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+/* Window definitions for GNU Emacs.
+   Copyright (C) 1985, 1986 Free Software Foundation, Inc.
+
+This file is part of GNU Emacs.
+
+GNU Emacs is free software; you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation; either version 1, or (at your option)
+any later version.
+
+GNU Emacs is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with GNU Emacs; see the file COPYING.  If not, write to
+the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.  */
+
+
+/* Windows are allocated as if they were vectors, but then the
+Lisp data type is changed to Lisp_Window.  They are garbage
+collected along with the vectors.
+
+All windows in use are arranged into a tree, with pointers up and down.
+
+Windows that are leaves of the tree are actually displayed
+and show the contents of buffers.  Windows that are not leaves
+are used for representing the way groups of leaf windows are
+arranged on the screen.  Leaf windows never become non-leaves.
+They are deleted only by calling delete-window on them (but
+this can be done implicitly).  Combination windows can be created
+and deleted at any time.
+
+A leaf window has a non-nil buffer field, and also
+ has markers in its start and pointm fields.  Non-leaf windows
+ have nil in these fields.
+
+Non-leaf windows are either vertical or horizontal combinations.
+
+A vertical combination window has children that are arranged on the screen
+one above the next.  Its vchild field points to the uppermost child.
+The parent field of each of the children points to the vertical
+combination window.  The next field of each child points to the
+child below it, or is nil for the lowest child.  The prev field
+of each child points to the child above it, or is nil for the
+highest child.
+
+A horizontal combination window has children that are side by side.
+Its hchild field points to the leftmost child.  In each child
+the next field points to the child to the right and the prev field
+points to the child to the left.
+
+The children of a vertical combination window may be leaf windows
+or horizontal combination windows.  The children of a horizontal
+combination window may be leaf windows or vertical combination windows.
+
+At the top of the tree are two windows which have nil as parent.
+The second of these is minibuf_window.  The first one manages all
+the screen area that is not minibuffer, and is called the root window.
+Different windows can be the root at different times;
+initially the root window is a leaf window, but if more windows
+are created then that leaf window ceases to be root and a newly
+made combination window becomes root instead.
+
+In any case, prev of the minibuf window is the root window and
+next of the root window is the minibuf window.  To find the
+root window at any time, do XWINDOW (minibuf_window)->prev.
+
+*/
+
+struct window
+  {
+    /* The first two fields are really the header of a vector */
+    /* The window code does not refer to them.  */
+    int size;
+    struct Lisp_Vector *vec_next;
+    /* The screen this window is on.  */
+    Lisp_Object screen;
+    /* t if this window is a minibuffer window.  */
+    Lisp_Object mini_p;
+    /* Following child (to right or down) at same level of tree */
+    Lisp_Object next;
+    /* Preceding child (to left or up) at same level of tree */
+    Lisp_Object prev;
+    /* First child of this window. */
+    /* vchild is used if this is a vertical combination,
+       hchild if this is a horizontal combination. */
+    Lisp_Object hchild, vchild;
+    /* The window this one is a child of. */
+    Lisp_Object parent;
+    /* The upper left corner coordinates of this window,
+       as integers relative to upper left corner of screen = 0, 0 */
+    Lisp_Object left;
+    Lisp_Object top;
+    /* The size of the window */
+    Lisp_Object height;
+    Lisp_Object width;
+    /* The buffer displayed in this window */
+    /* Of the fields vchild, hchild and buffer, only one is non-nil.  */
+    Lisp_Object buffer;
+    /* A marker pointing to where in the text to start displaying */
+    Lisp_Object start;
+    /* A marker pointing to where in the text point is in this window,
+       used only when the window is not selected.
+       This exists so that when multiple windows show one buffer
+       each one can have its own value of point.  */
+    Lisp_Object pointm;
+    /* Non-nil means next redisplay must use the value of start
+       set up for it in advance.  Set by scrolling commands.  */
+    Lisp_Object force_start;
+    /* Number of columns display within the window is scrolled to the left.  */
+    Lisp_Object hscroll;
+    /* Number saying how recently window was selected */
+    Lisp_Object use_time;
+    /* Unique number of window assigned when it was created */
+    Lisp_Object sequence_number;
+    /* No permanent meaning; used by save-window-excursion's bookkeeping */
+    Lisp_Object temslot;
+    /* text.modified of displayed buffer as of last time display completed */
+    Lisp_Object last_modified;
+    /* Value of point at that time */
+    Lisp_Object last_point;
+/* The rest are currently not used or only half used */
+    /* Screen coords of point at that time */
+    Lisp_Object last_point_x;
+    Lisp_Object last_point_y;
+    /* Screen coords of mark as of last time display completed */
+    /* May be nil if mark does not exist or was not on screen */
+    Lisp_Object last_mark_x;
+    Lisp_Object last_mark_y;
+    /* Number of characters in buffer past bottom of window,
+       as of last redisplay that finished. */
+    Lisp_Object window_end_pos;
+    /* t if window_end_pos is truly valid.
+       This is nil if nontrivial redisplay is preempted
+       since in that case the screen image that window_end_pos
+       did not get onto the screen.  */
+    Lisp_Object window_end_valid;
+    /* Vertical position (relative to window top) of that buffer position
+       of the first of those characters */
+    Lisp_Object window_end_vpos;
+    /* Non-nil means must regenerate mode line of this window */
+    Lisp_Object update_mode_line;
+    /* Non-nil means current value of `start'
+       was the beginning of a line when it was chosen.  */
+    Lisp_Object start_at_line_beg;
+    /* Display-table to use for displaying chars in this window.
+       Nil means use the buffer's own display-table.  */
+    Lisp_Object display_table;
+    /* Non-nil means window is marked as dedicated.  */
+    Lisp_Object dedicated;
+  };
+
+/* 1 if W is a minibuffer window.  */
+
+#define MINI_WINDOW_P(W)  (!EQ ((W)->mini_p, Qnil))
+
+/* This is the window in which the terminal's cursor should
+   be left when nothing is being done with it.  This must
+   always be a leaf window, and its buffer is selected by
+   the top level editing loop at the end of each command.
+
+   This value is always the same as
+    SCREEN_SELECTED_WINDOW (selected_screen).  */
+
+extern Lisp_Object selected_window;
+
+/* This is a time stamp for window selection, so we can find the least
+   recently used window.  Its only users are Fselect_window,
+   init_window_once, and make_screen.  */
+
+extern int window_select_count;
+
+/* The minibuffer window of the selected screen.
+   Note that you cannot test for minibufferness of an arbitrary window
+   by comparing against this; but you can test for minibufferness of
+   the selected window or of any window that is displayed.  */
+
+extern Lisp_Object minibuf_window;
+
+/* Non-nil => window to for C-M-v to scroll
+   when the minibuffer is selected.  */
+extern Lisp_Object Vminibuf_scroll_window;
+
+/* nil or a symbol naming the window system
+   under which emacs is running
+   ('x is the only current possibility) */
+extern Lisp_Object Vwindow_system;
+
+/* Version number of X windows: 10, 11 or nil.  */
+extern Lisp_Object Vwindow_system_version;
+
+/* Window that the mouse is over (nil if no mouse support).  */
+extern Lisp_Object Vmouse_window;
+
+/* Last mouse-click event (nil if no mouse support).  */
+extern Lisp_Object Vmouse_event;
+
+extern Lisp_Object Fnext_window ();
+extern Lisp_Object Fselect_window ();
+extern Lisp_Object Fdisplay_buffer ();
+extern Lisp_Object Fset_window_buffer ();
+
+/* Prompt to display in front of the minibuffer contents.  */
+extern char *minibuf_prompt;
+
+/* Message to display instead of minibuffer contents. 
+   This is what the functions error and message make,
+   and command echoing uses it as well. It overrides the
+   minibuf_prompt as well as the buffer.  */
+extern char *echo_area_glyphs;
+
+/* Depth in recursive edits.  */
+extern int command_loop_level;
+
+/* Depth in minibuffer invocations.  */
+extern int minibuf_level;
+
+/* true iff we should redraw the mode lines on the next redisplay.  */
+extern int update_mode_lines;
+
+/* Minimum value of GPT since last redisplay that finished.  */
+
+extern int beg_unchanged;
+
+/* Minimum value of Z - GPT since last redisplay that finished.  */
+
+extern int end_unchanged;
+
+/* MODIFF as of last redisplay that finished;
+   if it matches MODIFF, beg_unchanged and end_unchangedn
+   contain no useful information.  */
+extern int unchanged_modified;
+
+/* Nonzero if BEGV - BEG or Z - ZV of current buffer has changed
+   since last redisplay that finished.  */
+extern int clip_changed;
+
+/* Nonzero if window sizes or contents have changed
+ since last redisplay that finished */
+extern int windows_or_buffers_changed;
+
+/* Number of windows displaying the selected buffer.
+   Normally this is 1, but it can be more.  */
+extern int buffer_shared;