fix(button): set disabled attribute instead of tabindex #5765

Author: DiyanDimitrov

projects/igniteui-angular/src/lib/buttonGroup/buttongroup.component.spec.ts

@@ -244,9 +244,9 @@ describe('IgxButtonGroup', () => {
             expect(button.nativeElement.tagName).toBe('BUTTON');
 
             if (i < groupChildren.length - 1) {
-                expect(button.nativeElement.tabIndex).toBe(0);
+                expect(button.nativeElement.disabled).toBe(false);
             } else {
-                expect(button.nativeElement.tabIndex).toBe(-1);
+                expect(button.nativeElement.disabled).toBe(true);
             }
         }
     });

projects/igniteui-angular/src/lib/directives/button/button.directive.ts

@@ -191,9 +191,9 @@ export class IgxButtonDirective extends DisplayDensityBase {
     /**
      * @hidden
      */
-    @HostBinding('attr.tabIndex')
-    public get tabIndex(): number {
-        return this._disabled ? -1 : 0;
+    @HostBinding('attr.disabled')
+    public get disabledAttribute() {
+        return this._disabled ? this._disabled : null;
     }
 
     /**