Trace the many-phase gap to local-redraw deferral, not the placement pin

The experiment's `many` loss against e44dea3 came from `redraw` refusing
any widget whose given frame is not its offer and marking its parent
instead, which under the frame/extent protocol chains to the root because
a span gives its children its own placement across itself as their frame.
Record the trace, the measured bound with the pin off, the local re-ask
branch `wip/local-reask` and its two remaining depth-6 counterexamples,
and point readers at this section before the earlier explanations.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

docs/HANDOFF.md

@@ -18,14 +18,14 @@ present when checked on 2026-09-15.
 
 The separate frame/extent experiment now addresses the dominant size/resize
 redraw cascade through dependency tracking and invalidation, not glyph emission.
-Its current head is `0e107f0` in `/home/bob/repos/iris-layout-experiment`.
+Its current head is `34cafb6` on `wip/region-and-placement` in
-See **Frame and extent: retained prototype** for the mechanism, measured costs,
+`/home/bob/repos/iris-layout-experiment`. See **Frame and extent: retained
-and the remaining cold/many-update work. The app's framework pin is unchanged.
+prototype** for the mechanism. The app's framework pin is unchanged. **It is
-**It is not ready to replace #18 yet**: measured against `e44dea3` it wins on
+not ready to replace #18 yet**: against `e44dea3` it wins on `size` and
-`size` and `scroll` and loses badly on `many` -- 4.5x on seed 13 at depth 8
+`scroll` and loses on `many` and `resize`. The cause of the `many` loss was
-after the 2026-09-17 work -- and what is left of that is the measure/place
+traced on 2026-09-17 and is not what the earlier sections say; read **Where
-double draw rather than a conservative read. See **Measured against #18's
+the `many` gap actually comes from** before anything else in this document
-head, and the placement pin behind the gap** and the two sections after it.
+about performance, and take the earlier sections' explanations as history.
 
 **Widen one fuzzer axis at a time, and record which.** Seeds **1121** and
 **1839** at **depth 4** failed on `ea6dbae` and on every commit before it,
@@ -762,31 +762,79 @@ core tests, the oracle at 100 seeds, the shrinker at 400 trees of depth 5,
 five reference renders plus `tabs` resized to 900x1200 and `random` to
 1280x800, all byte-identical on Venus.
 
-### What is left of the gap, and it is not the placement pin
+### Where the `many` gap actually comes from (2026-09-17, traced)
 
-**The extent contract is what refuses most reuses.** The diagnostics rig
+The two sections above blame the placement pin and then the extent contract.
-now splits a failed reuse three ways. At seed 13, depth 8, `many`, of 1,203
+Neither is the cause. Disabling the pin (unsound, a bound) still redraws
-refusals a frame: **819 fail an extent range**, 379 a frame range and 310
+**487** distinct widgets a frame at seed 13 against `e44dea3`'s 159, and a
-the placement pin (they overlap). Seed 1: 55, 20 and 37 of 98.
+per-widget trace of one `many` frame shows what does it: **local redraws
+defer to the parent, and the deferrals chain to the root.**
 
-The point ranges are the tell -- `Holds { lo: 20, hi: 20 }`, `{ lo: 1916,
+`redraw` refuses a dirty widget whose given box is not as long as its offer
-hi: 1916 }`. `Painter::px_len` narrows the extent to exactly the length
+and marks its parent instead. Under this protocol that is nearly every
-read, and the measuring ask and the placing ask are two different extents:
+widget under a self-sized container. `Span` hands its children its own
-a widget that reads its pixel width while being measured cannot have that
+placement across itself as their *frame* (`UiRegion::from_axis(axis,
-drawing reused where it is finally placed. Wrapping text and `Scroll` both
+UiSpan::FULL, *own.axis(!axis))`), and that placement is `FULL` while the
-do. That is the measure/place double draw, and it is why the experiment
+span is being measured and its answer once it is placed, so a child's offer
-draws 508 distinct widgets of 583 active where `e44dea3` draws 159.
+frame across the span is the whole window and its given frame is the span's
+height or width. The trace has 43 deferrals in one frame, in chains such as
+424 → 425 → 437 → 449 → 483 → 487 → 491 with `given=(52, 40)
+offered=(1920, 40)` at every step. Each container reached that way runs its
+body at both placements, redrawing its subtree at two geometries.
+`e44dea3` defers under the same rule but its given and offer differ only by
+the widget's own alignment slack, so its chains stop after a level or two:
+19 deferrals, 159 distinct widgets.
 
-**`Span` is the one raw placement read left that costs anything.** Replacing
+**Re-asking locally at the offer** is the fix that follows, on branch
-`painter.placement()` with a non-recording read -- unsound, a bound only --
+`wip/local-reask` (one commit over `34cafb6`, pushed). `ActiveData` keeps
-takes seed 13's `many` from 4.73 ms to 3.03 ms; doing the same to `Scroll`
+`offer_region` beside `given_region`; `redraw` draws the widget once in the
-moves nothing. A span cannot use an extent ask as it stands: it hands each
+offer's frame at the offer's lengths and placement, and again at the given
-child the *frame* along the axis (so `rel(0.5)` is half the row) and a slot
+box where the two differ. Uninstrumented, 200 frames, seed and depth as
-cut from its own *extent*, and the slot is accumulated from lengths that are
+before, against `e44dea3`:
-fractions of the frame. Expressing those slots in extent coordinates needs
+
-the frame-to-extent ratio, which is the inverse division this branch
+| phase | `e44dea3` | `34cafb6` | `wip/local-reask` |
-removed. **Ask Bryan before designing that**; it is the shape of the
+| --- | ---: | ---: | ---: |
-protocol, not a cleanup.
+| `many`, seed 1, depth 8 | 0.263 ms / 157 draws / 95 distinct | 0.549 / 263 / 108 | 0.499 / 239 / 93 |
+| `many`, seed 13, depth 8 | 0.896 / 524 / 159 | 4.727 / 1904 / 508 | 1.282 / 647 / 294 |
+| `size`, seed 1 | 0.016 / 16 draws | | 0.010 / 3 |
+| `size`, seed 13 | 0.094 | | 0.003 |
+| `resize`, seed 1 | 0.019 / 13 draws | 27 draws | 0.028 / 22 |
+| `scroll`, `repaint` | 1 us | | 1 us |
+
+With the pin also disabled on that branch (bound), draws fall a further 15%
+and distinct widgets do not move. **What is left is the offer-then-place
+double pass**: a dirty container is drawn at its offer placement, where its
+children get the offer geometry, and again at its placed one, and any child
+whose `Holds` is a point (`Branch`, `Scroll`, wrapping text at a width it
+was not measured at) is drawn on both passes. That pass exists because the
+frame a span gives its children across itself is derived from the span's
+own answer, so it cannot be the same on both asks.
+
+**The branch is not sound.** The suite, the debug oracle, 100 seeds and the
+shrinker at 400 trees of depth 5 pass; the oracle at 1000 seeds of depth 6
+diverges on two. Both reduce to a self-sized container whose answer changes
+under a local redraw:
+
+- seed 532, `reorder`: `Pad{0} > Stack[ Span{RIGHT}[ Wrapped, Wrapped,
+  Rect[x:178px, y:123px] ] ]`, rotating the span's children. Warm places
+  a text at 947..1115 x 536..659, cold at 986..1076 x 483..712.
+- seed 398, `every-size`: `Span{LEFT}[ Rect, Rect, Stack[ Rect,
+  Branch{ probe: Wrapped, wide: Rect, narrow: Span{DOWN}[ Rect[x:129px,
+  y:leftover] ] } ] ]` with a size rule on the stack. Warm places a widget
+  at 11..183, cold at 6.5..187.5 on x.
+
+Reduce them with `SHRINK_SEED=<seed> SHRINK_DEPTH=6 SHRINK_CASE=<case>` on
+that branch before building on it.
+
+**The cross-axis frame is the protocol question, not the pin.** Bryan's
+rule that a widget's frame does not change between the measuring and the
+placing ask is broken across every span, because the only sensible
+reference for a child's fraction across a span is the span's own box, and
+that box is the span's answer. `e44dea3` has the same double pass and pays
+for it by deferring to the parent; what it does not have is a frame that
+changes under a widget. Whether to keep frame and placement apart at all
+turns on this and on nested-span semantics; see the 2026-09-17 assessment
+handed to Bryan.
 
 ## How layout is decided