Add observer to focus-trap

.changeset/curly-lions-cheat.md

@@ -0,0 +1,5 @@
+---
+'@primer/behaviors': patch
+---
+
+Adds mutation observer to `focus-trap` to ensure sentinel elements are always in the correct position

src/__tests__/focus-trap.test.tsx

@@ -240,3 +240,91 @@ it('Should handle dynamic content', async () => {
 
   controller?.abort()
 })
+
+it('should keep the sentinel elements at the start/end of the inner container', async () => {
+  const user = userEvent.setup()
+  const {container} = render(
+    <div>
+      <div id="trapContainer">
+        <button tabIndex={0}>Apple</button>
+        <button tabIndex={0}>Banana</button>
+        <button tabIndex={0}>Cantaloupe</button>
+      </div>
+      <button id="durian" tabIndex={0}>
+        Durian
+      </button>
+    </div>,
+  )
+
+  const trapContainer = container.querySelector<HTMLElement>('#trapContainer')!
+  const [firstButton, secondButton] = trapContainer.querySelectorAll('button')
+  const controller = focusTrap(trapContainer)
+
+  secondButton.focus()
+  await user.tab()
+  await user.tab()
+  expect(document.activeElement).toEqual(firstButton)
+
+  trapContainer.insertAdjacentHTML('afterbegin', '<button id="first" tabindex="0">New first button</button>')
+  const newFirstButton = trapContainer.querySelector('#first')
+
+  const sentinelStart = trapContainer.querySelector('.sentinel')
+
+  await user.tab({shift: true})
+  expect(trapContainer.firstElementChild).toEqual(sentinelStart)
+  expect(document.activeElement).toEqual(newFirstButton)
+
+  trapContainer.insertAdjacentHTML('beforeend', '<button id="last" tabindex="0">New last button</button>')
+  const newLastButton = trapContainer.querySelector('#last')
+
+  const sentinelEnd = trapContainer.querySelector('.sentinel')
+
+  await user.tab({shift: true})
+  expect(trapContainer.lastElementChild).toEqual(sentinelEnd)
+  expect(document.activeElement).toEqual(newLastButton)
+
+  controller?.abort()
+})
+
+it('should remove the mutation observer when the focus trap is released', async () => {
+  const user = userEvent.setup()
+  const {container} = render(
+    <div>
+      <div id="trapContainer">
+        <button tabIndex={0}>Apple</button>
+        <button tabIndex={0}>Banana</button>
+        <button tabIndex={0}>Cantaloupe</button>
+      </div>
+      <button id="durian" tabIndex={0}>
+        Durian
+      </button>
+    </div>,
+  )
+
+  const trapContainer = container.querySelector<HTMLElement>('#trapContainer')!
+  const [firstButton, secondButton] = trapContainer.querySelectorAll('button')
+  const controller = focusTrap(trapContainer)
+
+  secondButton.focus()
+  await user.tab()
+  await user.tab()
+  expect(document.activeElement).toEqual(firstButton)
+
+  trapContainer.insertAdjacentHTML('afterbegin', '<button id="first" tabindex="0">New first button</button>')
+  const newFirstButton = trapContainer.querySelector('#first')
+
+  const sentinelStart = trapContainer.querySelector('.sentinel')
+
+  await user.tab({shift: true})
+  expect(trapContainer.firstElementChild).toEqual(sentinelStart)
+  expect(document.activeElement).toEqual(newFirstButton)
+
+  controller?.abort()
+
+  trapContainer.insertAdjacentHTML('beforeend', '<button id="last" tabindex="0">New last button</button>')
+  const newLastButton = trapContainer.querySelector('#last')
+
+  await user.tab({shift: true})
+  expect(document.activeElement).not.toEqual(newLastButton)
+  expect(trapContainer.lastElementChild).toEqual(newLastButton)
+})

src/focus-trap.ts

@@ -30,6 +30,40 @@ function followSignal(signal: AbortSignal): AbortController {
   return controller
 }
 
+function observeFocusTrap(container: HTMLElement, sentinels: HTMLElement[]) {
+  const observer = new MutationObserver(mutations => {
+    for (const mutation of mutations) {
+      if (mutation.type === 'childList' && mutation.addedNodes.length) {
+        const sentinelChildren = Array.from(mutation.addedNodes).filter(
+          e => e instanceof HTMLElement && e.classList.contains('sentinel') && e.tagName === 'SPAN',
+        )
+
+        // If any of the added nodes are sentinels, don't do anything
+        if (sentinelChildren.length) {
+          return
+        }
+        // If the first and last children of container aren't sentinels, move them to the start and end
+        const firstChild = container.firstElementChild
+        const lastChild = container.lastElementChild
+
+        const [sentinelStart, sentinelEnd] = sentinels
+
+        // Adds back sentinel to correct position in the DOM
+        if (!firstChild?.classList.contains('sentinel')) {
+          container.insertAdjacentElement('afterbegin', sentinelStart)
+        }
+        if (!lastChild?.classList.contains('sentinel')) {
+          container.insertAdjacentElement('beforeend', sentinelEnd)
+        }
+      }
+    }
+  })
+
+  observer.observe(container, {childList: true})
+
+  return observer
+}
+
 /**
  * Traps focus within the given container
  * @param container The container in which to trap focus
@@ -67,6 +101,8 @@ export function focusTrap(
   container.prepend(sentinelStart)
   container.append(sentinelEnd)
 
+  const observer = observeFocusTrap(container, [sentinelStart, sentinelEnd])
+
   let lastFocusedChild: HTMLElement | undefined = undefined
   // Ensure focus remains in the trap zone by checking that a given recently-focused
   // element is inside the trap zone. If it isn't, redirect focus to a suitable
@@ -117,6 +153,7 @@ export function focusTrap(
     if (suspendedTrapIndex >= 0) {
       suspendedTrapStack.splice(suspendedTrapIndex, 1)
     }
+    observer.disconnect()
     tryReactivate()
   })