feat: add row focus mode to convey treegrid structure (#2434)

Author: tomivirkki

packages/vaadin-grid/src/vaadin-grid-a11y-mixin.js

@@ -74,6 +74,20 @@ export const A11yMixin = (superClass) =>
       Array.from(row.children).forEach((cell) => cell.setAttribute('aria-selected', Boolean(selected)));
     }
 
+    /**
+     * @param {!HTMLElement} row
+     * @protected
+     */
+    _a11yUpdateRowExpanded(row) {
+      if (this.__isRowExpandable(row)) {
+        row.setAttribute('aria-expanded', 'false');
+      } else if (this.__isRowCollapsible(row)) {
+        row.setAttribute('aria-expanded', 'true');
+      } else {
+        row.removeAttribute('aria-expanded');
+      }
+    }
+
     /**
      * @param {!HTMLElement} row
      * @param {number} level

packages/vaadin-grid/src/vaadin-grid-active-item-mixin.js

@@ -29,6 +29,7 @@ export const ActiveItemMixin = (superClass) =>
 
       this.$.scroller.addEventListener('click', this._onClick.bind(this));
       this.addEventListener('cell-activate', this._activateItem.bind(this));
+      this.addEventListener('row-activate', this._activateItem.bind(this));
     }
 
     /** @private */