docs: correct the multi-screen premise to per-window SDDM model

2 files changed, 32 insertions, 18 deletions

diff --git a/docs/superpowers/plans/2026-09-17-sddm-theme-udt.md b/docs/superpowers/plans/2026-09-17-sddm-theme-udt.md
index 1da3d20..81ada3d 100644
--- a/docs/superpowers/plans/2026-09-17-sddm-theme-udt.md
+++ b/docs/superpowers/plans/2026-09-17-sddm-theme-udt.md
@@ -10,6 +10,13 @@
 
 **Spec:** `docs/superpowers/specs/2026-09-17-sddm-theme-udt-design.md`
 
+> **Correction (post-implementation):** Task 1's `Main.qml` assumed one greeter
+> window spanning a union of all screens (`screenModel.geometry(-1)`). SDDM
+> 0.21 creates one window per screen, each with a single-row `screenModel` and a
+> boolean `primaryScreen` context property. The spec's multi-screen decision was
+> corrected; the implementation now gates the card per window. Read the spec,
+> not Task 1's original code, for the multi-screen model.
+
 ## Global Constraints
 
 - License: GPLv2 only. Every source file carries `SPDX-License-Identifier: GPL-2.0-only` and `Copyright (C) 2026 Danilo M. <danix@danix.xyz>`.
diff --git a/docs/superpowers/specs/2026-09-17-sddm-theme-udt-design.md b/docs/superpowers/specs/2026-09-17-sddm-theme-udt-design.md
index 34104ce..e3ddcc3 100644
--- a/docs/superpowers/specs/2026-09-17-sddm-theme-udt-design.md
+++ b/docs/superpowers/specs/2026-09-17-sddm-theme-udt-design.md
@@ -79,13 +79,17 @@ So the design needs a channel that is live, world-readable, and data-only.
    `/usr/share/sddm` (live edits, but lets user-writable QML run in the
    greeter).
 
-6. **GUI on one screen, wallpaper on all.** SDDM's root item spans the united
-   geometry of every screen; `screenModel` exposes each screen's `name`,
-   `geometry` and a `primary` index. The wallpaper is drawn once per screen via
-   a `Repeater`, the login card is centered inside the geometry of the selected
-   screen. The selected screen is `uiScreen` (an output name) falling back to
-   `screenModel.primary`. Rejected: mirroring the UI on every screen (what the
-   stock themes do, and what the user explicitly did not want).
+6. **GUI on one screen, wallpaper on all.** SDDM creates one greeter window per
+   screen, each with its own single-row `screenModel` and a boolean
+   `primaryScreen` context property (verified in upstream v0.21.0
+   `src/greeter/GreeterApp.cpp` and `ScreenModel.cpp`). So the theme decides per
+   window, not inside one union window: the wallpaper fills whatever window it
+   is in, and the login card is shown only where `uiScreen` (an output name)
+   matches, falling back to `primaryScreen` when `uiScreen` is empty. Rejected:
+   mirroring the UI on every screen (what the stock themes do, and what the user
+   explicitly did not want). Corrected during implementation: the original text
+   of this decision assumed a single union window and `screenModel.geometry(-1)`
+   covering all screens, which SDDM does not do.
 
 7. **Idle behaviour mirrors Breeze.** After `fadeoutMs` (default 60000) with no
    input the card fades out, leaving only the wallpaper; any key, click or mouse
@@ -125,10 +129,10 @@ root install (once, printed by UDT's install.sh)
       theme.conf.user -> ~/.local/share/udt/sddm-theme.conf
   /etc/sddm.conf.d/udt.conf            [Theme] Current=udt
 
-greeter start (as sddm)
+greeter start (as sddm), one window per screen
   reads theme.conf + theme.conf.user into `config`
   reads config.background directly
-  selects a screen from screenModel
+  shows the card only where uiScreen matches, else on the primaryScreen window
 ```
 
 Data flow is one-directional and needs no daemon: `udt-accent` writes a file,
@@ -201,13 +205,14 @@ parses and renders before any wallpaper has ever been set.
 
 ## Theme behaviour
 
-- **Background per screen.** `Repeater { model: screenModel }`, each delegate
-  positioned at `geometry` from the model, `Image` with
-  `fillMode: Image.PreserveAspectCrop`. A `base`-coloured overlay sits above it
-  for text contrast.
-- **Card on one screen.** `uiScreen` names an output (e.g. `DP-1`); empty or
-  unmatched means `screenModel.primary`. The card is centered in that screen's
-  geometry, so other screens show wallpaper only.
+- **Background per window.** Each greeter window is exactly one screen, so a
+  single `Image` fills it with `fillMode: Image.PreserveAspectCrop`. A
+  `base`-coloured overlay sits above it for text contrast.
+- **Card on one screen.** `uiScreen` names an output (e.g. `DP-1`); empty means
+  the window whose `primaryScreen` is true. The card is centered in its window,
+  so every other screen shows wallpaper only. A name that matches no screen
+  shows no card anywhere; set `uiScreen` in the root-owned `theme.conf`, never
+  in the regenerated `theme.conf.user`.
 - **Frosted card.** `ShaderEffectSource` samples the wallpaper at the card
   rectangle, `MultiEffect` (QtQuick.Effects, available on Qt 6.11) blurs it, and
   a rounded mask clips it to the card. If the effect fails to load, the card
@@ -293,8 +298,10 @@ The commands need no `sudo` because the user runs them in a root shell. The
 - **A wallpaper outside `~/Pictures` may not be world-readable.** The card then
   renders on `theme.conf`'s fallback colour. Documented, not defended against.
 - **`uiScreen` names a physical output.** Changing monitor layout or names needs
-  a one-line edit of the generated INI or the theme config; empty falls back to
-  primary, so the default never breaks.
+  a one-line edit of the root-owned `theme.conf`; empty falls back to the
+  primary window, so the default never breaks. A mis-spelled name matches no
+  window and shows no card anywhere, so change it deliberately and leave it
+  empty if unsure.
 - **QtQuick.Effects blur depends on a working GL context in the greeter.**
   Falls back to a flat translucent card, so a failure is cosmetic.
 - **SDDM version drift.** `config.stringValue` needs SDDM 0.20+ and