perf(virtual-core): skip sync DOM reads during normal scrolling

packages/react-virtual/e2e/app/scroll/main.tsx

@@ -21,10 +21,14 @@ const randomHeight = (() => {
 
 const App = () => {
   const parentRef = React.useRef<HTMLDivElement>(null)
+  const initialOffset = Number(
+    new URLSearchParams(window.location.search).get('initialOffset') ?? 0,
+  )
   const rowVirtualizer = useVirtualizer({
     count: 1002,
     getScrollElement: () => parentRef.current,
     estimateSize: () => 50,
+    initialOffset,
     debug: true,
   })
 

packages/react-virtual/e2e/app/test/scroll.spec.ts

@@ -51,6 +51,66 @@ test('scrolls to last item', async ({ page }) => {
   expect(atBottom).toBeLessThan(1.01)
 })
 
+test('renders correctly with initialOffset and user scroll up', async ({
+  page,
+}) => {
+  // Start at offset 5000 (no programmatic scrollToIndex)
+  await page.goto('/scroll/?initialOffset=5000')
+  await page.waitForTimeout(500)
+
+  // Items around offset 5000 should be visible
+  const visibleIndex = await page.evaluate(() => {
+    const container = document.querySelector('#scroll-container')
+    if (!container) throw new Error('Container not found')
+    const items = container.querySelectorAll('[data-index]')
+    const indices = Array.from(items).map((el) =>
+      Number(el.getAttribute('data-index')),
+    )
+    return Math.min(...indices)
+  })
+  expect(visibleIndex).toBeGreaterThan(0)
+
+  // Scroll up by 2000px (user scroll, not programmatic)
+  await page.evaluate(() => {
+    const container = document.querySelector('#scroll-container')
+    if (!container) throw new Error('Container not found')
+    container.scrollTop -= 2000
+  })
+  await page.waitForTimeout(500)
+
+  // After scroll up, items should be properly measured and positioned
+  // (no gaps, no overlaps) — verify consecutive items are contiguous
+  const layout = await page.evaluate(() => {
+    const container = document.querySelector('#scroll-container')
+    if (!container) throw new Error('Container not found')
+    const items = Array.from(container.querySelectorAll('[data-index]'))
+      .map((el) => {
+        const rect = el.getBoundingClientRect()
+        return {
+          index: Number(el.getAttribute('data-index')),
+          top: rect.top,
+          bottom: rect.bottom,
+          height: rect.height,
+        }
+      })
+      .sort((a, b) => a.index - b.index)
+
+    // Check that each item's top matches the previous item's bottom (within tolerance)
+    let maxGap = 0
+    for (let i = 1; i < items.length; i++) {
+      const gap = Math.abs(items[i].top - items[i - 1].bottom)
+      maxGap = Math.max(maxGap, gap)
+    }
+
+    return { items, maxGap }
+  })
+
+  expect(layout.items.length > 0).toBe(true)
+  expect(layout.items.length).toBeGreaterThan(3)
+  // Items should be contiguous — no gaps between consecutive items
+  expect(layout.maxGap).toBeLessThan(2)
+})
+
 test('scrolls to index 0', async ({ page }) => {
   await page.goto('/scroll/')
 

packages/react-virtual/tests/index.test.tsx

@@ -1,6 +1,6 @@
 import { beforeEach, test, expect, vi } from 'vitest'
 import * as React from 'react'
-import { render, screen, fireEvent } from '@testing-library/react'
+import { render, screen } from '@testing-library/react'
 
 import { useVirtualizer, Range } from '../src/index'
 
@@ -138,29 +138,6 @@ test('should render given dynamic size', async () => {
   expect(renderer).toHaveBeenCalledTimes(3)
 })
 
-test('should render given dynamic size after scroll', () => {
-  render(<List itemSize={100} dynamic />)
-
-  expect(screen.queryByText('Row 0')).toBeInTheDocument()
-  expect(screen.queryByText('Row 1')).toBeInTheDocument()
-  expect(screen.queryByText('Row 2')).toBeInTheDocument()
-  expect(screen.queryByText('Row 3')).not.toBeInTheDocument()
-
-  expect(renderer).toHaveBeenCalledTimes(3)
-  renderer.mockReset()
-
-  fireEvent.scroll(screen.getByTestId('scroller'), {
-    target: { scrollTop: 400 },
-  })
-
-  expect(screen.queryByText('Row 2')).not.toBeInTheDocument()
-  expect(screen.queryByText('Row 3')).toBeInTheDocument()
-  expect(screen.queryByText('Row 6')).toBeInTheDocument()
-  expect(screen.queryByText('Row 7')).not.toBeInTheDocument()
-
-  expect(renderer).toHaveBeenCalledTimes(2)
-})
-
 test('should use rangeExtractor', () => {
   render(<List rangeExtractor={() => [0, 1]} />)
 

packages/virtual-core/src/index.ts

@@ -409,7 +409,21 @@ export class Virtualizer<
       return (_ro = new this.targetWindow.ResizeObserver((entries) => {
         entries.forEach((entry) => {
           const run = () => {
-            this._measureElement(entry.target as TItemElement, entry)
+            const node = entry.target as TItemElement
+            const index = this.indexFromElement(node)
+
+            if (!node.isConnected) {
+              this.observer.unobserve(node)
+              this.elementsCache.delete(this.options.getItemKey(index))
+              return
+            }
+
+            if (this.shouldMeasureDuringScroll(index)) {
+              this.resizeItem(
+                index,
+                this.options.measureElement(node, entry, this),
+              )
+            }
           }
           this.options.useAnimationFrameWithResizeObserver
             ? requestAnimationFrame(run)
@@ -984,21 +998,19 @@ export class Virtualizer<
     return true
   }
 
-  private _measureElement = (
-    node: TItemElement,
-    entry: ResizeObserverEntry | undefined,
-  ) => {
-    if (!node.isConnected) {
-      this.observer.unobserve(node)
+  measureElement = (node: TItemElement | null) => {
+    if (!node) {
+      this.elementsCache.forEach((cached, key) => {
+        if (!cached.isConnected) {
+          this.observer.unobserve(cached)
+          this.elementsCache.delete(key)
+        }
+      })
       return
     }
 
     const index = this.indexFromElement(node)
-    const item = this.measurementsCache[index]
-    if (!item) {
-      return
-    }
-    const key = item.key
+    const key = this.options.getItemKey(index)
     const prevNode = this.elementsCache.get(key)
 
     if (prevNode !== node) {
@@ -1009,16 +1021,21 @@ export class Virtualizer<
       this.elementsCache.set(key, node)
     }
 
-    if (this.shouldMeasureDuringScroll(index)) {
-      this.resizeItem(index, this.options.measureElement(node, entry, this))
+    // Sync-measure when idle (initial render) or during programmatic scrolling
+    // (scrollToIndex/scrollToOffset) where reconcileScroll needs sizes in the same frame.
+    // During normal user scrolling, skip sync measurement — the RO callback handles it async.
+    if (
+      (!this.isScrolling || this.scrollState) &&
+      this.shouldMeasureDuringScroll(index)
+    ) {
+      this.resizeItem(index, this.options.measureElement(node, undefined, this))
     }
   }
 
   resizeItem = (index: number, size: number) => {
     const item = this.measurementsCache[index]
-    if (!item) {
-      return
-    }
+    if (!item) return
+
     const itemSize = this.itemSizeCache.get(item.key) ?? item.size
     const delta = size - itemSize
 
@@ -1045,20 +1062,6 @@ export class Virtualizer<
     }
   }
 
-  measureElement = (node: TItemElement | null | undefined) => {
-    if (!node) {
-      this.elementsCache.forEach((cached, key) => {
-        if (!cached.isConnected) {
-          this.observer.unobserve(cached)
-          this.elementsCache.delete(key)
-        }
-      })
-      return
-    }
-
-    this._measureElement(node, undefined)
-  }
-
   getVirtualItems = memo(
     () => [this.getVirtualIndexes(), this.getMeasurements()],
     (indexes, measurements) => {