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fix(react-headless-components-preview): reserve the scrollbar gutter while a dialog locks scroll #36666

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fix(react-headless-components-preview): reserve the scrollbar gutter …
ArrayKnight Aug 25, 2026
7d77229
chore: add change file
ArrayKnight Aug 31, 2026
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7 changes: 7 additions & 0 deletions change/@fluentui-react-headless-components-preview-4e33a94a-d467-47f7-87d9-2714995d6202.json
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Original file line number Diff line number Diff line change
@@ -0,0 +1,7 @@
{
"type": "patch",
"comment": "fix: reserve the scrollbar gutter while a modal dialog locks document scroll, so opening a dialog no longer shifts the page",
"packageName": "@fluentui/react-headless-components-preview",
"email": "array.knight@gmail.com",
"dependentChangeType": "patch"
}
86 changes: 86 additions & 0 deletions ...omponents/react-headless-components-preview/library/src/components/Dialog/Dialog.test.tsx
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Original file line number Diff line number Diff line change
Expand Up @@ -106,6 +106,92 @@ describe('Dialog', () => {
expect(dialog).not.toHaveAttribute('aria-labelledby');
});

describe('scroll lock', () => {
// jsdom reports clientWidth 0, which would read as a scrollbar on every page.
const setScrollbarWidth = (width: number) =>
Object.defineProperty(document.documentElement, 'clientWidth', {
configurable: true,
value: window.innerWidth - width,
});

afterEach(() => {
delete (document.documentElement as Partial<HTMLElement>).clientWidth;
document.documentElement.style.removeProperty('scrollbar-gutter');
document.body.style.removeProperty('overflow');
});

const renderModal = () =>
render(
<Dialog unmountOnClose={false}>
<DialogTrigger>
<button>Open dialog</button>
</DialogTrigger>
<DialogSurface>
<DialogTitle>Dialog title</DialogTitle>
<DialogActions>
<DialogTrigger>
<button>Close dialog</button>
</DialogTrigger>
</DialogActions>
</DialogSurface>
</Dialog>,
);

it('reserves the scrollbar gutter while a modal holds the lock', () => {
setScrollbarWidth(15);
const result = renderModal();

fireEvent.click(result.getByRole('button', { name: 'Open dialog' }));

expect(document.body.style.overflow).toBe('visible clip');
// On <html>, not <body>: scrollbar-gutter does not propagate to the viewport.
expect(document.documentElement.style.scrollbarGutter).toBe('stable');

fireEvent.click(result.getByRole('button', { name: 'Close dialog' }));

expect(document.documentElement.style.scrollbarGutter).toBe('');
});

it('reserves nothing when the scrollbar takes no layout width', () => {
setScrollbarWidth(0);
const result = renderModal();

fireEvent.click(result.getByRole('button', { name: 'Open dialog' }));

expect(document.body.style.overflow).toBe('visible clip');
expect(document.documentElement.style.scrollbarGutter).toBe('');
});

it('restores a gutter the host application had already set', () => {
setScrollbarWidth(15);
document.documentElement.style.scrollbarGutter = 'both-edges';
const result = renderModal();

fireEvent.click(result.getByRole('button', { name: 'Open dialog' }));

expect(document.documentElement.style.scrollbarGutter).toBe('stable');

fireEvent.click(result.getByRole('button', { name: 'Close dialog' }));

expect(document.documentElement.style.scrollbarGutter).toBe('both-edges');
});

it('leaves a non-modal dialog out of the lock entirely', () => {
setScrollbarWidth(15);
const result = render(
<Dialog defaultOpen modalType="non-modal">
<DialogSurface>
<DialogTitle>Non-modal title</DialogTitle>
</DialogSurface>
</Dialog>,
);

expect(result.container.querySelector('dialog')).toHaveAttribute('data-open');
expect(document.body.style.overflow).toBe('');
expect(document.documentElement.style.scrollbarGutter).toBe('');
});
});

it('keeps dialog mounted after close when unmountOnClose is false', () => {
const result = render(
<Dialog unmountOnClose={false}>
Expand Down
30 changes: 23 additions & 7 deletions ...omponents/react-headless-components-preview/library/src/components/Dialog/utils/scroll.ts
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Original file line number Diff line number Diff line change
@@ -1,14 +1,20 @@
type ScrollLockState = {
lockCount: number;
previousBodyOverflow: string;
previousScrollbarGutter: string;
};

const scrollLockStateByDocument = new WeakMap<Document, ScrollLockState>();

/**
* Prevents background scrolling while a modal/alert dialog is open by applying
* `overflow: hidden` to `<body>`. The `<html>` element is intentionally left
* untouched so host-application styles on the document element are preserved.
* `overflow: hidden` to `<body>`, and reserves the space the page scrollbar was
* occupying so nothing on the page moves sideways as it disappears.
*
* The gutter has to be reserved on `<html>`: `scrollbar-gutter` does not propagate
* from `<body>` to the viewport the way `overflow` does, so spelling it on `<body>`
* reserves nothing. It is written only when the scrollbar actually takes layout
* width, because `stable` otherwise reserves a gutter the page never had.
*
* Nested modal dialogs share a single lock via a reference count.
*/
Expand All @@ -19,18 +25,27 @@ export function lockDocumentScroll(targetDocument: Document): void {
return;
}

const { body, documentElement } = targetDocument;
// Read the scrollbar's layout width before the lock takes it away. Overlay
// scrollbars and unscrollable pages both measure 0, and both want no gutter.
const scrollbarWidth = (targetDocument.defaultView?.innerWidth ?? 0) - documentElement.clientWidth;

@dmytrokirpa Dmytro Kirpa (dmytrokirpa) Sep 1, 2026

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could we use useApplyScrollbarWidth or useScrollbarWidth hooks from @fluentui/react-utilities?

@ArrayKnight Ray Knight (ArrayKnight) Sep 1, 2026

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I looked at both hooks before settling on this shape — they solve a different half of the problem, so I kept the native gutter, but happy to switch if you prefer the tradeoff:

  • This fix reserves the gutter natively (scrollbar-gutter: stable on <html>) instead of compensating with a measured pixel width. Its only measurement, innerWidth - documentElement.clientWidth, asks whether this page, right now has a layout-consuming scrollbar.
  • useScrollbarWidth/useApplyScrollbarWidth measure a probe element (measureScrollbarWidth), which reports the UA's classic scrollbar width even when the page itself doesn't scroll — so on an unscrollable page a width/padding compensation would introduce the very shift this PR removes, and we'd still need the current-viewport check on top.
  • useApplyScrollbarWidth is a mount-only ref callback (it writes ${width}px on attach and early-returns on detach), so it can't express the lock's restore semantics — previous inline values plus the refcount for nested modals — and there's no natural ref to hand it for document.documentElement.

Adopting them would replace only the measurement while keeping all the lock/restore code, and would trade native reservation for pixel compensation. If you'd rather standardize on the shared utility regardless, I'm glad to rework it that way.


scrollLockStateByDocument.set(targetDocument, {
lockCount: 1,
previousBodyOverflow: targetDocument.body.style.overflow,
previousBodyOverflow: body.style.overflow,
previousScrollbarGutter: documentElement.style.scrollbarGutter,
});

targetDocument.body.style.overflow = 'visible clip';
body.style.overflow = 'visible clip';
if (scrollbarWidth > 0) {
documentElement.style.scrollbarGutter = 'stable';
}
}

/**
* Restores the document's scroll behavior by reverting the `overflow` style
* on the `<body>` element to its previous value. This function is typically
* called when a modal/alert dialog is closed.
* Restores the document's scroll behavior by reverting the `overflow` style on the
* `<body>` element and the reserved scrollbar gutter on `<html>` to their previous
* values. This function is typically called when a modal/alert dialog is closed.
*/
export function unlockDocumentScroll(targetDocument: Document): void {
const state = scrollLockStateByDocument.get(targetDocument);
Expand All @@ -44,5 +59,6 @@ export function unlockDocumentScroll(targetDocument: Document): void {
}

targetDocument.body.style.overflow = state.previousBodyOverflow;
targetDocument.documentElement.style.scrollbarGutter = state.previousScrollbarGutter;
scrollLockStateByDocument.delete(targetDocument);
}