fix(material/checkbox): focus not moved to input when clicking on touch target (#26545)

Fixes that while we were toggling the checked state of a checkbox when its touch target is clicked, we weren't moving focus to the internal `input`.

Fixes #26486.

(cherry picked from commit 48b4445)

Author: crisbeto

src/material/checkbox/checkbox.html

@@ -3,7 +3,7 @@
      (click)="_preventBubblingFromLabel($event)">
   <div #checkbox class="mdc-checkbox">
     <!-- Render this element first so the input is on top. -->
-    <div class="mat-mdc-checkbox-touch-target" (click)="_onInputClick()"></div>
+    <div class="mat-mdc-checkbox-touch-target" (click)="_onTouchTargetClick()"></div>
     <input #input
            type="checkbox"
            class="mdc-checkbox__native-control"

src/material/checkbox/checkbox.spec.ts

@@ -404,6 +404,20 @@ describe('MDC-based MatCheckbox', () => {
       expect(document.activeElement).toBe(inputElement);
     }));
 
+    it('should focus underlying input element when the touch target is clicked', fakeAsync(() => {
+      const touchTarget = checkboxElement.querySelector(
+        '.mat-mdc-checkbox-touch-target',
+      ) as HTMLElement;
+
+      expect(document.activeElement).not.toBe(inputElement);
+
+      touchTarget.click();
+      fixture.detectChanges();
+      flush();
+
+      expect(document.activeElement).toBe(inputElement);
+    }));
+
     it('should forward the value to input element', fakeAsync(() => {
       testComponent.checkboxValue = 'basic_checkbox';
       fixture.detectChanges();

src/material/checkbox/checkbox.ts

@@ -547,6 +547,16 @@ export class MatCheckbox
     super._handleInputClick();
   }
 
+  _onTouchTargetClick() {
+    super._handleInputClick();
+
+    if (!this.disabled) {
+      // Normally the input should be focused already, but if the click
+      // comes from the touch target, then we might have to focus it ourselves.
+      this._inputElement.nativeElement.focus();
+    }
+  }
+
   /**
    *  Prevent click events that come from the `<label/>` element from bubbling. This prevents the
    *  click handler on the host from triggering twice when clicking on the `<label/>` element. After

tools/public_api_guard/material/checkbox.md

@@ -58,6 +58,8 @@ export class MatCheckbox extends _MatCheckboxBase<MatCheckboxChange> implements
     protected _getAnimationTargetElement(): HTMLInputElement;
     // (undocumented)
     _onInputClick(): void;
+    // (undocumented)
+    _onTouchTargetClick(): void;
     _preventBubblingFromLabel(event: MouseEvent): void;
     // (undocumented)
     static ɵcmp: i0.ɵɵComponentDeclaration<MatCheckbox, "mat-checkbox", ["matCheckbox"], { "disableRipple": "disableRipple"; "color": "color"; "tabIndex": "tabIndex"; }, {}, never, ["*"], false, never>;