28.18 Atomic Windows

Atomic windows are rectangular compositions of at least two live windows. They have the following distinctive characteristics:

• The function split-window (see Splitting Windows), when applied to a constituent of an atomic window, will try to create the new window outside of the atomic window.
• The function delete-window (see Deleting Windows), when applied to a constituent of an atomic window, will try to delete the entire atomic window instead.
• The function delete-other-windows (see Deleting Windows), when applied to a constituent of an atomic window, will try to make the atomic window fill its frame or main window (see Side Windows).

This means that the basic groups of functions that alter the window structure treat an atomic window like a live one, thus preserving the internal structure of the atomic window.

Atomic windows are useful to construct and preserve window layouts that are meaningful only when all involved buffers are shown simultaneously in a specific manner, such as when showing differences between file revisions, or the same text in different languages or markups. They can also be used to permanently display information pertinent to a specific window in bars on that window's sides.

Atomic windows are implemented with the help of the reserved window-atom window parameter (see Window Parameters) and an internal window (see Basic Windows) called the root window of the atomic window. All windows that are part of the same atomic window have this root window as their common ancestor and are assigned a non-nil window-atom parameter.

The following function returns the root of the atomic window a specified window is part of:

The most simple approach to make a new atomic window is to take an existing internal window and apply the following function:

To create a new atomic window from an existing live window or to add a new window to an existing atomic window, the following buffer display action function (see Buffer Display Action Functions) can be used:

Note that the value of the window-atom parameter does not really matter as long as it is non-nil. The values assigned by display-buffer-in-atom-window just allow for easy retrieval of the original and the new window after that function has been applied. Note also that the window-atom parameter is the only window parameter assigned by display-buffer-in-atom-window. Further parameters have to be set by the application explicitly via a window-parameters entry in alist.

Atomic windows automatically cease to exist when one of their constituents gets deleted. To dissolve an atomic window manually, reset the window-atom parameter of its constituents—the root of the atomic window and all its descendants.

The following code snippet, when applied to a single-window frame, first splits the selected window and makes the selected and the new window constituents of an atomic window with their parent as root. It then displays the buffer *Messages* in a new window at the frame's bottom and makes that new window part of the atomic window just created.

     (let ((window (split-window-right)))
       (window-make-atom (window-parent window))
       (display-buffer-in-atom-window
        (get-buffer-create "*Messages*")
        `((window . ,(window-parent window)) (window-height . 5))))

At this moment typing C-x 2 in any window of that frame produces a new window at the bottom of the frame. Typing C-x 3 instead will put the new window at the frame's right. In either case, typing now C-x 1 in any window of the atomic window will remove the new window only. Typing C-x 0 in any window of the atomic window will make that new window fill the frame.