fix(dialog): hide all non-overlay content from assistive technology

Hides all non-overlay content from assistive technology by applying `aria-hidden` to it. This prevents users from being able to move focus out of the dialog using the screen reader navigational shortcuts.

Fixes angular#7787.

src/lib/dialog/dialog.spec.ts

@@ -658,6 +658,62 @@ describe('MatDialog', () => {
     expect(dialog.getDialogById('pizza')).toBe(dialogRef);
   });
 
+  it('should toggle `aria-hidden` on the overlay container siblings', fakeAsync(() => {
+    const sibling = document.createElement('div');
+    overlayContainerElement.parentNode!.appendChild(sibling);
+
+    const dialogRef = dialog.open(PizzaMsg, {viewContainerRef: testViewContainerRef});
+    viewContainerFixture.detectChanges();
+    flush();
+
+    expect(sibling.getAttribute('aria-hidden')).toBe('true', 'Expected sibling to be hidden');
+    expect(overlayContainerElement.hasAttribute('aria-hidden'))
+        .toBe(false, 'Expected overlay container not to be hidden.');
+
+    dialogRef.close();
+    viewContainerFixture.detectChanges();
+    flush();
+
+    expect(sibling.hasAttribute('aria-hidden'))
+        .toBe(false, 'Expected sibling to no longer be hidden.');
+    sibling.parentNode!.removeChild(sibling);
+  }));
+
+  it('should restore `aria-hidden` to the overlay container siblings on close', fakeAsync(() => {
+    const sibling = document.createElement('div');
+
+    sibling.setAttribute('aria-hidden', 'true');
+    overlayContainerElement.parentNode!.appendChild(sibling);
+
+    const dialogRef = dialog.open(PizzaMsg, {viewContainerRef: testViewContainerRef});
+    viewContainerFixture.detectChanges();
+    flush();
+
+    expect(sibling.getAttribute('aria-hidden')).toBe('true', 'Expected sibling to be hidden.');
+
+    dialogRef.close();
+    viewContainerFixture.detectChanges();
+    flush();
+
+    expect(sibling.getAttribute('aria-hidden')).toBe('true', 'Expected sibling to remain hidden.');
+    sibling.parentNode!.removeChild(sibling);
+  }));
+
+  it('should not set `aria-hidden` on `aria-live` elements', fakeAsync(() => {
+    const sibling = document.createElement('div');
+
+    sibling.setAttribute('aria-live', 'polite');
+    overlayContainerElement.parentNode!.appendChild(sibling);
+
+    dialog.open(PizzaMsg, {viewContainerRef: testViewContainerRef});
+    viewContainerFixture.detectChanges();
+    flush();
+
+    expect(sibling.hasAttribute('aria-hidden'))
+        .toBe(false, 'Expected live element not to be hidden.');
+    sibling.parentNode!.removeChild(sibling);
+  }));
+
   describe('disableClose option', () => {
     it('should prevent closing via clicks on the backdrop', () => {
       dialog.open(PizzaMsg, {

src/lib/dialog/dialog.ts

@@ -14,6 +14,7 @@ import {
   OverlayConfig,
   OverlayRef,
   ScrollStrategy,
+  OverlayContainer,
 } from '@angular/cdk/overlay';
 import {ComponentPortal, ComponentType, PortalInjector, TemplatePortal} from '@angular/cdk/portal';
 import {Location} from '@angular/common';
@@ -67,6 +68,7 @@ export class MatDialog {
   private _openDialogsAtThisLevel: MatDialogRef<any>[] = [];
   private _afterAllClosedAtThisLevel = new Subject<void>();
   private _afterOpenAtThisLevel = new Subject<MatDialogRef<any>>();
+  private _ariaHiddenElements = new Map<Element, string|null>();
 
   /** Keeps track of the currently-open dialogs. */
   get openDialogs(): MatDialogRef<any>[] {
@@ -96,7 +98,8 @@ export class MatDialog {
       private _injector: Injector,
       @Optional() location: Location,
       @Inject(MAT_DIALOG_SCROLL_STRATEGY) private _scrollStrategy,
-      @Optional() @SkipSelf() private _parentDialog: MatDialog) {
+      @Optional() @SkipSelf() private _parentDialog: MatDialog,
+      private _overlayContainer: OverlayContainer) {
 
     // Close all of the dialogs when the user goes forwards/backwards in history or when the
     // location hash changes. Note that this usually doesn't include clicking on links (unless
@@ -127,6 +130,11 @@ export class MatDialog {
     const dialogRef =
         this._attachDialogContent<T>(componentOrTemplateRef, dialogContainer, overlayRef, config);
 
+    // If this is the first dialog that we're opening, hide all the non-overlay content.
+    if (!this.openDialogs.length) {
+      this.__hideNonDialogContentFromAssistiveTechnology();
+    }
+
     this.openDialogs.push(dialogRef);
     dialogRef.afterClosed().subscribe(() => this._removeOpenDialog(dialogRef));
     this.afterOpen.next(dialogRef);
@@ -298,12 +306,44 @@ export class MatDialog {
     if (index > -1) {
       this.openDialogs.splice(index, 1);
 
-      // no open dialogs are left, call next on afterAllClosed Subject
+      // If all the dialogs were closed, remove/restore the `aria-hidden`
+      // to a the siblings and emit to the `afterAllClosed` stream.
       if (!this.openDialogs.length) {
+        this._ariaHiddenElements.forEach((previousValue, element) => {
+          if (previousValue) {
+            element.setAttribute('aria-hidden', previousValue);
+          } else {
+            element.removeAttribute('aria-hidden');
+          }
+        });
+
+        this._ariaHiddenElements.clear();
         this._afterAllClosed.next();
       }
     }
   }
+
+  /**
+   * Hides all of the content that isn't an overlay from assistive technology.
+   */
+  private __hideNonDialogContentFromAssistiveTechnology() {
+    const overlayContainer = this._overlayContainer.getContainerElement();
+    const siblings = overlayContainer.parentElement!.children;
+
+    for (let i = siblings.length - 1; i > -1; i--) {
+      let sibling = siblings[i];
+
+      if (sibling !== overlayContainer &&
+        sibling.nodeName !== 'SCRIPT' &&
+        sibling.nodeName !== 'STYLE' &&
+        !sibling.hasAttribute('aria-live')) {
+
+        this._ariaHiddenElements.set(sibling, sibling.getAttribute('aria-hidden'));
+        sibling.setAttribute('aria-hidden', 'true');
+      }
+    }
+  }
+
 }
 
 /**