Yep, that's Fluxbox tabs.

https://web.archive.org/web/20240202172611/http://fluxbox.org/features/

Tabbing is a nice feature that allows you to tab windows together. This can be combined with the "autogrouping" feature that is provided via the apps-file. This will make certain applications tab together by default.

Tabs can eiher be embedded into the window's titlebar as shown in the upper screenshot or they can appear as little tabs at the outside of a window such as the lower example. The position and size of the outside-tabs is customizable.