fix(material/autocomplete): allow overlay backdrop when opened

Currently when we open the panel, backdrop is not allowed and is not inline with `mat-select`, `mat-menu`, etc. This fix will align autocomplete with those components

Fixes #30457

Author: mistrykaran91

src/material/autocomplete/autocomplete-trigger.ts

@@ -900,6 +900,8 @@ export class MatAutocompleteTrigger
       scrollStrategy: this._scrollStrategy(),
       width: this._getPanelWidth(),
       direction: this._dir ?? undefined,
+      hasBackdrop: this._defaults?.hasBackdrop,
+      backdropClass: this._defaults?.backdropClass || 'cdk-overlay-transparent-backdrop',
       panelClass: this._defaults?.overlayPanelClass,
     });
   }

src/material/autocomplete/autocomplete.spec.ts

@@ -3014,6 +3014,66 @@ describe('MatAutocomplete', () => {
     });
   });
 
+  describe('with backdrop in default options', () => {
+    it('should contain backdrop by default', fakeAsync(() => {
+      const fixture = createComponent(SimpleAutocomplete, []);
+      fixture.detectChanges();
+      fixture.componentInstance.trigger.openPanel();
+      fixture.detectChanges();
+
+      tick(500);
+
+      expect(overlayContainerElement.querySelector('.cdk-overlay-backdrop')).toBeTruthy();
+    }));
+
+    it('should be able to remove the backdrop', fakeAsync(() => {
+      const fixture = createComponent(SimpleAutocomplete, [
+        {
+          provide: MAT_AUTOCOMPLETE_DEFAULT_OPTIONS,
+          useValue: {hasBackdrop: false},
+        },
+      ]);
+      fixture.detectChanges();
+      fixture.componentInstance.trigger.openPanel();
+      fixture.detectChanges();
+
+      tick(500);
+
+      expect(overlayContainerElement.querySelector('.cdk-overlay-backdrop')).toBeFalsy();
+    }));
+  });
+
+  describe('with backdropClass in default options', () => {
+    it('should contain default backdropClass', fakeAsync(() => {
+      const fixture = createComponent(SimpleAutocomplete, []);
+      fixture.detectChanges();
+      fixture.componentInstance.trigger.openPanel();
+      fixture.detectChanges();
+
+      tick(500);
+
+      const cdkPanelElement = overlayContainerElement.querySelector('.cdk-overlay-backdrop');
+      expect(cdkPanelElement?.classList).toContain('cdk-overlay-transparent-backdrop');
+    }));
+
+    it('should be able to remove the backdrop', fakeAsync(() => {
+      const fixture = createComponent(SimpleAutocomplete, [
+        {
+          provide: MAT_AUTOCOMPLETE_DEFAULT_OPTIONS,
+          useValue: {backdropClass: 'my-custom-backdrop-class', hasBackdrop: true},
+        },
+      ]);
+      fixture.detectChanges();
+      fixture.componentInstance.trigger.openPanel();
+      fixture.detectChanges();
+
+      tick(500);
+
+      const cdkPanelElement = overlayContainerElement.querySelector('.cdk-overlay-backdrop');
+      expect(cdkPanelElement?.classList).toContain('my-custom-backdrop-class');
+    }));
+  });
+
   describe('misc', () => {
     it('should allow basic use without any forms directives', () => {
       expect(() => {

src/material/autocomplete/autocomplete.ts

@@ -70,6 +70,12 @@ export interface MatAutocompleteDefaultOptions {
    */
   requireSelection?: boolean;
 
+  /** Class to be applied to the autocomplete's backdrop. */
+  backdropClass?: string;
+
+  /** Whether the autocomplete has a backdrop. */
+  hasBackdrop?: boolean;
+
   /** Class or list of classes to be applied to the autocomplete's overlay panel. */
   overlayPanelClass?: string | string[];
 
@@ -97,6 +103,8 @@ export function MAT_AUTOCOMPLETE_DEFAULT_OPTIONS_FACTORY(): MatAutocompleteDefau
     autoSelectActiveOption: false,
     hideSingleSelectionIndicator: false,
     requireSelection: false,
+    hasBackdrop: true,
+    backdropClass: 'cdk-overlay-transparent-backdrop',
   };
 }
 

tools/public_api_guard/material/autocomplete.md

@@ -135,6 +135,8 @@ export interface MatAutocompleteActivatedEvent {
 export interface MatAutocompleteDefaultOptions {
     autoActiveFirstOption?: boolean;
     autoSelectActiveOption?: boolean;
+    backdropClass?: string;
+    hasBackdrop?: boolean;
     hideSingleSelectionIndicator?: boolean;
     overlayPanelClass?: string | string[];
     requireSelection?: boolean;