dxGantt: fix context menu to make it accessible (T1269329) (#28774)

Author: Krijovnick

packages/devextreme/js/ui/gantt/ui.gantt.treelist.js

@@ -109,7 +109,7 @@ export class GanttTreeList {
         if(e.row?.rowType === 'data') {
             this.setOption('selectedRowKeys', [e.row.data[this._gantt.option('tasks.keyExpr')]]);
         }
-        e.items = [];
+
         const info = {
             cancel: false,
             event: e.event,
@@ -118,6 +118,7 @@ export class GanttTreeList {
             position: { x: e.event.pageX, y: e.event.pageY }
         };
         this._gantt._showPopupMenu(info);
+        e.event.preventDefault();
     }
 
     _getHeight() {

packages/devextreme/testing/tests/DevExpress.ui.widgets/ganttParts/contextMenu.tests.js

@@ -40,7 +40,7 @@ QUnit.module('Context Menu', moduleConfig, () => {
         assert.equal(getContextMenuElement().length, 0, 'menu is hidden on create');
         const $cellElement = $(this.instance._treeList.getCellElement(0, 0));
         $cellElement.trigger('contextmenu');
-        assert.equal(getContextMenuElement().length, 2, 'menu is visible after right click in tree list');
+        assert.equal(getContextMenuElement().length, 1, 'menu is visible after right click in tree list');
     });
     test('shown at correct position', function(assert) {
         this.createInstance(options.allSourcesOptions);