From 9817e1919d7fc19194c63e87ddafe1e3e04cb538 Mon Sep 17 00:00:00 2001
From: Mark Wooding <mdw@distorted.org.uk>
Date: Mon, 29 Apr 2024 11:03:33 +0100
Subject: [PATCH] dot/emacs, el/dot-emacs.el: Another `display-window' hack:
 fallback.

Allow a window in each frame to be declared as the `victim' for random
pop-up buffers, overriding the usual policy.
---
 dot/emacs       |  1 +
 el/dot-emacs.el | 46 ++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 47 insertions(+)

diff --git a/dot/emacs b/dot/emacs
index 5588a43..8749882 100644
--- a/dot/emacs
+++ b/dot/emacs
@@ -779,6 +779,7 @@
   (global-set-key [?\C-c ?w ?b] 'w3m-bookmark-view)
   (global-set-key [?\C-c ?w ?c] 'mdw-set-frame-colour)
   (global-set-key [?\C-c ?w ?d] 'mdw-divvy-window)
+  (global-set-key [?\C-c ?w ?f] 'mdw-set-fallback-window)
   (global-set-key [?\C-c ?w ?h] 'windmove-left)
   (global-set-key [?\C-c ?w ?j] 'windmove-down)
   (global-set-key [?\C-c ?w ?k] 'windmove-up)
diff --git a/el/dot-emacs.el b/el/dot-emacs.el
index 6cad98c..01d3ac5 100644
--- a/el/dot-emacs.el
+++ b/el/dot-emacs.el
@@ -772,6 +772,45 @@ Pretend they don't exist.  They might be on other display devices."
       even-window-heights nil
       display-buffer-reuse-frames nil)
 
+(defvar mdw-fallback-window-alist nil
+  "Alist mapping frames to fallback windows.")
+
+(defun mdw-cleanup-fallback-window-alist ()
+  "Remove entries for dead frames and windows from the fallback alist."
+  (let ((prev nil)
+	(cursor mdw-fallback-window-alist))
+    (while cursor
+      (let* ((assoc (car cursor))
+	     (tail (cdr cursor)))
+	(cond ((and (frame-live-p (car assoc))
+		    (window-live-p (cdr assoc)))
+	       (setq prev cursor))
+	      ((null prev)
+	       (setq mdw-fallback-window-alist tail))
+	      (t
+	       (setcdr prev tail)))
+	(setq cursor tail)))))
+
+(defun mdw-set-fallback-window (cancel)
+  "Prefer the selected window for pop-up buffers in this frame.
+With a prefix argument, clear the fallback window."
+  (interactive "P")
+  (let* ((frame (selected-frame)) (window (selected-window))
+	 (assoc (assq (selected-frame) mdw-fallback-window-alist)))
+    (cond (cancel
+	   (cond (assoc
+		  (setcdr assoc nil)
+		  (message "Fallback window cleared."))
+		 (t
+		  (message "No fallback window active in this frame."))))
+	  ((window-dedicated-p window)
+	   (error "Window is dedicated to its buffer."))
+	  (t
+	   (if assoc (setcdr assoc window)
+	     (push (cons frame window) mdw-fallback-window-alist))
+	   (message "Fallback window set.")))
+    (mdw-cleanup-fallback-window-alist)))
+
 (defun mdw-last-window-in-frame-p (window)
   "Return whether WINDOW is the last in its frame."
   (catch 'done
@@ -790,10 +829,17 @@ Begone, foul DWIMmerlaik!
 This is all totally subject to arbitrary change in the future, but the
 emphasis is on predictability rather than crazy DWIMmery."
   (let* ((selected (selected-window)) chosen
+	 (fallback (assq (selected-frame) mdw-fallback-window-alist))
 	 (full-height-p (window-full-height-p selected))
 	 (full-width-p (window-full-width-p selected)))
     (cond
 
+     ((and fallback (window-live-p (cdr fallback)))
+      ;; There's a fallback window set for this frame.  Use it.
+
+      (setq chosen (cdr fallback))
+      (display-buffer-record-window 'window chosen buffer))
+
      ((and full-height-p full-width-p)
       ;; We're basically the only window in the frame.  If we want to get
       ;; anywhere, we'll have to split the window.
-- 
2.11.0