IPanelDetailController is the interface used by those triangles in the panel tab.
The behaviour is up to you, I think kZoomSetsDetailControllerImpl switches visibility as you described.
A quick look at its uses shows mainly panels with more or fewer widgets in their details, such as align (Ausrichten), gradient (Verlauf), text wrap (Textumfluss).
The mechanism should also resize the panel as required, invoke AutoAttach/Detach and so forth.