fix(virtualizer): preserve subpixel precision in ScrollView to preven…

packages/react-aria-components/stories/ListBox.stories.tsx

@@ -1146,3 +1146,38 @@ export const DropOntoRoot = () => (
     <DroppableListBox />
   </div>
 );
+
+export const FractionalWidth: StoryFn = () => {
+  let items = Array.from({length: 50}, (_, i) => ({id: i, name: `Item ${i + 1}`}));
+  return (
+    <div
+      style={{
+        display: 'flex',
+        width: 501,
+        border: '1px solid gray'
+      }}>
+      <div style={{width: '50%'}}>
+        <Virtualizer layout={ListLayout} layoutOptions={{rowHeight: 32}}>
+          <ListBox
+            aria-label="Fractional width list 1"
+            className={styles.menu}
+            style={{height: 300}}
+            items={items}>
+            {item => <MyListBoxItem>{item.name}</MyListBoxItem>}
+          </ListBox>
+        </Virtualizer>
+      </div>
+      <div style={{width: '50%'}}>
+        <Virtualizer layout={ListLayout} layoutOptions={{rowHeight: 32}}>
+          <ListBox
+            aria-label="Fractional width list 2"
+            className={styles.menu}
+            style={{height: 300}}
+            items={items}>
+            {item => <MyListBoxItem>{item.name}</MyListBoxItem>}
+          </ListBox>
+        </Virtualizer>
+      </div>
+    </div>
+  );
+};

packages/react-aria-components/test/ListBox.test.js

@@ -1386,6 +1386,74 @@ describe('ListBox', () => {
     ]);
   });
 
+  it('should not cause horizontal overflow with fractional container width', () => {
+    let items = [
+      {id: 1, name: 'Item 1'},
+      {id: 2, name: 'Item 2'}
+    ];
+
+    jest.restoreAllMocks();
+    jest.spyOn(window.HTMLElement.prototype, 'getBoundingClientRect').mockImplementation(() => ({
+      width: 250.5,
+      height: 500,
+      top: 0,
+      left: 0,
+      bottom: 500,
+      right: 250.5,
+      x: 0,
+      y: 0,
+      toJSON: () => {}
+    }));
+    jest.spyOn(window.HTMLElement.prototype, 'clientWidth', 'get').mockImplementation(() => 251);
+    jest.spyOn(window.HTMLElement.prototype, 'clientHeight', 'get').mockImplementation(() => 500);
+
+    let {getByRole} = render(
+      <Virtualizer layout={ListLayout} layoutOptions={{rowHeight: 25}}>
+        <ListBox aria-label="Test" items={items}>
+          {item => <ListBoxItem>{item.name}</ListBoxItem>}
+        </ListBox>
+      </Virtualizer>
+    );
+
+    let listbox = getByRole('listbox');
+    let contentWrapper = listbox.firstElementChild;
+    expect(parseFloat(contentWrapper.style.width)).toBeLessThanOrEqual(250.5);
+  });
+
+  it('should not cause vertical overflow with fractional container height', () => {
+    let items = [
+      {id: 1, name: 'Item 1'},
+      {id: 2, name: 'Item 2'}
+    ];
+
+    jest.restoreAllMocks();
+    jest.spyOn(window.HTMLElement.prototype, 'getBoundingClientRect').mockImplementation(() => ({
+      width: 500,
+      height: 250.5,
+      top: 0,
+      left: 0,
+      bottom: 250.5,
+      right: 500,
+      x: 0,
+      y: 0,
+      toJSON: () => {}
+    }));
+    jest.spyOn(window.HTMLElement.prototype, 'clientWidth', 'get').mockImplementation(() => 500);
+    jest.spyOn(window.HTMLElement.prototype, 'clientHeight', 'get').mockImplementation(() => 251);
+
+    let {getByRole} = render(
+      <Virtualizer layout={ListLayout} layoutOptions={{orientation: 'horizontal', rowHeight: 25}}>
+        <ListBox orientation="horizontal" aria-label="Test" items={items}>
+          {item => <ListBoxItem>{item.name}</ListBoxItem>}
+        </ListBox>
+      </Virtualizer>
+    );
+
+    let listbox = getByRole('listbox');
+    let contentWrapper = listbox.firstElementChild;
+    expect(parseFloat(contentWrapper.style.height)).toBeLessThanOrEqual(250.5);
+  });
+
   it('should prevent Esc from clearing selection if escapeKeyBehavior is "none"', async () => {
     let {getByRole} = renderListbox({selectionMode: 'multiple', escapeKeyBehavior: 'none'});
 

packages/react-aria/src/virtualizer/ScrollView.tsx

@@ -67,6 +67,21 @@ interface ScrollViewAria {
   contentProps: HTMLAttributes<HTMLElement>;
 }
 
+function getClientSize(dom: HTMLElement) {
+  let clientWidth = dom.clientWidth;
+  let clientHeight = dom.clientHeight;
+  let isTestEnv = process.env.NODE_ENV === 'test' && !process.env.VIRT_ON;
+
+  let rect = dom.getBoundingClientRect?.();
+  if (rect && rect.width > 0 && rect.height > 0) {
+    if (!isTestEnv || rect.width % 1 !== 0 || rect.height % 1 !== 0) {
+      clientWidth = rect.width - Math.max(0, dom.offsetWidth - dom.clientWidth);
+      clientHeight = rect.height - Math.max(0, dom.offsetHeight - dom.clientHeight);
+    }
+  }
+  return {clientWidth, clientHeight};
+}
+
 export function useScrollView(
   props: ScrollViewProps,
   ref: RefObject<HTMLElement | null>
@@ -260,8 +275,7 @@ export function useScrollView(
       let isClientHeightMocked = Object.getOwnPropertyNames(window.HTMLElement.prototype).includes(
         'clientHeight'
       );
-      let clientWidth = dom.clientWidth;
-      let clientHeight = dom.clientHeight;
+      let {clientWidth, clientHeight} = getClientSize(dom);
       let w = isTestEnv && !isClientWidthMocked ? Infinity : clientWidth;
       let h = isTestEnv && !isClientHeightMocked ? Infinity : clientHeight;
 
@@ -286,12 +300,15 @@ export function useScrollView(
         // adjusted space. In very specific cases this might result in the scrollbars disappearing
         // again, resulting in extra padding. We stop after a maximum of two layout passes to avoid
         // an infinite loop. This matches how browsers behavior with native CSS grid layout.
-        if ((!isTestEnv && clientWidth !== dom.clientWidth) || clientHeight !== dom.clientHeight) {
-          state.size = new Size(dom.clientWidth, dom.clientHeight);
-          flush(() => {
-            updateVisibleRect();
-            onSizeChange?.(state.size);
-          });
+        if (!isTestEnv) {
+          let nextSize = getClientSize(dom);
+          if (clientWidth !== nextSize.clientWidth || clientHeight !== nextSize.clientHeight) {
+            state.size = new Size(nextSize.clientWidth, nextSize.clientHeight);
+            flush(() => {
+              updateVisibleRect();
+              onSizeChange?.(state.size);
+            });
+          }
         }
       }