fix(tabs): always defaulting focusIndex to 0 on initialization (#20384)

We were always defaulting the `focusIndex` to 0 on init, rather than taking the index of the selected tab. We actually had tests for this behavior, but they were all testing against 0 so we never caught the issue.

Fixes #20374.

(cherry picked from commit a0b44d4)

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

@@ -248,12 +248,12 @@ describe('MDC-based MatTabGroup', () => {
 
       expect(fixture.componentInstance.handleFocus).toHaveBeenCalledTimes(0);
 
-      tabLabels[1].nativeElement.click();
+      tabLabels[2].nativeElement.click();
       fixture.detectChanges();
 
       expect(fixture.componentInstance.handleFocus).toHaveBeenCalledTimes(1);
       expect(fixture.componentInstance.handleFocus)
-        .toHaveBeenCalledWith(jasmine.objectContaining({index: 1}));
+        .toHaveBeenCalledWith(jasmine.objectContaining({index: 2}));
     });
 
     it('should emit focusChange on arrow key navigation', () => {
@@ -267,8 +267,8 @@ describe('MDC-based MatTabGroup', () => {
       expect(fixture.componentInstance.handleFocus).toHaveBeenCalledTimes(0);
 
       // In order to verify that the `focusChange` event also fires with the correct
-      // index, we focus the second tab before testing the keyboard navigation.
-      tabLabels[1].nativeElement.click();
+      // index, we focus the third tab before testing the keyboard navigation.
+      tabLabels[2].nativeElement.click();
       fixture.detectChanges();
 
       expect(fixture.componentInstance.handleFocus).toHaveBeenCalledTimes(1);
@@ -277,7 +277,7 @@ describe('MDC-based MatTabGroup', () => {
 
       expect(fixture.componentInstance.handleFocus).toHaveBeenCalledTimes(2);
       expect(fixture.componentInstance.handleFocus)
-        .toHaveBeenCalledWith(jasmine.objectContaining({index: 0}));
+        .toHaveBeenCalledWith(jasmine.objectContaining({index: 1}));
     });
 
     it('should clean up the tabs QueryList on destroy', () => {

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

@@ -63,8 +63,15 @@ describe('MDC-based MatTabHeader', () => {
     });
 
     it('should initialize to the selected index', () => {
+      // Recreate the fixture so we can test that it works with a non-default selected index
+      fixture.destroy();
+      fixture = TestBed.createComponent(SimpleTabHeaderApp);
+      fixture.componentInstance.selectedIndex = 1;
       fixture.detectChanges();
-      expect(appComponent.tabHeader.focusIndex).toBe(appComponent.selectedIndex);
+      appComponent = fixture.componentInstance;
+      tabListContainer = appComponent.tabHeader._tabListContainer.nativeElement;
+
+      expect(appComponent.tabHeader.focusIndex).toBe(1);
     });
 
     it('should send focus change event', () => {

src/material/tabs/paginated-tab-header.ts

@@ -196,7 +196,7 @@ export abstract class MatPaginatedTabHeader implements AfterContentChecked, Afte
       .withHomeAndEnd()
       .withWrap();
 
-    this._keyManager.updateActiveItem(0);
+    this._keyManager.updateActiveItem(this._selectedIndex);
 
     // Defer the first call in order to allow for slower browsers to lay out the elements.
     // This helps in cases where the user lands directly on a page with paginated tabs.

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

@@ -247,12 +247,12 @@ describe('MatTabGroup', () => {
 
       expect(fixture.componentInstance.handleFocus).toHaveBeenCalledTimes(0);
 
-      tabLabels[1].nativeElement.click();
+      tabLabels[2].nativeElement.click();
       fixture.detectChanges();
 
       expect(fixture.componentInstance.handleFocus).toHaveBeenCalledTimes(1);
       expect(fixture.componentInstance.handleFocus)
-        .toHaveBeenCalledWith(jasmine.objectContaining({index: 1}));
+        .toHaveBeenCalledWith(jasmine.objectContaining({index: 2}));
     });
 
     it('should emit focusChange on arrow key navigation', () => {
@@ -266,8 +266,8 @@ describe('MatTabGroup', () => {
       expect(fixture.componentInstance.handleFocus).toHaveBeenCalledTimes(0);
 
       // In order to verify that the `focusChange` event also fires with the correct
-      // index, we focus the second tab before testing the keyboard navigation.
-      tabLabels[1].nativeElement.click();
+      // index, we focus the third tab before testing the keyboard navigation.
+      tabLabels[2].nativeElement.click();
       fixture.detectChanges();
 
       expect(fixture.componentInstance.handleFocus).toHaveBeenCalledTimes(1);
@@ -276,7 +276,7 @@ describe('MatTabGroup', () => {
 
       expect(fixture.componentInstance.handleFocus).toHaveBeenCalledTimes(2);
       expect(fixture.componentInstance.handleFocus)
-        .toHaveBeenCalledWith(jasmine.objectContaining({index: 0}));
+        .toHaveBeenCalledWith(jasmine.objectContaining({index: 1}));
     });
 
     it('should clean up the tabs QueryList on destroy', () => {

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

@@ -65,8 +65,15 @@ describe('MatTabHeader', () => {
     });
 
     it('should initialize to the selected index', () => {
+      // Recreate the fixture so we can test that it works with a non-default selected index
+      fixture.destroy();
+      fixture = TestBed.createComponent(SimpleTabHeaderApp);
+      fixture.componentInstance.selectedIndex = 1;
       fixture.detectChanges();
-      expect(appComponent.tabHeader.focusIndex).toBe(appComponent.selectedIndex);
+      appComponent = fixture.componentInstance;
+      tabListContainer = appComponent.tabHeader._tabListContainer.nativeElement;
+
+      expect(appComponent.tabHeader.focusIndex).toBe(1);
     });
 
     it('should send focus change event', () => {