fix: initialScrollTop not working w/o initialItemCount

Fixes #883

e2e/initial-scroll-top.test.ts

@@ -8,6 +8,7 @@ test.describe('initial scroll top', () => {
   })
 
   test('starts from 50px', async ({ page }) => {
+    await page.waitForTimeout(100)
     const scrollTop = await page.evaluate(() => {
       const scroller = document.querySelectorAll('[data-test-id=virtuoso-scroller]')[0]
       return scroller.scrollTop

examples/initial-scroll-top.tsx

@@ -5,7 +5,6 @@ export function Example() {
   return (
     <Virtuoso
       computeItemKey={(key) => `item-${key}`}
-      initialItemCount={30}
       totalCount={100}
       initialScrollTop={50}
       itemContent={(index) => <div style={{ height: 30 }}>Item {index}</div>}

examples/initial-topmost-item.tsx

@@ -3,7 +3,6 @@ import { useState } from 'react'
 import { IndexLocationWithAlign, Virtuoso } from '../src'
 import { IndexLocation } from '../src/scrollToIndexSystem'
 
-// @ts-expect-error I know
 globalThis['VIRTUOSO_LOG_LEVEL'] = 0
 
 const itemContent = (index: number) => <div style={{ height: index % 2 ? 30 : 20, background: 'white' }}>Item {index}</div>

examples/iti-multiple.tsx

@@ -0,0 +1,51 @@
+import * as React from 'react'
+import {  Virtuoso } from '../src'
+
+const itemContent = (index: number) => <div style={{ height: index % 2 ? 30 : 20, background: 'white' }}>Item {index}</div>
+
+export function App() {
+  const data = Array(50)
+  .fill(undefined)
+  .map((_, i) => i);
+
+  return (
+    <div className="App">
+      <div style={{ display: "flex", flexDirection: "row", gap: 10 }}>
+        {Array(20)
+          .fill(undefined)
+          .map((_, i) => (
+            <Virtuoso
+              key={i}
+              style={{
+                flex: 1,
+                height: 400,
+                border: "2px black solid",
+              }}
+              data={data}
+              itemContent={(_, i) => (
+                <div
+                  style={{ backgroundColor: i == 0 ? "red" : "transparent" }}
+                >
+                  {i}
+                </div>
+              )}
+              //initialScrollTop={200}
+              initialTopMostItemIndex={5}
+            />
+          ))}
+      </div>
+    </div>
+  );
+}
+export function Example() {
+
+  return (
+    <>
+      <div style={{ display: 'flex', flexDirection: 'row', height: 400}}>
+        {Array.from({length: 30}).map((_, key) => {
+          return <Virtuoso key={key} totalCount={100} itemContent={itemContent} initialTopMostItemIndex={20} style={{ height: 300, minWidth: '3rem', fontSize: '7px', flex: 1 }} />
+        })}
+      </div>
+    </>
+  )
+}

package.json

@@ -88,7 +88,7 @@
     "vite": "^4.0.0",
     "vitest": "^0.26.2"
   },
-  "packageManager": "pnpm@7.21.0",
+  "packageManager": "pnpm@8.3.0",
   "resolutions": {
     "@types/react": "^16.14.2"
   },