fix(virtual-core): measure the visible scroll viewport

.changeset/tidy-client-viewport.md

@@ -0,0 +1,5 @@
+---
+'@tanstack/virtual-core': patch
+---
+
+Measure the scroll viewport using client dimensions so borders and scrollbars do not obscure end-aligned items.

docs/api/virtualizer.md

@@ -177,6 +177,8 @@ observeElementRect: (
 
 An optional function that if provided is called when the scrollElement changes and should implement the initial measurement and continuous monitoring of the scrollElement's `Rect` (an object with `width` and `height`). It's called with the instance (which also gives you access to the scrollElement via `instance.scrollElement`. Built-in implementations are exported as `observeElementRect` and `observeWindowRect` which are automatically configured for you by your framework adapter's exported functions like `useVirtualizer` or `useWindowVirtualizer`.
 
+The built-in `observeElementRect` measures the visible client area (`clientWidth` and `clientHeight`), excluding borders and scrollbars. Custom implementations should report the same viewport dimensions.
+
 ### `observeElementOffset`
 
 ```tsx

packages/marko-virtual/tests/setup.ts

@@ -31,8 +31,8 @@ if (!global.requestAnimationFrame) {
   })
 
 // ---------------------------------------------------------------------------
-// offsetHeight / offsetWidth — always 0 in jsdom (no CSS layout engine).
-// observeElementRect calls getRect(element) = { offsetWidth, offsetHeight }.
+// Element dimensions are always 0 in jsdom (no CSS layout engine).
+// The viewport uses client dimensions, while rows use offset dimensions.
 // Without this, the virtualizer sees a 0-height viewport and renders no items.
 // 400px ÷ 50px default estimateSize = 8 visible + 5 overscan each side ≈ 18.
 // ---------------------------------------------------------------------------
@@ -44,3 +44,12 @@ Object.defineProperty(HTMLElement.prototype, 'offsetWidth', {
   configurable: true,
   get: () => 400,
 })
+
+Object.defineProperty(HTMLElement.prototype, 'clientHeight', {
+  configurable: true,
+  get: () => 400,
+})
+Object.defineProperty(HTMLElement.prototype, 'clientWidth', {
+  configurable: true,
+  get: () => 400,
+})

packages/virtual-core/src/index.ts

@@ -74,8 +74,8 @@ export interface Rect {
 //
 
 const getRect = (element: HTMLElement): Rect => {
-  const { offsetWidth, offsetHeight } = element
-  return { width: offsetWidth, height: offsetHeight }
+  const { clientWidth, clientHeight } = element
+  return { width: clientWidth, height: clientHeight }
 }
 
 // Animation frames for `useAnimationFrameWithResizeObserver`, requested from
@@ -141,16 +141,8 @@ export const observeElementRect = <T extends Element>(
   }
 
   const frames = createFrameQueue(targetWindow)
-  const observer = new targetWindow.ResizeObserver((entries) => {
+  const observer = new targetWindow.ResizeObserver(() => {
     const run = () => {
-      const entry = entries[0]
-      if (entry?.borderBoxSize) {
-        const box = entry.borderBoxSize[0]
-        if (box) {
-          handler({ width: box.inlineSize, height: box.blockSize })
-          return
-        }
-      }
       handler(getRect(element as unknown as HTMLElement))
     }
 
@@ -159,7 +151,7 @@ export const observeElementRect = <T extends Element>(
       : run()
   })
 
-  observer.observe(element, { box: 'border-box' })
+  observer.observe(element, { box: 'content-box' })
 
   return () => {
     observer.unobserve(element)

packages/virtual-core/tests/measurement.test.ts

@@ -877,7 +877,8 @@ function createOwnWindow() {
     frames.clear()
     queued.forEach((cb) => cb(0))
   }
-  const resize = (target: Element, blockSize: number) =>
+  const resize = (target: Element, blockSize: number) => {
+    Object.assign(target, { clientHeight: blockSize })
     roCallback!(
       [
         {
@@ -887,7 +888,10 @@ function createOwnWindow() {
       ],
       {} as ResizeObserver,
     )
+  }
   const el = {
+    clientWidth: 400,
+    clientHeight: 600,
     scrollTop: 0,
     scrollLeft: 0,
     scrollWidth: 400,
@@ -994,3 +998,81 @@ test('observeElementRect defers to a frame of the scroll element window and canc
     expect(globalRaf).not.toHaveBeenCalled()
   })
 })
+
+test.each([false, true])(
+  'viewport measurements exclude borders and scrollbars (horizontal: %s)',
+  (horizontal) => {
+    const { el, resize } = createOwnWindow()
+    Object.assign(el, {
+      clientWidth: 370,
+      clientHeight: 570,
+      scrollWidth: 5000,
+    })
+    const virtualizer = new Virtualizer({
+      count: 100,
+      horizontal,
+      estimateSize: () => 50,
+      getScrollElement: () => el,
+      scrollToFn: vi.fn(),
+      observeElementRect,
+      observeElementOffset: (_instance, cb) => {
+        cb(0, false)
+      },
+    })
+    virtualizer._willUpdate()
+    expect(virtualizer.scrollRect).toEqual({ width: 370, height: 570 })
+    expect(virtualizer.getOffsetForIndex(10, 'end')?.[0]).toBe(
+      horizontal ? 180 : 0,
+    )
+    resize(el, 470)
+    expect(virtualizer.scrollRect).toEqual({ width: 370, height: 470 })
+    expect(virtualizer.getOffsetForIndex(10, 'end')?.[0]).toBe(
+      horizontal ? 180 : 80,
+    )
+    virtualizer._didMount()()
+  },
+)
+
+test('refreshes horizontal end alignment when a scrollbar toggles', () => {
+  const { el } = createOwnWindow()
+  let deliverResize: ResizeObserverCallback
+  let observedBox: ResizeObserverBoxOptions | undefined
+  el.ownerDocument.defaultView!.ResizeObserver = vi.fn(function (cb) {
+    deliverResize = cb
+    return {
+      observe: (_target: Element, options?: ResizeObserverOptions) => {
+        observedBox = options?.box
+      },
+      unobserve: vi.fn(),
+      disconnect: vi.fn(),
+    }
+  }) as unknown as typeof ResizeObserver
+  Object.assign(el, { scrollWidth: 5000 })
+  const virtualizer = new Virtualizer({
+    count: 100,
+    horizontal: true,
+    estimateSize: () => 50,
+    getScrollElement: () => el,
+    scrollToFn: vi.fn(),
+    observeElementRect,
+    observeElementOffset: (_instance, cb) => {
+      cb(0, false)
+    },
+  })
+  virtualizer._willUpdate()
+  expect(virtualizer.getOffsetForIndex(10, 'end')?.[0]).toBe(150)
+  for (const width of [385, 400]) {
+    // A classic scrollbar changes the content box, leaving the border box fixed.
+    Object.assign(el, { clientWidth: width })
+    if (observedBox !== 'border-box') {
+      deliverResize!(
+        [{ target: el } as ResizeObserverEntry],
+        {} as ResizeObserver,
+      )
+    }
+    expect(el.offsetWidth).toBe(400)
+    expect(virtualizer.scrollRect?.width).toBe(width)
+    expect(virtualizer.getOffsetForIndex(10, 'end')?.[0]).toBe(550 - width)
+  }
+  virtualizer._didMount()()
+})