diff --git a/builtin/runtime/zoom.lua b/builtin/runtime/zoom.lua
index a211520..f54b265 100644
--- a/builtin/runtime/zoom.lua
+++ b/builtin/runtime/zoom.lua
@@ -77,12 +77,16 @@ end
 -- workaround for one.
 --
 -- It also RESTORES the round-trip contract, which a raw step breaks:
--- with 0.015 the sequence is 16.00 -> 16.02 -> 16.01, because each
--- operation rounds independently and 16.015 and 16.005 round in
--- opposite directions. Quantizing first makes every step exact
--- addition in the quantized domain, so n in and n out returns to the
--- starting value for ANY accepted step, not only for the ones that
--- happened to be representable.
+-- with 0.015 the sequence is 16.00 -> 16.02 -> 16.01. Each operation
+-- rounds independently, and both intermediates land on an EXACT tie ---
+-- 16.015 and 16.005 are 1601.5 and 1600.5 centi-px --- which this
+-- half-up quantizer sends UP. Half-up is not symmetric under negation:
+-- rounding up on the way in adds half a centi-pixel, and rounding up on
+-- the way out adds another, so the two errors accumulate instead of
+-- cancelling. Quantizing first makes every step exact addition in the
+-- quantized domain, so n in and n out returns to the starting value for
+-- ANY accepted step, not only for the ones that happened to be
+-- representable.
 local function effective_step()
   return quantize(pmacs.config.get("ui.gpu-zoom-step"))
 end
diff --git a/docs/gui-zoom-framing.md b/docs/gui-zoom-framing.md
index b7bb5eb..97fff50 100644
--- a/docs/gui-zoom-framing.md
+++ b/docs/gui-zoom-framing.md
@@ -133,9 +133,13 @@ validates **finiteness and bounds and nothing else**
 *after* a value is stored — they cannot veto. So `0.015` is a
 perfectly settable step, and nothing in the registry can refuse it.
 
-Used raw it breaks the guarantee below, because each operation rounds
-independently and `16.015` and `16.005` round in **opposite
-directions**:
+Used raw it breaks the guarantee below. Each operation rounds
+independently, and both intermediates land on an **exact tie** —
+`16.015` and `16.005` are `1601.5` and `1600.5` centi-pixels — which the
+half-up quantizer sends **up**. Half-up is not symmetric under negation:
+rounding up on the way in adds half a centi-pixel, rounding up on the
+way out adds another, so the two errors accumulate instead of
+cancelling:
 
 ```
 step 0.015:  16.00 -> 16.02 -> 16.01     round trip broken
diff --git a/tests/gui_zoom_acceptance.rs b/tests/gui_zoom_acceptance.rs
index 0d7a50d..6fe4925 100644
--- a/tests/gui_zoom_acceptance.rs
+++ b/tests/gui_zoom_acceptance.rs
@@ -128,8 +128,10 @@ fn n_steps_in_then_n_out_returns_exactly() {
 /// The registry accepts any finite number in range — `ConfigKind::Number`
 /// validates finiteness and bounds and nothing else, and `on_change`
 /// cannot veto — so 0.015 is a settable step. Used raw it breaks the
-/// contract: each operation rounds independently, 16.015 rounds up and
-/// 16.005 rounds down, giving 16.00 -> 16.02 -> 16.01.
+/// contract: each operation rounds independently, and both intermediates
+/// land on an exact tie — 16.015 and 16.005 — which half-up rounding
+/// sends *up*. Up on the way in and up on the way out accumulate rather
+/// than cancel, giving 16.00 -> 16.02 -> 16.01.
 ///
 /// Quantizing the step at the point of use restores exactness for every
 /// accepted step, not just the representable ones.