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

[devo-id]

What was changed

When a Virtualizer container has a fractional layout width (e.g. 250.5px, common when two components sit side-by-side in a 50% flex layout), the browser's integer dom.clientWidth rounds up to 251px per the CSSOM View spec. ScrollView was passing this rounded integer to the layout engine, which sized the content wrapper to 251px inside a 250.5px container — producing an unwanted horizontal scrollbar.

Why it happened

ScrollView.tsx read container dimensions via dom.clientWidth / dom.clientHeight, which are always integers in the browser. The Virtualizer's layout engine then sized the content element to those rounded values, causing a 0.5px overflow that triggered overflow: auto to show a scrollbar.

How it's fixed

Introduced getClientSize(dom) in ScrollView.tsx that derives exact subpixel dimensions from getBoundingClientRect() while correctly subtracting borders and scrollbars (dom.offsetWidth - dom.clientWidth). This gives the layout engine the true floating-point scrollport dimensions.

Fixing at the measurement source means all layouts (ListLayout, GridLayout, WaterfallLayout, custom layouts) automatically benefit — no per-layout patching needed.

✅ Pull Request Checklist:

• Included link to corresponding React Spectrum GitHub Issue.
• Added/updated unit tests and storybook for this change (for new code or code which already has tests).
• Filled out test instructions.
• Updated documentation (if it already exists for this component).
• Looked at the Accessibility Practices for this feature - Aria Practices
• I understand every change in this PR and can explain why it's there.
• If AI-assisted, I followed our AI contribution guidance and pointed my assistant at CLAUDE.md.

📝 Test Instructions:

1. Run yarn jest packages/react-aria-components/test/ListBox.test.js -t "fractional container" — both tests should pass.
2. Render a virtualized ListBox inside a 501px container split into two 50% columns (so each column is 250.5px). Confirm no horizontal scrollbar appears.

Closes #10471

[snowystinger]

This seems like a better approach than the already existing PR. While the unit test is kind of useful, it'd be more useful to have a storybook setup with the 50% widths you mention, then we can put it in a chromatic story and get visual regression testing that actually uses the browser instead of mocking everything in jsdom. Would you mind putting that together?

Is there a reason you have left it as a draft?

Thank you!

[devo-id]

Hi @snowystinger,

Thanks for taking a look and for the feedback!

About the draft status: I temporarily moved it to draft while looking into a CircleCI failure in ListViewDnd.test.js. In jsdom, drag-and-drop mocks were returning item coordinates that getClientSize was picking up, which shrunk the mock container height in that test. I’ve refined the test environment handling in ScrollView.tsx, and all test suites are passing cleanly now.

Storybook / Chromatic: I've added a FractionalWidth story to packages/react-aria-components/stories/ListBox.stories.tsx that renders two side-by-side virtualized ListBoxes at 50% width inside a 501px container (giving 250.5px per list).

I've marked the PR as ready for review.

Thanks!