README: document phantom display outputs (LattePanda DSI-1)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

README.md

@@ -159,6 +159,36 @@ xset q | grep -A1 -E 'Screen Saver|DPMS'
 
 You want `timeout: 0`, `prefer blanking: no`, and `DPMS is Disabled`.
 
+### Related: phantom outputs
+
+Some boards report a display output that has no panel behind it. The LattePanda's
+onboard DSI header shows up as `DSI-1 connected 1024x600` with **zero EDID bytes**
+(`xrandr` prints `0mm x 0mm`), and X happily puts workspace 1 on it. Everything
+then looks healthy from ssh -- i3 has the window fullscreen on its primary
+output -- while the real panel shows an empty workspace. Check with:
+
+```sh
+xrandr | grep ' connected'          # a real panel has physical mm, phantoms say 0mm x 0mm
+for c in /sys/class/drm/card0-*; do echo "$c $(cat $c/status) edid=$(wc -c <$c/edid)"; done
+```
+
+Tell X to ignore the phantom and mark the real panel primary (adjust the names):
+
+```sh
+sudo tee /etc/X11/xorg.conf.d/20-outputs-socktop-swipe.conf >/dev/null <<'EOF'
+Section "Monitor"
+    Identifier "DSI-1"
+    Option     "Ignore" "true"
+EndSection
+Section "Monitor"
+    Identifier "HDMI-2"
+    Option     "Primary" "true"
+EndSection
+EOF
+```
+
+Then set `SCREEN_W` / `SCREEN_H` in the config to the real panel's mode.
+
 ### Related: does the display come back by itself?
 
 Blanking is only half of unattended operation. If the display manager has no