fix(material-experimental/mdc-chips): don't use button element if chip row isn't editable

Currently we always use a `button` element inside the `mat-chip-row` which can be confusing for users if the row isn't editable, because the button won't do anything.

These changes switch to using a `span` whose `role` we toggle based on whether it's editable.

The change ended up trickier than expected, because:
1. The focus behavior of the `span` is slightly different from the button we had before.
2. The focus restoration after an edit is finished is currently working by accident. When editing starts, we set `isInteractive = false` on the primary action which clears its `tabindex`. When editing is finished, we reset the flag, but change detection hasn't had the chance to run so the element won't actually have a `tabindex` at the moment when we try to restore focus. It worked by accident because a `button` is still focusable even without a `tabindex`.

An alternative approach I was testing out was to swap between a `span` and `button` element based on the `editable` input, but that didn't work, because we have a `ViewChild` query for the primary action which wasn't being updated by the framework.

Author: crisbeto

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

@@ -25,15 +25,15 @@ <h4>With avatar, icons, and color</h4>
         <mat-chip disabled>
           <mat-icon matChipAvatar>home</mat-icon>
           Home
-          <button matChipRemove>
+          <button matChipRemove aria-label="Remove chip">
             <mat-icon>cancel</mat-icon>
           </button>
         </mat-chip>
 
         <mat-chip highlighted="true" color="accent">
           <mat-chip-avatar>P</mat-chip-avatar>
           Portel
-          <button matChipRemove>
+          <button matChipRemove aria-label="Remove chip">
             <mat-icon>cancel</mat-icon>
           </button>
         </mat-chip>
@@ -45,7 +45,7 @@ <h4>With avatar, icons, and color</h4>
 
         <mat-chip>
           Koby
-          <button matChipRemove>
+          <button matChipRemove aria-label="Remove chip">
             <mat-icon>cancel</mat-icon>
           </button>
         </mat-chip>
@@ -85,7 +85,7 @@ <h4>With Events</h4>
         <mat-chip highlighted="true" color="warn" *ngIf="visible"
                  (destroyed)="displayMessage('chip destroyed')" (removed)="toggleVisible()">
           With Events
-          <button matChipRemove>
+          <button matChipRemove aria-label="Remove chip">
             <mat-icon>cancel</mat-icon>
           </button>
         </mat-chip>
@@ -152,7 +152,7 @@ <h4>Input is last child of chip grid</h4>
                    (removed)="remove(person)"
                    (edited)="edit(person, $event)">
             {{person.name}}
-            <button matChipRemove>
+            <button matChipRemove aria-label="Remove contributor">
               <mat-icon>cancel</mat-icon>
             </button>
           </mat-chip-row>
@@ -171,7 +171,7 @@ <h4>Input is next sibling child of chip grid</h4>
         <mat-chip-grid #chipGrid2 [(ngModel)]="selectedPeople" required [disabled]="disableInputs">
           <mat-chip-row *ngFor="let person of people" (removed)="remove(person)">
             {{person.name}}
-            <button matChipRemove>
+            <button matChipRemove aria-label="Remove contributor">
               <mat-icon>cancel</mat-icon>
             </button>
           </mat-chip-row>

src/material-experimental/mdc-chips/chip-row.html

@@ -8,8 +8,9 @@
 
 
 <span class="mdc-evolution-chip__cell mdc-evolution-chip__cell--primary" role="gridcell">
-  <button
+  <span
     matChipAction
+    [attr.role]="editable ? 'button' : null"
     [tabIndex]="tabIndex"
     [disabled]="disabled"
     [attr.aria-label]="ariaLabel">
@@ -27,7 +28,7 @@
 
       <span class="mat-mdc-chip-primary-focus-indicator mat-mdc-focus-indicator"></span>
     </span>
-  </button>
+  </span>
 </span>
 
 <span

src/material-experimental/mdc-chips/chip-row.spec.ts

@@ -259,6 +259,16 @@ describe('MDC-based Row Chips', () => {
         return chipNativeElement.querySelector('.mat-chip-edit-input')!;
       }
 
+      it('should set the role of the primary action based on whether it is editable', () => {
+        testComponent.editable = false;
+        fixture.detectChanges();
+        expect(primaryAction.hasAttribute('role')).toBe(false);
+
+        testComponent.editable = true;
+        fixture.detectChanges();
+        expect(primaryAction.getAttribute('role')).toBe('button');
+      });
+
       it('should not delete the chip on DELETE or BACKSPACE', () => {
         spyOn(testComponent, 'chipDestroy');
         keyDownOnPrimaryAction(DELETE, 'Delete');

src/material-experimental/mdc-chips/chip-row.ts

@@ -81,6 +81,13 @@ export interface MatChipEditedEvent extends MatChipEvent {
 export class MatChipRow extends MatChip implements AfterViewInit {
   protected override basicChipAttrName = 'mat-basic-chip-row';
 
+  /**
+   * The editing action has to be triggered in a timeout. While we're waiting on it, a blur
+   * event might occur which will interrupt the editing. This flag is used to avoid interruptions
+   * while the editing action is being initialized.
+   */
+  private _editStartPending = false;
+
   @Input() editable: boolean = false;
 
   /** Emitted when the chip is edited. */
@@ -120,7 +127,7 @@ export class MatChipRow extends MatChip implements AfterViewInit {
 
     this.role = 'row';
     this._onBlur.pipe(takeUntil(this.destroyed)).subscribe(() => {
-      if (this._isEditing) {
+      if (this._isEditing && !this._editStartPending) {
         this._onEditFinish();
       }
     });
@@ -175,18 +182,19 @@ export class MatChipRow extends MatChip implements AfterViewInit {
     // The value depends on the DOM so we need to extract it before we flip the flag.
     const value = this.value;
 
-    // Make the primary action non-interactive so that it doesn't
-    // navigate when the user presses the arrow keys while editing.
-    this.primaryAction.isInteractive = false;
     this._isEditing = true;
+    this._editStartPending = true;
 
     // Defer initializing the input so it has time to be added to the DOM.
-    setTimeout(() => this._getEditInput().initialize(value));
+    setTimeout(() => {
+      this._getEditInput().initialize(value);
+      this._editStartPending = false;
+    });
   }
 
   private _onEditFinish() {
     this._isEditing = false;
-    this.primaryAction.isInteractive = true;
+    this._editStartPending = false;
     this.edited.emit({chip: this, value: this._getEditInput().getValue()});
 
     // If the edit input is still focused or focus was returned to the body after it was destroyed,