fix(tabs): don't fire change event when amount of tabs changes (#12097)

Fixes the tab group firing its `selectedTabChange` event when the amount of tabs changes, which might throw the consumer into an infinite loop if they're adding new tabs inside the callback.

Fixes #12084.

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

@@ -403,6 +403,20 @@ describe('MatTabGroup', () => {
       expect(component._tabs.toArray()[1].isActive).toBe(true);
     });
 
+    it('should not fire `selectedTabChange` when the amount of tabs changes', fakeAsync(() => {
+      fixture.detectChanges();
+      fixture.componentInstance.selectedIndex = 1;
+      fixture.detectChanges();
+
+      // Add a new tab at the beginning.
+      spyOn(fixture.componentInstance, 'handleSelection');
+      fixture.componentInstance.tabs.unshift({label: 'New tab', content: 'at the start'});
+      fixture.detectChanges();
+      tick();
+      fixture.detectChanges();
+
+      expect(fixture.componentInstance.handleSelection).not.toHaveBeenCalled();
+    }));
 
   });
 

src/lib/tabs/tab-group.ts

@@ -205,7 +205,10 @@ export class MatTabGroup extends _MatTabGroupMixinBase implements AfterContentIn
       // Maintain the previously-selected tab if a new tab is added or removed.
       for (let i = 0; i < tabs.length; i++) {
         if (tabs[i].isActive) {
-          this._indexToSelect = i;
+          // Assign both to the `_indexToSelect` and `_selectedIndex` so we don't fire a changed
+          // 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;
           break;
         }
       }