feat: in-page navigation

src/components/InPageNavigation/InPageNavigation.module.scss

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+@media not (prefers-reduced-motion) {
+  html {
+    scroll-behavior: smooth;
+  }
+}
+
+:target {
+  scroll-margin-top: var(--margin-offset);
+}

src/components/InPageNavigation/InPageNavigation.stories.tsx

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+import React from 'react'
+import { InPageNavigation } from './InPageNavigation'
+import { CONTENT } from './content'
+import { HeadingLevel } from '../../types/headingLevel'
+import classNames from 'classnames'
+
+export default {
+  title: 'Components/In-Page Navigation',
+  component: InPageNavigation,
+  argTypes: {
+    headingLevel: {
+      control: 'select',
+      options: ['h1', 'h2', 'h3', 'h4', 'h5', 'h6'],
+    },
+    rootMargin: {
+      control: 'text',
+    },
+    threshold: {
+      control: { type: 'range', min: 0, max: 1, step: 0.01 },
+    },
+    title: {
+      control: 'text',
+    },
+  },
+  args: {
+    headingLevel: 'h4',
+    rootMargin: '0px 0px 0px 0px',
+    threshold: 1,
+    title: 'On this page',
+  },
+  parameters: {
+    docs: {
+      description: {
+        component: `
+### USWDS 3.0 In-Page Navigation component
+
+Source: https://designsystem.digital.gov/components/in-page-navigation/
+`,
+      },
+    },
+  },
+}
+
+type StorybookArguments = {
+  headingLevel: HeadingLevel
+  rootMargin: string
+  scrollOffset: string
+  threshold: number
+  title: string
+}
+
+export const Default = (argTypes: StorybookArguments): React.ReactElement => (
+  <InPageNavigation
+    content={CONTENT}
+    headingLevel={argTypes.headingLevel}
+    mainProps={{ className: 'usa-prose' }}
+    rootMargin={argTypes.rootMargin}
+    threshold={argTypes.threshold}
+    title={argTypes.title}
+  />
+)
+
+// Storybook seems to force anchor links to open in a new window,
+// so this story is just to demonstrate how the scroll offset works
+export const ScrollOffset = (
+  argTypes: StorybookArguments
+): React.ReactElement => (
+  <InPageNavigation
+    content={CONTENT}
+    headingLevel={argTypes.headingLevel}
+    mainProps={{ className: 'usa-prose' }}
+    rootMargin={argTypes.rootMargin}
+    scrollOffset="2rem"
+    threshold={argTypes.threshold}
+    title={argTypes.title}
+  />
+)

src/components/InPageNavigation/InPageNavigation.test.tsx

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+import React from 'react'
+import { screen, render, getByRole } from '@testing-library/react'
+import userEvent from '@testing-library/user-event'
+import { InPageNavigation } from './InPageNavigation'
+import { HeadingLevel } from '../../types/headingLevel'
+import { CONTENT } from './content'
+
+describe('InPageNavigation component', () => {
+  const props = {
+    content: CONTENT,
+    headingLevel: 'h1' as HeadingLevel,
+    title: 'What do we have <i>here</i>?',
+  }
+
+  const setup = (plain?: boolean) => {
+    const utils = plain
+      ? render(<InPageNavigation content={props.content} />)
+      : render(
+          <InPageNavigation
+            content={props.content}
+            headingLevel={props.headingLevel}
+            title={props.title}
+          />
+        )
+    const nav = screen.getByTestId('InPageNavigation')
+    const user = userEvent.setup()
+    return {
+      nav,
+      user,
+      ...utils,
+    }
+  }
+
+  beforeEach(() => {
+    // IntersectionObserver isn't available in test environment
+    const mockIntersectionObserver = jest.fn()
+    mockIntersectionObserver.mockReturnValue({
+      observe: () => null,
+      unobserve: () => null,
+      disconnect: () => null,
+    })
+    window.IntersectionObserver = mockIntersectionObserver
+  })
+
+  it('renders without errors', () => {
+    const { nav } = setup(true)
+    expect(nav).toBeInTheDocument()
+    const heading = getByRole(nav, 'heading', {
+      level: 4,
+      name: 'On this page',
+    })
+    expect(heading).toBeInTheDocument()
+  })
+
+  it('sets the heading and title', () => {
+    const { nav } = setup()
+    const heading = getByRole(nav, 'heading', {
+      level: Number(props.headingLevel.slice(-1)),
+      name: props.title,
+    })
+    expect(heading).toBeInTheDocument()
+  })
+})

src/components/InPageNavigation/InPageNavigation.tsx

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+import React, { useEffect, useState } from 'react'
+import classnames from 'classnames'
+import { HeadingLevel } from '../../types/headingLevel'
+import { Link } from '../Link/Link'
+import styles from './InPageNavigation.module.scss'
+
+type InPageNavigationProps = {
+  className?: string
+  content: JSX.Element
+  headingLevel?: HeadingLevel
+  mainProps?: JSX.IntrinsicElements['main']
+  navProps?: JSX.IntrinsicElements['nav']
+  rootMargin?: string
+  scrollOffset?: string
+  threshold?: number
+  title?: string
+}
+
+export const InPageNavigation = ({
+  className,
+  content,
+  headingLevel = 'h4',
+  mainProps,
+  navProps,
+  rootMargin = '0px 0px 0px 0px',
+  scrollOffset,
+  threshold = 1,
+  title = 'On this page',
+  ...divProps
+}: InPageNavigationProps &
+  JSX.IntrinsicElements['div']): React.ReactElement => {
+  const classes = classnames('usa-in-page-nav', styles.target, className)
+  const { className: navClassName, ...remainingNavProps } = navProps || {}
+  const navClasses = classnames('usa-in-page-nav__nav', navClassName)
+  const { className: mainClassName, ...remainingMainProps } = mainProps || {}
+  const mainClasses = classnames('main-content', mainClassName)
+  const Heading = headingLevel
+  const offsetStyle = {
+    '--margin-offset': scrollOffset,
+  } as React.CSSProperties
+  const [currentSection, setCurrentSection] = useState('')
+  const sectionHeadings: JSX.Element[] = content.props.children.filter(
+    (el: JSX.Element) => el.type === 'h2' || el.type === 'h3'
+  )
+  const handleIntersection = (entries: IntersectionObserverEntry[]) => {
+    entries.forEach((entry) => {
+      if (entry.isIntersecting) {
+        setCurrentSection(entry.target.id)
+      }
+    })
+  }
+  const observerOptions = {
+    root: null,
+    rootMargin: rootMargin,
+    threshold: [threshold],
+  }
+  const observer = new IntersectionObserver(handleIntersection, observerOptions)
+  useEffect(() => {
+    document.querySelectorAll('h2,h3').forEach((h) => observer.observe(h))
+  })
+
+  return (
+    <div className="usa-in-page-nav-container" {...divProps}>
+      <aside
+        className={classes}
+        aria-label={title}
+        data-testid="InPageNavigation">
+        <nav className={navClasses} {...remainingNavProps}>
+          <Heading className="usa-in-page-nav__heading" tabIndex={0}>
+            {title}
+          </Heading>
+          <ul className="usa-in-page-nav__list">
+            {sectionHeadings.map((el: JSX.Element) => {
+              const heading: JSX.Element = el.props.children
+              const href: string = el.props.id
+              const hClass = classnames('usa-in-page-nav__item', {
+                'usa-in-page-nav__item--sub-item': el.type === 'h3',
+              })
+              const lClass = classnames('usa-in-page-nav__link', {
+                'usa-current': href === currentSection,
+              })
+              return (
+                <li key={`usa-in-page-nav__item_${heading}`} className={hClass}>
+                  <Link href={`#${href}`} className={lClass}>
+                    {heading}
+                  </Link>
+                </li>
+              )
+            })}
+          </ul>
+        </nav>
+      </aside>
+      <main
+        id="main-content"
+        className={mainClasses}
+        {...remainingMainProps}
+        style={scrollOffset ? offsetStyle : undefined}>
+        {content}
+      </main>
+    </div>
+  )
+}
+
+export default InPageNavigation