tools/magit: split sub-windows to the side

When a log or diff buffer is opened from the magit status buffer, it
would display them in a random window (or split, if none were
available). This changes forces it to predictably open to the right of
the magit status window (using the next window over, if available).

modules/tools/magit/autoload.el

@@ -53,7 +53,29 @@
                                magit-diff-mode
                                magit-stash-mode
                                magit-status-mode))))
-              '(display-buffer-same-window))))))
+              '(display-buffer-same-window))
+
+             ('(+magit--display-buffer-in-direction))))))
+
+(defun +magit--display-buffer-in-direction (buffer alist)
+  "`display-buffer-alist' handler that opens BUFFER in a direction.
+
+This differs from `display-buffer-in-direction' in one way: it will try to use a
+window that already exists in that direction. It will split otherwise."
+  (let ((direction (or (alist-get 'direction alist)
+                       +magit-open-windows-in-direction)))
+    (if-let (window (window-in-direction direction))
+        (select-window window)
+      (if-let (window (window-in-direction
+                       (pcase direction
+                         (`right 'left)
+                         (`left 'right)
+                         ((or `up `above) 'down)
+                         ((or `down `below) 'up))))
+          (select-window window)
+        (split-window-horizontally)))
+    (switch-to-buffer buffer)
+    (selected-window)))
 
 
 ;;

modules/tools/magit/config.el

@@ -1,5 +1,13 @@
 ;;; tools/magit/config.el -*- lexical-binding: t; -*-
 
+(defvar +magit-open-windows-in-direction 'right
+  "What direction to open new windows from the status buffer.
+For example, diffs and log buffers. Accepts `left', `right', `up', and `down'.")
+
+
+;;
+;;; Packages
+
 (use-package! magit
   :commands magit-file-delete
   :defer-incrementally (dash f s with-editor git-commit package eieio lv transient)