refactor(cdk/platform): add common utility for resolving focused element

Based on a discussion on an earlier PR, these changes move the logic for resolving the `activeElement` while piercing through the shadow DOM into a common helper. Furthermore, they expand the logic to pierce through multiple layers of shadow DOM.

Author: crisbeto

src/cdk/a11y/focus-trap/focus-trap.ts

@@ -7,6 +7,7 @@
  */
 
 import {BooleanInput, coerceBooleanProperty} from '@angular/cdk/coercion';
+import {_getFocusedElementPierceShadowDom} from '@angular/cdk/platform';
 import {DOCUMENT} from '@angular/common';
 import {
   AfterContentInit,
@@ -454,11 +455,7 @@ export class CdkTrapFocus implements OnDestroy, AfterContentInit, OnChanges, DoC
   }
 
   private _captureFocus() {
-    // If the `activeElement` is inside a shadow root, `document.activeElement` will
-    // point to the shadow root so we have to descend into it ourselves.
-    const activeElement = this._document?.activeElement as HTMLElement|null;
-    this._previouslyFocusedElement =
-      activeElement?.shadowRoot?.activeElement as HTMLElement || activeElement;
+    this._previouslyFocusedElement = _getFocusedElementPierceShadowDom();
     this.focusTrap.focusInitialElementWhenReady();
   }
 

src/cdk/platform/features/shadow-dom.ts

@@ -32,3 +32,23 @@ export function _getShadowRoot(element: HTMLElement): ShadowRoot | null {
 
   return null;
 }
+
+/**
+ * Gets the currently-focused element on the page while
+ * also piercing through Shadow DOM boundaries.
+ */
+export function _getFocusedElementPierceShadowDom(): HTMLElement | null {
+  let activeElement = typeof document !== 'undefined' && document ?
+    document.activeElement as HTMLElement | null : null;
+
+  while (activeElement && activeElement.shadowRoot) {
+    const newActiveElement = activeElement.shadowRoot.activeElement as HTMLElement | null;
+    if (newActiveElement === activeElement) {
+      break;
+    } else {
+      activeElement = newActiveElement;
+    }
+  }
+
+  return activeElement;
+}

src/material/bottom-sheet/BUILD.bazel

@@ -27,6 +27,7 @@ ng_module(
         "//src/cdk/keycodes",
         "//src/cdk/layout",
         "//src/cdk/overlay",
+        "//src/cdk/platform",
         "//src/cdk/portal",
         "//src/material/core",
         "@npm//@angular/animations",

src/material/bottom-sheet/bottom-sheet-container.ts

@@ -34,6 +34,7 @@ import {matBottomSheetAnimations} from './bottom-sheet-animations';
 import {Subscription} from 'rxjs';
 import {DOCUMENT} from '@angular/common';
 import {FocusTrap, FocusTrapFactory} from '@angular/cdk/a11y';
+import {_getFocusedElementPierceShadowDom} from '@angular/cdk/platform';
 
 // TODO(crisbeto): consolidate some logic between this, MatDialog and MatSnackBar
 
@@ -207,7 +208,7 @@ export class MatBottomSheetContainer extends BasePortalOutlet implements OnDestr
     if (this.bottomSheetConfig.autoFocus) {
       this._focusTrap.focusInitialElementWhenReady();
     } else {
-      const activeElement = this._getActiveElement();
+      const activeElement = _getFocusedElementPierceShadowDom();
 
       // Otherwise ensure that focus is on the container. It's possible that a different
       // component tried to move focus while the open animation was running. See:
@@ -226,7 +227,7 @@ export class MatBottomSheetContainer extends BasePortalOutlet implements OnDestr
 
     // We need the extra check, because IE can set the `activeElement` to null in some cases.
     if (this.bottomSheetConfig.restoreFocus && toFocus && typeof toFocus.focus === 'function') {
-      const activeElement = this._getActiveElement();
+      const activeElement = _getFocusedElementPierceShadowDom();
       const element = this._elementRef.nativeElement;
 
       // Make sure that focus is still inside the bottom sheet or is on the body (usually because a
@@ -246,19 +247,11 @@ export class MatBottomSheetContainer extends BasePortalOutlet implements OnDestr
 
   /** Saves a reference to the element that was focused before the bottom sheet was opened. */
   private _savePreviouslyFocusedElement() {
-    this._elementFocusedBeforeOpened = this._getActiveElement();
+    this._elementFocusedBeforeOpened = _getFocusedElementPierceShadowDom();
 
     // The `focus` method isn't available during server-side rendering.
     if (this._elementRef.nativeElement.focus) {
       Promise.resolve().then(() => this._elementRef.nativeElement.focus());
     }
   }
-
-  /** Gets the currently-focused element on the page. */
-  private _getActiveElement(): HTMLElement | null {
-    // If the `activeElement` is inside a shadow root, `document.activeElement` will
-    // point to the shadow root so we have to descend into it ourselves.
-    const activeElement = this._document.activeElement;
-    return activeElement?.shadowRoot?.activeElement as HTMLElement || activeElement;
-  }
 }

src/material/datepicker/BUILD.bazel

@@ -32,6 +32,7 @@ ng_module(
         "//src/cdk/coercion",
         "//src/cdk/keycodes",
         "//src/cdk/overlay",
+        "//src/cdk/platform",
         "//src/cdk/portal",
         "//src/material/button",
         "//src/material/core",

src/material/datepicker/datepicker-base.ts

@@ -50,6 +50,7 @@ import {
 } from '@angular/material/core';
 import {merge, Subject, Observable, Subscription} from 'rxjs';
 import {filter, take} from 'rxjs/operators';
+import {_getFocusedElementPierceShadowDom} from '@angular/cdk/platform';
 import {MatCalendar, MatCalendarView} from './calendar';
 import {matDatepickerAnimations} from './datepicker-animations';
 import {createMissingDateImplError} from './datepicker-errors';
@@ -553,11 +554,7 @@ export abstract class MatDatepickerBase<C extends MatDatepickerControl<D>, S,
       throw Error('Attempted to open an MatDatepicker with no associated input.');
     }
 
-    // If the `activeElement` is inside a shadow root, `document.activeElement` will
-    // point to the shadow root so we have to descend into it ourselves.
-    const activeElement: HTMLElement|null = this._document?.activeElement;
-    this._focusedElementBeforeOpen =
-      activeElement?.shadowRoot?.activeElement as HTMLElement || activeElement;
+    this._focusedElementBeforeOpen = _getFocusedElementPierceShadowDom();
     this._openOverlay();
     this._opened = true;
     this.openedStream.emit();

src/material/dialog/BUILD.bazel

@@ -26,6 +26,7 @@ ng_module(
         "//src/cdk/bidi",
         "//src/cdk/keycodes",
         "//src/cdk/overlay",
+        "//src/cdk/platform",
         "//src/cdk/portal",
         "//src/material/core",
         "@npm//@angular/animations",

src/material/dialog/dialog-container.ts

@@ -8,6 +8,7 @@
 
 import {AnimationEvent} from '@angular/animations';
 import {FocusMonitor, FocusOrigin, FocusTrap, FocusTrapFactory} from '@angular/cdk/a11y';
+import {_getFocusedElementPierceShadowDom} from '@angular/cdk/platform';
 import {
   BasePortalOutlet,
   CdkPortalOutlet,
@@ -182,7 +183,7 @@ export abstract class _MatDialogContainerBase extends BasePortalOutlet {
     // We need the extra check, because IE can set the `activeElement` to null in some cases.
     if (this._config.restoreFocus && previousElement &&
         typeof previousElement.focus === 'function') {
-      const activeElement = this._getActiveElement();
+      const activeElement = _getFocusedElementPierceShadowDom();
       const element = this._elementRef.nativeElement;
 
       // Make sure that focus is still inside the dialog or is on the body (usually because a
@@ -213,7 +214,7 @@ export abstract class _MatDialogContainerBase extends BasePortalOutlet {
   /** Captures the element that was focused before the dialog was opened. */
   private _capturePreviouslyFocusedElement() {
     if (this._document) {
-      this._elementFocusedBeforeDialogWasOpened = this._getActiveElement() as HTMLElement;
+      this._elementFocusedBeforeDialogWasOpened = _getFocusedElementPierceShadowDom();
     }
   }
 
@@ -228,17 +229,9 @@ export abstract class _MatDialogContainerBase extends BasePortalOutlet {
   /** Returns whether focus is inside the dialog. */
   private _containsFocus() {
     const element = this._elementRef.nativeElement;
-    const activeElement = this._getActiveElement();
+    const activeElement = _getFocusedElementPierceShadowDom();
     return element === activeElement || element.contains(activeElement);
   }
-
-  /** Gets the currently-focused element on the page. */
-  private _getActiveElement(): Element | null {
-    // If the `activeElement` is inside a shadow root, `document.activeElement` will
-    // point to the shadow root so we have to descend into it ourselves.
-    const activeElement = this._document.activeElement;
-    return activeElement?.shadowRoot?.activeElement as HTMLElement || activeElement;
-  }
 }
 
 /**

tools/public_api_guard/cdk/platform.d.ts

@@ -1,3 +1,5 @@
+export declare function _getFocusedElementPierceShadowDom(): HTMLElement | null;
+
 export declare function _getShadowRoot(element: HTMLElement): ShadowRoot | null;
 
 export declare function _supportsShadowDom(): boolean;