When you move/hover/navigate around the menus, if the number of items within this menu is big thus the menu items can not fit the screen, the menu items displayed will flow up to the menu boundary. These makes the menus covered (shielded) by the menu items shown.

Steps to Reproduce:
1. Put your mouse on the "File" menu.

Current Result:
The "File" menu and the other menus next to it are invisible. If you want to move to another menu, you have to firstly move you mouse cursor to a blank area.

Expected:
The menu items shown should never cover the menus.

Further information:
The behaviour in Firefox is good, menu items are shown below the menu. If there are many menu items so they can not fit to the screen, an up or down triangle button is shown.

Version: 6.1.0.0.alpha0+
Build ID：ff8826d438a9fd080e8e61f8e66b3d3b2c8d752b
CPU 线程：2; 操作系统：Linux 4.14; UI 渲染：默认; VCL: gtk3; 
区域语言：zh-CN (zh_CN.UTF-8); Calc: group threaded

版本：5.4.4.2
Build ID：5.4.4.2-1.fc27
CPU 线程：2; 操作系统：Linux 4.14; UI 渲染：默认; VCL: gtk3; 
区域语言：zh-CN (zh_CN.UTF-8); Calc: group

Fedora 27, Gnome Desktop