fix(material/sidenav): incorrectly trapping focus in some cases (#32699)

In #27355 the sidenav was changed to trap focus if the container has a backdrop. This can be incorrect, because the sidenav that's causing the backdrop to be shown might not be open.

These changes switch to enabling focus trapping if the backdrop is actually visible.

Fixes #32661.

(cherry picked from commit c70a473)

Author: crisbeto

src/material/sidenav/drawer.spec.ts

@@ -714,6 +714,22 @@ describe('MatDrawer', () => {
       flush();
       expect(anchors.map(anchor => anchor.getAttribute('tabindex'))).toEqual([null, null]);
     }));
+
+    it('should not trap focus if the drawer container has a backdrop, but is not showing it', fakeAsync(() => {
+      fixture.destroy();
+
+      const hiddenBackdropFixture = TestBed.createComponent(DrawerWithSideAndHiddenBackdrop);
+      hiddenBackdropFixture.detectChanges();
+      tick();
+      hiddenBackdropFixture.detectChanges();
+
+      const anchors = Array.from<HTMLElement>(
+        hiddenBackdropFixture.nativeElement.querySelectorAll('.cdk-focus-trap-anchor'),
+      );
+
+      expect(anchors.length).toBeGreaterThan(0);
+      expect(anchors.every(anchor => !anchor.hasAttribute('tabindex'))).toBe(true);
+    }));
   });
 
   it('should mark the drawer content as scrollable', () => {
@@ -1418,3 +1434,16 @@ class NestedDrawerContainers {
   @ViewChild('innerContainer') innerContainer!: MatDrawerContainer;
   @ViewChild('innerDrawer') innerDrawer!: MatDrawer;
 }
+
+@Component({
+  template: `
+    <mat-sidenav-container>
+      <mat-sidenav opened mode="side">
+        <button>Button inside</button>
+      </mat-sidenav>
+      <mat-sidenav mode="over" position="end">End content</mat-sidenav>
+    </mat-sidenav-container>
+  `,
+  imports: [MatSidenavModule],
+})
+class DrawerWithSideAndHiddenBackdrop {}

src/material/sidenav/drawer.ts

@@ -609,7 +609,7 @@ export class MatDrawer implements AfterViewInit, OnDestroy {
     if (this._focusTrap) {
       // Trap focus only if the backdrop is enabled. Otherwise, allow end user to interact with the
       // sidenav content.
-      this._focusTrap.enabled = !!this._container?.hasBackdrop && this.opened;
+      this._focusTrap.enabled = this.opened && !!this._container?._isShowingBackdrop();
     }
   }