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diff --git a/tests/installer-steps/test_dotfiles_dependency_packages.py b/tests/installer-steps/test_dotfiles_dependency_packages.py
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+++ b/tests/installer-steps/test_dotfiles_dependency_packages.py
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+"""Pin the packages the dotfiles assume are present.
+
+Three packages are not optional extras: something the dotfiles ship depends
+on each one, and when the package is missing the dependent silently does the
+wrong thing rather than failing.
+
+- libreoffice-fresh :: common/.config/mimeapps.list maps presentations,
+  documents and spreadsheets to libreoffice-impress/-writer/-calc. With the
+  package absent those .desktop files don't exist, so xdg-mime falls through
+  to the next application claiming the type — on a machine with the winvm
+  dotfiles that is powerpoint.desktop, which boots a Windows VM to open a
+  deck (2026-09-18).
+- imv :: gui-open --image execs imv, and without it every agent-side image
+  render fails with "required application is unavailable" (2026-09-18).
+- git-lfs :: a repo tracking globs in LFS fails every checkout and merge
+  with "smudge filter lfs failed" (2026-09-20).
+
+All three were installed before the 2026-08-13 rebuild and absent after it,
+which is the regression this pins: they are dependencies of shipped defaults,
+so the installer has to declare them rather than leave them to whatever a
+machine happens to carry.
+
+Method mirrors test_required_software: sed-extract supplemental_software from
+the real `archsetup`, stub pacman_install as a recorder, run it, and assert
+against what it actually invoked. Running the function beats matching its
+source text, because the property under test is "every machine installs this"
+and only a run resolves the conditionals that could make that false. The
+function is run once per desktop environment for the same reason.
+
+Run from repo root:
+    python3 -m unittest tests.installer-steps.test_dotfiles_dependency_packages
+"""
+
+import os
+import subprocess
+import textwrap
+import unittest
+
+REPO_ROOT = os.path.abspath(os.path.join(os.path.dirname(__file__), "..", ".."))
+ARCHSETUP = os.path.join(REPO_ROOT, "archsetup")
+
+DEPENDENCY_PACKAGES = ("libreoffice-fresh", "imv", "git-lfs")
+
+# Every desktop environment the installer branches on inside this function.
+DESKTOP_ENVS = ("dwm", "hyprland")
+
+
+def declared_packages(desktop_env):
+    """Return (exit_code, [pacman package, ...]) for one desktop environment.
+
+    Everything the function calls besides pacman_install is stubbed to a no-op,
+    so the run records package declarations and nothing else. aur_install is
+    deliberately separate: these three are pacman packages, and folding the two
+    recorders together would let an AUR declaration satisfy the pin.
+    """
+    script = textwrap.dedent(f"""\
+        desktop_env={desktop_env}
+        display() {{ :; }}
+        aur_install() {{ :; }}
+        mask_fwupd_passim() {{ :; }}
+        run_task() {{ :; }}
+        error_warn() {{ :; }}
+        pacman_install() {{ echo "$1"; }}
+        source <(sed -n '/^supplemental_software() {{/,/^}}/p' "{ARCHSETUP}")
+        supplemental_software
+    """)
+    result = subprocess.run(
+        ["bash", "-c", script], capture_output=True, text=True, timeout=30,
+    )
+    return result.returncode, result.stdout.split()
+
+
+class DotfilesDependencyPackages(unittest.TestCase):
+    # ------------------------------------------------------------ normal ----
+    def test_each_dependency_package_is_installed(self):
+        rc, pkgs = declared_packages("hyprland")
+        self.assertEqual(rc, 0)
+        for package in DEPENDENCY_PACKAGES:
+            with self.subTest(package=package):
+                self.assertIn(
+                    package, pkgs,
+                    f"{package} is a dependency of a shipped dotfiles default "
+                    "and must be declared",
+                )
+
+    # ---------------------------------------------------------- boundary ----
+    def test_each_is_declared_exactly_once(self):
+        # A second declaration is dead weight and drifts out of sync with the
+        # first when one of them is edited.
+        rc, pkgs = declared_packages("hyprland")
+        self.assertEqual(rc, 0)
+        for package in DEPENDENCY_PACKAGES:
+            with self.subTest(package=package):
+                self.assertEqual(pkgs.count(package), 1)
+
+    # ------------------------------------------------------------- error ----
+    def test_none_is_gated_behind_a_desktop_environment(self):
+        # The dotfiles defaults that need these apply on every DE, so a
+        # declaration reachable under only one of them would leave the same
+        # hole on the other.
+        for env in DESKTOP_ENVS:
+            rc, pkgs = declared_packages(env)
+            self.assertEqual(rc, 0)
+            for package in DEPENDENCY_PACKAGES:
+                with self.subTest(desktop_env=env, package=package):
+                    self.assertIn(package, pkgs)
+
+    def test_harness_observes_desktop_environment_gating(self):
+        # The control for the test above: ranger IS gated to dwm on purpose, so
+        # if this run can't see that, the DE-independence assertion is vacuous
+        # and would pass against a gated package too.
+        _, dwm = declared_packages("dwm")
+        _, hyprland = declared_packages("hyprland")
+        self.assertIn("ranger", dwm)
+        self.assertNotIn("ranger", hyprland)
+
+
+if __name__ == "__main__":
+    unittest.main()