feat(material/tabs): allow for content tabindex to be customized

In some cases, adding a `tabindex` to a `tabpanel` can improve accessibility, however that
is currently impossible with our setup, because the element isn't exposed. These changes
add an input that allows consumers to customize the `tabindex` based on their use case.

We tried to tackle this in an earlier PR (#14808), but it was decided not to proceed,
because the `tabindex` isn't relevant for all use cases. This approach should be a bit
more flexible since it allows users to opt into it.

Fixes #21819.

Author: crisbeto

src/material-experimental/mdc-tabs/tab-body.scss

@@ -7,6 +7,7 @@
   @include mat-fill;
   display: block;
   overflow: hidden;
+  outline: 0;
 
   // Fix for auto content wrapping in IE11
   flex-basis: 100%;

src/material-experimental/mdc-tabs/tab-group.html

@@ -54,8 +54,9 @@
   <mat-tab-body role="tabpanel"
                *ngFor="let tab of _tabs; let i = index"
                [id]="_getTabContentId(i)"
+               [attr.tabindex]="(contentTabIndex != null && selectedIndex === i) ? contentTabIndex : null"
                [attr.aria-labelledby]="_getTabLabelId(i)"
-               [class.mat-mdc-tab-body-active]="selectedIndex == i"
+               [class.mat-mdc-tab-body-active]="selectedIndex === i"
                [content]="tab.content!"
                [position]="tab.position!"
                [origin]="tab.origin"

src/material-experimental/mdc-tabs/tab-group.spec.ts

@@ -327,6 +327,20 @@ describe('MDC-based MatTabGroup', () => {
         .toHaveBeenCalledWith(jasmine.objectContaining({index: 2}));
     }));
 
+    it('should be able to set a tabindex on the inner content element', () => {
+      fixture.componentInstance.contentTabIndex = 1;
+      fixture.detectChanges();
+      const contentElements = Array.from<HTMLElement>(fixture.nativeElement
+          .querySelectorAll('mat-tab-body'));
+
+      expect(contentElements.map(e => e.getAttribute('tabindex'))).toEqual([null, '1', null]);
+
+      fixture.componentInstance.selectedIndex = 0;
+      fixture.detectChanges();
+
+      expect(contentElements.map(e => e.getAttribute('tabindex'))).toEqual(['1', null, null]);
+    });
+
   });
 
   describe('aria labelling', () => {
@@ -808,6 +822,7 @@ describe('MatTabNavBar with a default config', () => {
         [(selectedIndex)]="selectedIndex"
         [headerPosition]="headerPosition"
         [disableRipple]="disableRipple"
+        [contentTabIndex]="contentTabIndex"
         (animationDone)="animationDone()"
         (focusChange)="handleFocus($event)"
         (selectedTabChange)="handleSelection($event)">
@@ -833,6 +848,7 @@ class SimpleTabsTestApp {
   focusEvent: any;
   selectEvent: any;
   disableRipple: boolean = false;
+  contentTabIndex: number | null = null;
   headerPosition: MatTabHeaderPosition = 'above';
   handleFocus(event: any) {
     this.focusEvent = event;

src/material/tabs/tab-config.ts

@@ -26,6 +26,9 @@ export interface MatTabsConfig {
 
   /** Whether the tab group should grow to the size of the active tab. */
   dynamicHeight?: boolean;
+
+  /** `tabindex` to be set on the inner element that wraps the tab content. */
+  contentTabIndex?: number;
 }
 
 /** Injection token that can be used to provide the default options the tabs module. */

src/material/tabs/tab-group.html

@@ -40,8 +40,9 @@
   <mat-tab-body role="tabpanel"
                *ngFor="let tab of _tabs; let i = index"
                [id]="_getTabContentId(i)"
+               [attr.tabindex]="(contentTabIndex != null && selectedIndex === i) ? contentTabIndex : null"
                [attr.aria-labelledby]="_getTabLabelId(i)"
-               [class.mat-tab-body-active]="selectedIndex == i"
+               [class.mat-tab-body-active]="selectedIndex === i"
                [content]="tab.content!"
                [position]="tab.position!"
                [origin]="tab.origin"

src/material/tabs/tab-group.scss

@@ -50,6 +50,7 @@
   @include mat-fill;
   display: block;
   overflow: hidden;
+  outline: 0;
 
   // Fix for auto content wrapping in IE11
   flex-basis: 100%;

src/material/tabs/tab-group.spec.ts

@@ -326,6 +326,20 @@ describe('MatTabGroup', () => {
         .toHaveBeenCalledWith(jasmine.objectContaining({index: 2}));
     }));
 
+    it('should be able to set a tabindex on the inner content element', () => {
+      fixture.componentInstance.contentTabIndex = 1;
+      fixture.detectChanges();
+      const contentElements = Array.from<HTMLElement>(fixture.nativeElement
+          .querySelectorAll('mat-tab-body'));
+
+      expect(contentElements.map(e => e.getAttribute('tabindex'))).toEqual([null, '1', null]);
+
+      fixture.componentInstance.selectedIndex = 0;
+      fixture.detectChanges();
+
+      expect(contentElements.map(e => e.getAttribute('tabindex'))).toEqual(['1', null, null]);
+    });
+
   });
 
   describe('aria labelling', () => {
@@ -752,6 +766,7 @@ describe('nested MatTabGroup with enabled animations', () => {
         [(selectedIndex)]="selectedIndex"
         [headerPosition]="headerPosition"
         [disableRipple]="disableRipple"
+        [contentTabIndex]="contentTabIndex"
         (animationDone)="animationDone()"
         (focusChange)="handleFocus($event)"
         (selectedTabChange)="handleSelection($event)">
@@ -777,6 +792,7 @@ class SimpleTabsTestApp {
   focusEvent: any;
   selectEvent: any;
   disableRipple: boolean = false;
+  contentTabIndex: number | null = null;
   headerPosition: MatTabHeaderPosition = 'above';
   handleFocus(event: any) {
     this.focusEvent = event;

src/material/tabs/tab-group.ts

@@ -131,6 +131,19 @@ export abstract class _MatTabGroupBase extends _MatTabGroupMixinBase implements
   }
   private _animationDuration: string;
 
+  /**
+   * `tabindex` to be set on the inner element that wraps the tab content. Can be used for improved
+   * accessibility when the tab does not have focusable elements or if it has scrollable content.
+   * The `tabindex` will be removed automatically for inactive tabs.
+   * Read more at https://www.w3.org/TR/wai-aria-practices/examples/tabs/tabs-2/tabs.html
+   */
+  @Input()
+  get contentTabIndex(): number | null { return this._contentTabIndex; }
+  set contentTabIndex(value: number | null) {
+    this._contentTabIndex = coerceNumberProperty(value, null);
+  }
+  private _contentTabIndex: number | null;
+
   /**
    * Whether pagination should be disabled. This can be used to avoid unnecessary
    * layout recalculations if it's known that pagination won't be required.
@@ -182,6 +195,7 @@ export abstract class _MatTabGroupBase extends _MatTabGroupMixinBase implements
         defaultConfig.disablePagination : false;
     this.dynamicHeight = defaultConfig && defaultConfig.dynamicHeight != null ?
         defaultConfig.dynamicHeight : false;
+    this.contentTabIndex = defaultConfig?.contentTabIndex ?? null;
   }
 
   /**
@@ -385,6 +399,7 @@ export abstract class _MatTabGroupBase extends _MatTabGroupMixinBase implements
   static ngAcceptInputType_animationDuration: NumberInput;
   static ngAcceptInputType_selectedIndex: NumberInput;
   static ngAcceptInputType_disableRipple: BooleanInput;
+  static ngAcceptInputType_contentTabIndex: BooleanInput;
 }
 
 /**

tools/public_api_guard/material/tabs.d.ts

@@ -41,6 +41,8 @@ export declare abstract class _MatTabGroupBase extends _MatTabGroupMixinBase imp
     set animationDuration(value: string);
     get backgroundColor(): ThemePalette;
     set backgroundColor(value: ThemePalette);
+    get contentTabIndex(): number | null;
+    set contentTabIndex(value: number | null);
     disablePagination: boolean;
     get dynamicHeight(): boolean;
     set dynamicHeight(value: boolean);
@@ -64,10 +66,11 @@ export declare abstract class _MatTabGroupBase extends _MatTabGroupMixinBase imp
     ngOnDestroy(): void;
     realignInkBar(): void;
     static ngAcceptInputType_animationDuration: NumberInput;
+    static ngAcceptInputType_contentTabIndex: BooleanInput;
     static ngAcceptInputType_disableRipple: BooleanInput;
     static ngAcceptInputType_dynamicHeight: BooleanInput;
     static ngAcceptInputType_selectedIndex: NumberInput;
-    static ɵdir: i0.ɵɵDirectiveDefWithMeta<_MatTabGroupBase, never, never, { "dynamicHeight": "dynamicHeight"; "selectedIndex": "selectedIndex"; "headerPosition": "headerPosition"; "animationDuration": "animationDuration"; "disablePagination": "disablePagination"; "backgroundColor": "backgroundColor"; }, { "selectedIndexChange": "selectedIndexChange"; "focusChange": "focusChange"; "animationDone": "animationDone"; "selectedTabChange": "selectedTabChange"; }, never>;
+    static ɵdir: i0.ɵɵDirectiveDefWithMeta<_MatTabGroupBase, never, never, { "dynamicHeight": "dynamicHeight"; "selectedIndex": "selectedIndex"; "headerPosition": "headerPosition"; "animationDuration": "animationDuration"; "contentTabIndex": "contentTabIndex"; "disablePagination": "disablePagination"; "backgroundColor": "backgroundColor"; }, { "selectedIndexChange": "selectedIndexChange"; "focusChange": "focusChange"; "animationDone": "animationDone"; "selectedTabChange": "selectedTabChange"; }, never>;
     static ɵfac: i0.ɵɵFactoryDef<_MatTabGroupBase, [null, null, { optional: true; }, { optional: true; }]>;
 }
 
@@ -250,6 +253,7 @@ export declare const matTabsAnimations: {
 
 export interface MatTabsConfig {
     animationDuration?: string;
+    contentTabIndex?: number;
     disablePagination?: boolean;
     dynamicHeight?: boolean;
     fitInkBarToContent?: boolean;