@leocg
I designed and programmed GTK and QT apps. I know what are widgets and containers.
Workspace concept is not Group.
Workspace is a new space. Groups share same space. So tabs and groups are in the same space.
It may seem same to you because it is a browser, and all you see are tabs. Actually, your Groups and tabs are inside a workspace. The browser cannot do much in terms of workspace, is not fully utilised, that is why you think is a group.
A better example is the Desktop workspace. Although developers set launcher and wallpaper to shared across workspace, it can be different all together. Can have different widgets on differet workspaces. Not just grouping apps. Is space, same as when you have two tables. Each table has a box(group as in browser) of papers, but box is not a table.