fix(tabs): detach tab portal when tab hides from view (#8486)

src/lib/tabs/tab-body.ts

@@ -81,15 +81,19 @@ export class MatTabBodyPortal extends _MatTabBodyPortalBaseClass implements OnIn
   ngOnInit(): void {
     if (this._host._isCenterPosition(this._host._position)) {
       this.attach(this._host._content);
-    } else {
-      this._centeringSub = this._host._beforeCentering.subscribe(() => {
-        this.attach(this._host._content);
-        this._centeringSub.unsubscribe();
-      });
     }
+    this._centeringSub = this._host._beforeCentering.subscribe((isCentering: boolean) => {
+      if (isCentering) {
+        if (!this.hasAttached()) {
+          this.attach(this._host._content);
+        }
+      } else {
+        this.detach();
+      }
+    });
   }
 
-  /** Clean up subscription if necessary. */
+  /** Clean up centering subscription. */
   ngOnDestroy(): void {
     if (this._centeringSub && !this._centeringSub.closed) {
       this._centeringSub.unsubscribe();
@@ -136,7 +140,7 @@ export class MatTabBody implements OnInit {
   @Output() _onCentering: EventEmitter<number> = new EventEmitter<number>();
 
   /** Event emitted before the centering of the tab begins. */
-  @Output() _beforeCentering: EventEmitter<number> = new EventEmitter<number>();
+  @Output() _beforeCentering: EventEmitter<boolean> = new EventEmitter<boolean>();
 
   /** Event emitted when the tab completes its animation towards the center. */
   @Output() _onCentered: EventEmitter<void> = new EventEmitter<void>(true);
@@ -185,8 +189,9 @@ export class MatTabBody implements OnInit {
   }
 
   _onTranslateTabStarted(e: AnimationEvent): void {
-    if (this._isCenterPosition(e.toState)) {
-      this._beforeCentering.emit();
+    const isCentering = this._isCenterPosition(e.toState);
+    this._beforeCentering.emit(isCentering);
+    if (isCentering) {
       this._onCentering.emit(this._elementRef.nativeElement.clientHeight);
     }
   }

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

@@ -350,6 +350,19 @@ describe('MatTabGroup', () => {
       expect(fixture.componentInstance.legumes).toBeTruthy();
     });
 
+    it('should only have the active tab in the DOM', async(() => {
+      expect(fixture.nativeElement.textContent).toContain('Pizza, fries');
+      expect(fixture.nativeElement.textContent).not.toContain('Peanuts');
+
+      tabGroup.selectedIndex = 3;
+      fixture.detectChanges();
+      // Use whenStable to wait for async observables and change detection to run in content.
+      fixture.whenStable().then(() => {
+        expect(fixture.nativeElement.textContent).not.toContain('Pizza, fries');
+        expect(fixture.nativeElement.textContent).toContain('Peanuts');
+      });
+    }));
+
     it('should support setting the header position', () => {
       let tabGroupNode = fixture.debugElement.query(By.css('mat-tab-group')).nativeElement;