fix(menu): panel positions not changing if position is updated after first open

src/lib/menu/menu-trigger.ts

@@ -195,6 +195,7 @@ export class MatMenuTrigger implements AfterContentInit, OnDestroy {
     }
 
     const overlayRef = this._createOverlay();
+    this._setPosition(overlayRef.getConfig().positionStrategy as FlexibleConnectedPositionStrategy);
     overlayRef.attach(this._portal);
 
     if (this.menu.lazyContent) {
@@ -349,7 +350,9 @@ export class MatMenuTrigger implements AfterContentInit, OnDestroy {
    */
   private _getOverlayConfig(): OverlayConfig {
     return new OverlayConfig({
-      positionStrategy: this._getPosition(),
+      positionStrategy: this._overlay.position()
+          .flexibleConnectedTo(this._element)
+          .withTransformOriginOn('.mat-menu-panel'),
       hasBackdrop: this.menu.hasBackdrop == null ? !this.triggersSubmenu() : this.menu.hasBackdrop,
       backdropClass: this.menu.backdropClass || 'cdk-overlay-transparent-backdrop',
       scrollStrategy: this._scrollStrategy(),
@@ -374,11 +377,11 @@ export class MatMenuTrigger implements AfterContentInit, OnDestroy {
   }
 
   /**
-   * This method builds the position strategy for the overlay, so the menu is properly connected
-   * to the trigger.
-   * @returns ConnectedPositionStrategy
+   * Sets the appropriate positions on a position strategy
+   * so the overlay connects with the trigger correctly.
+   * @param positionStrategy Strategy whose position to update.
    */
-  private _getPosition(): FlexibleConnectedPositionStrategy {
+  private _setPosition(positionStrategy: FlexibleConnectedPositionStrategy) {
     let [originX, originFallbackX]: HorizontalConnectionPos[] =
         this.menu.xPosition === 'before' ? ['end', 'start'] : ['start', 'end'];
 
@@ -400,27 +403,24 @@ export class MatMenuTrigger implements AfterContentInit, OnDestroy {
       originFallbackY = overlayFallbackY === 'top' ? 'bottom' : 'top';
     }
 
-    return this._overlay.position()
-        .flexibleConnectedTo(this._element)
-        .withTransformOriginOn('.mat-menu-panel')
-        .withPositions([
-          {originX, originY, overlayX, overlayY, offsetY},
-          {originX: originFallbackX, originY, overlayX: overlayFallbackX, overlayY, offsetY},
-          {
-            originX,
-            originY: originFallbackY,
-            overlayX,
-            overlayY: overlayFallbackY,
-            offsetY: -offsetY
-          },
-          {
-            originX: originFallbackX,
-            originY: originFallbackY,
-            overlayX: overlayFallbackX,
-            overlayY: overlayFallbackY,
-            offsetY: -offsetY
-          }
-        ]);
+    positionStrategy.withPositions([
+      {originX, originY, overlayX, overlayY, offsetY},
+      {originX: originFallbackX, originY, overlayX: overlayFallbackX, overlayY, offsetY},
+      {
+        originX,
+        originY: originFallbackY,
+        overlayX,
+        overlayY: overlayFallbackY,
+        offsetY: -offsetY
+      },
+      {
+        originX: originFallbackX,
+        originY: originFallbackY,
+        overlayX: overlayFallbackX,
+        overlayY: overlayFallbackY,
+        offsetY: -offsetY
+      }
+    ]);
   }
 
   /** Cleans up the active subscriptions. */

src/lib/menu/menu.spec.ts

@@ -494,27 +494,28 @@ describe('MatMenu', () => {
 
   describe('positions', () => {
     let fixture: ComponentFixture<PositionedMenu>;
-    let panel: HTMLElement;
+    let trigger: HTMLElement;
 
     beforeEach(() => {
       fixture = createComponent(PositionedMenu);
       fixture.detectChanges();
 
-      const trigger = fixture.componentInstance.triggerEl.nativeElement;
+      trigger = fixture.componentInstance.triggerEl.nativeElement;
 
       // Push trigger to the bottom edge of viewport,so it has space to open "above"
       trigger.style.position = 'fixed';
       trigger.style.top = '600px';
 
       // Push trigger to the right, so it has space to open "before"
       trigger.style.left = '100px';
+    });
 
+    it('should append mat-menu-before if the x position is changed', () => {
       fixture.componentInstance.trigger.openMenu();
       fixture.detectChanges();
-      panel = overlayContainerElement.querySelector('.mat-menu-panel') as HTMLElement;
-    });
 
-    it('should append mat-menu-before if the x position is changed', () => {
+      const panel = overlayContainerElement.querySelector('.mat-menu-panel') as HTMLElement;
+
       expect(panel.classList).toContain('mat-menu-before');
       expect(panel.classList).not.toContain('mat-menu-after');
 
@@ -526,6 +527,11 @@ describe('MatMenu', () => {
     });
 
     it('should append mat-menu-above if the y position is changed', () => {
+      fixture.componentInstance.trigger.openMenu();
+      fixture.detectChanges();
+
+      const panel = overlayContainerElement.querySelector('.mat-menu-panel') as HTMLElement;
+
       expect(panel.classList).toContain('mat-menu-above');
       expect(panel.classList).not.toContain('mat-menu-below');
 
@@ -546,12 +552,41 @@ describe('MatMenu', () => {
       newFixture.detectChanges();
       newFixture.componentInstance.trigger.openMenu();
       newFixture.detectChanges();
-      panel = overlayContainerElement.querySelector('.mat-menu-panel') as HTMLElement;
+      const panel = overlayContainerElement.querySelector('.mat-menu-panel') as HTMLElement;
 
       expect(panel.classList).toContain('mat-menu-below');
       expect(panel.classList).toContain('mat-menu-after');
     });
 
+    it('should be able to update the position after the first open', () => {
+      trigger.style.position = 'fixed';
+      trigger.style.top = '200px';
+
+      fixture.componentInstance.yPosition = 'above';
+      fixture.detectChanges();
+
+      fixture.componentInstance.trigger.openMenu();
+      fixture.detectChanges();
+
+      let panel = overlayContainerElement.querySelector('.mat-menu-panel') as HTMLElement;
+
+      expect(Math.floor(panel.getBoundingClientRect().bottom))
+          .toBe(Math.floor(trigger.getBoundingClientRect().bottom), 'Expected menu to open above');
+
+      fixture.componentInstance.trigger.closeMenu();
+      fixture.detectChanges();
+
+      fixture.componentInstance.yPosition = 'below';
+      fixture.detectChanges();
+
+      fixture.componentInstance.trigger.openMenu();
+      fixture.detectChanges();
+      panel = overlayContainerElement.querySelector('.mat-menu-panel') as HTMLElement;
+
+      expect(Math.floor(panel.getBoundingClientRect().top))
+          .toBe(Math.floor(trigger.getBoundingClientRect().top), 'Expected menu to open below');
+    });
+
   });
 
   describe('fallback positions', () => {