fix(material/sidenav): only trap focus when backdrop is enabled

Correct when Sidenav enabled focus trapping. When backdrop is show, trap
focus. Do no trap focus when backdrop is not shown.

Existing behavior is that Sidenav traps focus whenever it is not in side
mode. This causes the end user to not be able to interact with the
sidenav content when the mode is push/over, backdrop is disabled and
using ConfigurableFocusTrapFactory (angular#26572).

With this commit applied, Sidenav always traps focus when backdrop is
shown. Sidenav never traps focus when backdrop is not shown, regardless
of what mode the sidenav is in, focus trapping will respect if the
backdrop is shown or not shown.

Fix this issue by correcting boolean logic for detecting if backdrop is
enabled and using that logic to determine when to trap focus. Add an
example that injects ConfigurableFocusTrapFactory.

Fix angular#26572

src/components-examples/material/sidenav/index.ts

@@ -3,7 +3,7 @@ export {SidenavBackdropExample} from './sidenav-backdrop/sidenav-backdrop-exampl
 export {SidenavDisableCloseExample} from './sidenav-disable-close/sidenav-disable-close-example';
 export {SidenavDrawerOverviewExample} from './sidenav-drawer-overview/sidenav-drawer-overview-example';
 export {SidenavFixedExample} from './sidenav-fixed/sidenav-fixed-example';
-export {SidenavModeExample} from './sidenav-mode/sidenav-mode-example';
+export {SidenavConfigurableFocusTrapExample} from './sidenav-configurable-focus-trap/sidenav-configurable-focus-trap-example';
 export {SidenavOpenCloseExample} from './sidenav-open-close/sidenav-open-close-example';
 export {SidenavOverviewExample} from './sidenav-overview/sidenav-overview-example';
 export {SidenavPositionExample} from './sidenav-position/sidenav-position-example';

src/components-examples/material/sidenav/sidenav-configurable-focus-trap/sidenav-configurable-focus-trap-example.css

@@ -0,0 +1,14 @@
+.example-container {
+  position: absolute;
+  top: 0;
+  bottom: 0;
+  left: 0;
+  right: 0;
+}
+
+.example-radio-group {
+  display: block;
+  border: 1px solid #555;
+  margin: 20px;
+  padding: 10px;
+}

src/components-examples/material/sidenav/sidenav-configurable-focus-trap/sidenav-configurable-focus-trap-example.html

@@ -0,0 +1,36 @@
+<mat-sidenav-container class="example-container" *ngIf="shouldRun" [hasBackdrop]="hasBackdrop.value">
+  <mat-sidenav #sidenav [mode]="mode.value!" [position]="position.value!">
+    <p><button mat-button (click)="sidenav.toggle()">Toggle</button></p>
+    <p>
+      <label>Test input for drawer<input/></label>
+    </p>
+  </mat-sidenav>
+
+  <mat-sidenav-content>
+    <p><button mat-button (click)="sidenav.toggle()">Toggle</button></p>
+    <p>
+      <mat-radio-group class="example-radio-group" [formControl]="mode">
+        <label>Mode:</label>
+        <mat-radio-button value="over">Over</mat-radio-button>
+        <mat-radio-button value="side">Side</mat-radio-button>
+        <mat-radio-button value="push">Push</mat-radio-button>
+      </mat-radio-group>
+      <mat-radio-group class="example-radio-group" [formControl]="hasBackdrop">
+        <label>Has Backdrop:</label>
+        <mat-radio-button [value]="null">Default</mat-radio-button>
+        <mat-radio-button [value]="true">true</mat-radio-button>
+        <mat-radio-button [value]="false">false</mat-radio-button>
+      </mat-radio-group>
+      <mat-radio-group class="example-radio-group" [formControl]="position">
+        <label>Position:</label>
+        <mat-radio-button value="start">Start</mat-radio-button>
+        <mat-radio-button value="end">End</mat-radio-button>
+      </mat-radio-group>
+    </p>
+    <p>
+      <label>Test input for drawer content<input/></label>
+    </p>
+  </mat-sidenav-content>
+</mat-sidenav-container>
+
+<div *ngIf="!shouldRun">Please open on Stackblitz to see result</div>

src/components-examples/material/sidenav/sidenav-configurable-focus-trap/sidenav-configurable-focus-trap-example.ts

@@ -0,0 +1,31 @@
+import {Component} from '@angular/core';
+import {NgIf} from '@angular/common';
+import {FormControl, FormsModule, ReactiveFormsModule} from '@angular/forms';
+import {MatDrawerMode, MatSidenavModule} from '@angular/material/sidenav';
+import {MatRadioModule} from '@angular/material/radio';
+import {MatButtonModule} from '@angular/material/button';
+import {ConfigurableFocusTrapFactory, FocusTrapFactory} from '@angular/cdk/a11y';
+
+/** @title Sidenav using injected ConfigurableFocusTrap */
+@Component({
+  selector: 'sidenav-configurable-focus-trap-example',
+  templateUrl: 'sidenav-configurable-focus-trap-example.html',
+  styleUrls: ['sidenav-configurable-focus-trap-example.css'],
+  standalone: true,
+  imports: [
+    NgIf,
+    MatSidenavModule,
+    MatButtonModule,
+    MatRadioModule,
+    FormsModule,
+    ReactiveFormsModule,
+  ],
+  providers: [{provide: FocusTrapFactory, useClass: ConfigurableFocusTrapFactory}],
+})
+export class SidenavConfigurableFocusTrapExample {
+  mode = new FormControl('over' as MatDrawerMode);
+  hasBackdrop = new FormControl(null as null | boolean);
+  position = new FormControl('start' as 'start' | 'end');
+
+  shouldRun = /(^|.)(stackblitz|webcontainer).(io|com)$/.test(window.location.host);
+}

src/material/sidenav/drawer.spec.ts

@@ -567,6 +567,19 @@ describe('MatDrawer', () => {
       expect(document.activeElement).toBe(firstFocusableElement);
     }));
 
+    it('should trap focus when opened in "side" mode if backdrop is explicitly enabled', fakeAsync(() => {
+      testComponent.mode = 'push';
+      testComponent.hasBackdrop = true;
+      fixture.detectChanges();
+      lastFocusableElement.focus();
+
+      drawer.open();
+      fixture.detectChanges();
+      tick();
+
+      expect(document.activeElement).toBe(firstFocusableElement);
+    }));
+
     it('should not auto-focus by default when opened in "side" mode', fakeAsync(() => {
       testComponent.mode = 'side';
       fixture.detectChanges();
@@ -596,6 +609,23 @@ describe('MatDrawer', () => {
       }),
     );
 
+    it(
+      'should auto-focus to first tabbable element when opened in "push" mode' +
+        'when backdrop is enabled explicitly',
+      fakeAsync(() => {
+        testComponent.mode = 'push';
+        testComponent.hasBackdrop = true;
+        fixture.detectChanges();
+        lastFocusableElement.focus();
+
+        drawer.open();
+        fixture.detectChanges();
+        tick();
+
+        expect(document.activeElement).toBe(firstFocusableElement);
+      }),
+    );
+
     it('should focus the drawer if there are no focusable elements', fakeAsync(() => {
       fixture.destroy();
 
@@ -1229,7 +1259,7 @@ class DrawerDynamicPosition {
   // Note: we use inputs here, because they're guaranteed
   // to be focusable across all platforms.
   template: `
-    <mat-drawer-container>
+    <mat-drawer-container [hasBackdrop]="hasBackdrop">
       <mat-drawer position="start" [mode]="mode">
         <input type="text" class="input1"/>
       </mat-drawer>
@@ -1238,6 +1268,7 @@ class DrawerDynamicPosition {
 })
 class DrawerWithFocusableElements {
   mode: string = 'over';
+  hasBackdrop: boolean | null = null;
 }
 
 @Component({

src/material/sidenav/drawer.ts

@@ -595,8 +595,9 @@ export class MatDrawer implements AfterViewInit, AfterContentChecked, OnDestroy
   /** Updates the enabled state of the focus trap. */
   private _updateFocusTrapState() {
     if (this._focusTrap) {
-      // The focus trap is only enabled when the drawer is open in any mode other than side.
-      this._focusTrap.enabled = this.opened && this.mode !== 'side';
+      // Trap focus only if the backdrop is enabled. Otherwise, allow end user to interact with the
+      // sidenav content.
+      this._focusTrap.enabled = !!this._container?.hasBackdrop;
     }
   }
 
@@ -698,7 +699,9 @@ export class MatDrawerContainer implements AfterContentInit, DoCheck, OnDestroy
   @Input()
   get hasBackdrop(): boolean {
     if (this._backdropOverride == null) {
-      return !this._start || this._start.mode !== 'side' || !this._end || this._end.mode !== 'side';
+      const startHasBackdrop = !!this._start && this._start.mode !== 'side';
+      const endHasBackdrop = !!this._end && this._end.mode !== 'side';
+      return startHasBackdrop || endHasBackdrop;
     }
 
     return this._backdropOverride;
@@ -1004,21 +1007,17 @@ export class MatDrawerContainer implements AfterContentInit, DoCheck, OnDestroy
   _closeModalDrawersViaBackdrop() {
     // Close all open drawers where closing is not disabled and the mode is not `side`.
     [this._start, this._end]
-      .filter(drawer => drawer && !drawer.disableClose && this._canHaveBackdrop(drawer))
+      .filter(drawer => drawer && !drawer.disableClose && this.hasBackdrop)
       .forEach(drawer => drawer!._closeViaBackdropClick());
   }
 
   _isShowingBackdrop(): boolean {
     return (
-      (this._isDrawerOpen(this._start) && this._canHaveBackdrop(this._start)) ||
-      (this._isDrawerOpen(this._end) && this._canHaveBackdrop(this._end))
+      (this._isDrawerOpen(this._start) && this.hasBackdrop) ||
+      (this._isDrawerOpen(this._end) && this.hasBackdrop)
     );
   }
 
-  private _canHaveBackdrop(drawer: MatDrawer): boolean {
-    return drawer.mode !== 'side' || !!this._backdropOverride;
-  }
-
   private _isDrawerOpen(drawer: MatDrawer | null): drawer is MatDrawer {
     return drawer != null && drawer.opened;
   }