The idea is this:

• The menu is displayed horizontally in the titlebar by default with this feature enabled.
• The user can toggle the menu at any time to hide it with the title bar application menu button.
• When the window becomes too thin to fit the menu in the titlebar, the menu will deactivate.
• When the window is too thin to fit the menu in the titlebar, the menu button functions like the current title bar application menu button with a vertical menu.

This mockup is using settings that I personally use such as Breeze Dark, the titlebar button configuration and the left aligned window title, but those are not actually part of my idea. It was just easier to use my own settings in the mockup than to switch to a different user with default settings.