fix(material/stepper): fix nested tabpanel role in stepper

Updates Angular Components Stepper component to fix the nested
aria-role tabpanel in the vertical stepper.

Fixes b/361783174

Author: essjay05

src/material/stepper/step-header.ts

@@ -35,7 +35,6 @@ import {_CdkPrivateStyleLoader, _VisuallyHiddenLoader} from '@angular/cdk/privat
   host: {
     'class': 'mat-step-header',
     '[class]': '"mat-" + (color || "primary")',
-    'role': 'tab',
   },
   encapsulation: ViewEncapsulation.None,
   changeDetection: ChangeDetectionStrategy.OnPush,

src/material/stepper/stepper.html

@@ -11,9 +11,10 @@
 @switch (orientation) {
   @case ('horizontal') {
     <div class="mat-horizontal-stepper-wrapper">
-      <div class="mat-horizontal-stepper-header-container">
+      <div class="mat-horizontal-stepper-header-container" role="tablist">
         @for (step of steps; track step) {
           <ng-container
+            role="tab"
             [ngTemplateOutlet]="stepTemplate"
             [ngTemplateOutletContext]="{step, i: $index}"/>
           @if (!$last) {
@@ -40,28 +41,29 @@
   }
 
   @case ('vertical') {
-    @for (step of steps; track step) {
-      <div class="mat-step">
-        <ng-container
-          [ngTemplateOutlet]="stepTemplate"
-          [ngTemplateOutletContext]="{step, i: $index}"/>
-        <div
-          #animatedContainer
-          class="mat-vertical-content-container"
-          [class.mat-stepper-vertical-line]="!$last"
-          [class.mat-vertical-content-container-active]="selectedIndex === $index"
-          [attr.inert]="selectedIndex === $index ? null : ''">
-          <div class="mat-vertical-stepper-content"
-            role="tabpanel"
-            [id]="_getStepContentId($index)"
-            [attr.aria-labelledby]="_getStepLabelId($index)">
-            <div class="mat-vertical-content">
-              <ng-container [ngTemplateOutlet]="step.content"/>
+    <div class="mat-vertical-stepper-wrapper" role="tree">
+      @for (step of steps; track step) {
+        <div class="mat-step" role="treeitem">
+          <ng-container
+            [ngTemplateOutlet]="stepTemplate"
+            [ngTemplateOutletContext]="{step, i: $index}"/>
+          <div
+            #animatedContainer
+            class="mat-vertical-content-container"
+            [class.mat-stepper-vertical-line]="!$last"
+            [class.mat-vertical-content-container-active]="selectedIndex === $index"
+            [attr.inert]="selectedIndex === $index ? null : ''">
+            <div class="mat-vertical-stepper-content"
+              [id]="_getStepContentId($index)"
+              [attr.aria-labelledby]="_getStepLabelId($index)">
+              <div class="mat-vertical-content">
+                <ng-container [ngTemplateOutlet]="step.content"/>
+              </div>
             </div>
           </div>
         </div>
-      </div>
-    }
+      }
+    </div>
   }
 }
 

src/material/stepper/stepper.ts

@@ -132,7 +132,6 @@ export class MatStep extends CdkStep implements ErrorStateMatcher, AfterContentI
     '[class.mat-stepper-animating]': '_isAnimating()',
     '[style.--mat-stepper-animation-duration]': '_getAnimationDuration()',
     '[attr.aria-orientation]': 'orientation',
-    'role': 'tablist',
   },
   providers: [{provide: CdkStepper, useExisting: MatStepper}],
   encapsulation: ViewEncapsulation.None,