[lispm]

In former times that was the screen in front of you. With its own window manager, menus, scrolling, mouse controls, etc.

If one looks at current development environments, many of them have their own single window (with multiple subpanes and window layouts) and often a not-so-native user experience. Many cross-platform toolkits also add an additional layer. What is different here, is that the window is a new window system, where we often find in modern IDEs the IDE of subpanes, instead of subwindows/...

The Symbolics Genera UI OTOH favors multiple (mostly) full screen applications with subpanes. One switches between them via keyboard commands: select-e is the editor, select-l the listener, select-d the documentation reader, select-m the mailer, ... you get the idea.