fix(ui): tab component (#50183)

## What kind of change does this PR introduce?
bug fix on tab component + a small refactor

## What is the current behavior?

the active tab in an underline list gets border-b-2 while its siblings
get nothing, so it's 2px taller and its label sits higher than the rest
causing a smol layout shift within docs

## What is the new behavior?

- adds one absolute positioned bar that slides between tabs so nothing
moves
- favors track under an underline as an inset shadow vs a border

| state | preview |
| -------|------|
| before | <video
src="https://github.com/user-attachments/assets/b73820a6-2994-46d1-aa98-452681f0fef2"
/> |
| after | <video
src="https://github.com/user-attachments/assets/054cd67d-a50c-4aef-9340-a1e1d515047b"
/> |


## Test
1. visit [api
reference](https://docs-git-antlio-ui-components-tabs-supabase.vercel.app/docs/reference/javascript/installing?platform=npm&queryGroups=platform)
2. visit a [guide
](https://docs-git-antlio-ui-components-tabs-supabase.vercel.app/docs/guides/database/prisma)


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

- **New Features**
- Added an animated tab indicator that follows the active tab and adapts
to layout changes.
  - Added support for customizing tab indicator styling.
- Respects reduced-motion preferences by disabling indicator transitions
when appropriate.

- **Style**
  - Streamlined tab borders, spacing, and underlined-tab styling.
- Improved tab panel spacing and standardized tab behavior in
documentation examples.
- Centralized easing behavior for smoother overlays, dropdowns, slides,
and panels.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: Danny White <3104761+dnywh@users.noreply.github.com>

apps/design-system/components/mdx-components.tsx

@@ -17,6 +17,7 @@ import {
   cn,
   Tabs,
   TabsContent,
+  TabsIndicator,
   TabsList,
   TabsTrigger,
 } from 'ui'
@@ -226,16 +227,26 @@ const components = {
   Tabs: ({ className, ...props }: React.ComponentProps<typeof Tabs>) => (
     <Tabs className={cn('relative mt-6 w-full', className)} {...props} />
   ),
-  TabsList: ({ className, ...props }: React.ComponentProps<typeof TabsList>) => (
+  TabsList: ({ className, children, ...props }: React.ComponentProps<typeof TabsList>) => (
     <TabsList
-      className={cn('w-full justify-start rounded-none border-b bg-transparent p-0', className)}
+      className={cn(
+        'w-full justify-start rounded-none bg-transparent p-0',
+        'ps-4 -ms-4 [--tab-track-inset:--spacing(4)]',
+        className
+      )}
       {...props}
-    />
+    >
+      {children}
+      <TabsIndicator />
+    </TabsList>
   ),
   TabsTrigger: ({ className, ...props }: React.ComponentProps<typeof TabsTrigger>) => (
     <TabsTrigger
       className={cn(
-        'relative h-9 rounded-none border-b-2 border-b-transparent bg-transparent px-4 pb-3 pt-2 font-semibold text-muted-foreground shadow-none transition-none data-[state=active]:border-b-primary data-[state=active]:text-foreground data-[state=active]:shadow-none',
+        'relative h-9 rounded-none bg-transparent px-4 pb-3 pt-2 font-semibold text-muted-foreground shadow-none transition-none data-[state=active]:text-foreground data-[state=active]:shadow-none',
+        // The first label lines up with the surrounding content, keeping its
+        // padding so the focus ring sits off the glyphs
+        'first:-ms-4',
         className
       )}
       {...props}

apps/design-system/registry/default/example/tabs-demo.tsx

@@ -10,6 +10,7 @@ import {
   Label,
   Tabs,
   TabsContent,
+  TabsIndicator,
   TabsList,
   TabsTrigger,
 } from 'ui'
@@ -20,6 +21,7 @@ export default function TabsDemo() {
       <TabsList className="grid w-full grid-cols-2">
         <TabsTrigger value="account">Account</TabsTrigger>
         <TabsTrigger value="password">Password</TabsTrigger>
+        <TabsIndicator />
       </TabsList>
       <TabsContent value="account">
         <Card>

apps/docs/features/ui/PromptPanel.tsx

@@ -11,7 +11,7 @@ import {
   type ReactNode,
 } from 'react'
 // shadcn tabs from packages/ui/src/components/shadcn/ui/tabs.tsx
-import { cn, copyToClipboard, Tabs, TabsContent, TabsList, TabsTrigger } from 'ui'
+import { cn, copyToClipboard, Tabs, TabsContent, TabsIndicator, TabsList, TabsTrigger } from 'ui'
 
 type PromptTitleProps = {
   children: ReactNode
@@ -248,12 +248,13 @@ function PromptPanel({ children, className }: PromptPanelProps) {
   const header = (
     <div className="flex h-11 items-center justify-between border-b bg-surface-75 px-4">
       {hasTabs ? (
-        <TabsList className="h-full gap-5 border-0">
+        <TabsList className="h-full gap-5 border-0 [--tab-track:transparent]">
           {prompts.map((prompt) => (
             <TabsTrigger key={prompt.value} value={prompt.value} className={tabTriggerClassName}>
               <TabLabel icon={prompt.icon}>{prompt.title}</TabLabel>
             </TabsTrigger>
           ))}
+          <TabsIndicator />
         </TabsList>
       ) : (
         <span

apps/docs/features/ui/Tabs.tsx

@@ -1,7 +1,6 @@
 'use client'
 
 import { cva } from 'class-variance-authority'
-import { Tabs as TabsPrimitive } from 'radix-ui'
 import {
   Children,
   isValidElement,
@@ -13,7 +12,7 @@ import {
   useState,
   type KeyboardEvent,
 } from 'react'
-import { cn } from 'ui'
+import { cn, TabsContent, TabsIndicator, TabsList, Tabs as TabsRoot, TabsTrigger } from 'ui'
 
 import { useTocRerenderTrigger } from '../docs/GuidesMdx.state'
 import { useStickyTabs, UseStickyTabsOptions } from './useStickyTabs'
@@ -39,10 +38,15 @@ export interface TabsProps {
 export const tabsListVariants = cva(cn('flex'), {
   variants: {
     type: {
-      pills: 'space-x-1',
-      underlined: 'items-center border-b border-secondary',
-      cards: '',
-      'rounded-pills': 'flex-wrap gap-2',
+      pills: 'border-b-0 space-x-1',
+      // fix focus ring so it is not clipped
+      underlined: cn(
+        'relative items-center',
+        'ps-(--tab-lead) -ms-(--tab-lead) [--tab-track-inset:var(--tab-lead)]',
+        '[--tab-track:var(--border-secondary)]'
+      ),
+      cards: 'border-b-0',
+      'rounded-pills': 'border-b-0 flex-wrap gap-2',
     },
     scrollable: {
       true: 'overflow-auto whitespace-nowrap no-scrollbar mask-fadeout-right',
@@ -55,15 +59,18 @@ export const tabsListVariants = cva(cn('flex'), {
 
 export const tabsTriggerListVariants = cva(
   cn(
-    'relative cursor-pointer flex items-center space-x-2 text-center transition-colors focus-ring [&_img]:m-0 [&_img]:size-3.5 [&_svg]:size-3.5'
+    'relative cursor-pointer flex items-center gap-2 text-center transition-colors',
+    'focus-inset rounded-md',
+    '[&_img]:m-0 [&_img]:size-3.5 [&_svg]:size-3.5'
   ),
   {
     variants: {
       type: {
-        pills: 'shadow-xs rounded-sm border',
-        underlined: 'text-foreground-lighter',
-        cards: '',
-        'rounded-pills': 'shadow-xs rounded-full',
+        pills: 'border-b-0 data-[state=active]:border-b-stronger shadow-xs rounded-sm border',
+        underlined: 'text-foreground-lighter data-[state=active]:shadow-none',
+        cards: 'border-b-0',
+        'rounded-pills':
+          'border-b-0 data-[state=active]:border-b-foreground shadow-xs rounded-full',
       },
       size: {
         tiny: 'text-xs px-2.5 py-1',
@@ -94,8 +101,9 @@ export const tabsTriggerListVariants = cva(
       {
         type: 'underlined',
         isActive: true,
-        className: '!text-foreground border-b-2 border-foreground',
+        className: '!text-foreground',
       },
+      { type: 'underlined', className: 'py-3 first:-ms-(--tab-lead)' },
       {
         type: 'rounded-pills',
         isActive: true,
@@ -111,6 +119,14 @@ export const tabsTriggerListVariants = cva(
   }
 )
 
+const TAB_LEAD: Record<NonNullable<TabsProps['size']>, string> = {
+  tiny: 'calc(var(--spacing) * 2.5)',
+  small: 'calc(var(--spacing) * 3)',
+  medium: 'calc(var(--spacing) * 4)',
+  large: 'calc(var(--spacing) * 4)',
+  xlarge: 'calc(var(--spacing) * 6)',
+}
+
 const isString = (maybeStr: unknown): maybeStr is string => typeof maybeStr === 'string'
 
 export const Tabs = ({
@@ -208,12 +224,13 @@ export const Tabs = ({
     ]
   )
   return (
-    <TabsPrimitive.Root
+    <TabsRoot
       value={activeTab}
       className={cn('w-full justify-between space-y-4', baseClassNames)}
       ref={observedRef}
     >
-      <TabsPrimitive.List
+      <TabsList
+        style={{ '--tab-lead': TAB_LEAD[size] } as React.CSSProperties}
         className={tabsListVariants({
           type,
           scrollable,
@@ -227,7 +244,7 @@ export const Tabs = ({
           const isActive = activeTab === tab.props.id
 
           return (
-            <TabsPrimitive.Trigger
+            <TabsTrigger
               onKeyDown={(e: KeyboardEvent<HTMLButtonElement>) => {
                 if (e.key === 'Enter') {
                   e.preventDefault()
@@ -242,13 +259,14 @@ export const Tabs = ({
               {tab.props.icon}
               <span>{tab.props.label}</span>
               {tab.props.iconRight}
-            </TabsPrimitive.Trigger>
+            </TabsTrigger>
           )
         })}
         {addOnAfter}
-      </TabsPrimitive.List>
+        {type === 'underlined' ? <TabsIndicator /> : null}
+      </TabsList>
       {childrenArr}
-    </TabsPrimitive.Root>
+    </TabsRoot>
   )
 }
 
@@ -263,12 +281,8 @@ interface TabPanelProps {
 
 export const TabPanel = ({ children, id, className }: TabPanelProps) => {
   return (
-    <TabsPrimitive.Content
-      value={id}
-      className={cn('focus:outline-hidden transition-height', className)}
-      tabIndex={-1}
-    >
+    <TabsContent value={id} className={cn('mt-0 focus:outline-hidden', className)} tabIndex={-1}>
       {children}
-    </TabsPrimitive.Content>
+    </TabsContent>
   )
 }

packages/config/css/animations.css

@@ -10,36 +10,39 @@
  */
 
 @theme {
+  --ease-enter: cubic-bezier(0.16, 1, 0.3, 1);
+  --ease-move: cubic-bezier(0.87, 0, 0.13, 1);
+
   --animate-fade-in: fadeIn 300ms both;
   --animate-fade-out: fadeOut 300ms both;
 
   /* may be unused — verify before pruning */
-  --animate-dropdown-content-show: overlayContentShow 100ms cubic-bezier(0.16, 1, 0.3, 1);
+  --animate-dropdown-content-show: overlayContentShow 100ms var(--ease-enter);
   /* may be unused — verify before pruning */
-  --animate-dropdown-content-hide: overlayContentHide 100ms cubic-bezier(0.16, 1, 0.3, 1);
+  --animate-dropdown-content-hide: overlayContentHide 100ms var(--ease-enter);
 
-  --animate-overlay-show: overlayContentShow 300ms cubic-bezier(0.16, 1, 0.3, 1);
-  --animate-overlay-hide: overlayContentHide 300ms cubic-bezier(0.16, 1, 0.3, 1);
+  --animate-overlay-show: overlayContentShow 300ms var(--ease-enter);
+  --animate-overlay-hide: overlayContentHide 300ms var(--ease-enter);
 
   /* may be unused — verify before pruning */
   --animate-fade-in-overlay-bg: fadeInOverlayBg 300ms;
   /* may be unused — verify before pruning */
   --animate-fade-out-overlay-bg: fadeOutOverlayBg 300ms;
 
-  --animate-slide-down: slideDown 300ms cubic-bezier(0.87, 0, 0.13, 1);
-  --animate-slide-up: slideUp 300ms cubic-bezier(0.87, 0, 0.13, 1);
+  --animate-slide-down: slideDown 300ms var(--ease-move);
+  --animate-slide-up: slideUp 300ms var(--ease-move);
 
-  --animate-slide-down-normal: slideDownNormal 300ms cubic-bezier(0.87, 0, 0.13, 1);
-  --animate-slide-up-normal: slideUpNormal 300ms cubic-bezier(0.87, 0, 0.13, 1);
+  --animate-slide-down-normal: slideDownNormal 300ms var(--ease-move);
+  --animate-slide-up-normal: slideUpNormal 300ms var(--ease-move);
 
   /* may be unused — verify before pruning */
-  --animate-panel-slide-left-out: panelSlideLeftOut 200ms cubic-bezier(0.87, 0, 0.13, 1);
+  --animate-panel-slide-left-out: panelSlideLeftOut 200ms var(--ease-move);
   /* may be unused — verify before pruning */
-  --animate-panel-slide-left-in: panelSlideLeftIn 250ms cubic-bezier(0.87, 0, 0.13, 1);
+  --animate-panel-slide-left-in: panelSlideLeftIn 250ms var(--ease-move);
   /* may be unused — verify before pruning */
-  --animate-panel-slide-right-out: panelSlideRightOut 200ms cubic-bezier(0.87, 0, 0.13, 1);
+  --animate-panel-slide-right-out: panelSlideRightOut 200ms var(--ease-move);
   /* may be unused — verify before pruning */
-  --animate-panel-slide-right-in: panelSlideRightIn 250ms cubic-bezier(0.87, 0, 0.13, 1);
+  --animate-panel-slide-right-in: panelSlideRightIn 250ms var(--ease-move);
 
   --animate-line-loading: lineLoading 1.8s infinite;
   --animate-line-loading-slower: lineLoading 2.3s infinite;

packages/ui/index.tsx

@@ -113,6 +113,7 @@ export * from './src/components/ShadowScrollArea'
 export * from './src/components/shadcn/ui/collapsible'
 
 export * from './src/components/shadcn/ui/tabs'
+export * from './src/components/shadcn/ui/useTabIndicator'
 
 export * from './src/components/shadcn/ui/tooltip'
 

packages/ui/src/components/shadcn/ui/tabs.test.tsx

@@ -0,0 +1,85 @@
+import { render, screen, waitFor } from '@testing-library/react'
+import { describe, expect, it } from 'vitest'
+
+import { Tabs, TabsContent, TabsIndicator, TabsList, TabsTrigger } from './tabs'
+
+const renderTabs = ({ indicator = false } = {}) =>
+  render(
+    <Tabs defaultValue="one">
+      <TabsList>
+        <TabsTrigger value="one">One</TabsTrigger>
+        <TabsTrigger value="two">Two</TabsTrigger>
+        {indicator ? <TabsIndicator /> : null}
+      </TabsList>
+      <TabsContent value="one">First</TabsContent>
+      <TabsContent value="two">Second</TabsContent>
+    </Tabs>
+  )
+
+describe('Tabs', () => {
+  it('borders each trigger when no indicator is rendered', () => {
+    const { container } = renderTabs()
+
+    expect(screen.getByRole('tab', { name: 'One' })).toHaveClass('border-b-2')
+    expect(container.querySelector('[data-tab-indicator]')).toBeNull()
+  })
+
+  it('measures the active tab only for lists that render an indicator', async () => {
+    const withoutIndicator = renderTabs().container.querySelector<HTMLElement>('[role="tablist"]')
+    const withIndicator = renderTabs({ indicator: true }).container.querySelector<HTMLElement>(
+      '[role="tablist"]'
+    )
+
+    await waitFor(() => expect(withIndicator?.dataset.tabIndicatorReady).toBe(''))
+    expect(withoutIndicator?.dataset.tabIndicatorReady).toBeUndefined()
+  })
+
+  it('hides the underline when no trigger is active', async () => {
+    const { container, rerender } = render(
+      <Tabs value="one">
+        <TabsList>
+          <TabsTrigger value="one">One</TabsTrigger>
+          <TabsIndicator />
+        </TabsList>
+        <TabsContent value="one">First</TabsContent>
+      </Tabs>
+    )
+    const list = container.querySelector<HTMLElement>('[role="tablist"]')
+    await waitFor(() => expect(list?.dataset.tabIndicatorReady).toBe(''))
+
+    rerender(
+      <Tabs value="gone">
+        <TabsList>
+          <TabsTrigger value="one">One</TabsTrigger>
+          <TabsIndicator />
+        </TabsList>
+        <TabsContent value="one">First</TabsContent>
+      </Tabs>
+    )
+
+    await waitFor(() => expect(list?.dataset.tabIndicatorReady).toBeUndefined())
+  })
+
+  it('renders the first trigger as the first element in the list', () => {
+    const { container } = renderTabs({ indicator: true })
+
+    expect(container.querySelector('[role="tablist"]')?.firstElementChild).toBe(
+      screen.getByRole('tab', { name: 'One' })
+    )
+  })
+
+  it('passes a caller ref through to the list element', () => {
+    const ref = { current: null as HTMLDivElement | null }
+
+    render(
+      <Tabs defaultValue="one">
+        <TabsList ref={ref}>
+          <TabsTrigger value="one">One</TabsTrigger>
+        </TabsList>
+        <TabsContent value="one">First</TabsContent>
+      </Tabs>
+    )
+
+    expect(ref.current).toBe(screen.getByRole('tablist'))
+  })
+})

packages/ui/src/components/shadcn/ui/tabs.tsx

@@ -1,46 +1,87 @@
 'use client'
 
 import { Tabs as TabsPrimitive } from 'radix-ui'
-import * as React from 'react'
+import { useRef, type ComponentPropsWithRef } from 'react'
 
 import { cn } from '../../../lib/utils/cn'
+import { useTabIndicator } from './useTabIndicator'
 
 const Tabs = TabsPrimitive.Root
 
-const TabsList = React.forwardRef<
-  React.ElementRef<typeof TabsPrimitive.List>,
-  React.ComponentPropsWithoutRef<typeof TabsPrimitive.List>
->(({ className, ...props }, ref) => (
-  <TabsPrimitive.List
-    ref={ref}
-    className={cn('flex items-center border-b', className)}
+const trackClasses = cn(
+  'has-[[data-tab-indicator]]:border-b-0',
+  'has-[[data-tab-indicator]]:after:pointer-events-none has-[[data-tab-indicator]]:after:absolute',
+  'has-[[data-tab-indicator]]:after:bottom-0 has-[[data-tab-indicator]]:after:h-px',
+  'has-[[data-tab-indicator]]:after:left-[var(--tab-track-inset,0px)]',
+  'has-[[data-tab-indicator]]:after:right-0',
+  'has-[[data-tab-indicator]]:after:bg-[var(--tab-track,var(--border-default))]'
+)
+
+const TabsList = ({
+  className,
+  children,
+  ref,
+  ...props
+}: ComponentPropsWithRef<typeof TabsPrimitive.List>) => {
+  const listRef = useRef<HTMLDivElement>(null)
+  useTabIndicator(listRef)
+
+  return (
+    <TabsPrimitive.List
+      ref={(node) => {
+        listRef.current = node
+        if (typeof ref === 'function') ref(node)
+        else if (ref) ref.current = node
+      }}
+      className={cn('group/list relative flex items-center border-b', trackClasses, className)}
+      {...props}
+    >
+      {children}
+    </TabsPrimitive.List>
+  )
+}
+
+const TabsIndicator = ({ className, ...props }: ComponentPropsWithRef<'span'>) => (
+  <span
+    aria-hidden
+    data-tab-indicator
+    className={cn(
+      'pointer-events-none absolute bottom-0 left-0 h-px bg-foreground',
+      'w-[var(--active-tab-width,0)] translate-x-[var(--active-tab-left,0)]',
+      'transition-none opacity-0',
+      'group-data-[tab-indicator-ready]/list:opacity-100',
+      'group-data-[tab-indicator-ready]/list:transition-[translate,width]',
+      'group-data-[tab-indicator-ready]/list:duration-[250ms]',
+      'group-data-[tab-indicator-ready]/list:ease-move',
+      'motion-reduce:transition-none',
+      className
+    )}
     {...props}
   />
-))
-TabsList.displayName = TabsPrimitive.List.displayName
+)
 
-const TabsTrigger = React.forwardRef<
-  React.ElementRef<typeof TabsPrimitive.Trigger>,
-  React.ComponentPropsWithoutRef<typeof TabsPrimitive.Trigger>
->(({ className, ...props }, ref) => (
+const TabsTrigger = ({
+  className,
+  ...props
+}: ComponentPropsWithRef<typeof TabsPrimitive.Trigger>) => (
   <TabsPrimitive.Trigger
-    ref={ref}
     className={cn(
-      'inline-flex cursor-pointer items-center justify-center whitespace-nowrap py-1.5 text-sm transition-colors focus-ring disabled:pointer-events-none disabled:opacity-50 data-[state=active]:text-foreground data-[state=active]:shadow-xs text-foreground-lighter hover:text-foreground data-[state=active]:border-foreground border-b-2 border-transparent',
+      'inline-flex cursor-pointer items-center justify-center whitespace-nowrap py-1.5 text-sm transition-colors disabled:pointer-events-none disabled:opacity-50 data-[state=active]:text-foreground data-[state=active]:shadow-xs text-foreground-lighter hover:text-foreground',
+      'focus-inset',
+      'border-b-2 border-b-transparent data-[state=active]:border-b-foreground',
+      'group-has-[[data-tab-indicator]]/list:border-b-0',
       'group',
       className
     )}
     {...props}
   />
-))
-TabsTrigger.displayName = TabsPrimitive.Trigger.displayName
+)
 
-const TabsContent = React.forwardRef<
-  React.ElementRef<typeof TabsPrimitive.Content>,
-  React.ComponentPropsWithoutRef<typeof TabsPrimitive.Content>
->(({ className, ...props }, ref) => (
-  <TabsPrimitive.Content ref={ref} className={cn('mt-4 focus-ring', className)} {...props} />
-))
-TabsContent.displayName = TabsPrimitive.Content.displayName
+const TabsContent = ({
+  className,
+  ...props
+}: ComponentPropsWithRef<typeof TabsPrimitive.Content>) => (
+  <TabsPrimitive.Content className={cn('mt-4 focus-ring', className)} {...props} />
+)
 
-export { Tabs, TabsContent, TabsList, TabsTrigger }
+export { Tabs, TabsContent, TabsIndicator, TabsList, TabsTrigger }

packages/ui/src/components/shadcn/ui/useTabIndicator.ts

@@ -0,0 +1,66 @@
+'use client'
+
+import { useLayoutEffect, type RefObject } from 'react'
+
+export const useTabIndicator = (listRef: RefObject<HTMLElement | null>) => {
+  useLayoutEffect(() => {
+    const list = listRef.current
+    if (!list || !list.querySelector(':scope > [data-tab-indicator]')) return
+
+    let cancelled = false
+
+    const measure = () => {
+      const trigger = list.querySelector<HTMLElement>(':scope > [role="tab"][data-state="active"]')
+      if (cancelled) return
+      if (!trigger) {
+        delete list.dataset.tabIndicatorReady
+        return
+      }
+
+      const styles = getComputedStyle(trigger)
+      const paddingLeft = parseFloat(styles.paddingLeft) || 0
+      const paddingRight = parseFloat(styles.paddingRight) || 0
+
+      list.style.setProperty('--active-tab-left', `${trigger.offsetLeft + paddingLeft}px`)
+      list.style.setProperty(
+        '--active-tab-width',
+        `${trigger.offsetWidth - paddingLeft - paddingRight}px`
+      )
+      if (list.dataset.tabIndicatorReady === undefined) {
+        // makes the bar already sitting on the active tab rather than sliding in
+        requestAnimationFrame(() => {
+          if (!cancelled) list.dataset.tabIndicatorReady = ''
+        })
+      }
+    }
+
+    const resizes = typeof ResizeObserver === 'undefined' ? undefined : new ResizeObserver(measure)
+    const sync = () => {
+      resizes?.disconnect()
+      resizes?.observe(list)
+      Array.from(list.children).forEach((child) => resizes?.observe(child))
+      measure()
+    }
+
+    sync()
+
+    const mutations =
+      typeof MutationObserver === 'undefined' ? undefined : new MutationObserver(sync)
+    mutations?.observe(list, {
+      attributes: true,
+      attributeFilter: ['data-state'],
+      subtree: true,
+      childList: true,
+    })
+
+    // label widths shift when webfonts land
+    document.fonts?.ready.then(measure).catch(() => {})
+
+    return () => {
+      cancelled = true
+      mutations?.disconnect()
+      resizes?.disconnect()
+      delete list.dataset.tabIndicatorReady
+    }
+  }, [listRef])
+}