fix(virtual-core): Fix #1258

packages/virtual-core/src/index.ts

@@ -1707,9 +1707,16 @@ export class Virtualizer<
     }
   }
 
+  private getVirtualMaxScrollOffset = () => {
+    return Math.max(
+      this.getTotalSize() - this.options.paddingEnd - this.getSize(),
+      0,
+    )
+  }
+
   private getVirtualDistanceFromEnd = () => {
     return Math.max(
-      this.getTotalSize() - this.getSize() - this.getScrollOffset(),
+      this.getVirtualMaxScrollOffset() - this.getScrollOffset(),
       0,
     )
   }
@@ -1878,7 +1885,7 @@ export class Virtualizer<
       return
     }
 
-    this.scrollToOffset(Math.max(this.getTotalSize() - this.getSize(), 0), {
+    this.scrollToOffset(this.getVirtualMaxScrollOffset(), {
       behavior,
     })
   }

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

@@ -2879,13 +2879,15 @@ function createChatVirtualizer({
   itemSize = 50,
   followOnAppend = false,
   threshold = 1,
+  paddingEnd = 0,
 }: {
   messages: Array<{ id: string }>
   offset: number
   viewportSize?: number
   itemSize?: number
   followOnAppend?: boolean | 'auto' | 'smooth' | 'instant'
   threshold?: number
+  paddingEnd?: number
 }) {
   let currentMessages = messages
   const scrollToFn = vi.fn()
@@ -2941,6 +2943,7 @@ function createChatVirtualizer({
       anchorTo: 'end' as const,
       followOnAppend,
       scrollEndThreshold: threshold,
+      paddingEnd,
     }
   }
 
@@ -3753,3 +3756,33 @@ test('#1218: first measurement of a spanning item still compensates', () => {
 
   expect(v.scrollOffset).toBe(before + 70)
 })
+
+// ─── #1258: paddingEnd must not break anchorTo:'end' streaming growth ─────────
+// When paddingEnd > 0, getVirtualDistanceFromEnd() was including paddingEnd in
+// its calculation, making it always report a distance >= paddingEnd even when
+// the viewport was scrolled to the absolute bottom. This caused wasAtEnd to be
+// false in resizeItem, so streaming growth was never followed.
+
+test('#1258: anchorTo:end keeps a pinned streaming message pinned as it grows with paddingEnd', () => {
+  // 5 items × 50px = 250px scrollHeight, 200px viewport → maxScrollOffset = 50
+  // paddingEnd = 80, scrollEndThreshold = 0 (default)
+  // User is at the DOM bottom: scrollTop = 50, getDistanceFromEnd() = 0
+  // Without the fix: getVirtualDistanceFromEnd() = getTotalSize(330) - size(200) - offset(50)
+  //   = 80 > threshold(0), so wasAtEnd = false and resizeItem skips the
+  //   end-anchor adjustment — the viewport drifts as the item grows.
+  const messages = Array.from({ length: 5 }, (_, i) => ({ id: `m-${i}` }))
+  const { virtualizer, scrollElement, scrollToFn } = createChatVirtualizer({
+    messages,
+    offset: 50,
+    paddingEnd: 80,
+    threshold: 0,
+  })
+
+  // Last item grows from 50 → 120px (+70); scrollHeight reflects the growth
+  ;(scrollElement as any).scrollHeight = 320
+  virtualizer.resizeItem(4, 120)
+
+  // resizeItem end-anchor must fire the scroll adjustment
+  expect(scrollToFn).toHaveBeenCalledTimes(1)
+  expect(scrollToFn.mock.calls[0]![1].adjustments).toBe(70)
+})