fix(material/menu): adjust overlay size when amount of items changes

Currently we lock the menu into a position after it is opened so that it doesn't jump
around when the user scrolls, but this means that if the amount of items changes,
it might not be the optimal position anymore.

These changes add some code to re-calculate the position if the amount of items changes.

Fixes angular#21456.

src/material-experimental/mdc-menu/menu.spec.ts

@@ -41,7 +41,7 @@ import {
   MockNgZone,
 } from '@angular/cdk/testing/private';
 import {Subject} from 'rxjs';
-import {ScrollDispatcher} from '@angular/cdk/scrolling';
+import {ScrollDispatcher, ViewportRuler} from '@angular/cdk/scrolling';
 import {FocusMonitor} from '@angular/cdk/a11y';
 import {
   MAT_MENU_SCROLL_STRATEGY,
@@ -58,6 +58,7 @@ describe('MDC-based MatMenu', () => {
   let overlayContainer: OverlayContainer;
   let overlayContainerElement: HTMLElement;
   let focusMonitor: FocusMonitor;
+  let viewportRuler: ViewportRuler;
 
   function createComponent<T>(component: Type<T>,
                               providers: Provider[] = [],
@@ -68,11 +69,13 @@ describe('MDC-based MatMenu', () => {
       providers
     }).compileComponents();
 
-    inject([OverlayContainer, FocusMonitor], (oc: OverlayContainer, fm: FocusMonitor) => {
-      overlayContainer = oc;
-      overlayContainerElement = oc.getContainerElement();
-      focusMonitor = fm;
-    })();
+    inject([OverlayContainer, FocusMonitor, ViewportRuler],
+      (oc: OverlayContainer, fm: FocusMonitor, vr: ViewportRuler) => {
+        overlayContainer = oc;
+        overlayContainerElement = oc.getContainerElement();
+        focusMonitor = fm;
+        viewportRuler = vr;
+      })();
 
     return TestBed.createComponent<T>(component);
   }
@@ -1059,6 +1062,28 @@ describe('MDC-based MatMenu', () => {
     expect(document.activeElement).toBe(items[3], 'Expected fourth item to be focused');
   }));
 
+  it('should keep the panel in the viewport when more items are added while open', () => {
+    const fixture = createComponent(SimpleMenu, [], [FakeIcon]);
+    fixture.detectChanges();
+
+    const triggerEl = fixture.componentInstance.triggerEl.nativeElement;
+    triggerEl.style.position = 'absolute';
+    triggerEl.style.left = '200px';
+    triggerEl.style.bottom = '300px';
+    triggerEl.click();
+    fixture.detectChanges();
+
+    const panel = overlayContainerElement.querySelector('.mat-mdc-menu-panel')!;
+    const viewportHeight = viewportRuler.getViewportSize().height;
+    let panelRect = panel.getBoundingClientRect();
+    expect(Math.floor(panelRect.bottom)).toBeLessThan(viewportHeight);
+
+    fixture.componentInstance.extraItems = new Array(50).fill('Hello there');
+    fixture.detectChanges();
+    panelRect = panel.getBoundingClientRect();
+    expect(Math.floor(panelRect.bottom)).toBe(viewportHeight);
+  });
+
   describe('lazy rendering', () => {
     it('should be able to render the menu content lazily', fakeAsync(() => {
       const fixture = createComponent(SimpleLazyMenu);

src/material/menu/menu-trigger.ts

@@ -247,8 +247,9 @@ export class MatMenuTrigger implements AfterContentInit, OnDestroy {
 
     const overlayRef = this._createOverlay();
     const overlayConfig = overlayRef.getConfig();
+    const positionStrategy = overlayConfig.positionStrategy as FlexibleConnectedPositionStrategy;
 
-    this._setPosition(overlayConfig.positionStrategy as FlexibleConnectedPositionStrategy);
+    this._setPosition(positionStrategy);
     overlayConfig.hasBackdrop = this.menu.hasBackdrop == null ? !this.triggersSubmenu() :
         this.menu.hasBackdrop;
     overlayRef.attach(this._getPortal());
@@ -262,6 +263,12 @@ export class MatMenuTrigger implements AfterContentInit, OnDestroy {
 
     if (this.menu instanceof _MatMenuBase) {
       this.menu._startAnimation();
+      this.menu._directDescendantItems.changes.pipe(takeUntil(this.menu.close)).subscribe(() => {
+        // Re-adjust the position without locking when the amount of items
+        // changes so that the overlay is allowed to pick a new optimal position.
+        positionStrategy.withLockedPosition(false).reapplyLastPosition();
+        positionStrategy.withLockedPosition(true);
+      });
     }
   }
 

src/material/menu/menu.spec.ts

@@ -11,7 +11,7 @@ import {
   TAB,
 } from '@angular/cdk/keycodes';
 import {Overlay, OverlayContainer} from '@angular/cdk/overlay';
-import {ScrollDispatcher} from '@angular/cdk/scrolling';
+import {ScrollDispatcher, ViewportRuler} from '@angular/cdk/scrolling';
 import {
   createKeyboardEvent,
   createMouseEvent,
@@ -59,6 +59,7 @@ describe('MatMenu', () => {
   let overlayContainer: OverlayContainer;
   let overlayContainerElement: HTMLElement;
   let focusMonitor: FocusMonitor;
+  let viewportRuler: ViewportRuler;
 
   function createComponent<T>(component: Type<T>,
                               providers: Provider[] = [],
@@ -69,11 +70,13 @@ describe('MatMenu', () => {
       providers
     }).compileComponents();
 
-    inject([OverlayContainer, FocusMonitor], (oc: OverlayContainer, fm: FocusMonitor) => {
-      overlayContainer = oc;
-      overlayContainerElement = oc.getContainerElement();
-      focusMonitor = fm;
-    })();
+    inject([OverlayContainer, FocusMonitor, ViewportRuler],
+      (oc: OverlayContainer, fm: FocusMonitor, vr: ViewportRuler) => {
+        overlayContainer = oc;
+        overlayContainerElement = oc.getContainerElement();
+        focusMonitor = fm;
+        viewportRuler = vr;
+      })();
 
     return TestBed.createComponent<T>(component);
   }
@@ -1002,6 +1005,28 @@ describe('MatMenu', () => {
     expect(document.activeElement).toBe(items[3], 'Expected fourth item to be focused');
   }));
 
+  it('should keep the panel in the viewport when more items are added while open', () => {
+    const fixture = createComponent(SimpleMenu, [], [FakeIcon]);
+    fixture.detectChanges();
+
+    const triggerEl = fixture.componentInstance.triggerEl.nativeElement;
+    triggerEl.style.position = 'absolute';
+    triggerEl.style.left = '200px';
+    triggerEl.style.bottom = '300px';
+    triggerEl.click();
+    fixture.detectChanges();
+
+    const panel = overlayContainerElement.querySelector('.mat-menu-panel')!;
+    const viewportHeight = viewportRuler.getViewportSize().height;
+    let panelRect = panel.getBoundingClientRect();
+    expect(Math.floor(panelRect.bottom)).toBeLessThan(viewportHeight);
+
+    fixture.componentInstance.extraItems = new Array(50).fill('Hello there');
+    fixture.detectChanges();
+    panelRect = panel.getBoundingClientRect();
+    expect(Math.floor(panelRect.bottom)).toBe(viewportHeight);
+  });
+
   describe('lazy rendering', () => {
     it('should be able to render the menu content lazily', fakeAsync(() => {
       const fixture = createComponent(SimpleLazyMenu);

src/material/menu/menu.ts

@@ -111,7 +111,7 @@ export class _MatMenuBase implements AfterContentInit, MatMenuPanel<MatMenuItem>
   @ContentChildren(MatMenuItem, {descendants: true}) _allItems: QueryList<MatMenuItem>;
 
   /** Only the direct descendant menu items. */
-  private _directDescendantItems = new QueryList<MatMenuItem>();
+  _directDescendantItems = new QueryList<MatMenuItem>();
 
   /** Subscription to tab events on the menu panel */
   private _tabSubscription = Subscription.EMPTY;

tools/public_api_guard/material/menu.d.ts

@@ -4,6 +4,7 @@ export declare class _MatMenuBase implements AfterContentInit, MatMenuPanel<MatM
     _classList: {
         [key: string]: boolean;
     };
+    _directDescendantItems: QueryList<MatMenuItem>;
     _isAnimating: boolean;
     _panelAnimationState: 'void' | 'enter';
     ariaDescribedby: string;