fix: floor fractional layout widths to prevent horizontal scrollbar in Virtualizer

packages/@react-spectrum/ai/src/ListLayout.ts

@@ -584,7 +584,7 @@ export class ListLayout<T, O extends ListLayoutOptions = ListLayoutOptions>
     }
 
     let contentHeight = Math.max(contentLength, this.virtualizer!.size.height);
-    this.contentSize = new Size(this.virtualizer!.size.width, contentHeight);
+    this.contentSize = new Size(Math.floor(this.virtualizer!.size.width), contentHeight);
 
     // Iterate last → first so the last item in the collection (newest) is placed at the visual
     // bottom and written to nodes[0] (first in DOM) for screen-reader accessibility.

packages/react-stately/src/layout/GridLayout.ts

@@ -277,7 +277,7 @@ export class GridLayout<T, O extends GridLayoutOptions = GridLayoutOptions>
     }
 
     this.layoutInfos = newLayoutInfos;
-    this.contentSize = new Size(this.virtualizer!.size.width, y);
+    this.contentSize = new Size(Math.floor(this.virtualizer!.size.width), y);
   }
 
   getLayoutInfo(key: Key): LayoutInfo | null {

packages/react-stately/src/layout/ListLayout.ts

@@ -444,8 +444,8 @@ export class ListLayout<T, O extends ListLayoutOptions = ListLayoutOptions>
     offset += isEmptyOrLoading ? 0 : this.padding;
     this.contentSize =
       this.orientation === 'horizontal'
-        ? new Size(offset, this.virtualizer!.size.height)
-        : new Size(this.virtualizer!.size.width, offset);
+        ? new Size(offset, Math.floor(this.virtualizer!.size.height))
+        : new Size(Math.floor(this.virtualizer!.size.width), offset);
 
     return nodes;
   }
@@ -519,8 +519,8 @@ export class ListLayout<T, O extends ListLayoutOptions = ListLayoutOptions>
 
   protected buildSection(node: Node<T>, x: number, y: number): LayoutNode {
     let collection = this.virtualizer!.collection;
-    let width = this.virtualizer!.size.width - this.padding - x;
-    let height = this.virtualizer!.size.height - this.padding - y;
+    let width = Math.floor(this.virtualizer!.size.width - this.padding - x);
+    let height = Math.floor(this.virtualizer!.size.height - this.padding - y);
     let rect =
       this.orientation === 'horizontal' ? new Rect(x, y, 0, height) : new Rect(x, y, width, 0);
     let layoutInfo = new LayoutInfo(node.type, node.key, rect);
@@ -576,10 +576,10 @@ export class ListLayout<T, O extends ListLayoutOptions = ListLayoutOptions>
   protected buildSectionHeader(node: Node<T>, x: number, y: number): LayoutNode {
     let widthProperty = this.orientation === 'horizontal' ? 'height' : 'width';
     let heightProperty = this.orientation === 'horizontal' ? 'width' : 'height';
-    let width =
+    let width = Math.floor(
       this.virtualizer!.size[widthProperty] -
       this.padding -
-      (this.orientation === 'horizontal' ? y : x);
+      (this.orientation === 'horizontal' ? y : x));
     let rectHeight = this.headingSize;
     let isEstimated = false;
 
@@ -626,10 +626,10 @@ export class ListLayout<T, O extends ListLayoutOptions = ListLayoutOptions>
     let widthProperty = this.orientation === 'horizontal' ? 'height' : 'width';
     let heightProperty = this.orientation === 'horizontal' ? 'width' : 'height';
 
-    let width =
+    let width = Math.floor(
       this.virtualizer!.size[widthProperty] -
       this.padding -
-      (this.orientation === 'horizontal' ? y : x);
+      (this.orientation === 'horizontal' ? y : x));
     let rectHeight = this.rowSize;
     let isEstimated = false;
 

packages/react-stately/src/layout/WaterfallLayout.ts

@@ -247,7 +247,7 @@ export class WaterfallLayout<
       maxHeight = layoutInfo.rect.maxY;
     }
 
-    this.contentSize = new Size(this.virtualizer!.size.width, maxHeight);
+    this.contentSize = new Size(Math.floor(this.virtualizer!.size.width), maxHeight);
     this.layoutInfos = newLayoutInfos;
     this.numColumns = numColumns;
   }

packages/react-stately/test/virtualizer/ListLayout.test.ts

@@ -0,0 +1,120 @@
+/*
+ * Copyright 2026 Adobe. All rights reserved.
+ * This file is licensed to you under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License. You may obtain a copy
+ * of the License at http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software distributed under
+ * the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR REPRESENTATIONS
+ * OF ANY KIND, either express or implied. See the License for the specific language
+ * governing permissions and limitations under the License.
+ */
+
+import {Key, Node} from '@react-types/shared';
+import {ListLayout, ListLayoutOptions} from '../../src/layout/ListLayout';
+import {Rect} from '../../src/virtualizer/Rect';
+import {Size} from '../../src/virtualizer/Size';
+
+/**
+ * Creates a minimal mock virtualizer and collection, then calls layout.update()
+ * so the layout has valid internal state for contentSize verification.
+ */
+function setupListLayout(
+  options: ListLayoutOptions = {},
+  itemCount = 4,
+  viewportWidth = 250.5,
+  viewportHeight = 600
+) {
+  let layout = new ListLayout<Node<unknown>, ListLayoutOptions>();
+
+  // Build a minimal collection
+  let items: Node<unknown>[] = [];
+  for (let i = 0; i < itemCount; i++) {
+    items.push({
+      type: 'item',
+      key: `item-${i}`,
+      value: null,
+      level: 0,
+      hasChildNodes: false,
+      rendered: null,
+      textValue: `Item ${i}`,
+      'aria-label': undefined,
+      index: i,
+      parentKey: null,
+      prevKey: i > 0 ? `item-${i - 1}` : null,
+      nextKey: i < itemCount - 1 ? `item-${i + 1}` : null,
+      childNodes: [],
+      props: {}
+    } as unknown as Node<unknown>);
+  }
+
+  let collection = {
+    size: items.length,
+    getItem(key: Key) {
+      return items.find(i => i.key === key) ?? null;
+    },
+    getFirstKey() {
+      return items[0]?.key ?? null;
+    },
+    getLastKey() {
+      return items[items.length - 1]?.key ?? null;
+    },
+    getKeyBefore(key: Key) {
+      let idx = items.findIndex(i => i.key === key);
+      return idx > 0 ? items[idx - 1].key : null;
+    },
+    getKeyAfter(key: Key) {
+      let idx = items.findIndex(i => i.key === key);
+      return idx < items.length - 1 ? items[idx + 1].key : null;
+    },
+    [Symbol.iterator]() {
+      return items[Symbol.iterator]();
+    }
+  };
+
+  // Attach a mock virtualizer
+  (layout as any).virtualizer = {
+    collection,
+    visibleRect: new Rect(0, 0, viewportWidth, viewportHeight),
+    size: new Size(viewportWidth, viewportHeight),
+    isPersistedKey: () => false
+  };
+
+  // Run layout update
+  layout.update({
+    layoutOptions: {
+      rowSize: 40,
+      ...options
+    },
+    sizeChanged: true,
+    offsetChanged: false,
+    layoutOptionsChanged: true
+  });
+
+  return layout;
+}
+
+describe('ListLayout', () => {
+  it('floors the contentSize width when the viewport width is fractional', () => {
+    let layout = setupListLayout();
+    let contentSize = layout.getContentSize();
+    // Viewport width 250.5 should be rounded down to 250 so no content
+    // overflows the container, which would produce a horizontal scrollbar.
+    expect(contentSize.width).toBe(250);
+    expect(Number.isInteger(contentSize.width)).toBe(true);
+  });
+
+  it('does not change contentSize for integer viewport widths', () => {
+    let layout = setupListLayout({}, 4, 300);
+    let contentSize = layout.getContentSize();
+    expect(contentSize.width).toBe(300);
+  });
+
+  it('floors section and item rect widths to avoid fractional overflow', () => {
+    let layout = setupListLayout();
+    let itemInfo = layout.getLayoutInfo('item-0');
+    expect(itemInfo).not.toBeNull();
+    expect(itemInfo!.rect.width).toBeLessThanOrEqual(250);
+    expect(Number.isInteger(itemInfo!.rect.width)).toBe(true);
+  });
+});