fix: always set tabIndex to restore keydown a11y (#6871)

Skipping the tabIndex property on created elements due to #6145
optimizations blocks them from receiving keyboard events, due to not
being focusable; for example this breaks closing ModalDialog elements by
pressing Escape.
Fix this by always setting tabIndex, as the element may return the same
value even though the property has not been explicitly set.

Fixes #6870

src/js/utils/dom.js

@@ -165,7 +165,7 @@ export function createEl(tagName = 'div', properties = {}, attributes = {}, cont
     // method for it.
     } else if (propName === 'textContent') {
       textContent(el, val);
-    } else if (el[propName] !== val) {
+    } else if (el[propName] !== val || propName === 'tabIndex') {
       el[propName] = val;
     }
   });