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27.12 Choosing a Window for Display

The command display-buffer flexibly chooses a window for display, and displays a specified buffer in that window. It can be called interactively, via the key binding C-x 4 C-o. It is also used as a subroutine by many functions and commands, including switch-to-buffer and pop-to-buffer (see Switching Buffers).

This command performs several complex steps to find a window to display in. These steps are described by means of display actions, which have the form (function . alist). Here, function is either a function or a list of functions, which we refer to as action functions; alist is an association list, which we refer to as action alists.

An action function accepts two arguments: the buffer to display and an action alist. It attempts to display the buffer in some window, picking or creating a window according to its own criteria. If successful, it returns the window; otherwise, it returns nil. See Display Action Functions, for a list of predefined action functions.

display-buffer works by combining display actions from several sources, and calling the action functions in turn, until one of them manages to display the buffer and returns a non-nil value.

— Command: display-buffer buffer-or-name &optional action frame

This command makes buffer-or-name appear in some window, without selecting the window or making the buffer current. The argument buffer-or-name must be a buffer or the name of an existing buffer. The return value is the window chosen to display the buffer.

The optional argument action, if non-nil, should normally be a display action (described above). display-buffer builds a list of action functions and an action alist, by consolidating display actions from the following sources (in order):

Each action function is called in turn, passing the buffer as the first argument and the combined action alist as the second argument, until one of the functions returns non-nil. The caller can pass (allow-no-window . t) as an element of the action alist to indicate its readiness to handle the case of not displaying the buffer in a window.

The argument action can also have a non-nil, non-list value. This has the special meaning that the buffer should be displayed in a window other than the selected one, even if the selected window is already displaying it. If called interactively with a prefix argument, action is t.

The optional argument frame, if non-nil, specifies which frames to check when deciding whether the buffer is already displayed. It is equivalent to adding an element (reusable-frames . frame) to the action alist of action. See Display Action Functions.

— Variable: display-buffer-overriding-action

The value of this variable should be a display action, which is treated with the highest priority by display-buffer. The default value is empty, i.e., (nil . nil).

— User Option: display-buffer-alist

The value of this option is an alist mapping conditions to display actions. Each condition may be either a regular expression matching a buffer name or a function that takes two arguments: a buffer name and the action argument passed to display-buffer. If the name of the buffer passed to display-buffer either matches a regular expression in this alist or the function specified by a condition returns non-nil, then display-buffer uses the corresponding display action to display the buffer.

— User Option: display-buffer-base-action

The value of this option should be a display action. This option can be used to define a “standard” display action for calls to display-buffer.

— Constant: display-buffer-fallback-action

This display action specifies the fallback behavior for display-buffer if no other display actions are given.