diff --git a/agent/BACKLOG.md b/agent/BACKLOG.md
index 394e9c6..cc2f463 100644
--- a/agent/BACKLOG.md
+++ b/agent/BACKLOG.md
@@ -214,20 +214,48 @@ the existing `refresh_callback` (already running at 50 fps) whenever
 included), entry, lock, any live bullets, progress box/bar. Keep it a
 canvas-change check, not an every-frame recompute.
 
-**Testing — the rig exists, and it needs root.** nixpkgs' plymouth ships
-`x11.so`, and plymouth has a built-in fake dual-head mode:
-`PLY_CREATE_FAKE_MULTI_HEAD_SETUP=1` (`renderers/x11/plugin.c:294`) creates
-800×600 + 640×480 heads, so the splash can be rendered in a window with no
-DRM and no reboot. Caveat: that fake path builds both heads up front, so it
-proves *rendering and centring*, not the canvas-change bug itself — for that,
-plug/unplug while the splash is up, or compare a boot with and without the
-external. `plymouthd` wants root and its theme comes from
-`/etc/plymouth/plymouthd.conf`, so this needs a sudo session.
+**The rig works, needs no root, and is in `scratchpad/ply-rig.sh`** (recipe
+below — rebuild it, it is ~20 lines). Rendered on the dev box 2026-07-16:
 
-**Do not ship this blind:** the same script draws the **LUKS passphrase box**,
-this machine is encrypted, and a script that fails to parse leaves a prompt you
-cannot see. Pass = with the rig, a two-head render centred on both; then a real
-docked boot + shutdown, undocked unchanged.
+- `unshare -rm` gives a user + mount namespace: root *in there* only, every
+  mount invisible to the real system and gone when it exits.
+- `/run/plymouth` is BOTH the daemon's socket dir **and** its compiled-in
+  plugin/theme path (`strings plymouthd` → `/run/plymouth/plugins/`). Bind a
+  writable dir over it **and recreate its three symlinks** (`plugins`,
+  `themes`, `plymouthd.defaults` → `/etc/plymouth/…`) or plymouth cannot find
+  `x11.so`, silently skips X11 and reaches for a tty.
+- **Do not pass `--tty`**: `main.c:2133` only takes the X11 path while
+  `default_tty` is unset. Bind an empty dir over `/dev/dri` too — with X11
+  first in the renderer order (`ply-renderer.c:283`) DRM is never used, and the
+  mask makes that a guarantee rather than a hope.
+- `PLY_CREATE_FAKE_MULTI_HEAD_SETUP=1` → 800×600 + 640×480 windows.
+  `plymouth show-splash`, then `plymouth ask-for-password` renders the LUKS
+  dialog. Debug: `--kernel-command-line="plymouth.debug=stream:/path splash"`
+  (**`stream:`**, not `file:` — `file:` buffers and you get nothing).
+- Fast loop: copy the theme into the rig's `themes/` and sed its `.plymouth`
+  `ScriptFile=`/`ImageDir=` to the copy — then editing the script is a 10s
+  render instead of a 3min `nixos-rebuild`.
+
+**What the rig already proved (2026-07-16):** with a *stable* two-head canvas
+the **current** theme centres correctly on both heads — logo and progress bar
+each in their own head's middle. So the geometry is not the bug, which is the
+evidence for the timing diagnosis above. The fake path builds both heads up
+front, so it cannot reproduce a canvas *change*: for that, plug/unplug while
+the splash is up, or compare a boot with and without the external.
+
+**Attempt 1 failed, and the rig caught it — that is the point.** Hoisting the
+positions into a `layout()` called from `refresh_callback` rendered the
+background on both heads and **nothing else**: no logo, no progress bar. The
+script plugin logs `starting boot animation` and **no parse error**, so it
+fails *silently* — reverted rather than shipped (that script draws the LUKS box
+on an encrypted machine). Next session: bisect with the fast loop. Prime
+suspect is scope — plymouth script assigns into `local` unless the name already
+exists, so a `logo.image = …` inside a function may not be the `logo` the rest
+of the script sees, even though the same pattern works in `update_progress_bar`.
+Test that hypothesis *first*, in isolation, before rewriting anything.
+
+Pass = with the rig, a two-head render centred on both **plus the password
+dialog visible**; then a real docked boot + shutdown, undocked unchanged.
 
 ### 143. Rofi keeps the selected *row number* when the search changes, so the highlight lands on an unrelated entry
 
diff --git a/docs/TESTING.md b/docs/TESTING.md
index 92e1ab5..105ceb3 100644
--- a/docs/TESTING.md
+++ b/docs/TESTING.md
@@ -108,6 +108,19 @@ unattended env it uses is in the script, and the same flow works
 interactively from the live terminal. If the desktop comes up unthemed,
 read `/var/log/nomarchy-hm-preactivate.log` on the installed system.
 
+`tools/plymouth-preview.sh` renders the **boot splash** — including the LUKS
+password dialog — into X11 windows in about ten seconds: no reboot, no root,
+and no DRM. It runs plymouthd inside `unshare -rm` (a user + mount namespace),
+binds a writable dir over `/run/plymouth` (which is both the daemon's socket
+dir *and* its compiled-in plugin path), masks `/dev/dri` so a preview can never
+mode-set a real output, and sets `PLY_CREATE_FAKE_MULTI_HEAD_SETUP` for plymouth's
+built-in fake dual head. Pass it a theme store path to iterate on the script
+without a rebuild. Use it before touching `modules/nixos/plymouth/`: that script
+draws the passphrase box, a failure there is silent (the plugin logs
+`starting boot animation` and no error), and "type your LUKS password blind" is
+a bad way to find out. Its limit: the fake heads exist from the start, so it
+cannot reproduce a head arriving *mid-splash* (#137).
+
 Two full-desktop VM harnesses (softGL Hyprland, too heavy for `checks.*`)
 live next to the scripts: `tools/theme-shot.nix` (themed-desktop
 screenshots) and `tools/monitor-fallback.nix` (KVM display transitions:
diff --git a/tools/plymouth-preview.sh b/tools/plymouth-preview.sh
new file mode 100755
index 0000000..8041212
--- /dev/null
+++ b/tools/plymouth-preview.sh
@@ -0,0 +1,64 @@
+#!/usr/bin/env bash
+# Render the Plymouth splash in X11 windows — no root, no reboot, no DRM (#137).
+#
+#   tools/plymouth-preview.sh [<theme-store-path>]
+#
+# Plymouth is otherwise untestable without rebooting, which is a poor loop for
+# a script that draws the LUKS passphrase box on an encrypted machine. This
+# gives you a real render in seconds:
+#
+#   * `unshare -rm` = user + mount namespace. We are root only in here, and
+#     every mount below dies with the process.
+#   * /run/plymouth is both the daemon's socket dir AND its compiled-in plugin
+#     path (`strings plymouthd` → /run/plymouth/plugins/). So we bind a
+#     writable dir over it and recreate its symlinks — miss those and plymouth
+#     cannot find x11.so, silently skips X11, and goes looking for a tty.
+#   * X11 is first in plymouth's renderer order (ply-renderer.c:283) and is
+#     taken only while default_tty is unset — so never pass --tty. /dev/dri is
+#     masked as well: a preview must not be able to mode-set the real outputs.
+#   * PLY_CREATE_FAKE_MULTI_HEAD_SETUP (renderers/x11/plugin.c) fakes two heads
+#     (800x600 + 640x480) — the multi-monitor case, on a single-monitor desk.
+#
+# Caveat: the fake heads are created up front, so this proves rendering and
+# centring, NOT #137's canvas-change bug (a head arriving mid-splash).
+set -u
+THEME="${1:-}"
+PLY=$(nix build --no-link --print-out-paths nixpkgs#plymouth 2>/dev/null | head -1)
+[ -n "$PLY" ] || { echo "could not build plymouth" >&2; exit 1; }
+[ -n "${DISPLAY:-}" ] || { echo "no DISPLAY — the x11 renderer needs Xwayland" >&2; exit 1; }
+
+export PLY THEME
+unshare -rm -- bash -euc '
+  rm -rf /tmp/plyrig; mkdir -p /tmp/plyrig/run /tmp/plyrig/dri /tmp/plyrig/themes
+  if [ -n "$THEME" ]; then
+    cp -rL "$THEME/share/plymouth/themes/nomarchy" /tmp/plyrig/themes/
+    # The .plymouth bakes absolute store paths at build time; point them at the
+    # copy so the script here is the one that renders (edit + rerun = seconds).
+    sed -i "s|^ScriptFile=.*|ScriptFile=/tmp/plyrig/themes/nomarchy/nomarchy.script|;
+            s|^ImageDir=.*|ImageDir=/tmp/plyrig/themes/nomarchy|" \
+      /tmp/plyrig/themes/nomarchy/nomarchy.plymouth
+    ln -s /tmp/plyrig/themes /tmp/plyrig/run/themes
+  else
+    ln -s /etc/plymouth/themes /tmp/plyrig/run/themes
+  fi
+  ln -s /etc/plymouth/plugins            /tmp/plyrig/run/plugins
+  ln -s /etc/plymouth/plymouthd.defaults /tmp/plyrig/run/plymouthd.defaults
+  mount --bind /tmp/plyrig/run /run/plymouth
+  mount --bind /tmp/plyrig/dri /dev/dri 2>/dev/null || true
+
+  export PLY_CREATE_FAKE_MULTI_HEAD_SETUP=1
+  "$PLY/sbin/plymouthd" --no-daemon --mode=boot --no-boot-log \
+    --kernel-command-line="plymouth.debug=stream:/tmp/plyrig/dbg.log splash" \
+    >/dev/null 2>&1 &
+  d=$!
+  sleep 2
+  "$PLY/bin/plymouth" show-splash >/dev/null 2>&1
+  echo "splash up — two windows (800x600, 640x480). Ctrl+C or wait 20s."
+  sleep 8
+  "$PLY/bin/plymouth" ask-for-password --prompt="Unlock" --number-of-tries=1 >/dev/null 2>&1 &
+  echo "password dialog requested (the LUKS path)"
+  sleep 10
+  "$PLY/bin/plymouth" quit >/dev/null 2>&1; sleep 1; kill $d 2>/dev/null
+  cp /tmp/plyrig/dbg.log /tmp/plymouth-preview.log 2>/dev/null
+  echo "log: /tmp/plymouth-preview.log"
+'