feat(system-utils): tint the *scratch* background a shade lighter

A buffer-local face remap lightens the *scratch* default background by cj/scratch-background-lighten percent (default 5) so it reads as the scratch buffer, applied on emacs-startup-hook. The colour math is display-dependent (verified live); the pure helper's guard contract is unit-tested.

1 files changed, 29 insertions, 2 deletions

diff --git a/modules/system-utils.el b/modules/system-utils.el
index 7cf958674..b3e038ef0 100644
--- a/modules/system-utils.el
+++ b/modules/system-utils.el
@@ -157,12 +157,39 @@ detached from Emacs."
 ;; Set scratch buffer to org-mode
 (setopt initial-major-mode 'org-mode)
 
-;; Move cursor to end of scratch buffer on startup
+;; Tint the *scratch* background a shade lighter than the default so it reads
+;; as the scratch buffer at a glance.  Buffer-local face remap, recomputed from
+;; whatever theme is loaded.
+(require 'color)
+
+(defcustom cj/scratch-background-lighten 5
+  "Percent to lighten the *scratch* background above the default background.
+Aesthetic; tune to taste."
+  :type 'integer
+  :group 'convenience)
+
+(defun cj/--scratch-lightened-background (bg)
+  "Return BG lightened by `cj/scratch-background-lighten' percent.
+Return nil when BG is not a usable color string (e.g. `unspecified')."
+  (when (and (stringp bg) (color-name-to-rgb bg))
+    (color-lighten-name bg cj/scratch-background-lighten)))
+
+(defun cj/scratch-apply-background ()
+  "Remap the *scratch* buffer background a shade lighter than the default."
+  (when (get-buffer "*scratch*")
+    (with-current-buffer "*scratch*"
+      (let ((lighter (cj/--scratch-lightened-background
+                      (face-attribute 'default :background nil t))))
+        (when lighter
+          (face-remap-add-relative 'default :background lighter))))))
+
+;; Move cursor to end of scratch buffer on startup, and tint its background
 (add-hook 'emacs-startup-hook
           (lambda ()
             (when (get-buffer "*scratch*")
               (with-current-buffer "*scratch*"
-                (goto-char (point-max))))))
+                (goto-char (point-max))))
+            (cj/scratch-apply-background)))
 
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