fix(dialog): recapture focus when clicking on backdrop when cl… (#18826)

`MatDialog` has an option to disable closing by clicking on the backdrop, but this can lead to focus being bumped back to the `body` and allowing users to tab past the dialog. These changes add some extra logic to recapture focus when the user clicks on the backdrop.

Fixes #18799.

src/material/dialog/dialog-container.ts

@@ -148,30 +148,31 @@ export class MatDialogContainer extends BasePortalOutlet {
     return this._portalOutlet.attachDomPortal(portal);
   }
 
-  /** Moves the focus inside the focus trap. */
-  private _trapFocus() {
-    const element = this._elementRef.nativeElement;
+  /** Moves focus back into the dialog if it was moved out. */
+  _recaptureFocus() {
+    if (!this._containsFocus()) {
+      const focusWasTrapped = this._getFocusTrap().focusInitialElement();
 
-    if (!this._focusTrap) {
-      this._focusTrap = this._focusTrapFactory.create(element);
+      if (!focusWasTrapped) {
+        this._elementRef.nativeElement.focus();
+      }
     }
+  }
 
+  /** Moves the focus inside the focus trap. */
+  private _trapFocus() {
     // If we were to attempt to focus immediately, then the content of the dialog would not yet be
     // ready in instances where change detection has to run first. To deal with this, we simply
     // wait for the microtask queue to be empty.
     if (this._config.autoFocus) {
-      this._focusTrap.focusInitialElementWhenReady();
-    } else {
-      const activeElement = this._document.activeElement;
-
+      this._getFocusTrap().focusInitialElementWhenReady();
+    } else if (!this._containsFocus()) {
       // Otherwise ensure that focus is on the dialog container. It's possible that a different
       // component tried to move focus while the open animation was running. See:
       // https://github.com/angular/components/issues/16215. Note that we only want to do this
       // if the focus isn't inside the dialog already, because it's possible that the consumer
       // turned off `autoFocus` in order to move focus themselves.
-      if (activeElement !== element && !element.contains(activeElement)) {
-        element.focus();
-      }
+      this._elementRef.nativeElement.focus();
     }
   }
 
@@ -214,6 +215,22 @@ export class MatDialogContainer extends BasePortalOutlet {
     }
   }
 
+  /** Returns whether focus is inside the dialog. */
+  private _containsFocus() {
+    const element = this._elementRef.nativeElement;
+    const activeElement = this._document.activeElement;
+    return element === activeElement || element.contains(activeElement);
+  }
+
+  /** Gets the focus trap associated with the dialog. */
+  private _getFocusTrap() {
+    if (!this._focusTrap) {
+      this._focusTrap = this._focusTrapFactory.create(this._elementRef.nativeElement);
+    }
+
+    return this._focusTrap;
+  }
+
   /** Callback, invoked whenever an animation on the host completes. */
   _onAnimationDone(event: AnimationEvent) {
     if (event.toState === 'enter') {

src/material/dialog/dialog-ref.ts

@@ -94,6 +94,14 @@ export class MatDialogRef<T, R = any> {
         event.preventDefault();
         this.close();
       });
+
+    _overlayRef.backdropClick().subscribe(() => {
+      if (this.disableClose) {
+        this._containerInstance._recaptureFocus();
+      } else {
+        this.close();
+      }
+    });
   }
 
   /**

src/material/dialog/dialog.spec.ts

@@ -973,6 +973,29 @@ describe('MatDialog', () => {
 
       expect(overlayContainerElement.querySelector('mat-dialog-container')).toBeFalsy();
     }));
+
+    it('should recapture focus when clicking on the backdrop', fakeAsync(() => {
+      dialog.open(PizzaMsg, {
+        disableClose: true,
+        viewContainerRef: testViewContainerRef
+      });
+
+      viewContainerFixture.detectChanges();
+      flushMicrotasks();
+
+      let backdrop = overlayContainerElement.querySelector('.cdk-overlay-backdrop') as HTMLElement;
+      let input = overlayContainerElement.querySelector('input') as HTMLInputElement;
+
+      expect(document.activeElement).toBe(input, 'Expected input to be focused on open');
+
+      input.blur(); // Programmatic clicks might not move focus so we simulate it.
+      backdrop.click();
+      viewContainerFixture.detectChanges();
+      flush();
+
+      expect(document.activeElement).toBe(input, 'Expected input to stay focused after click');
+    }));
+
   });
 
   describe('hasBackdrop option', () => {

src/material/dialog/dialog.ts

@@ -248,15 +248,6 @@ export class MatDialog implements OnDestroy {
     const dialogRef =
         new MatDialogRef<T, R>(overlayRef, dialogContainer, config.id);
 
-    // When the dialog backdrop is clicked, we want to close it.
-    if (config.hasBackdrop) {
-      overlayRef.backdropClick().subscribe(() => {
-        if (!dialogRef.disableClose) {
-          dialogRef.close();
-        }
-      });
-    }
-
     if (componentOrTemplateRef instanceof TemplateRef) {
       dialogContainer.attachTemplatePortal(
         new TemplatePortal<T>(componentOrTemplateRef, null!,

tools/public_api_guard/material/dialog.d.ts

@@ -99,6 +99,7 @@ export declare class MatDialogContainer extends BasePortalOutlet {
     _config: MatDialogConfig);
     _onAnimationDone(event: AnimationEvent): void;
     _onAnimationStart(event: AnimationEvent): void;
+    _recaptureFocus(): void;
     _startExitAnimation(): void;
     attachComponentPortal<T>(portal: ComponentPortal<T>): ComponentRef<T>;
     attachTemplatePortal<C>(portal: TemplatePortal<C>): EmbeddedViewRef<C>;