diff --git a/docs/re/structures/title-residual-tone-vs-geometry.md b/docs/re/structures/title-residual-tone-vs-geometry.md
index 7c54b6b5..02fde31b 100644
--- a/docs/re/structures/title-residual-tone-vs-geometry.md
+++ b/docs/re/structures/title-residual-tone-vs-geometry.md
@@ -36,8 +36,29 @@ So fitting a LUT *on the screen itself* and measuring what remains gives an
 ## The positive control
 
 The method must be able to close a residual that really is tone. On the **main
-menu** — where the port measures only 0.06 % of pixels differing, so geometry is
-essentially right — it does:
+menu** it does:
+
+> 🔴 **THE PREMISE OF THIS CONTROL WAS WRONG, corrected 2026-08-31.** It read
+> *"where the port measures only 0.06 % of pixels differing, so geometry is
+> essentially right"*. That 0.06 % is `verify-capture`'s, and it counts pixels
+> surviving `-threshold 25%` — differing by more than ~64 levels. **It is a
+> gross-displacement detector by design**, blind to sub-pixel offsets and
+> antialiasing, which are exactly what a per-level LUT also cannot close. It says
+> *no gross displacement*, not *geometry is right*, and I read one as the other.
+>
+> `sylpheed-port` measured the screen directly (`DECISIONS.md` at `5738b83`): after
+> this LUT, the menu's residual is **6.94 on edge pixels against 2.20 on flat —
+> 3.2×**, with a known negative (a render against itself under a pure gamma)
+> leaving exactly **0.00**, since a per-level LUT inverts a per-level effect
+> perfectly. **So the menu carries spatial error and is not a geometry-free
+> control.** ⚠️ I have **not** reproduced that split — it needs their render beside
+> the capture — and it is recorded as their measurement, not mine.
+>
+> ✅ **The result below does not rest on this control.** *"Whatever a fitted
+> per-level LUT cannot close is, by construction, not a per-pixel-level effect"* is
+> stated as the method's basis above and stands on its own. What the control
+> actually shows is narrower than claimed: the LUT closes 70 % of the menu's
+> residual — evidence it works on tone, not evidence the menu is tone-only.
 
 | | mean abs difference | closed |
 |---|---|---|