feat(autocomplete): allow panel to have a width value of auto

src/lib/autocomplete/autocomplete-trigger.ts

@@ -523,13 +523,13 @@ export class MatAutocompleteTrigger implements ControlValueAccessor, OnDestroy {
       if (this._viewportRuler) {
         this._viewportSubscription = this._viewportRuler.change().subscribe(() => {
           if (this.panelOpen && this._overlayRef) {
-            this._overlayRef.updateSize({width: this._getHostWidth()});
+            this._overlayRef.updateSize({width: this._getPanelWidth()});
           }
         });
       }
     } else {
       // Update the panel width and direction, in case anything has changed.
-      this._overlayRef.updateSize({width: this._getHostWidth()});
+      this._overlayRef.updateSize({width: this._getPanelWidth()});
     }
 
     if (this._overlayRef && !this._overlayRef.hasAttached()) {
@@ -553,7 +553,7 @@ export class MatAutocompleteTrigger implements ControlValueAccessor, OnDestroy {
     return new OverlayConfig({
       positionStrategy: this._getOverlayPosition(),
       scrollStrategy: this._scrollStrategy(),
-      width: this._getHostWidth(),
+      width: this._getPanelWidth(),
       direction: this._dir
     });
   }
@@ -579,6 +579,10 @@ export class MatAutocompleteTrigger implements ControlValueAccessor, OnDestroy {
     return this._formField ? this._formField.getConnectedOverlayOrigin() : this._element;
   }
 
+  private _getPanelWidth(): number | string {
+    return this.autocomplete.panelWidth || this._getHostWidth();
+  }
+
   /** Returns the width of the input element, so the panel width can match it. */
   private _getHostWidth(): number {
     return this._getConnectedElement().nativeElement.getBoundingClientRect().width;

src/lib/autocomplete/autocomplete.spec.ts

@@ -1922,6 +1922,50 @@ describe('MatAutocomplete', () => {
     expect(Math.ceil(parseFloat(overlayPane.style.width as string))).toBe(400);
   }));
 
+  it('should have panel width match host width by default', () => {
+    const widthFixture = createComponent(SimpleAutocomplete);
+
+    widthFixture.componentInstance.width = 300;
+    widthFixture.detectChanges();
+
+    widthFixture.componentInstance.trigger.openPanel();
+    widthFixture.detectChanges();
+
+    const overlayPane = overlayContainerElement.querySelector('.cdk-overlay-pane') as HTMLElement;
+
+    expect(Math.ceil(parseFloat(overlayPane.style.width as string))).toBe(300);
+  });
+
+  it('should have panel width set to string value', () => {
+    const widthFixture = createComponent(SimpleAutocomplete);
+
+    widthFixture.componentInstance.width = 300;
+    widthFixture.detectChanges();
+
+    widthFixture.componentInstance.trigger.autocomplete.panelWidth = 'auto';
+    widthFixture.componentInstance.trigger.openPanel();
+    widthFixture.detectChanges();
+
+    const overlayPane = overlayContainerElement.querySelector('.cdk-overlay-pane') as HTMLElement;
+
+    expect(overlayPane.style.width).toBe('auto');
+  });
+
+  it('should have panel width set to number value', () => {
+    const widthFixture = createComponent(SimpleAutocomplete);
+
+    widthFixture.componentInstance.width = 300;
+    widthFixture.detectChanges();
+
+    widthFixture.componentInstance.trigger.autocomplete.panelWidth = 400;
+    widthFixture.componentInstance.trigger.openPanel();
+    widthFixture.detectChanges();
+
+    const overlayPane = overlayContainerElement.querySelector('.cdk-overlay-pane') as HTMLElement;
+
+    expect(Math.ceil(parseFloat(overlayPane.style.width as string))).toBe(400);
+  });
+
   it('should show the panel when the options are initialized later within a component with ' +
     'OnPush change detection', fakeAsync(() => {
       let fixture = createComponent(AutocompleteWithOnPushDelay);

src/lib/autocomplete/autocomplete.ts

@@ -126,6 +126,11 @@ export class MatAutocomplete extends _MatAutocompleteMixinBase implements AfterC
   }
   private _autoActiveFirstOption: boolean;
 
+  /**
+   * Specify the width of the autocomplete panel.  Can be any CSS sizing value, otherwise it will
+   * match the width of its host.
+   */
+  @Input() panelWidth: string | number;
 
   /** Event that is emitted whenever an option from the list is selected. */
   @Output() readonly optionSelected: EventEmitter<MatAutocompleteSelectedEvent> =