fix(tabs): maintain selected tab when new tabs are added or removed

Maintains the `selectedIndex` of the current tab when a new tab is added or removed. Previously, changing the amount of tabs would shift the array, causing a different tab to be selected.

Fixes #7738.

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

@@ -278,10 +278,12 @@ describe('MatTabGroup', () => {
       expect(tabs[3].origin).toBeGreaterThanOrEqual(0);
 
       // Add a new tab in the beginning and select it, expect an origin < than 0 (animate left)
-      fixture.componentInstance.tabs.push({label: 'New tab', content: 'to left of index'});
       fixture.componentInstance.selectedIndex = 0;
       fixture.detectChanges();
 
+      fixture.componentInstance.tabs.push({label: 'New tab', content: 'to left of index'});
+      fixture.detectChanges();
+
       tabs = component._tabs.toArray();
       expect(tabs[0].origin).toBeLessThan(0);
     });
@@ -303,6 +305,47 @@ describe('MatTabGroup', () => {
       expect(component.selectedIndex).toBe(numberOfTabs - 2);
     });
 
+
+    it('should maintain the selected tab if a new tab is added', () => {
+      fixture.detectChanges();
+      const component: MatTabGroup =
+          fixture.debugElement.query(By.css('mat-tab-group')).componentInstance;
+
+      fixture.componentInstance.selectedIndex = 1;
+      fixture.detectChanges();
+
+      // Add a new tab at the beginning.
+      fixture.componentInstance.tabs.unshift({label: 'New tab', content: 'at the start'});
+      fixture.detectChanges();
+
+      expect(component.selectedIndex).toBe(2);
+      expect(component._tabs.toArray()[2].isActive).toBe(true);
+    });
+
+
+    it('should maintain the selected tab if a tab is removed', () => {
+      // Add a couple of tabs so we have more to work with.
+      fixture.componentInstance.tabs.push(
+        {label: 'New tab', content: 'with new content'},
+        {label: 'Another new tab', content: 'with newer content'}
+      );
+
+      // Select the second-to-last tab.
+      fixture.componentInstance.selectedIndex = 3;
+      fixture.detectChanges();
+
+      const component: MatTabGroup =
+          fixture.debugElement.query(By.css('mat-tab-group')).componentInstance;
+
+      // Remove a tab right before the selected one.
+      fixture.componentInstance.tabs.splice(2, 1);
+      fixture.detectChanges();
+
+      expect(component.selectedIndex).toBe(1);
+      expect(component._tabs.toArray()[1].isActive).toBe(true);
+    });
+
+
   });
 
   describe('async tabs', () => {

src/lib/tabs/tab-group.ts

@@ -210,6 +210,16 @@ export class MatTabGroup extends _MatTabGroupMixinBase implements AfterContentIn
     // Subscribe to changes in the amount of tabs, in order to be
     // able to re-render the content as new tabs are added or removed.
     this._tabsSubscription = this._tabs.changes.subscribe(() => {
+      const tabs = this._tabs.toArray();
+
+      // 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;
+          break;
+        }
+      }
+
       this._subscribeToTabLabels();
       this._changeDetectorRef.markForCheck();
     });