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### 27.2 Windows and Frames

Each window belongs to exactly one frame (see Frames).

Function: window-frame &optional window

  This function returns the frame that the window window belongs to. If window is `nil`, it defaults to the selected window.

Function: window-list &optional frame minibuffer window

  This function returns a list of live windows belonging to the frame frame. If frame is omitted or `nil`, it defaults to the selected frame.

  The optional argument minibuffer specifies whether to include the minibuffer window in the returned list. If minibuffer is `t`, the minibuffer window is included. If minibuffer is `nil` or omitted, the minibuffer window is included only if it is active. If minibuffer is neither `nil` nor `t`, the minibuffer window is never included.

  The optional argument window, if non-`nil`, should be a live window on the specified frame; then window will be the first element in the returned list. If window is omitted or `nil`, the window selected within the frame is the first element.

Windows in the same frame are organized into a window tree, whose leaf nodes are the live windows. The internal nodes of a window tree are not live; they exist for the purpose of organizing the relationships between live windows. The root node of a window tree is called the root window. It can be either a live window (if the frame has just one window), or an internal window.

A minibuffer window (see Minibuffer Windows) is not part of its frame’s window tree unless the frame is a minibuffer-only frame. Nonetheless, most of the functions in this section accept the minibuffer window as an argument. Also, the function `window-tree` described at the end of this section lists the minibuffer window alongside the actual window tree.

Function: frame-root-window &optional frame-or-window

  This function returns the root window for frame-or-window. The argument frame-or-window should be either a window or a frame; if omitted or `nil`, it defaults to the selected frame. If frame-or-window is a window, the return value is the root window of that window’s frame.

When a window is split, there are two live windows where previously there was one. One of these is represented by the same Lisp window object as the original window, and the other is represented by a newly-created Lisp window object. Both of these live windows become leaf nodes of the window tree, as child windows of a single internal window. If necessary, Emacs automatically creates this internal window, which is also called the parent window, and assigns it to the appropriate position in the window tree. A set of windows that share the same parent are called siblings.

Function: window-parent &optional window

  This function returns the parent window of window. If window is omitted or `nil`, it defaults to the selected window. The return value is `nil` if window has no parent (i.e., it is a minibuffer window or the root window of its frame).

Each internal window always has at least two child windows. If this number falls to one as a result of window deletion, Emacs automatically deletes the internal window, and its sole remaining child window takes its place in the window tree.

Each child window can be either a live window, or an internal window (which in turn would have its own child windows). Therefore, each internal window can be thought of as occupying a certain rectangular screen area—the union of the areas occupied by the live windows that are ultimately descended from it.

For each internal window, the screen areas of the immediate children are arranged either vertically or horizontally (never both). If the child windows are arranged one above the other, they are said to form a vertical combination; if they are arranged side by side, they are said to form a horizontal combination. Consider the following example:

         ______________________________________
        | ______  ____________________________ |
        ||      || __________________________ ||
        ||      |||                          |||
        ||      |||                          |||
        ||      |||                          |||
        ||      |||____________W4____________|||
        ||      || __________________________ ||
        ||      |||                          |||
        ||      |||                          |||
        ||      |||____________W5____________|||
        ||__W2__||_____________W3_____________ |
        |__________________W1__________________|

The root window of this frame is an internal window, W1. Its child windows form a horizontal combination, consisting of the live window W2 and the internal window W3. The child windows of W3 form a vertical combination, consisting of the live windows W4 and W5. Hence, the live windows in this window tree are W2, W4, and W5.

The following functions can be used to retrieve a child window of an internal window, and the siblings of a child window.

Function: window-top-child &optional window

  This function returns the topmost child window of window, if window is an internal window whose children form a vertical combination. For any other type of window, the return value is `nil`.

Function: window-left-child &optional window

  This function returns the leftmost child window of window, if window is an internal window whose children form a horizontal combination. For any other type of window, the return value is `nil`.

Function: window-child window

  This function returns the first child window of the internal window window—the topmost child window for a vertical combination, or the leftmost child window for a horizontal combination. If window is a live window, the return value is `nil`.

Function: window-combined-p &optional window horizontal

  This function returns a non-`nil` value if and only if window is part of a vertical combination. If window is omitted or `nil`, it defaults to the selected one.

  If the optional argument horizontal is non-`nil`, this means to return non-`nil` if and only if window is part of a horizontal combination.

Function: window-next-sibling &optional window

  This function returns the next sibling of the window window. If omitted or `nil`, window defaults to the selected window. The return value is `nil` if window is the last child of its parent.

Function: window-prev-sibling &optional window

  This function returns the previous sibling of the window window. If omitted or `nil`, window defaults to the selected window. The return value is `nil` if window is the first child of its parent.

The functions `window-next-sibling` and `window-prev-sibling` should not be confused with the functions `next-window` and `previous-window`, which return the next and previous window, respectively, in the cyclic ordering of windows (see Cyclic Window Ordering).

You can use the following functions to find the first live window on a frame and the window nearest to a given window.

Function: frame-first-window &optional frame-or-window

  This function returns the live window at the upper left corner of the frame specified by frame-or-window. The argument frame-or-window must denote a window or a live frame and defaults to the selected frame. If frame-or-window specifies a window, this function returns the first window on that window’s frame. Under the assumption that the frame from our canonical example is selected `(frame-first-window)` returns W2.

Function: window-in-direction direction &optional window ignore sign wrap mini

  This function returns the nearest live window in direction direction as seen from the position of `window-point` in window window. The argument direction must be one of `above`, `below`, `left` or `right`. The optional argument window must denote a live window and defaults to the selected one.

  This function does not return a window whose `no-other-window` parameter is non-`nil` (see Window Parameters). If the nearest window’s `no-other-window` parameter is non-`nil`, this function tries to find another window in the indicated direction whose `no-other-window` parameter is `nil`. If the optional argument ignore is non-`nil`, a window may be returned even if its `no-other-window` parameter is non-`nil`.

  If the optional argument sign is a negative number, it means to use the right or bottom edge of window as reference position instead of `window-point`. If sign is a positive number, it means to use the left or top edge of window as reference position.

  If the optional argument wrap is non-`nil`, this means to wrap direction around frame borders. For example, if window is at the top of the frame and direction is `above`, then this function usually returns the frame’s minibuffer window if it’s active and a window at the bottom of the frame otherwise.

  If the optional argument mini is `nil`, this means to return the minibuffer window if and only if it is currently active. If mini is non-`nil`, this function may return the minibuffer window even when it’s not active. However, if wrap is non-`nil`, it always acts as if mini were `nil`.

  If it doesn’t find a suitable window, this function returns `nil`.

The following function allows the entire window tree of a frame to be retrieved:

Function: window-tree &optional frame

  This function returns a list representing the window tree for frame frame. If frame is omitted or `nil`, it defaults to the selected frame.

  The return value is a list of the form `(root mini)`, where root represents the window tree of the frame’s root window, and mini is the frame’s minibuffer window.

  If the root window is live, root is that window itself. Otherwise, root is a list `(dir edges w1 w2 ...)` where dir is `nil` for a horizontal combination and `t` for a vertical combination, edges gives the size and position of the combination, and the remaining elements are the child windows. Each child window may again be a window object (for a live window) or a list with the same format as above (for an internal window). The edges element is a list `(left top right bottom)`, similar to the value returned by `window-edges` (see Coordinates and Windows).

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