
 import hairlineWidth = StyleSheet.hairlineWidth
-import {isWeb} from '#/platform/detection'
 
 const EMPTY_FEED_ITEM = {_reactKey: '__empty__'}
 const LOAD_MORE_ERROR_ITEM = {_reactKey: '__load_more_error__'}
@@ -183,15 +182,7 @@ export function Feed({
         refreshing={isPTRing}
         onRefresh={onRefresh}
         onEndReached={onEndReached}
-        onEndReachedThreshold={
-          /*
-          NOTE:
-          web's intersection observer struggles with the 2x threshold
-          and leads to missed pagination, so we keep it <1
-          -prf
-          */
-          isWeb ? 0.6 : 2
-        }
+        onEndReachedThreshold={2}
         onScrolledDownChange={onScrolledDownChange}
         contentContainerStyle={s.contentContainer}
         // @ts-ignore our .web version only -prf
diff --git a/src/view/com/util/List.web.tsx b/src/view/com/util/List.web.tsx
index e917ab1d32..6493abf652 100644
--- a/src/view/com/util/List.web.tsx
+++ b/src/view/com/util/List.web.tsx
@@ -63,53 +63,7 @@ function ListImpl<ItemT>(
   }: ListProps<ItemT>,
   ref: React.Ref<ListMethods>,
 ) {
-  const contextScrollHandlers = useScrollHandlers()
-  const pal = usePalette('default')
-  const {isMobile} = useWebMediaQueries()
-  if (!isMobile) {
-    contentContainerStyle = addStyle(
-      contentContainerStyle,
-      styles.containerScroll,
-    )
-  }
-
-  const isEmpty = !data || data.length === 0
-
-  let headerComponent: JSX.Element | null = null
-  if (ListHeaderComponent != null) {
-    if (isValidElement(ListHeaderComponent)) {
-      headerComponent = ListHeaderComponent
-    } else {
-      // @ts-ignore Nah it's fine.
-      headerComponent = <ListHeaderComponent />
-    }
-  }
-
-  let footerComponent: JSX.Element | null = null
-  if (ListFooterComponent != null) {
-    if (isValidElement(ListFooterComponent)) {
-      footerComponent = ListFooterComponent
-    } else {
-      // @ts-ignore Nah it's fine.
-      footerComponent = <ListFooterComponent />
-    }
-  }
-
-  let emptyComponent: JSX.Element | null = null
-  if (ListEmptyComponent != null) {
-    if (isValidElement(ListEmptyComponent)) {
-      emptyComponent = ListEmptyComponent
-    } else {
-      // @ts-ignore Nah it's fine.
-      emptyComponent = <ListEmptyComponent />
-    }
-  }
-
-  if (headerOffset != null) {
-    style = addStyle(style, {
-      paddingTop: headerOffset,
-    })
-  }
+  const [prevDataLength, setPrevDataLength] = React.useState(data?.length)
 
   const getScrollableNode = React.useCallback(() => {
     if (containWeb) {
@@ -184,6 +138,77 @@ function ListImpl<ItemT>(
     }
   }, [containWeb])
 
+  // Whenever we get new data, we want to make sure that the onEndReached threshold will be *below* the current scroll
+  // bottom position. If it is not the IntersectionObserver will never fire the onEndReached threshold for us - since
+  // we've already scrolled past the component.
+  const callOnEndReachedIfNeeded = () => {
+    const node = getScrollableNode()
+    if (!node || !onEndReached || !onEndReachedThreshold || !data?.length) {
+      return
+    }
+
+    const scrollViewHeight = node.clientHeight
+    const scrollContainerHeight = node.scrollHeight
+    const scrollPos = node.scrollY
+    const thresholdPx = onEndReachedThreshold * scrollViewHeight
+
+    if (scrollContainerHeight - thresholdPx <= scrollPos) {
+      onEndReached({distanceFromEnd: 0})
+    }
+    setPrevDataLength(data?.length)
+  }
+  if (data?.length !== prevDataLength) {
+    callOnEndReachedIfNeeded()
+  }
+
+  const contextScrollHandlers = useScrollHandlers()
+  const pal = usePalette('default')
+  const {isMobile} = useWebMediaQueries()
+  if (!isMobile) {
+    contentContainerStyle = addStyle(
+      contentContainerStyle,
+      styles.containerScroll,
+    )
+  }
+
+  const isEmpty = !data || data.length === 0
+
+  let headerComponent: JSX.Element | null = null
+  if (ListHeaderComponent != null) {
+    if (isValidElement(ListHeaderComponent)) {
+      headerComponent = ListHeaderComponent
+    } else {
+      // @ts-ignore Nah it's fine.
+      headerComponent = <ListHeaderComponent />
+    }
+  }
+
+  let footerComponent: JSX.Element | null = null
+  if (ListFooterComponent != null) {
+    if (isValidElement(ListFooterComponent)) {
+      footerComponent = ListFooterComponent
+    } else {
+      // @ts-ignore Nah it's fine.
+      footerComponent = <ListFooterComponent />
+    }
+  }
+
+  let emptyComponent: JSX.Element | null = null
+  if (ListEmptyComponent != null) {
+    if (isValidElement(ListEmptyComponent)) {
+      emptyComponent = ListEmptyComponent
+    } else {
+      // @ts-ignore Nah it's fine.
+      emptyComponent = <ListEmptyComponent />
+    }
+  }
+
+  if (headerOffset != null) {
+    style = addStyle(style, {
+      paddingTop: headerOffset,
+    })
+  }
+
   const nativeRef = React.useRef<HTMLDivElement>(null)
   React.useImperativeHandle(
     ref,