fix(material/chips): allow focusing disabled listbox options (#25771)

Allow user to focus to disabled listbox options. Unlike other chips,
disabled `MatChipOption` remains in the tab order, but it cannot be
clicked. This aligns with WAI ARIA documented best practice to allow
focusing disabled listbox options. Fix issue where screen reader does
not announce disabled item in single selection list.

Summary of API and behavior changes:
 - when disabled, `MatChipOption` sets `aria-disabled="true"` and omits
   `disabled` attribute.
 - Add private `@Input _alowFocusWithDisabled` to `MatChipAction` to
   support focusing the action when it is disabled.
 - `MatChipSet` defines `_skipPredicate` as an instance member which
   dervied classes may override.

`_alowFocusWithDisabled` and `_skipPredicate` are internal API to the
chips.

src/dev-app/chips/chips-demo.html

@@ -108,7 +108,7 @@ <h4>Single selection</h4>
       <mat-chip-listbox multiple="false" [disabled]="disabledListboxes">
         <mat-chip-option>Extra Small</mat-chip-option>
         <mat-chip-option>Small</mat-chip-option>
-        <mat-chip-option>Medium</mat-chip-option>
+        <mat-chip-option disabled>Medium</mat-chip-option>
         <mat-chip-option>Large</mat-chip-option>
       </mat-chip-listbox>
 

src/material/chips/chip-action.ts

@@ -32,8 +32,8 @@ const _MatChipActionMixinBase = mixinTabIndex(_MatChipActionBase, -1);
     // in order to avoid some super-specific `:hover` styles from MDC.
     '[class.mdc-evolution-chip__action--presentational]': '_isPrimary',
     '[class.mdc-evolution-chip__action--trailing]': '!_isPrimary',
-    '[attr.tabindex]': '(disabled || !isInteractive) ? null : tabIndex',
-    '[attr.disabled]': "disabled ? '' : null",
+    '[attr.tabindex]': '_getTabindex()',
+    '[attr.disabled]': '_getDisabledAttribute()',
     '[attr.aria-disabled]': 'disabled',
     '(click)': '_handleClick($event)',
     '(keydown)': '_handleKeydown($event)',
@@ -56,6 +56,30 @@ export class MatChipAction extends _MatChipActionMixinBase implements HasTabInde
   }
   private _disabled = false;
 
+  /**
+   * Private API to allow focusing this chip when it is disabled.
+   */
+  @Input()
+  private _allowFocusWhenDisabled = false;
+
+  /**
+   * Determine the value of the disabled attribute for this chip action.
+   */
+  protected _getDisabledAttribute(): string | null {
+    // When this chip action is disabled and focusing disabled chips is not permitted, return empty
+    // string to indicate that disabled attribute should be included.
+    return this.disabled && !this._allowFocusWhenDisabled ? '' : null;
+  }
+
+  /**
+   * Determine the value of the tabindex attribute for this chip action.
+   */
+  protected _getTabindex(): string | null {
+    return (this.disabled && !this._allowFocusWhenDisabled) || !this.isInteractive
+      ? null
+      : this.tabIndex.toString();
+  }
+
   constructor(
     public _elementRef: ElementRef<HTMLElement>,
     @Inject(MAT_CHIP)

src/material/chips/chip-listbox.ts

@@ -7,6 +7,7 @@
  */
 
 import {BooleanInput, coerceBooleanProperty} from '@angular/cdk/coercion';
+import {MatChipAction} from './chip-action';
 import {TAB} from '@angular/cdk/keycodes';
 import {
   AfterContentInit,
@@ -364,4 +365,21 @@ export class MatChipListbox
       return this.selected;
     }
   }
+
+  /**
+   * Determines if key manager should avoid putting a given chip action in the tab index. Skip
+   * non-interactive actions since the user can't do anything with them.
+   */
+  protected override _skipPredicate(action: MatChipAction): boolean {
+    // Override the skip predicate in the base class to avoid skipping disabled chips. Allow
+    // disabled chip options to receive focus to align with WAI ARIA recommendation. Normally WAI
+    // ARIA's instructions are to exclude disabled items from the tab order, but it makes a few
+    // exceptions for compound widgets.
+    //
+    // From [Developing a Keyboard Interface](
+    // https://www.w3.org/WAI/ARIA/apg/practices/keyboard-interface/):
+    //   "For the following composite widget elements, keep them focusable when disabled: Options in a
+    //   Listbox..."
+    return !action.isInteractive;
+  }
 }

src/material/chips/chip-option.html

@@ -8,7 +8,7 @@
   <button
     matChipAction
     [tabIndex]="tabIndex"
-    [disabled]="disabled"
+    [_allowFocusWhenDisabled]="true"
     [attr.aria-selected]="ariaSelected"
     [attr.aria-label]="ariaLabel"
     role="option">

src/material/chips/chip-option.ts

@@ -32,8 +32,10 @@ export class MatChipSelectionChange {
 }
 
 /**
- * An extension of the MatChip component that supports chip selection.
- * Used with MatChipListbox.
+ * An extension of the MatChip component that supports chip selection. Used with MatChipListbox.
+ *
+ * Unlike other chips, the user can focus on disabled chip options inside a MatChipListbox. The
+ * user cannot click disabled chips.
  */
 @Component({
   selector: 'mat-basic-chip-option, mat-chip-option',

src/material/chips/chip-set.ts

@@ -249,8 +249,7 @@ export class MatChipSet
       .withVerticalOrientation()
       .withHorizontalOrientation(this._dir ? this._dir.value : 'ltr')
       .withHomeAndEnd()
-      // Skip non-interactive and disabled actions since the user can't do anything with them.
-      .skipPredicate(action => !action.isInteractive || action.disabled);
+      .skipPredicate(action => this._skipPredicate(action));
 
     // Keep the manager active index in sync so that navigation picks
     // up from the current chip if the user clicks into the list directly.
@@ -267,6 +266,16 @@ export class MatChipSet
       .subscribe(direction => this._keyManager.withHorizontalOrientation(direction));
   }
 
+  /**
+   * Determines if key manager should avoid putting a given chip action in the tab index. Skip
+   * non-interactive and disabled actions since the user can't do anything with them.
+   */
+  protected _skipPredicate(action: MatChipAction): boolean {
+    // Skip chips that the user cannot interact with. `mat-chip-set` does not permit focusing disabled
+    // chips.
+    return !action.isInteractive || action.disabled;
+  }
+
   /** Listens to changes in the chip set and syncs up the state of the individual chips. */
   private _trackChipSetChanges() {
     this._chips.changes.pipe(startWith(null), takeUntil(this._destroyed)).subscribe(() => {

tools/public_api_guard/material/chips.md

@@ -310,6 +310,7 @@ export class MatChipListbox extends MatChipSet implements AfterContentInit, OnDe
     get selected(): MatChipOption[] | MatChipOption;
     setDisabledState(isDisabled: boolean): void;
     _setSelectionByValue(value: any, isUserInput?: boolean): void;
+    protected _skipPredicate(action: MatChipAction): boolean;
     get value(): any;
     set value(value: any);
     // (undocumented)
@@ -449,6 +450,7 @@ export class MatChipSet extends _MatChipSetMixinBase implements AfterViewInit, H
     protected _originatesFromChip(event: Event): boolean;
     get role(): string | null;
     set role(value: string | null);
+    protected _skipPredicate(action: MatChipAction): boolean;
     protected _syncChipsState(): void;
     // (undocumented)
     static ɵcmp: i0.ɵɵComponentDeclaration<MatChipSet, "mat-chip-set", never, { "disabled": "disabled"; "role": "role"; }, {}, ["_chips"], ["*"], false, never>;