fix(material/tabs): focus wrapping back to selected label when using shift + tab (#14194)

Currently the `tabindex` of each of the tab labels is determined by the selected index. This means that if the user tabbed into the header, pressed the right arrow and then pressed shift + tab, their focus would end up on the selected tab. These changes switch to basing the `tabindex` on the focused index which is tied both to the selected index and the user's keyboard navigation.

(cherry picked from commit b9bfaee)

Author: crisbeto

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

@@ -1,4 +1,4 @@
-import {LEFT_ARROW} from '@angular/cdk/keycodes';
+import {LEFT_ARROW, RIGHT_ARROW} from '@angular/cdk/keycodes';
 import {dispatchFakeEvent, dispatchKeyboardEvent} from '../../cdk/testing/private';
 import {Component, DebugElement, OnInit, QueryList, ViewChild, ViewChildren} from '@angular/core';
 import {
@@ -379,6 +379,23 @@ describe('MDC-based MatTabGroup', () => {
 
       expect(contentElements.map(e => e.getAttribute('tabindex'))).toEqual(['1', null, null]);
     });
+
+    it('should update the tabindex of the labels when navigating via keyboard', () => {
+      fixture.detectChanges();
+
+      const tabLabels = fixture.debugElement
+        .queryAll(By.css('.mat-mdc-tab'))
+        .map(label => label.nativeElement);
+      const tabLabelContainer = fixture.debugElement.query(By.css('.mat-mdc-tab-label-container'))
+        .nativeElement as HTMLElement;
+
+      expect(tabLabels.map(label => label.getAttribute('tabindex'))).toEqual(['-1', '0', '-1']);
+
+      dispatchKeyboardEvent(tabLabelContainer, 'keydown', RIGHT_ARROW);
+      fixture.detectChanges();
+
+      expect(tabLabels.map(label => label.getAttribute('tabindex'))).toEqual(['-1', '-1', '0']);
+    });
   });
 
   describe('aria labelling', () => {

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

@@ -1,4 +1,4 @@
-import {LEFT_ARROW} from '@angular/cdk/keycodes';
+import {LEFT_ARROW, RIGHT_ARROW} from '@angular/cdk/keycodes';
 import {dispatchFakeEvent, dispatchKeyboardEvent} from '../../cdk/testing/private';
 import {Component, DebugElement, OnInit, QueryList, ViewChild, ViewChildren} from '@angular/core';
 import {
@@ -378,6 +378,23 @@ describe('MatTabGroup', () => {
 
       expect(contentElements.map(e => e.getAttribute('tabindex'))).toEqual(['1', null, null]);
     });
+
+    it('should update the tabindex of the labels when navigating via keyboard', () => {
+      fixture.detectChanges();
+
+      const tabLabels = fixture.debugElement
+        .queryAll(By.css('.mat-tab-label'))
+        .map(label => label.nativeElement);
+      const tabLabelContainer = fixture.debugElement.query(By.css('.mat-tab-label-container'))
+        .nativeElement as HTMLElement;
+
+      expect(tabLabels.map(label => label.getAttribute('tabindex'))).toEqual(['-1', '0', '-1']);
+
+      dispatchKeyboardEvent(tabLabelContainer, 'keydown', RIGHT_ARROW);
+      fixture.detectChanges();
+
+      expect(tabLabels.map(label => label.getAttribute('tabindex'))).toEqual(['-1', '-1', '0']);
+    });
   });
 
   describe('aria labelling', () => {

src/material/tabs/tab-group.ts

@@ -99,6 +99,9 @@ export abstract class _MatTabGroupBase
   /** The tab index that should be selected after the content has been checked. */
   private _indexToSelect: number | null = 0;
 
+  /** Index of the tab that was focused last. */
+  private _lastFocusedTabIndex: number | null = null;
+
   /** Snapshot of the height of the tab body wrapper before another tab is activated. */
   private _tabBodyWrapperHeight: number = 0;
 
@@ -267,6 +270,7 @@ export abstract class _MatTabGroupBase
 
     if (this._selectedIndex !== indexToSelect) {
       this._selectedIndex = indexToSelect;
+      this._lastFocusedTabIndex = null;
       this._changeDetectorRef.markForCheck();
     }
   }
@@ -292,6 +296,7 @@ export abstract class _MatTabGroupBase
             // event, otherwise the consumer may end up in an infinite loop in some edge cases like
             // adding a tab within the `selectedIndexChange` event.
             this._indexToSelect = this._selectedIndex = i;
+            this._lastFocusedTabIndex = null;
             selectedTab = tabs[i];
             break;
           }
@@ -366,6 +371,7 @@ export abstract class _MatTabGroupBase
   }
 
   _focusChanged(index: number) {
+    this._lastFocusedTabIndex = index;
     this.focusChange.emit(this._createChangeEvent(index));
   }
 
@@ -448,11 +454,12 @@ export abstract class _MatTabGroupBase
   }
 
   /** Retrieves the tabindex for the tab. */
-  _getTabIndex(tab: MatTab, idx: number): number | null {
+  _getTabIndex(tab: MatTab, index: number): number | null {
     if (tab.disabled) {
       return null;
     }
-    return this.selectedIndex === idx ? 0 : -1;
+    const targetIndex = this._lastFocusedTabIndex ?? this.selectedIndex;
+    return index === targetIndex ? 0 : -1;
   }
 
   /** Callback for when the focused state of a tab has changed. */

tools/public_api_guard/material/tabs.md

@@ -240,7 +240,7 @@ export abstract class _MatTabGroupBase extends _MatTabGroupMixinBase implements
     _focusChanged(index: number): void;
     focusTab(index: number): void;
     _getTabContentId(i: number): string;
-    _getTabIndex(tab: MatTab, idx: number): number | null;
+    _getTabIndex(tab: MatTab, index: number): number | null;
     _getTabLabelId(i: number): string;
     _handleClick(tab: MatTab, tabHeader: MatTabGroupBaseHeader, index: number): void;
     headerPosition: MatTabHeaderPosition;