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

Add an updatePosition() method to MatAutocompleteTrigger that re-evaluates the position of the
autocomplete panel overlay and falls back to the above position if necessary. This is useful if
enough options have been added to the panel while it is open to cause it to exceed the viewport.

Fixes #11492

Author: benelliott

src/lib/autocomplete/autocomplete-trigger.ts

@@ -224,6 +224,16 @@ export class MatAutocompleteTrigger implements ControlValueAccessor, OnDestroy {
     }
   }
 
+  /**
+   * Updates the position of the autocomplete suggestion panel to ensure that its position allows
+   * for all suggestions to be displayed 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

@@ -1478,6 +1478,54 @@ 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();
+
+      Promise.resolve().then(() => {
+        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();
+        fixture.detectChanges();
+        tick();
+
+        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', () => {
@@ -2198,6 +2246,7 @@ class AutocompleteWithNumbers {
   `
 })
 class AutocompleteWithOnPushDelay implements OnInit {
+  @ViewChild(MatAutocompleteTrigger) trigger: MatAutocompleteTrigger;
   options: string[];
 
   ngOnInit() {