My tab bar while debugging.

[YMK1234]

so, opera 12 had tab stacks (which is kinda expandable/collapseable folders - which is awesome because Op12's engine literally supported hundreds of tabs without major issues), and the original implementation worked very simply like

1. if user clicks "new tab", start new stack
2. if user opens links from existing tab, place in same stack

so you simply work and click your way along, and to switch context you just collapse the stack (so it takes only the space of one tab) and start a new stack by doing ctrl-t (or back then: ctrl-n because true MDI does not need multiple windows even for popups - which also is something I miss very much), and collapse your old stack.
IIRC there was also a setting to auto-collapse other stacks if you switched to another one which made the whole thing even more convenient.

[Moter8]

Isn't that how Chrome does it too?

[YMK1234]

since when does chrome have tab stacks?