Originally an extension for MacOs, and later built in to the system: the windowing behaviour whereby clicking a widget built into the title bar of a window, or double-clicking the main part of the title bar, causes the window to "collapse" and display just the title bar on-screen. The window can still be focussed and brought to front by the normal rules, independantly of the collapsed state: clicking a title bar of a collapsed window focusses and brings to front, and clicking elsewhere unfocusses it.

Compare/contrast with the "minimize" behaviour of Windows or "iconify" behaviour of some Linux-based window managers.