Trap focus when connected in `<details open>`

When the `<details-dialog>` element is connected to the document as a
child of a `<details>` element rendered with `[open]`, trap focus
immediately.

In support of this change, extract the `trapFocus` and `releaseFocus`
functions, then call `trapFocus` within the `connectedCallback` if the
ancestor detail is `[open]`.

src/index.ts

@@ -95,6 +95,25 @@ function onSummaryClick(event: Event): void {
   }
 }
 
+function trapFocus(dialog: DetailsDialogElement, details: Element): void {
+  const root = 'getRootNode' in dialog ? (dialog.getRootNode() as Document | ShadowRoot) : document
+  if (root.activeElement instanceof HTMLElement) {
+    initialized.set(dialog, {details, activeElement: root.activeElement})
+  }
+
+  autofocus(dialog)
+  ;(details as HTMLElement).addEventListener('keydown', keydown)
+}
+
+function releaseFocus(dialog: DetailsDialogElement, details: Element): void {
+  for (const form of dialog.querySelectorAll('form')) {
+    form.reset()
+  }
+  const focusElement = findFocusElement(details, dialog)
+  if (focusElement) focusElement.focus()
+  ;(details as HTMLElement).removeEventListener('keydown', keydown)
+}
+
 function toggle(event: Event): void {
   const details = event.currentTarget
   if (!(details instanceof Element)) return
@@ -102,20 +121,9 @@ function toggle(event: Event): void {
   if (!(dialog instanceof DetailsDialogElement)) return
 
   if (details.hasAttribute('open')) {
-    const root = 'getRootNode' in dialog ? (dialog.getRootNode() as Document | ShadowRoot) : document
-    if (root.activeElement instanceof HTMLElement) {
-      initialized.set(dialog, {details, activeElement: root.activeElement})
-    }
-
-    autofocus(dialog)
-    ;(details as HTMLElement).addEventListener('keydown', keydown)
+    trapFocus(dialog, details)
   } else {
-    for (const form of dialog.querySelectorAll('form')) {
-      form.reset()
-    }
-    const focusElement = findFocusElement(details, dialog)
-    if (focusElement) focusElement.focus()
-    ;(details as HTMLElement).removeEventListener('keydown', keydown)
+    releaseFocus(dialog, details)
   }
 }
 
@@ -249,6 +257,7 @@ class DetailsDialogElement extends HTMLElement {
 
     details.addEventListener('toggle', toggle)
     state.details = details
+    if (details.hasAttribute('open')) trapFocus(this, details)
 
     updateIncludeFragmentEventListeners(details, this.src, this.preload)
   }

test/test.js

@@ -289,6 +289,48 @@ describe('details-dialog-element', function() {
     })
   })
 
+  describe('connected as a child of an already [open] <details> element', function () {
+    let details
+    let dialog
+    let summary
+    let close
+
+    beforeEach(function() {
+      const container = document.createElement('div')
+      container.innerHTML = `
+        <details open>
+          <summary>Click</summary>
+          <details-dialog>
+            <button data-button>Button</button>
+            <button ${CLOSE_ATTR}>Goodbye</button>
+          </details-dialog>
+        </details>
+      `
+      document.body.append(container)
+
+      details = document.querySelector('details')
+      dialog = details.querySelector('details-dialog')
+      summary = details.querySelector('summary')
+      close = dialog.querySelector(CLOSE_SELECTOR)
+    })
+
+    afterEach(function() {
+      document.body.innerHTML = ''
+    })
+
+    it('manages focus', async function() {
+      assert.equal(document.activeElement, dialog)
+      triggerKeydownEvent(document.activeElement, 'Tab', true)
+      assert.equal(document.activeElement, document.querySelector(`[${CLOSE_ATTR}]`))
+      triggerKeydownEvent(document.activeElement, 'Tab')
+      assert.equal(document.activeElement, document.querySelector(`[data-button]`))
+      triggerKeydownEvent(document.activeElement, 'Tab')
+      assert.equal(document.activeElement, document.querySelector(`[${CLOSE_ATTR}]`))
+      triggerKeydownEvent(document.activeElement, 'Tab')
+      assert.equal(document.activeElement, document.querySelector(`[data-button]`))
+    })
+  })
+
   describe('shadow DOM context', function() {
     let shadowRoot, details, summary, dialog
     beforeEach(function() {