fix(virtual-core): apply the whole shrink while a clamped end write is pending

If an end-pinned item grew before the sizer did, its compensation write is
clamped and waits to be retried once the sizer grows. A scroll element shrink
in that window was capped at the DOM distance to the end, which is stale
until the sizer commits, so the shrink was dropped and the retry landed short
of the new end. Apply the whole shrink while a clamped write is pending; the
retry lands it.

Author: langleyd

packages/virtual-core/src/index.ts

@@ -903,8 +903,13 @@ export class Virtualizer<
             this.getVirtualDistanceFromEnd() <= this.options.scrollEndThreshold
           this.scrollRect = rect
           if (wasAtEnd && prevSize > this.getSize()) {
+            // While a clamped write waits for the sizer to grow, the DOM end
+            // is stale; apply the whole shrink and let the retry land it.
+            const shrink = prevSize - this.getSize()
             this.applyScrollAdjustment(
-              Math.min(prevSize - this.getSize(), this.getDistanceFromEnd()),
+              this._clampedAdjustment !== null
+                ? shrink
+                : Math.min(shrink, this.getDistanceFromEnd()),
             )
           }
           this.maybeNotify()

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

@@ -3318,6 +3318,22 @@ test('anchorTo:end keeps the distance from the end when pinned within the thresh
   expect(virtualizer.getVirtualDistanceFromEnd()).toBe(5)
 })
 
+test('anchorTo:end reaches the end when the scroll element shrinks while a clamped write waits for the sizer', () => {
+  const messages = Array.from({ length: 10 }, (_, i) => ({ id: `m-${i}` }))
+  const { virtualizer, scrollElement, scrollToFn, resizeViewport } =
+    createChatVirtualizer({ messages, offset: 300 })
+
+  // The last item grows before the sizer does, so the end write is clamped.
+  virtualizer.resizeItem(9, 120)
+  resizeViewport(150)
+  // The sizer commits and the clamped write is retried.
+  ;(scrollElement as any).scrollHeight = 570
+  virtualizer._willUpdate()
+
+  const [offset, { adjustments }] = scrollToFn.mock.calls.at(-1)!
+  expect(offset + (adjustments ?? 0)).toBe(420)
+})
+
 test.each(['scroll element first', 'item first'])(
   'anchorTo:end stays pinned when an item above the viewport shrinks in the same frame as the scroll element (%s)',
   (order) => {