alt Hacker News> By default, minimize button not present in titlebars
This is explained by the ElementaryOS H.I.G.:
> Apps should save their current state when closed so they can be reopened right to where the user left off. Typically, closing and reopening an app should be indistinguishable from the legacy concept of minimizing and unminimizing an app; that is, all elements should be saved including open documents, scroll position, undo history, etc.
> Because of the strong convention of saved state, elementary OS does not expose or optimize for legacy minimize behavior; e.g. there is no minimize button, and the Multitasking View does not distinguish minimized windows.