Keep menus alive for submenu interaction

crates/egui/src/containers/menu.rs

@@ -515,6 +515,13 @@ impl SubMenu {
                 content(ui)
             });
 
+        if let Some(_) = &popup_response {
+            // Keep the submenu alive so `from_id`'s staleness check doesn't close it.
+            // Without this, menus whose content has no further SubMenuButtons never update
+            // `last_visible_pass`, causing the parent to reset `open_item` after ~2 frames.
+            MenuState::mark_shown(ui.ctx(), id);
+        }
+
         if let Some(popup_response) = &popup_response {
             // If no child sub menu is open means we must be the deepest child sub menu.
             let is_deepest_submenu = MenuState::is_deepest_open_sub_menu(ui.ctx(), id);