From e582713e23cb8845c123bf301392eaa870617d39 Mon Sep 17 00:00:00 2001
From: Dan Abramov <dan.abramov@gmail.com>
Date: Sat, 23 Dec 2023 18:16:44 +0000
Subject: [PATCH] Snap the header scroll on the web

---
 src/lib/ScrollContext.tsx                | 23 +++++++++++---
 src/view/com/util/List.web.tsx           | 20 ++++++++++--
 src/view/com/util/MainScrollProvider.tsx | 40 +++++++++++++++++-------
 3 files changed, 64 insertions(+), 19 deletions(-)

diff --git a/src/lib/ScrollContext.tsx b/src/lib/ScrollContext.tsx
index 00b197bedc..447db74e0e 100644
--- a/src/lib/ScrollContext.tsx
+++ b/src/lib/ScrollContext.tsx
@@ -1,17 +1,28 @@
 import React, {createContext, useContext, useMemo} from 'react'
-import {ScrollHandlers} from 'react-native-reanimated'
+import {ScrollHandler} from 'react-native-reanimated'
+import {NativeScrollEvent} from 'react-native'
 
-const ScrollContext = createContext<ScrollHandlers<any>>({
+type ScrollHandlers = {
+  onBeginDrag: undefined | ScrollHandler
+  onEndDrag: undefined | ScrollHandler<any>
+  onScroll: undefined | ScrollHandler<any>
+  onScrollEndWeb:
+    | undefined
+    | ((e: Pick<NativeScrollEvent, 'contentOffset'>) => void) // Web-only.
+}
+
+const ScrollContext = createContext<ScrollHandlers>({
   onBeginDrag: undefined,
   onEndDrag: undefined,
   onScroll: undefined,
+  onScrollEndWeb: undefined,
 })
 
-export function useScrollHandlers(): ScrollHandlers<any> {
+export function useScrollHandlers(): ScrollHandlers {
   return useContext(ScrollContext)
 }
 
-type ProviderProps = {children: React.ReactNode} & ScrollHandlers<any>
+type ProviderProps = {children: React.ReactNode} & ScrollHandlers
 
 // Note: this completely *overrides* the parent handlers.
 // It's up to you to compose them with the parent ones via useScrollHandlers() if needed.
@@ -20,14 +31,16 @@ export function ScrollProvider({
   onBeginDrag,
   onEndDrag,
   onScroll,
+  onScrollEndWeb,
 }: ProviderProps) {
   const handlers = useMemo(
     () => ({
       onBeginDrag,
       onEndDrag,
       onScroll,
+      onScrollEndWeb,
     }),
-    [onBeginDrag, onEndDrag, onScroll],
+    [onBeginDrag, onEndDrag, onScroll, onScrollEndWeb],
   )
   return (
     <ScrollContext.Provider value={handlers}>{children}</ScrollContext.Provider>
diff --git a/src/view/com/util/List.web.tsx b/src/view/com/util/List.web.tsx
index 10c515f2ac..d29b912b19 100644
--- a/src/view/com/util/List.web.tsx
+++ b/src/view/com/util/List.web.tsx
@@ -109,7 +109,7 @@ function ListImpl<ItemT>(
   const containerRef = useRef(null)
   useResizeObserver(containerRef, onContentSizeChange)
 
-  // --- onScroll ---
+  // --- onScroll & onScrollEndWeb ---
   const [isInsideVisibleTree, setIsInsideVisibleTree] = React.useState(false)
   const handleWindowScroll = useNonReactiveCallback(() => {
     if (isInsideVisibleTree) {
@@ -124,6 +124,16 @@ function ListImpl<ItemT>(
       )
     }
   })
+  const handleWindowScrollEnd = useNonReactiveCallback(() => {
+    if (isInsideVisibleTree) {
+      contextScrollHandlers.onScrollEndWeb?.({
+        contentOffset: {
+          x: window.scrollX,
+          y: window.scrollY,
+        },
+      })
+    }
+  })
   React.useEffect(() => {
     if (!isInsideVisibleTree) {
       // Prevents hidden tabs from firing scroll events.
@@ -131,8 +141,12 @@ function ListImpl<ItemT>(
       return
     }
     window.addEventListener('scroll', handleWindowScroll)
-    return () => window.removeEventListener('scroll', handleWindowScroll)
-  }, [isInsideVisibleTree, handleWindowScroll])
+    window.addEventListener('scrollend', handleWindowScrollEnd)
+    return () => {
+      window.removeEventListener('scroll', handleWindowScroll)
+      window.removeEventListener('scrollend', handleWindowScrollEnd)
+    }
+  }, [isInsideVisibleTree, handleWindowScroll, handleWindowScrollEnd])
 
   // --- onScrolledDownChange ---
   const isScrolledDown = useRef(false)
diff --git a/src/view/com/util/MainScrollProvider.tsx b/src/view/com/util/MainScrollProvider.tsx
index 15aefb5659..d3dd185f02 100644
--- a/src/view/com/util/MainScrollProvider.tsx
+++ b/src/view/com/util/MainScrollProvider.tsx
@@ -28,6 +28,21 @@ export function MainScrollProvider({children}: {children: React.ReactNode}) {
     }
   })
 
+  const snapToClosestMode = React.useCallback(
+    (scrollY: number) => {
+      startDragOffset.value = null
+      startMode.value = null
+      if (scrollY < headerHeight.value / 2) {
+        // If we're close to the top, show the shell.
+        setMode(false)
+      } else {
+        // Snap to whichever state is the closest.
+        setMode(Math.round(mode.value) === 1)
+      }
+    },
+    [startDragOffset, startMode, headerHeight, mode, setMode],
+  )
+
   const onBeginDrag = useCallback(
     (e: NativeScrollEvent) => {
       'worklet'
@@ -40,17 +55,19 @@ export function MainScrollProvider({children}: {children: React.ReactNode}) {
   const onEndDrag = useCallback(
     (e: NativeScrollEvent) => {
       'worklet'
-      startDragOffset.value = null
-      startMode.value = null
-      if (e.contentOffset.y < headerHeight.value / 2) {
-        // If we're close to the top, show the shell.
-        setMode(false)
-      } else {
-        // Snap to whichever state is the closest.
-        setMode(Math.round(mode.value) === 1)
-      }
+      snapToClosestMode(e.contentOffset.y)
     },
-    [startDragOffset, startMode, setMode, mode, headerHeight],
+    [snapToClosestMode],
+  )
+
+  // On the web, we don't get the begin/end drag events,
+  // but we still need to expand or collapse the header.
+  const onScrollEndWeb = useCallback(
+    (e: Pick<NativeScrollEvent, 'contentOffset'>) => {
+      'worklet'
+      snapToClosestMode(e.contentOffset.y)
+    },
+    [snapToClosestMode],
   )
 
   const onScroll = useCallback(
@@ -100,7 +117,8 @@ export function MainScrollProvider({children}: {children: React.ReactNode}) {
     <ScrollProvider
       onBeginDrag={onBeginDrag}
       onEndDrag={onEndDrag}
-      onScroll={onScroll}>
+      onScroll={onScroll}
+      onScrollEndWeb={onScrollEndWeb}>
       {children}
     </ScrollProvider>
   )