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+;;; 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