fix: initialItemCount no longer blocks data change

Fixes #801

examples/grid-initial-item-count.tsx

@@ -0,0 +1,31 @@
+import * as React from 'react'
+import { VirtuosoGrid } from '../src'
+
+function generateItems(length: number) {
+  return Array.from({ length }, (_, index) => `My Item ${index}`)
+}
+
+const itemContent = (_: number, data: string) => {
+  return <div style={{ height: 30 }}>{data}</div>
+}
+
+export function Example() {
+  const [data, setData] = React.useState(() => generateItems(100))
+
+  const onEndReached = () => {
+    setData((prevData) => {
+      return generateItems(prevData.length + 100)
+    })
+  }
+
+  return (
+    <VirtuosoGrid
+      useWindowScroll
+      initialItemCount={100}
+      endReached={onEndReached}
+      data={data}
+      itemContent={itemContent}
+      style={{ height: 300 }}
+    />
+  )
+}

src/gridSystem.ts

@@ -99,8 +99,9 @@ export const gridSystem = /*#__PURE__*/ u.system(
 
     u.connect(
       u.pipe(
-        u.combineLatest(didMount, initialItemCount, data),
-        u.filter(([, count]) => count !== 0),
+        didMount,
+        u.withLatestFrom(initialItemCount, data),
+        u.filter(([didMount, count]) => didMount && count !== 0),
         u.map(([, count, data]) => {
           return {
             items: buildItems(0, count - 1, data),
@@ -188,12 +189,25 @@ export const gridSystem = /*#__PURE__*/ u.system(
       listBoundary
     )
 
+    const hasScrolled = u.statefulStream(false)
+
+    u.connect(
+      u.pipe(
+        scrollTop,
+        u.withLatestFrom(hasScrolled),
+        u.map(([scrollTop, hasScrolled]) => {
+          return hasScrolled || scrollTop !== 0
+        })
+      ),
+      hasScrolled
+    )
+
     const endReached = u.streamFromEmitter(
       u.pipe(
         u.duc(gridState),
         u.filter(({ items }) => items.length > 0),
-        u.withLatestFrom(totalCount),
-        u.filter(([{ items }, totalCount]) => items[items.length - 1].index === totalCount - 1),
+        u.withLatestFrom(totalCount, hasScrolled),
+        u.filter(([{ items }, totalCount, hasScrolled]) => hasScrolled && items[items.length - 1].index === totalCount - 1),
         u.map(([, totalCount]) => totalCount - 1),
         u.distinctUntilChanged()
       )