feat(autocomplete): add updatePosition() method to MatAutocompleteTrigger (#11495)

src/lib/autocomplete/autocomplete-trigger.ts

@@ -230,6 +230,16 @@ export class MatAutocompleteTrigger implements ControlValueAccessor, OnDestroy {
     }
   }
 
+  /**
+   * Updates the position of the autocomplete suggestion panel to ensure that it fits all options
+   * within the viewport.
+   */
+  updatePosition(): void {
+    if (this._overlayAttached) {
+      this._overlayRef!.updatePosition();
+    }
+  }
+
   /**
    * A stream of actions that should close the autocomplete panel, including
    * when an option is selected, on blur, and when TAB is pressed.

src/lib/autocomplete/autocomplete.spec.ts

@@ -1506,6 +1506,50 @@ describe('MatAutocomplete', () => {
           .toEqual(Math.floor(panelBottom), `Expected panel to stay aligned after filtering.`);
     }));
 
+    it('should fall back to above position when requested if options are added while ' +
+        'the panel is open', fakeAsync(() => {
+      let fixture = createComponent(AutocompleteWithOnPushDelay);
+      fixture.detectChanges();
+
+      let inputEl = fixture.debugElement.query(By.css('input')).nativeElement;
+      let inputReference = fixture.debugElement.query(By.css('.mat-form-field-flex')).nativeElement;
+
+      // Push the element down so it has a little bit of space, but not enough to render.
+      inputReference.style.bottom = '10px';
+      inputReference.style.position = 'fixed';
+
+      // Focus the input to load the deferred options.
+      dispatchFakeEvent(inputEl, 'focusin');
+      tick(1000);
+
+      fixture.detectChanges();
+      tick();
+
+      const inputBottom = inputReference.getBoundingClientRect().bottom;
+      const panel = overlayContainerElement.querySelector('.mat-autocomplete-panel')!;
+      const panelTop = panel.getBoundingClientRect().top;
+
+      expect(Math.floor(inputBottom))
+        .toEqual(Math.floor(panelTop),
+          `Expected panel top to be below input before repositioning.`);
+
+      // Request a position update now that there are too many suggestions to fit in the viewport.
+      fixture.componentInstance.trigger.updatePosition();
+
+      const inputTop = inputReference.getBoundingClientRect().top;
+      const panelBottom = panel.getBoundingClientRect().bottom;
+
+      expect(Math.floor(inputTop))
+        .toEqual(Math.floor(panelBottom),
+          `Expected panel to fall back to above position after repositioning.`);
+    }));
+
+    it('should not throw if a panel reposition is requested while the panel is closed', () => {
+        let fixture = createComponent(SimpleAutocomplete);
+        fixture.detectChanges();
+
+        expect(() => fixture.componentInstance.trigger.updatePosition()).not.toThrow();
+    });
   });
 
   describe('Option selection', () => {
@@ -2303,6 +2347,7 @@ class AutocompleteWithNumbers {
   `
 })
 class AutocompleteWithOnPushDelay implements OnInit {
+  @ViewChild(MatAutocompleteTrigger) trigger: MatAutocompleteTrigger;
   options: string[];
 
   ngOnInit() {