Fix to #263546, for submenu of treeView view/item/context z-index iss… (#263555)

Fix to #263546, for submenu of treeView view/item/context z-index issue - ensure submenu appears above other elements

Author: yiliang114

src/vs/base/browser/ui/menu/menu.ts

@@ -903,6 +903,8 @@ class SubmenuMenuActionViewItem extends BaseMenuActionViewItem {
 			this.submenuContainer.style.position = 'fixed';
 			this.submenuContainer.style.top = '0';
 			this.submenuContainer.style.left = '0';
+			// Fix to #263546, for submenu of treeView view/item/context z-index issue - ensure submenu appears above other elements
+			this.submenuContainer.style.zIndex = '1';
 
 			this.parentData.submenu = new Menu(this.submenuContainer, this.submenuActions.length ? this.submenuActions : [new EmptySubmenuAction()], this.submenuOptions, this.menuStyle);