FocusScope's Tab interception defeats browser-native keyboard-focusable scroll containers, scrollable content in a Modal Dialog cannot be scrolled by keyboard (WCAG 2.1.1)

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Evaluación

Dificultad
4/5
Tiempo estimado
3-5 días
Aptitud para principiantes
52/100
Tipo de issue
Error
Claridad
Bastante claro
Estado de actividad
Tranquilo
Stack tecnológico
react, typescript
Área
accessibility, frontend

Línea de trabajo

Comienza con el tabbable-tree walker de FocusScope y ejecuta la reproducción vinculada de StackBlitz en Chrome o Firefox. Compara el comportamiento de tabulación del cuerpo desplazable del diálogo con y sin tabIndex={0}; después, inspecciona el comportamiento relacionado de FocusScope señalado en las issues #6729 y #9570. Se considera terminado cuando los usuarios de teclado pueden acceder a un contenedor de diálogo desbordado y desplazarse por él de forma nativa sin requerir un tabIndex permanente innecesario.

Escrito por el modelo de indexación a partir del texto del issue.

Descripción

Provide a general summary of the issue here

Since Chrome 127 and Firefox, browsers automatically include scrollable containers without focusable children in the sequential tab order, so keyboard users can focus them and scroll with arrow keys (Chrome feature: keyboard-focusable scrollers). Inside a Dialog, FocusScope intercepts every Tab keypress (preventDefault) and moves focus itself using its own focusable-tree walker. That walker only recognizes conventionally focusable elements, so an implicitly keyboard-focusable scroll container is skipped entirely. As a result, a dialog with a scrollable content region (pinned header/footer with overflow: auto body, a very common dialog layout) is impossible to scroll with the keyboard: focus can never reach the scroller, and there is nothing arrow keys will scroll. This fails WCAG 2.1.1 (Keyboard) and axe's scrollable-region-focusable rule.

🤔 Expected Behavior?

Tab (as managed by FocusScope inside a Dialog) should be able to reach a scrollable container that the browser itself would place in the tab order, matching native behavior outside the focus trap. Once focused, arrow keys / PageUp / PageDown / Home / End scroll it natively.

😯 Current Behavior

Tab cycles only between conventionally focusable elements (buttons, inputs, [tabindex]). The scrollable region is unreachable; keyboard scrolling is impossible.

Reproduced by Claude Fable via iinstrumented reproduction in Chromium 149:

Without tabIndex on the scroller: Tab visits only the dialog's buttons (keydown for Tab arrives with defaultPrevented: true at the document, FocusScope moved focus itself). After ArrowDown ×2 + PageDown, the scroller's scrollTop is still 0.
With an explicit tabIndex={0} on the same element: Tab reaches it and native scrolling works immediately (ArrowDown → 40, PageDown → 550, End → bottom).
The same markup outside a FocusScope is keyboard-scrollable in Chrome 127+/Firefox with no tabindex, because native Tab handling includes the scroller.

💁 Possible Solution

Teach the focusable/tabbable tree walker used by FocusScope to treat an element as tabbable when the browser would: a scroll container (computed overflow scrollable and scrollHeight > clientHeight / scrollWidth > clientWidth) containing no other focusable elements, i.e. mirror Chrome's keyboard-focusable-scrollers heuristic. Alternatively (or until then), document that scrollable regions inside Dialog require an explicit tabIndex={0}.

Workaround we use today: hardcode tabIndex={0} on the dialog body. It works, but it adds a permanent tab stop even when the content doesn't overflow, which browsers' native heuristic avoids.

🔦 Context

We build a design-system Dialog on react-aria-components with a pinned header/footer and a scrolling body

.dialog {
  display: flex;
  flex-direction: column;
  max-height: 80dvh;
  overflow: hidden;
}

.dialog__body {
  overflow-y: auto;
}

Our Storybook axe checks flag scrollable-region-focusable, and manual testing confirms keyboard users genuinely cannot scroll long dialog content (e.g. terms of service, multi-step forms). Comparable dialog implementations whose focus traps preserve native Tab handling via sentinel/guard elements get this right automatically in Chrome/Firefox — the regression is specific to focus traps that re-implement Tab navigation, which is what react-aria's FocusScope does. Related: #6729 (native video controls skipped by the same walker), #9570 (pointer interaction with scrollable content under contain).

🖥️ Steps to Reproduce

Created with the help of Claude AI.

https://stackblitz.com/edit/rac-dialog-scroll-issue?file=src%2FApp.tsx

  1. Open the dialog with the keyboard (Enter on the trigger).
  2. Press Tab repeatedly — focus only ever reaches the Close button; the scrollable is never focused.
  3. Press ArrowDown / PageDown — nothing scrolls (scrollTop stays 0).
  4. Control: render the same scrollable outside the Modal in Chrome 127+ — Tab focuses it natively and arrow keys scroll it.
  5. Control 2: add tabIndex={0} to the inside the Modal — Tab now reaches it and keyboard scrolling works.
Version

react-aria-components: 1.19.0 (react-aria 3.50.0)

What browsers are you seeing the problem on?

Chrome, Firefox

If other, please specify.

Browsers: Chromium 149.0.7827.55 (behavior applies to Chrome 127+ and Firefox, which ship keyboard-focusable scrollers; Safari lacks the native feature so it's equally broken there either way)

What operating system are you using?

Windows 11

🧢 Your Company/Team

No response

🕷 Tracking Issue

No response

Lenguaje dominante
TypeScript
Estrellas
16k
Forks
1.6k
Merge medio
2 d 19 h
PR fusionados (30 d)
41

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