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28.29 Window Parameters

This section describes how window parameters can be used to associate additional information with windows.

— Function: window-parameter window parameter

This function returns window's value for parameter. The default for window is the selected window. If window has no setting for parameter, this function returns nil.

— Function: window-parameters &optional window

This function returns all parameters of window and their values. The default for window is the selected window. The return value is either nil, or an association list whose elements have the form (parameter . value).

— Function: set-window-parameter window parameter value

This function sets window's value of parameter to value and returns value. The default for window is the selected window.

By default, the functions that save and restore window configurations or the states of windows (see Window Configurations) do not care about window parameters. This means that when you change the value of a parameter within the body of a save-window-excursion, the previous value is not restored when that macro exits. It also means that when you restore via window-state-put a window state saved earlier by window-state-get, all cloned windows have their parameters reset to nil. The following variable allows you to override the standard behavior:

— Variable: window-persistent-parameters

This variable is an alist specifying which parameters get saved by current-window-configuration and window-state-get, and subsequently restored by set-window-configuration and window-state-put. See Window Configurations.

The car of each entry of this alist is a symbol specifying the parameter. The cdr should be one of the following:

This value means the parameter is saved neither by window-state-get nor by current-window-configuration.
This value specifies that the parameter is saved by current-window-configuration and (provided its writable argument is nil) by window-state-get.
This means that the parameter is saved unconditionally by both current-window-configuration and window-state-get. This value should not be used for parameters whose values do not have a read syntax. Otherwise, invoking window-state-put in another session may fail with an invalid-read-syntax error.

Some functions (notably delete-window, delete-other-windows and split-window), may behave specially when the window specified by their window argument has a parameter whose name is equal to the function's name. You can override such special behavior by binding the following variable to a non-nil value:

— Variable: ignore-window-parameters

If this variable is non-nil, some standard functions do not process window parameters. The functions currently affected by this are split-window, delete-window, delete-other-windows, and other-window.

An application can bind this variable to a non-nil value around calls to these functions. If it does so, the application is fully responsible for correctly assigning the parameters of all involved windows when exiting that function.

The following parameters are currently used by the window management code:

This parameter affects the execution of delete-window (see Deleting Windows).
This parameter affects the execution of delete-other-windows (see Deleting Windows).
This parameter marks the window as not deletable by delete-other-windows (see Deleting Windows).
This parameter affects the execution of split-window (see Splitting Windows).
This parameter affects the execution of other-window (see Cyclic Window Ordering).
This parameter marks the window as not selectable by other-window (see Cyclic Window Ordering).
This parameter specifies the window that this one has been cloned from. It is installed by window-state-get (see Window Configurations).
This parameter specifies a buffer, a direction where nil means vertical and t horizontal, and a size in pixels. If this window displays the specified buffer and its size in the indicated direction equals the size specified by this parameter, then Emacs will try to preserve the size of this window in the indicated direction. This parameter is installed and updated by the function window-preserve-size (see Preserving Window Sizes).
This parameter is installed by the buffer display functions (see Choosing Window) and consulted by quit-restore-window (see Quitting Windows). It contains four elements:

The first element is one of the symbols window, meaning that the window has been specially created by display-buffer; frame, a separate frame has been created; same, the window has only ever displayed this buffer; or other, the window showed another buffer before. frame and window affect how the window is quit, while same and other affect the redisplay of buffers previously shown in this window.

The second element is either one of the symbols window or frame, or a list whose elements are the buffer shown in the window before, that buffer's window start and window point positions, and the window's height at that time. If that buffer is still live when the window is quit, then the function quit-restore-window reuses the window to display the buffer.

The third element is the window selected at the time the parameter was created. If quit-restore-window deletes the window passed to it as argument, it then tries to reselect this window.

The fourth element is the buffer whose display caused the creation of this parameter. quit-restore-window deletes the specified window only if it still shows that buffer.

See the description of quit-restore-window in Quitting Windows for details.

window-side window-slot
These parameters are used for implementing side windows (see Side Windows).
This parameter is used for implementing atomic windows, see Atomic Windows.
This parameter replaces the value of the buffer-local variable mode-line-format (see Mode Line Basics) of this window's buffer whenever this window is displayed. The symbol none means to suppress display of a mode line for this window. Display and contents of the mode line on other windows showing this buffer are not affected.
This parameter replaces the value of the buffer-local variable header-line-format (see Mode Line Basics) of this window's buffer whenever this window is displayed. The symbol none means to suppress display of a header line for this window. Display and contents of the header line on other windows showing this buffer are not affected.
The value of this parameter is a cons cell whose car and cdr, if non-nil, specify the minimum values (in columns) for the left and right margin of this window. When present, Emacs will use these values instead of the actual margin widths for determining whether a window can be split or shrunk horizontally.

Emacs never auto-adjusts the margins of any window after splitting or resizing it. It is the sole responsibility of any application setting this parameter to adjust the margins of this window as well as those of any new window that inherits this window's margins due to a split. Both window-configuration-change-hook and window-size-change-functions (see Window Hooks) should be employed for this purpose.

This parameter was introduced in Emacs version 25.1 to support applications that use large margins to center buffer text within a window and should be used, with due care, exclusively by those applications. It might be replaced by an improved solution in future versions of Emacs.