On some pages, like Home or My page or plugin using main menu entries the project menu is empty but the div html is still present.

It's a problem when you a background image in CSS, because you see the background image without the menu items. Also its a problem if you use contect menu slider as done in plugin "redmine sidebar hide":https://github.com/ries-tech/sidebar_hide where additional space is made availaibe if collapsed

Could you remove this div or add a CSS class to identify an empty menu so i am able to unset css main.margin-left property, for view here no project context and aka no project menu entries are available.

The fix #3259 doesn't fix the problem, because i still have empty "main-menu" divs in my rendered site