fix(dashboard): exempt the banner buffer from auto-dim

The butterfly banner is a transparent PNG. On this X11 build Emacs composites image alpha against one background color and caches the flat pixmap. So when auto-dim remaps a non-selected dashboard's background to near-black, the cached image keeps its old composite and the transparent edges show as a lighter rectangle.

I exempted the *dashboard* buffer from dimming through the fork's never-dim-buffer hook, so its background never shifts. Live alpha compositing would need a pgtk build, which is out because of its fractional-scaling input lag, and every theme-level workaround changes dimming for all buffers. Scoping the exemption to one short-lived buffer is the narrow fix. The trade is no focus cue when the dashboard is shown in a split.

I also dropped the :mask heuristic prop from the prior banner commit. The PNG already carries a real alpha channel, so heuristic masking was the wrong tool. Once the background is stable, the native alpha over the theme background reads clean on its own. I added Normal/Boundary/Error tests for the predicate.

1 files changed, 26 insertions, 0 deletions

diff --git a/tests/test-auto-dim-config.el b/tests/test-auto-dim-config.el
index f33a475ab..4648f062c 100644
--- a/tests/test-auto-dim-config.el
+++ b/tests/test-auto-dim-config.el
@@ -132,5 +132,31 @@
                    '((term 25 97)
                      (term 24 97))))))
 
+(ert-deftest test-auto-dim-config-never-dim-dashboard-exempts-dashboard ()
+  "Normal: the *dashboard* buffer is exempt from dimming."
+  (skip-unless (file-directory-p test-auto-dim--fork))
+  (require 'auto-dim-config)
+  (let* ((existed (get-buffer "*dashboard*"))
+         (buffer (or existed (get-buffer-create "*dashboard*"))))
+    (unwind-protect
+        (should (cj/auto-dim--never-dim-dashboard-p buffer))
+      (unless existed (kill-buffer buffer)))))
+
+(ert-deftest test-auto-dim-config-never-dim-dashboard-near-miss-name-dims ()
+  "Boundary: a buffer whose name only resembles the dashboard is not exempt."
+  (skip-unless (file-directory-p test-auto-dim--fork))
+  (require 'auto-dim-config)
+  (let ((buffer (get-buffer-create "dashboard")))
+    (unwind-protect
+        (should-not (cj/auto-dim--never-dim-dashboard-p buffer))
+      (kill-buffer buffer))))
+
+(ert-deftest test-auto-dim-config-never-dim-dashboard-other-buffer-dims ()
+  "Error: an ordinary buffer is not exempt from dimming."
+  (skip-unless (file-directory-p test-auto-dim--fork))
+  (require 'auto-dim-config)
+  (with-temp-buffer
+    (should-not (cj/auto-dim--never-dim-dashboard-p (current-buffer)))))
+
 (provide 'test-auto-dim-config)
 ;;; test-auto-dim-config.el ends here