Record the leftover boundary, and close the reorder case

The shrinker's `reorder` case is no longer red: its two seeds were a span
deciding whether anything was left over on a box length its own answer had
produced, which comes back a few bits off. `5ed9e87` in `/home/bob/repos/
iris-pr18` moves that boundary by `HOLDS_EPSILON_PX` of room instead of
softening the comparison, so the validity range stays exact where generated
seed 16 needs it.

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

docs/HANDOFF.md

@@ -8,7 +8,7 @@ log.
 
 Canonical Iris `main` is **`ca2b4b2`** (#17, the headless rig). **#18
 `split/18-position-chain`** is open in `/home/bob/repos/iris-pr18`; its local
-head is **`d3b0ebf`**, fifty commits. Built-in alignment is complete there;
+head is **`5ed9e87`**, fifty-one commits. Built-in alignment is complete there;
 see "Built-in alignment" below for the retained-layout details. No PR reviews
 were present when checked on 2026-09-15.
 
@@ -65,20 +65,25 @@ rewrite:
 - `Span`'s leftover/no-leftover split is a strict layout decision, not a
   rounding tolerance. Its `Holds` range must use the same exact divided
   boundary as drawing; a tolerant endpoint retained zero-height children at
-  the boundary in generated seed 16.
+  the boundary in generated seed 16. `5ed9e87` moves that boundary rather
+  than softening it -- see "The leftover boundary" below.
 
 `core/src/ui/holds.rs`, the retained tests, and the generated cold-layout oracle
 pin those rules. Seeds 10 and 86 are now in the ordinary generated set.
 
-## Verification at `d3b0ebf`
+## Verification at `5ed9e87`
 
 - `cargo fmt --all --check`
 - `cargo build --workspace --all-features`
 - `cargo clippy --workspace --all-targets --all-features -- -D warnings`
-- `cargo test --workspace --all-features`: 88 passed, 10 ignored
-- The release generated cold-layout oracle passed 100 seeds in 65.5 s.
-- The release shrinker passed both `repaint` and `resize` at 300 seeds each:
-  26,001 widgets per case, largest tree 331 widgets.
+- `cargo test --workspace --all-features`: 89 passed, 10 ignored
+- The release generated cold-layout oracle passed 100 seeds in 68.5 s.
+- The release shrinker passed **all five** cases -- `resize`, `repaint`,
+  `resize-repaint`, `reorder`, `size-change` -- at 1000 seeds of depth 6:
+  159,024 widgets per case, largest tree 587 widgets. At the ordinary 300
+  seeds of depth 5 it is 26,001 widgets per case, largest tree 331.
+- `tabs`, `view`, `minimal`, `text` and `random` render byte-identical at
+  1920x1200 against the same worktree without `5ed9e87`.
 - `tests/drift.rs` passed its 20,000-move exactness check in release mode.
 - The seeded `random` example, live-resized from 1920x1200 to 1280x800, is
   byte-identical to a cold 1280x800 render. Both PNGs hash to
@@ -175,11 +180,12 @@ tighter figures.
 - Text shaping is retained separately from line breaking. A greedy line break
   remains valid from its longest produced line through the width at which it
   was made, and `TextView` reports that interval through `Painter::holds`.
-- `Span`'s decision to distribute `leftover` is a pixel question. Its validity
-  interval is split exactly at the length where fixed parts fill the box;
-  pure `leftover` children are undrawn on the no-space side. Exact/open ranges
-  are for hard widget decisions; the ordinary tolerant ranges remain for
-  accumulated coordinate rounding.
+- `Span`'s decision to distribute `leftover` is a pixel question. The room to
+  divide is `len * fixed - total.px`, and under `HOLDS_EPSILON_PX` of it is
+  none; pure `leftover` children are undrawn on the no-room side. The validity
+  interval is split exactly at that moved boundary. Exact/open ranges are for
+  hard widget decisions; the ordinary tolerant ranges remain for accumulated
+  coordinate rounding.
 - `Scroll` reports its content's first measured size. Its drawing can survive
   container-length changes only over the interval in which clamping and its
   current offset do not change.
@@ -199,9 +205,7 @@ tighter figures.
 
 ## Built-in alignment
 
-Committed as `d3b0ebf` in `/home/bob/repos/iris-pr18`. The shrinker's
-`reorder` case remains red at its base `8220a78`, so that failure is
-pre-existing and unrelated.
+Committed as `d3b0ebf` in `/home/bob/repos/iris-pr18`.
 
 What is in it: `align` is a widget property beside `region_node` and the size
 rule, `Aligned` is deleted, `.align()`/`.center()` set the property, and the
@@ -288,6 +292,35 @@ length the box takes and the drawing holds for that length alone.
 A `debug_assert` that a reported non-leftover size does not exceed the box it
 drew in would have caught both immediately, and is still worth adding.
 
+## The leftover boundary
+
+Fixed in `5ed9e87`, which closed the shrinker's `reorder` case. Neither of its
+two red seeds was about reordering.
+
+A span's box in pixels, compared with what its fixed and relative children
+fill, is the same number whenever the parent sized that box from the span's own
+answer -- and the box comes back through the chain a few bits off. So
+`0.00003 px` decided whether a `leftover`-only child existed: warm rounded
+under and left it undrawn, cold rounded over and drew it at zero length. Both
+layouts are stable and the pixels are identical either way, which is why only
+the warm-against-cold oracle could see it.
+
+**A structural decision may not be taken where boxes structurally land.** The
+fix is not a tolerant comparison -- that is what generated seed 16 punished --
+but a boundary moved by `HOLDS_EPSILON_PX` of room, with the validity range
+split exactly at the moved boundary. What it gives up is a share of under a
+twentieth of a pixel. `tests/unsettled.rs`'s
+`a_box_that_only_rounds_past_its_fixed_children_leaves_nothing_over` is the
+six-widget regression, shrunk from 266; it needs the span above the one that
+divides, because without a box composed through it both trees round the same
+way.
+
+`Scroll`'s `content_len <= container_len` sits on the same coincidence but is
+continuous there -- it chooses between two `Holds` ranges that both contain the
+current length, so a rounding difference costs a redraw rather than a
+different layout. Checked while fixing this; nothing else reads a box in
+pixels to decide something structural.
+
 ## Rigs and reproduction
 
 Ordinary framework verification:
@@ -349,15 +382,13 @@ child-placement API. `docs/LAYOUT.md` §2 is stale: it still describes
 
 **Built-in alignment and size goes on #18 rather than after it** (Bryan,
 2026-09-15: #18 is unreviewed and already large enough that most lines get read
-anyway). The size half landed as `8220a78`; the alignment half is in the
-working tree and described below.
+anyway). The size half landed as `8220a78`, the alignment half as `d3b0ebf`.
 
 Do not restore `OnResize::Translate`; retained translation is now expressed by
 the same `Holds` contract and box chain.
 
 Queued from this work, in order:
 
-- The shrinker's `repaint` case, via the replacement placement design above.
 - Delete `OrthoSize`. It is redundant now that size is built in: a span should
   read its own rule on the orthogonal axis and, where that is fixed, skip
   reading its children's orthogonal sizes entirely, which is what `Full` does
@@ -369,10 +400,10 @@ Queued from this work, in order:
   clamp resolved where the box is decided also fixes `MaxSize`'s reading of
   `px_size`, which pinned its interval to one exact box on both axes and
   redrew its whole subtree on any resize. The clamp boundary is a hard layout
-  decision, not a tolerance: its `Holds` range must be exact and split at the
-  crossover, the way generated seed 16 taught for `Span`.
-- The shrinker's `reorder` case, which is red at `8220a78` and was not
-  introduced by any of this.
+  decision: its `Holds` range must be exact and split at the crossover, the
+  way generated seed 16 taught for `Span` -- and the crossover itself has to
+  sit off the length a box structurally lands on, the way `5ed9e87` taught.
+  See "The leftover boundary".
 
 Other queued work, in dependency order: