I know this has been asked before, and the answer was GDL, but it now seems unused and outdated, so I thought it would be wise to ask it again.

The language of my choice is D, but it shouldn’t matter much; I can also use C++ if it’s really needed.

I want an interface with dockable panels, very similar to Inkscape and IntelliJ. I know Inkscape is also GTK-based, but I don’t know what they do to acheive this.

With drag-and-drop, one should be able to detach docks from the left/right/bottom panes, split docks (top-left/top-right) and detach tabs from the main app interface. Docks should be closed by clicking somewhere.