chore(tabs): minor code/visibility refactors

Changes:
- Use ElementInternals for aria roles outside of Firefox
- Don't export Tabs interface used by tab
- Get parent tabs element with closest() rather than
- Remove md-tabs delegate focus (not used)
- Remove selectedAttribute property (always 'selected')
- Made focusedItem private
PiperOrigin-RevId: 561287221

tabs/internal/tab.ts

@@ -17,10 +17,7 @@ import {requestUpdateOnAriaChange} from '../../internal/aria/delegate.js';
 import {dispatchActivationClick, isActivationClick} from '../../internal/controller/events.js';
 import {EASING} from '../../internal/motion/animation.js';
 
-/**
- * An element that can select items.
- */
-export interface Tabs extends HTMLElement {
+interface Tabs extends HTMLElement {
   selected?: number;
   selectedItem?: Tab;
   previousSelectedItem?: Tab;
@@ -132,10 +129,6 @@ export class Tab extends LitElement {
     dispatchActivationClick(this.button);
   };
 
-  private get tabs() {
-    return this.parentElement as Tabs;
-  }
-
   private animateSelected() {
     this.indicator.getAnimations().forEach(a => {
       a.cancel();
@@ -152,9 +145,12 @@ export class Tab extends LitElement {
     if (!this.selected) {
       return reduceMotion ? [{'opacity': 1}, {'transform': 'none'}] : null;
     }
+
+    // TODO(b/298105040): avoid hardcoding selector
+    const tabs = this.closest<Tabs>('md-tabs');
     const from: Keyframe = {};
     const fromRect =
-        (this.tabs?.previousSelectedItem?.indicator.getBoundingClientRect() ??
+        (tabs?.previousSelectedItem?.indicator.getBoundingClientRect() ??
          ({} as DOMRect));
     const fromPos = fromRect.left;
     const fromExtent = fromRect.width;

tabs/internal/tabs.ts

@@ -41,12 +41,6 @@ const NAVIGATION_KEYS = new Map([
  *
  */
 export class Tabs extends LitElement {
-  /** @nocollapse */
-  static override readonly shadowRootOptions = {
-    ...LitElement.shadowRootOptions,
-    delegatesFocus: true
-  };
-
   /**
    * Index of the selected item.
    */
@@ -71,8 +65,6 @@ export class Tabs extends LitElement {
   // be kept in sync
   @state() private itemsDirty = false;
 
-  private readonly selectedAttribute = `selected`;
-
   /**
    * The item currently selected.
    */
@@ -90,10 +82,13 @@ export class Tabs extends LitElement {
   /**
    * The item currently focused.
    */
-  protected get focusedItem() {
+  private get focusedItem() {
     return this.items.find((el: HTMLElement) => el.matches(':focus-within'));
   }
 
+  private readonly internals =
+      (this as HTMLElement /* needed for closure */).attachInternals();
+
   constructor() {
     super();
     if (!isServer) {
@@ -105,7 +100,14 @@ export class Tabs extends LitElement {
 
   override connectedCallback() {
     super.connectedCallback();
-    this.setAttribute('role', 'tablist');
+    // Firefox does not support ElementInternals aria yet, so we need to hydrate
+    // an attribute.
+    if (!('role' in this.internals)) {
+      this.setAttribute('role', 'tablist');
+      return;
+    }
+
+    this.internals.role = 'tablist';
   }
 
   // focus item on keydown and optionally select it
@@ -218,8 +220,8 @@ export class Tabs extends LitElement {
     }
     if (itemsChanged || changed.has('selected')) {
       if (this.previousSelectedItem !== this.selectedItem) {
-        this.previousSelectedItem?.removeAttribute(this.selectedAttribute);
-        this.selectedItem?.setAttribute(this.selectedAttribute, '');
+        this.previousSelectedItem?.removeAttribute('selected');
+        this.selectedItem?.setAttribute('selected', '');
       }
       if (this.selectedItem !== this.focusedItem) {
         this.updateFocusableItem(this.selectedItem);