diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3a9858e..615ddd7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,7 @@ one per merged pull request.
 ## 2.9.7 - 2026-10-01
 
 - **feat: the SteamOS conversion fixes a corrupted gaming mode screen with an NVIDIA GPU: nvidia-drm.modeset=1 and fbdev=1 on the kernel command line, NVIDIA modules in the initramfs (lib/nvidia.sh)**
+- **test: tests/nvidia-test.sh runs the NVIDIA fix against a fake RTX 5080 beside an iGPU, a stub modinfo and temp copies of the Limine, systemd-boot and GRUB files (no root, VM or hardware); NVIDIA_DRM_DIR points detection at a fake sysfs tree**
 
 ## 2.9.6 - 2026-09-30
 
diff --git a/lib/nvidia.sh b/lib/nvidia.sh
index ef08329..c7d0cb9 100644
--- a/lib/nvidia.sh
+++ b/lib/nvidia.sh
@@ -10,11 +10,13 @@
 NVIDIA_PARAMS="nvidia-drm.modeset=1 nvidia-drm.fbdev=1"
 NVIDIA_INITRAMFS_CONF=/etc/mkinitcpio.conf.d/90-steamify-nvidia.conf
 NVIDIA_MARK="# steamify-nvidia"
+# The DRM devices in sysfs; the tests point this at a fake tree.
+NVIDIA_DRM_DIR=${NVIDIA_DRM_DIR:-/sys/class/drm}
 
 nvidia_present() {
     # An NVIDIA GPU (PCI vendor 0x10de, display class) and its kernel driver.
     local d
-    for d in /sys/class/drm/card[0-9]*/device; do
+    for d in "$NVIDIA_DRM_DIR"/card[0-9]*/device; do
         [[ "$(cat "$d/vendor" 2>/dev/null)" == 0x10de ]] || continue
         [[ "$(cat "$d/class" 2>/dev/null)" == 0x03* ]] && modinfo nvidia_drm >/dev/null 2>&1 && return 0
     done
@@ -65,11 +67,11 @@ nvidia_enable() {
             info "NVIDIA GPU found: adding $NVIDIA_PARAMS to the kernel command line (fixes a corrupted screen in gaming mode)..."
             backup_file "$f"
             case "$f" in
-                /etc/default/limine)
+                */limine)
                     printf 'KERNEL_CMDLINE[default]+=" %s" %s\n' "$NVIDIA_PARAMS" "$NVIDIA_MARK" | sudo tee -a "$f" >/dev/null ;;
-                /etc/sdboot-manage.conf)
+                */sdboot-manage.conf)
                     sudo sed -i -E "s/^(LINUX_OPTIONS=\"[^\"]*)\"/\1 $NVIDIA_PARAMS\"/" "$f" ;;
-                /etc/default/grub)
+                */grub)
                     sudo sed -i -E "s/^(GRUB_CMDLINE_LINUX_DEFAULT=\"[^\"]*)\"/\1 $NVIDIA_PARAMS\"/" "$f" ;;
             esac
             changed=1
@@ -91,7 +93,7 @@ nvidia_disable() {
     local f changed=0; f="$(nvidia_boot_file)"
     if [[ -n "$f" ]] && grep -q "$NVIDIA_PARAMS" "$f" 2>/dev/null; then
         case "$f" in
-            /etc/default/limine) sudo sed -i "/$NVIDIA_MARK\$/d" "$f" ;;
+            */limine) sudo sed -i "/$NVIDIA_MARK\$/d" "$f" ;;
             *) sudo sed -i "s/ \?$NVIDIA_PARAMS//" "$f" ;;
         esac
         changed=1
diff --git a/tests/nvidia-test.sh b/tests/nvidia-test.sh
new file mode 100644
index 0000000..2a9a0d4
--- /dev/null
+++ b/tests/nvidia-test.sh
@@ -0,0 +1,48 @@
+#!/bin/bash
+# Tests lib/nvidia.sh against a fake RTX 5080 (PCI 10de:2c02) beside an
+# Intel iGPU, a stub modinfo, and temp copies of the boot loader files. No
+# root, VM or NVIDIA hardware needed: bash tests/nvidia-test.sh
+cd "$(dirname "$0")/.." || exit 1
+T="$(mktemp -d)"; trap 'rm -rf "$T"' EXIT
+fail=0
+check() { if eval "$2"; then echo "ok   $1"; else echo "FAIL $1"; fail=1; fi; }
+
+source lib/common.sh; source lib/hdmi-refresh.sh; source lib/nvidia.sh
+sudo() { "$@"; }
+info() { :; }; ok() { :; }; warn() { :; }; err() { echo "ERR $*"; }
+backup_file() { [[ -f "$1.bak" ]] || cp "$1" "$1.bak"; }
+modinfo() { [[ "$FAKE_DRIVER" == 1 ]]; }
+REBUILDS=0; nvidia_rebuild_boot() { REBUILDS=$((REBUILDS + 1)); }
+NVIDIA_INITRAMFS_CONF="$T/90-steamify-nvidia.conf"
+mkdir -p "$T/drm/card0/device" "$T/drm/card1/device"
+echo 0x8086 > "$T/drm/card0/device/vendor"; echo 0x030000 > "$T/drm/card0/device/class"
+echo 0x10de > "$T/drm/card1/device/vendor"; echo 0x030000 > "$T/drm/card1/device/class"
+echo 0x2c02 > "$T/drm/card1/device/device"
+NVIDIA_DRM_DIR="$T/drm"
+
+FAKE_DRIVER=1; check "finds the NVIDIA card as card1 beside an iGPU" nvidia_present
+FAKE_DRIVER=0; check "skips when the driver isn't installed" '! nvidia_present'
+FAKE_DRIVER=1; NVIDIA_DRM_DIR="$T/none"; check "skips without an NVIDIA GPU" '! nvidia_present'
+NVIDIA_DRM_DIR="$T/drm"
+
+for loader in limine sdboot grub; do
+    case $loader in
+        limine) F="$T/limine"; printf 'KERNEL_CMDLINE[default]="quiet splash"\n' > "$F" ;;
+        sdboot) F="$T/sdboot-manage.conf"; printf 'LINUX_OPTIONS="quiet splash"\n' > "$F" ;;
+        grub)   F="$T/grub";   printf 'GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"\n' > "$F" ;;
+    esac
+    nvidia_boot_file() { echo "$F"; }
+    rm -f "$NVIDIA_INITRAMFS_CONF"; REBUILDS=0; ORIG="$(cat "$F")"
+    nvidia_enable
+    check "$loader: parameters added" 'grep -q "nvidia-drm.modeset=1" "$F" && grep -q "nvidia-drm.fbdev=1" "$F"'
+    check "$loader: initramfs drop-in written" 'grep -q nvidia_drm "$NVIDIA_INITRAMFS_CONF"'
+    check "$loader: backup made, rebuilt once" '[[ -f "$F.bak" && $REBUILDS == 1 ]]'
+    SUM="$(cat "$F")"; nvidia_enable
+    check "$loader: second run changes nothing" '[[ "$(cat "$F")" == "$SUM" && $REBUILDS == 1 ]]'
+    nvidia_disable
+    check "$loader: disable restores the file and drops the drop-in" '[[ "$(cat "$F")" == "$ORIG" && ! -f "$NVIDIA_INITRAMFS_CONF" ]]'
+done
+
+FAKE_DRIVER=0; printf 'LINUX_OPTIONS="quiet"\n' > "$T/sdboot"; F="$T/sdboot"; REBUILDS=0; nvidia_enable
+check "no NVIDIA driver: nothing touched" '[[ "$(cat "$F")" == "LINUX_OPTIONS=\"quiet\"" && $REBUILDS == 0 ]]'
+exit $fail