test(host-environment): cover laptop/desktop, platform, display, timezone predicates

Four new test files extending the existing test-host-environment.el
(which already covered the two battery helpers).

- platform-predicates: env-linux-p, env-bsd-p, env-macos-p, env-windows-p
  walked across every supported system-type value. 8 tests.
- display-predicates: env-x-p, env-x11-p, env-wayland-p, env-terminal-p,
  env-gui-p exercised under every relevant combination of window-system,
  WAYLAND_DISPLAY, and display-graphic-p. 13 tests.
- env-laptop-p: composition over the helpers, with Linux dispatch
  isolated from non-Linux dispatch via system-type binding. 8 tests
  including env-desktop-p as the inverse. battery-status-function is
  forward-declared in this test file (initialized to nil) so cl-letf's
  symbol-value place can read the prior value without hitting
  void-variable.
- detect-system-timezone: the four-method priority chain. Mocks
  cj/match-localtime-to-zoneinfo and getenv at the boundary; uses
  cl-letf on file-exists-p / insert-file-contents to exercise the
  /etc/timezone fall-through without touching real system files.
  5 tests.

34 new tests for host-environment, all passing. Full suite green.

1 files changed, 112 insertions, 0 deletions

diff --git a/tests/test-host-environment--display-predicates.el b/tests/test-host-environment--display-predicates.el
new file mode 100644
index 00000000..15dff2ef
--- /dev/null
+++ b/tests/test-host-environment--display-predicates.el
@@ -0,0 +1,112 @@
+;;; test-host-environment--display-predicates.el --- Tests for env-x/x11/wayland/terminal/gui-p -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Tests for the display-environment predicates in host-environment.el.
+;; Mocks `window-system', `getenv', and `display-graphic-p' at the
+;; framework boundary so each predicate can be exercised under every
+;; relevant combination of (window-system, WAYLAND_DISPLAY, graphic-p).
+
+;;; Code:
+
+(require 'ert)
+(require 'cl-lib)
+
+(add-to-list 'load-path (expand-file-name "modules" user-emacs-directory))
+(require 'host-environment)
+
+(defmacro test-host-env--with-display (window-system-value
+                                       wayland-display
+                                       graphic-p
+                                       &rest body)
+  "Run BODY with display state stubbed.
+WINDOW-SYSTEM-VALUE becomes the return of `(window-system)'.
+WAYLAND-DISPLAY becomes the value of $WAYLAND_DISPLAY (nil for unset).
+GRAPHIC-P becomes the return of `(display-graphic-p)'."
+  (declare (indent 3) (debug t))
+  `(cl-letf (((symbol-function 'window-system)
+              (lambda (&optional _) ,window-system-value))
+             ((symbol-function 'getenv)
+              (lambda (name)
+                (when (string= name "WAYLAND_DISPLAY") ,wayland-display)))
+             ((symbol-function 'display-graphic-p)
+              (lambda (&optional _) ,graphic-p)))
+     ,@body))
+
+;;; env-x-p
+
+(ert-deftest test-host-environment-x-p-true-when-window-system-is-x ()
+  "Normal: env-x-p returns t when window-system is `x'."
+  (test-host-env--with-display 'x nil t
+    (should (env-x-p))))
+
+(ert-deftest test-host-environment-x-p-false-when-window-system-is-pgtk ()
+  "Boundary: env-x-p returns nil when window-system is `pgtk'."
+  (test-host-env--with-display 'pgtk nil t
+    (should-not (env-x-p))))
+
+(ert-deftest test-host-environment-x-p-false-when-no-window-system ()
+  "Boundary: env-x-p returns nil under no window system (terminal)."
+  (test-host-env--with-display nil nil nil
+    (should-not (env-x-p))))
+
+;;; env-x11-p
+
+(ert-deftest test-host-environment-x11-p-true-on-x-without-wayland-display ()
+  "Normal: env-x11-p returns t under X with no WAYLAND_DISPLAY."
+  (test-host-env--with-display 'x nil t
+    (should (env-x11-p))))
+
+(ert-deftest test-host-environment-x11-p-false-under-xwayland ()
+  "Boundary: env-x11-p returns nil when running through XWayland.
+Detected via WAYLAND_DISPLAY being set even though window-system is `x'."
+  (test-host-env--with-display 'x "wayland-0" t
+    (should-not (env-x11-p))))
+
+(ert-deftest test-host-environment-x11-p-false-on-pgtk ()
+  "Boundary: env-x11-p returns nil under pure-GTK Wayland."
+  (test-host-env--with-display 'pgtk "wayland-0" t
+    (should-not (env-x11-p))))
+
+;;; env-wayland-p
+
+(ert-deftest test-host-environment-wayland-p-true-when-wayland-display-set ()
+  "Normal: env-wayland-p returns t when WAYLAND_DISPLAY is set."
+  (test-host-env--with-display 'pgtk "wayland-0" t
+    (should (env-wayland-p))))
+
+(ert-deftest test-host-environment-wayland-p-false-when-wayland-display-unset ()
+  "Boundary: env-wayland-p returns nil when WAYLAND_DISPLAY is unset."
+  (test-host-env--with-display 'x nil t
+    (should-not (env-wayland-p))))
+
+(ert-deftest test-host-environment-wayland-p-true-under-xwayland ()
+  "Boundary: env-wayland-p returns t even under XWayland.
+WAYLAND_DISPLAY is set by the compositor regardless of which X surface
+Emacs uses, so XWayland counts as Wayland by this predicate."
+  (test-host-env--with-display 'x "wayland-0" t
+    (should (env-wayland-p))))
+
+;;; env-terminal-p / env-gui-p
+
+(ert-deftest test-host-environment-terminal-p-true-when-not-graphical ()
+  "Normal: env-terminal-p returns t when display-graphic-p is nil."
+  (test-host-env--with-display nil nil nil
+    (should (env-terminal-p))))
+
+(ert-deftest test-host-environment-terminal-p-false-when-graphical ()
+  "Boundary: env-terminal-p returns nil under a graphical frame."
+  (test-host-env--with-display 'x nil t
+    (should-not (env-terminal-p))))
+
+(ert-deftest test-host-environment-gui-p-true-when-graphical ()
+  "Normal: env-gui-p returns t under a graphical frame."
+  (test-host-env--with-display 'x nil t
+    (should (env-gui-p))))
+
+(ert-deftest test-host-environment-gui-p-false-in-terminal ()
+  "Boundary: env-gui-p returns nil under a terminal frame."
+  (test-host-env--with-display nil nil nil
+    (should-not (env-gui-p))))
+
+(provide 'test-host-environment--display-predicates)
+;;; test-host-environment--display-predicates.el ends here