>From b211505e9ddc4a2685f55f2afb43bcf5c4a7b860 Mon Sep 17 00:00:00 2001
From: Hugo Heagren <hugo@heagren.com>
Date: Tue, 26 Jul 2022 21:12:03 +0100
Subject: [PATCH 1/6] split-window-below: Add WINDOW-TO-SPLIT argument

* lisp/window.el (split-window-below): Add optional argument to
control which window is split (previously, would only split selected
window).  Update docstring.
* doc/lispref/windows.texi (Splitting Windows): Update docs for
`split-window-below'.
---
 doc/lispref/windows.texi |  9 +++++----
 lisp/window.el           | 26 +++++++++++++-------------
 2 files changed, 18 insertions(+), 17 deletions(-)

diff --git a/doc/lispref/windows.texi b/doc/lispref/windows.texi
index 704ed30366..255e33ad0d 100644
--- a/doc/lispref/windows.texi
+++ b/doc/lispref/windows.texi
@@ -1477,10 +1477,11 @@ Splitting Windows
 negative, the right window gets @minus{}@var{size} columns.
 @end deffn
 
-@deffn Command split-window-below &optional size
-This function splits the selected window into two windows, one above
-the other, leaving the upper window selected.  If @var{size} is
-positive, the upper window gets @var{size} lines; if @var{size} is
+@deffn Command split-window-below &optional size window-to-split
+This function splits the window @var{window-to-split} into two
+windows, one above the other, leaving the upper window selected.
+@var{window-to-split} defaults to the selected window.  If @var{size}
+is positive, the upper window gets @var{size} lines; if @var{size} is
 negative, the lower window gets @minus{}@var{size} lines.
 @end deffn
 
diff --git a/lisp/window.el b/lisp/window.el
index eba888a89d..69089e63f5 100644
--- a/lisp/window.el
+++ b/lisp/window.el
@@ -5672,9 +5672,9 @@ split-window-keep-point
   :type 'boolean
   :group 'windows)
 
-(defun split-window-below (&optional size)
-  "Split the selected window into two windows, one above the other.
-The selected window is above.  The newly split-off window is
+(defun split-window-below (&optional size window-to-split)
+  "Split WINDOW-TO-SPLIT into two windows, one above the other.
+WINDOW-TO-SPLIT is above.  The newly split-off window is
 below and displays the same buffer.  Return the new window.
 
 If optional argument SIZE is omitted or nil, both windows get the
@@ -5683,22 +5683,22 @@ split-window-below
 lower (new) window gets -SIZE lines.
 
 If the variable `split-window-keep-point' is non-nil, both
-windows get the same value of point as the selected window.
+windows get the same value of point as the WINDOW-TO-SPLIT.
 Otherwise, the window starts are chosen so as to minimize the
 amount of redisplay; this is convenient on slow terminals."
-  (interactive "P")
-  (let ((old-window (selected-window))
-	(old-point (window-point))
-	(size (and size (prefix-numeric-value size)))
+  (interactive `(,(when current-prefix-arg
+                    (prefix-numeric-value current-prefix-arg))
+                 ,(selected-window)))
+  (let ((old-point (window-point))
         moved-by-window-height moved new-window bottom)
     (when (and size (< size 0) (< (- size) window-min-height))
       ;; `split-window' would not signal an error here.
       (error "Size of new window too small"))
-    (setq new-window (split-window nil size))
+    (setq new-window (split-window window-to-split size))
     (unless split-window-keep-point
-      (with-current-buffer (window-buffer)
+      (with-current-buffer (window-buffer window-to-split)
 	;; Use `save-excursion' around vertical movements below
-	;; (Bug#10971).  Note: When the selected window's buffer has a
+	;; (Bug#10971).  Note: When WINDOW-TO-SPLIT's buffer has a
 	;; header line, up to two lines of the buffer may not show up
 	;; in the resulting configuration.
 	(save-excursion
@@ -5713,13 +5713,13 @@ split-window-below
 	  (setq bottom (point)))
 	(and moved-by-window-height
 	     (<= bottom (point))
-	     (set-window-point old-window (1- bottom)))
+	     (set-window-point window-to-split (1- bottom)))
 	(and moved-by-window-height
 	     (<= (window-start new-window) old-point)
 	     (set-window-point new-window old-point)
 	     (select-window new-window))))
     ;; Always copy quit-restore parameter in interactive use.
-    (let ((quit-restore (window-parameter old-window 'quit-restore)))
+    (let ((quit-restore (window-parameter window-to-split 'quit-restore)))
       (when quit-restore
 	(set-window-parameter new-window 'quit-restore quit-restore)))
     new-window))
-- 
2.20.1