Headertabs

This is a proposal on how tabs in the headerbar could be implemented and used. The most important benefits of this implementation, and the use of tabs in the headerbar in general, is:

Better overview and faster navigation of an app's structure. Although breadcrumbs (which aren't really tabs) stands for much of the work in this respect.
A simple solution for project-based apps. The concept of a tab and the concept of a project comes closer to each other. Creating a project is the same as opening a tab, and vice versa.
Saves space.
Allows for more flexibility in managing windows.

Controlling tabs

Scrolling on the headerbar - switching tabs
Middle mouse button - close tab
Left mouse button - select tab in unselected, if selected, opens the tab-menu
Right mouse button - opens tab-menu
Left mouse button dragging - moves the window
Right mouse button dragging - moves the tab

The tab-menu is a popover that opens when you click on an already selected tab or right click it. It contains basic items such as close and save, but can contain almost anything. In the case of gnome web, the popover needs some styling.

Discrete tabs

Discrete tabs are for apps that are not focused on being able to use tabs. The tab-behavior only kicks in when two tabs are open, but if only one "tab" is open in that window, it will only show as a normal title.

Discrete tabs

Discrete tabs with breadcrumbs

Discrete tabs with overview-tab and breadcrumbs

Static tabs

Static tabs are tabs that are, well, static.

Static tabs

Static tabs with breadcrumbs

Dynamic tabs

Dynamic tabs are meant for project-based apps, for example gimp, libreoffice writer and so on.