ai-app/docs/HANDOFF.md

# Handoff

Where the work in flight stands for a session picking it up cold. Keep current
invariants, measurements, and failed hypotheses here; this is not a decisions
log. Pruned on 2026-09-17: the chronicle of closed defects, superseded
verification lists and cross-fixture tables went, the rules and the lessons
stayed.

## Where things stand

Canonical Iris `main` is **`ca2b4b2`** (#17, the headless rig). **#18
`split/18-position-chain`** is open in `/home/bob/repos/iris-pr18`, head
**`25e456e`**, pushed. It holds LAYOUT.md §2's position chain, `leftover`,
the `Holds` retained-layout contract, region nodes, built-in alignment and
size rules, fixed-point layout, a box in pixels threaded down the draw, and
a report read as a fraction of the containing widget. No PR review was
present when checked on 2026-09-15.

**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,
and nothing in the routine verification reached them: the fast oracle takes
ten seeds, the shrinker 400 at depth 5 and the long oracle 1000 at depth 6,
so a defect past seed 400 at depth 4 had nowhere to show. Both are fixed by
`4bd8607`. The scan that found them, worth running again after any change
to layout:

```rust
// tests/scan.rs, deleted once it had done its job
over_seeds((1..=2000).collect(), |seed| {
    let grown = plan(seed, 4, &Edits::default());
    for &case in ALL.iter() {
        if let Some(how) = diverges(&grown, case, seed) {
            println!("HIT seed {seed} case {} {how}", case.name());
        }
    }
});
```

261 s for 2000 seeds at depth 4 over all fifteen cases, and clean at
`4bd8607`. `Rng::new` is `seed | 1`, so an even seed and the odd one above
it are one tree: 1120 and 1121 are the same counterexample, as are 1838 and
1839.

**Two ideas outrank everything else in this document** (Bryan, 2026-09-17).
First, a changed tree lays out exactly as if it had been drawn that way from
the start; that is what the retained machinery is for and what the oracle
and shrinker check. Second, widgets are predictable: `px` is that many
pixels, `rel(0.5)` is half of the containing widget's area however many
siblings there are and wherever it sits among them, and `leftover` is a
share of the room left once every sibling's `px` and `rel` are resolved. The
invariants listed further down were accumulated by agents chasing single
failures; any of them may be simplified or deleted if those two ideas still
hold.

## Review of 2026-09-17

A fresh read of `core/src/fixed.rs`, `orientation/`, `ui/holds.rs`,
`ui/painter.rs`, `ui/render_state.rs` and the position widgets, outside of
doing work on them, with each finding checked by a scratch test.

**Verdict.** The concepts are sound and stay. Fixed point on a `1/1024`
grid is the right base for a layout that decides "same box or not" by
equality. Threading the pixel box down the draw, with `Holds::through` the
exact preimage of that one multiply, is the strongest idea in the code:
layout has one route to every length and the reuse test is its exact
inverse. Offer, given and placed is the ordinary measure-then-arrange model.
What needed work was the bookkeeping around the second ask, one boundary in
`Span` computed by an expression other than the drawing it guards, and a
`rel` that meant two things. The two-step residual is structural and no
grid width fixes it; where it becomes visible is the shader's snap.

### Decided by Bryan

- **`rel` is a fraction of the containing widget's whole area.** In a span,
  `rel(0.5)` is half the span whatever else is in it and wherever it sits.
  It is never a fraction of what was left after earlier children. This
  supersedes the 2026-09-16 reading that a report is a fraction of the box
  the widget was given, wherever that box is a remainder rather than the
  child's whole area.
- **Splitting `Widget::draw` into a measure and a draw is acceptable.** His
  original reason against it: a span whose children have fixed sizes can
  place them exactly where they were measured and so draw once, and that
  must survive. The smaller items below come first.

### A report is a fraction of the containing widget (landed, `ffd79f3`)

**Superseded in part.** Bryan refined the rule twice on 2026-09-17: a
fraction means whatever the *parent* says it means, and it need not even
mean the same thing for two siblings -- "Main point is truly that rel is
decided by parent". The span conclusion below is unchanged and the reason
for it stands (a span's offer differs per child and the row does not), but
`reports_of` is the wrong shape for saying it; see the parked padding
branch.

`rel(0.5)` is half the span whatever else is in it and wherever the child
sits. A report used to come back composed through the box it was offered,
and a span offers each child the room from its cursor, so a nested span
taking half of what it was given took a quarter of a row whose first half
was spoken for -- where the same half written as a rule on the child took
half the row.

The offer is still the remainder, because a text has to wrap at the width
actually there. What separated from it is the base a report's fractions are
of, which the ask now carries as `reports_of`:

```rust
pub fn widget_at<'s, W: ?Sized>(
    &'s mut self,
    id: &'s StrongWidget<W>,
    region: UiRegion,
    reports_of: UiVec2,
    decided: [bool; 2],
) -> DrawResult<'s, 'a, W>
```

It is `region.size()` wherever the box offered is the child's whole area --
`Pad`'s inset, a `Stack` child, `Scroll`'s content -- and `Span` passes
`UiVec2::FULL_SIZE` along its row. `widget_decided` is gone; `widget_at`
says both things about an ask rather than one of them.

**A span can now overflow itself without bound**, which is the consequence
Bryan's rule asks for: two children reporting half each take the whole row
and a third starts past the end. Under the old reading `total.rel` could
not exceed one, so `Span`'s `fixed <= 0` branches were only reachable
through declared fractions; they are ordinary now, and with them boxes of
negative length passed down to children.

### An answer is not an answer while anything under it is dirty (`0e0d4af`)

`draw_inner` took an answer from `try_reuse`, which checks only whether the
widget itself is marked, where `retained_answer` beside it also refused one
while anything the widget read a size from was dirty. A widget whose drawing
happened to be reusable therefore handed back the answer it gave before that
descendant changed, and nothing puts it right: the comparison that tells a
reader its child's answer moved is in `redraw`, and a widget settled inside
its parent's own draw never goes through it. The placing ask redraws the
subtree, the descendant's mark is cleared there, and the parent keeps a
number the tree no longer agrees with.

So `dirty_size_under` is not the optimization its comment claimed. It is
what makes an answer an answer, and it is asked once in `draw_inner` for
both retained routes. Twenty-five rig counters are unchanged on `cold`,
`repaint`, `scroll`, `resize` and `size`; `many` makes 18 fewer reuse
attempts, 17 of which already reported "dirty".

Found at seed 564, depth 6, `shuffle-every-other`, reachable only once a
span could overflow itself. **The fast test for it is still owed**: the
divergence needs a reader drawing while a size dependency two levels under
it is dirty, which no hand-built tree has reproduced yet, and the seeds at
depth 4 above fail for some other reason.

### A text is handed back a box its own line fits in (landed, `4bd8607`)

A wrapping text reported the width it used through `Px::from_f32`, which
takes the nearest step and is under the line the shaper measured half the
time. A parent that sizes itself from that report -- a stack taking a span's
width, the span taking its widest child's -- then hands the text back a box
its own longest line does not fit in, and a greedy break there is one line
longer. Warm kept the break it had; cold made the narrower one.

Two tolerances were holding that together and both are gone:

- `TextBuffer::shape` answered any width within `BREAK_EPSILON_PX = 0.05` of
  the longest line from the break in hand. Fifty steps of the grid, and a
  structural decision taken on a hair's breadth -- the thing the `Span`
  boundary invariant below already forbids. It is `want >= layout.width()`
  now, exactly.
- The `Holds` range the text declares started at the nearest step to its
  longest line, so it admitted boxes that line does not fit in. It starts at
  `Px::ceil_from_f32` of it now.

Neither was the fix. **The fix is the report**: `Size::from_px(
PxVec2::ceil_from_f32(tex.size))`, the step at or above what was measured,
so the box that comes back fits. With it in place either tolerance could
have stayed and the case passes; both are wrong on their own terms, so both
went. `Fixed::ceil_from_f32` is new and is the only rounding on the grid
that is not to the nearest step.

The general shape, and the third time this branch has hit it: **a value that
comes back as a box has to be rounded away from the measurement, not to the
nearest step.** Rounding to nearest is right for a value being carried;
it is wrong for a bound.

### A frame settles strictly bottom-up (landed, `a92c6ac`)

The queue was already deepest-first, but a widget that could not settle
where it was called `redraw` on its parent from inside itself, which drew a
shallow widget while dirty widgets deeper in other subtrees were still
pending. A parent drawing over a subtree that has not settled reads answers
about to move, and the one that settles does so inside the parent's draw --
where its mark comes off and nothing compares what it now answers.

A widget that cannot settle defers instead: it marks its parent, stays
marked, and waits in `deferred` until the walk reaches the parent's depth,
which cannot happen before everything deeper has settled.

```rust
loop {
    let next = rsc.widgets().needs_redraw.iter().copied()
        .filter(|id| !self.deferred.contains(id))
        .max_by_key(|&id| self.depth(id));
    let Some(id) = next else { break };
    if !self.redraw(id, rsc) {
        self.deferred.insert(id);
    }
}
```

Bryan's, 2026-09-17, and the right answer where `0e0d4af` was a check:
"then that entire category of issue can't even occur".

**No fuzzer can tell whether `0e0d4af`'s guard still does anything.**
Dropping `dirty_size_under` from it passes the suite, the shrinker at 400
seeds of depth 5, the oracle at 1000 of depth 6 and 2000 seeds at depth 4.
It stays because `update` draws the root for a resize *before*
`redraw_updates` runs at all, which the ordering does not reach -- a hole
that is reasoned rather than measured, so either find the case or delete
the guard, but do not leave it on a hunch forever.

Drawn widgets, widget draws and primitive writes are unchanged on every rig
phase; `many` pays 51 queue pops for 27 and 1059 depth reads for 410.

### A span's leftover boundary is its own inverse (landed, `53b00c6`)

The decision used a rounded division where the room the children get is a
floored multiply, so the boundary and the drawing it guarded were two
expressions for one length. `room` is that length as a `Len`, `room.to_px`
is the multiply, and `Holds::through` is its exact preimage:

```rust
let room = Len::rel_max() - Len::from_parts(total.rel, total.px);
let mut shares = false;
if total.leftover > Weight::ZERO {
    shares = room.to_px(painter.px_len(axis)) > Px::ZERO;
    let holds = match shares {
        true => Holds::from(Px::STEP..=Px::MAX),
        false => Holds::from(Px::MIN..=Px::ZERO),
    };
    painter.holds(axis, holds.through(room));
}
```

The three branches were the sign of `1 - rel`, which `through` reads
already. Forty lines became twelve and one `div` left layout. The general
rule stands and is now demonstrated: **derive a boundary through the
inverse of the expression that draws, never by a second expression for the
same length.**

### Two branches parked, both real, neither ready

**`wip/stack-fraction-twice`.** A stack sized by a child that reports a
*fraction* applies that fraction twice: its parent places the stack at the
reported length, and `box_of(size)` then takes the same fraction of that
box. Half a row becomes a quarter. Pixels are idempotent under a second
application, so only a share ever shrank -- and warm and cold shrink alike,
so **no oracle can see it**. Bryan: "I believe this exact thing has come up
multiple times for some reason."

The fix is that every child gets the whole box, plus `widget_at` not
resolving a rule into a box already chosen from it. `Painter::box_of` is
deleted rather than guarded: a first attempt made it answer the whole box on
a `decided` axis, which fails because `place` can reuse the stack's
*measuring* drawing by remapping it, so `Stack::draw` never re-runs. **A
drawing has to be a function of its box alone** -- if `decided` is in it, a
reused drawing is wrong.

What stops it landing: seed 1091 at depth 4, `shuffle-swap-for-three`,
disagrees by three steps where the oracle tolerates two (warm 1053.9971
against cold 1054). `box_of` was also making placement *exact*, by handing a
child a box of exactly the length it asked for, and the whole box puts a
rounding back at each nesting level. Find that composition; do not widen
`AGREE_STEPS`.

**`wip/padding-outset-and-inset`.** Padding goes outside what it pads and
never insets the child (Bryan, 2026-09-17): otherwise a child's `rel` and
`leftover` would mean the inner box while its `px` meant the outer one.
`Padding::region` moves the child's box in rather than shrinking it, a new
`Inset` widget with `.inset()` is the old behaviour, and `Pad` is to be
renamed **`Outset`** with `.outset()` to match. `in_parent_frame`'s
composition and the `reports_of` argument go with it -- a report comes up
raw and the parent says what it is a fraction of, which `Inset` does for
itself.

What stops it landing: a child declaring `rel(0.5)` under an `Inset` comes
out 47.5 px wide of the 190 inside rather than 95, and the second halving is
unaccounted for. The `Pad` half is green on its own; three tests moved to
`.inset()` because they were using padding as scaffolding rather than
testing it.

### `Span`'s leftover boundary is a third expression for the room

The decision uses a rounded division, `total.px.div(fixed)`, while the room
the children get is a floored multiply, so the two disagree at the boundary.
Measured with 300 px, `rel(2/3)` and a `leftover` child:

| row width | leftover child | its threaded length |
| --- | --- | --- |
| 900.000 | undrawn | |
| 900.001 | drawn | 0 steps |
| 900.002 | drawn | 0 steps |

Harmless at two steps, but the three-branch block collapses into the inverse
that already exists. `room` is computed a few lines below the block as
`Len::rel_max() - Len::from_parts(total.rel, total.px)`, and its `to_px` is
exactly the threaded length the leftover children share:

```rust
let room = Len::rel_max() - Len::from_parts(total.rel, total.px);
let mut shares = false;
if total.leftover > Weight::ZERO {
    shares = room.to_px(painter.px_len(axis)) > Px::ZERO;
    let holds = match shares {
        true => Holds::from(Px::STEP..=Px::MAX),
        false => Holds::from(Px::MIN..=Px::ZERO),
    };
    painter.holds(axis, holds.through(room));
}
```

`through` already handles a negative fraction and a zero one, so the
`fixed < 0` and `fixed == 0` branches go with it. The general lesson: `mul`
floors while `div`, `div_int` and `ratio` round to nearest, so a boundary
derived with a division guards a drawing made with a multiply. Derive
boundaries through `through`, or make the grid floor everywhere.

### Where the residual comes from, and the snap

Both remaining steps are a symbolic region re-expressed by division rather
than recomputed the way a cold draw computes it: `AxisRemap::Scale` divides
to find a part's fraction of the old box, and `placed_box` scales a mixed
`Len` by the alignment. Neither touches the threaded pixel chain, so every
layout decision already agrees warm against cold; what differs is the
composed position the shader and hit testing see, by up to 0.002 px.

Closing `Scale` exactly is possible: keep each primitive's region in its
widget's own coordinates and recompose on a move with `within`, eight
multiplies and no division against the current four divisions and twelve
multiplies, exact by construction, sixteen bytes more per primitive. Closing
the alignment one means resolving alignment in pixels, which costs the
retained resize path. Neither is worth a thousandth of a pixel on its own.

Where it does matter is `snap_floor` in `prelude.wgsl`, which adds half a
layout step before flooring: that absorbs float error and not a layout
step, and truncation makes "one step under an integer" the common residue.
A third of 900 px is 299.999 on the grid and lands at 299 on screen, which
is the pixel `08c9d5a` moved `tabs`'s arcs by. Rounding to the nearest
pixel absorbs both the truncation and the two-step residual everywhere
except within two steps of a half pixel, where layout never lands on
purpose, and keeps integer widths for equal fractional parts:

```wgsl
fn snap_floor(v: vec2<f32>) -> vec2<f32> {
    return floor(v + 0.5);
}
```

Approved by Bryan on 2026-09-17, together with rounding on the CPU; see
item 5 under "Next" for why both, and what each does not fix. The check is
the reference render set plus the oracle; expect `tabs` to move its arcs
back.

### Smaller items

- The comment on the `local == UiRegion::FULL` shortcut in `widget_at` says
  composing through `FULL` "is not quite the identity in f32". On the grid
  it is exact; the shortcut is performance only now.
- An undrawn `leftover` child still contributes its gap, so a vanished
  child leaves a double gap.
- Nested spans pass `leftover` weight up, so three leftover children in one
  inner span beside one in another get three quarters to one quarter. No
  other layout system does that, and the doc's old example of two and two
  did not distinguish it from per-span division. Confirm it is wanted.
- `Fixed::div` by zero answers `MIN`/`MAX` while `ratio` answers `ZERO`;
  both are caller bugs under `debug_assert`, but the fallbacks differ.

### Measure and draw, later

Splitting `Widget::draw` into a measure and a draw would delete `at_offer`,
`offered`, `answers_offer`, the placing second ask and the reuse dance after
it. It costs two methods on every widget and is a rewrite of a core that
passes a thousand seeds at depth six. The one-draw property Bryan wants
kept holds either way: a child with a fixed size needs no measure, and a
measured child costs a measure plus a draw where it now costs a draw plus a
reuse or a redraw. Last in "Next".

## How layout is decided

### Fixed point

Decided with Bryan on 2026-09-15. Layout decides on a grid rather than in
floats.

- **`Fixed<SHIFT>` is an `i32` counting `1 / 2^SHIFT`.** Adding and
  subtracting are exact; `mul` drops to the step below (Bryan, 2026-09-16:
  truncation is preferable); `div`, `div_int` and `ratio` round to nearest;
  `to_scale` takes the nearest step. Two routes to one place that land on
  one number are the same place, so everything downstream compares for
  equality.
- **`Px` is `1/1024` px, `Rel` is `1/2^24` of a box, `Weight` is `1/65536`
  of a share.** `PX_SHIFT` and `REL_SHIFT` are the only statement of the
  first two; the shader's copy is prepended from them by
  `render::module_source`. `Px` was `1/64` first, where one rounding's
  residue was 0.016 px and enough to move a box. Range is +/-2.1M px and
  conversion to `f32` is exact to 16,384 px.
- A weight is not a fraction: a list divides its room by the total of its
  weights, and `Rel::ratio` turns two weights into a share on the finer
  grid.
- **Arithmetic wraps** (`4febabf`, Bryan: a coordinate past the range will
  not draw reasonably anyway, so wrap and break clearly). Saturating cost a
  twelfth of layout's instructions. `MIN` and `MAX` stand in for an
  unbounded end and are only ever compared against; `from_f32` is the one
  operation that clamps, and `Holds` keeps a saturating `narrow`.
- A pointer, a wheel notch, a shaped glyph advance and a window size arrive
  as floats and go on the grid where they arrive. `Vec2` is what the GPU
  and the platform speak; `PxVec2` is what layout decides in.
- **Do not widen the grid to chase a residue.** Every failure this branch
  saw was one value reached by two expressions, sitting on a boundary
  defined by the same value coming back the other way. No precision shrinks
  a residue that is the whole distance.

### A box in pixels is one multiply from its parent's

`ActiveData` keeps a widget's box as lengths of its parent's box --
`given_len`, and `offer_len` for the box it was first asked about --
`DrawInfo` carries the pixel lengths themselves (`px`, `offered_px`), and a
draw threads them down one `Len::to_px` at a time: the box its parent gave
it, then the part of that box its own answer placed its drawing in, which
`placed_lens` states once for both `placed_box` and the walk.
`Painter::px_size` and `px_len` read that value, and
`UiRenderState::asked_px` takes the same steps back up the parent chain
when a local redraw starts part-way down the tree. Neither chain has a
coordinate frame in it, so a region node cannot break either, and warm and
cold reach every length by the same expression.

- **`Holds::through` is the exact preimage of `px + floor(rel * box)`**:
  `floor(rel * B) >= lo - px` is `rel * B >= (lo - px) << REL` and
  `floor(rel * B) <= hi - px` is `rel * B < (hi - px + 1) << REL`, two
  `div_toward`s once the sign of `rel` has said which bound is which. The
  answer is an interval even for a single length, because a floor is not
  invertible. The range has to contain the box a drawing was made in (the
  `Holds` assertion in `draw_at`, debug only) and must not contain a box
  the drawing does not hold for (the oracle); being the preimage makes
  those one statement rather than a trade-off.
- **Symbolic regions are for the GPU, hit testing and remaps alone.**
  `Moves::resolve` is the only walk left and it is the vertex shader's.
  Nothing layout decides is composed back up the move chain.
- **`px` is not stored on `ActiveData`, deliberately.** A resize every
  widget's `Holds` admits redraws nothing, so a stored pixel length would
  be stale on every widget in the tree with nothing to say so. `asked_px`
  walks up only where a widget is already being redrawn; the mean chain is
  2.8 levels.
- **The window is not a move entry** (`5b78002`). A chain bottoms out in
  `MoveIdx::NONE`; the window is applied where a fraction becomes pixels,
  `to_px(output_size)` on the CPU and the uniform in the shader. A resize
  rewrites no retained entry and re-uploads nothing but the uniform; its
  cost is whatever `Holds` redraws.
- Failed hypothesis, kept as the shape of the mistake: an offer composed
  back up the chain fell back to `FULL` under a region node and was resolved
  against that node's *placed* box, so everything under a `Scroll` was
  re-asked at the content's width and confirmed its own answer. Pinned by
  `unsettled::a_widget_under_a_region_node_is_asked_in_the_box_that_node_was_offered`.
  The old chain with an allowance in `through` passed that case and the old
  chain with the exact `through` failed it; both halves had to land at once.

### What the fuzzers tolerate

`AGREE_STEPS` in `tests/scenario/mod.rs` is **2**, and both steps are
positions. One is a box centred in a fraction of its parent against the
same box centred in its own pixels, 0.001 px on a handful of seeds. The
other is `AxisRemap::Scale` re-expressing a part as a fraction of a box that
changed length; one step fails the 400-seed shrinker on `resize-size`,
seeds 384 and 162, by 0.002 px while passing the 100-seed oracle. Two of the
earlier sources were fixed rather than tolerated (`bdab558`): `Scroll` wrote
a box it had been given back out as its own length in pixels, and `Span`
placed each child a step from where the last ended rather than as the fixed
parts before it plus one rounded share. See "Where the residual comes from"
above for what closing the rest would cost.

## Retained-layout invariants

- `Holds` is the interval of box lengths for which a widget's drawing and
  reported size stay valid. Reading `Painter::px_len` or `px_size` narrows
  it to the length read; `Painter::holds` widens it. Parent validity is the
  intersection of what its children induce. The contract is trusted: a
  widget declaring a wrong range is a defective widget, and Iris adds no
  defensive work to recover from one.
- A retained drawing can be reused only when its `Holds` contains the new
  pixel box on both axes, its parent node is unchanged, its region-node
  choice matches the retained structure, it is on the layer it is asked
  for, and the widget is clean. A valid ordinary subtree moves without
  redrawing by recursive remap; a region node moves by one entry.
- **A retained drawing belongs to the layer it was made on.** A container
  that measures a child by drawing it measures on the layer that child will
  draw on -- `Painter::child_layer_at` -- or it pays two draws a frame.
- The first box a parent asks about is the offer; a later box chosen from
  the answer is the final box, not another answer. A dirty widget is
  re-asked in the box its parent gave it, and only where that box is as
  long as the offer; anything else is its parent's question, with the mark
  left on. Lengths and not whole boxes: what a drawing depends on is its
  lengths, so the same lengths elsewhere is the same question.
- An answer is reusable only where both its measurement and the drawing in
  its final placed box remain valid; the drawing's `Holds` is translated
  back through `placed_lens` and intersected with the answer's.
- `Painter::widget_decided(child, region, [bool; 2])` says the parent chose
  this box from the child's own answer along those axes, so the answer is
  not placed inside it again. A report of "half of what you give me" has no
  fixed point but zero, so the framework asks exactly twice: at the offer,
  and in the box chosen from the answer, final on the decided axes. `Span`
  decides the row axis, `Scroll` both, `Stack` both for its sizing child.
  `Pad` overrides nothing: its inset is exactly the inner where the box is
  its answer, and the slack is the inner's to sit in otherwise.
- **Placement cannot be applied after the fact.** Three attempts at "draw
  the widget, then move its drawing to where its alignment says" failed,
  because the move is a change of frame and no split of the stored state
  carries it: moving `ActiveData::region` with the drawing made a later
  local redraw ask a differently rounded question, and leaving it made
  `placed` accumulate without bound because `try_reuse` returns a clean
  subtree's size without walking into it. Alignment is applied where the
  size is known -- `declared_box` for a rule, the placing second ask
  otherwise.
- A widget that clips to its box reports its box: `Scroll` and `Masked`
  report `LEFTOVER` on both axes, and a `debug_assert` holds any widget
  that set a mask this draw to it. Overflowing is otherwise ordinary, which
  is why the assertion is narrowed to mask-setters. Where content shorter
  than a `Scroll`'s viewport sits is the scroll's own alignment, and its
  "fits at the start of any box" widening is gated on near alignment.
- **A widget's own mask is not the one it inherited.** `ActiveData` keeps
  both; they differ exactly where the widget called `set_mask`, which says
  whose mask a move rewrites, and a local redraw is handed the inherited
  one. Pinned by
  `retained::a_masked_widget_redrawn_on_its_own_sets_its_mask_again`.
- A span is as long across itself as its longest fixed child, unless a rule
  gives that length outright (`Painter::has_exact_size`), in which case it
  does not read its children there at all. Any relative or `leftover` child
  makes it report `leftover`. **Do not choose between a fixed and a relative
  child in pixels at the span's current width**: that admits multiple
  self-sizing fixed points, and generated seed 13 settled differently warm
  and cold under it. The same circularity is what a cap containing
  `leftover` would put into `SizeRule::Max`.
- `Span`'s leftover/no-leftover split is a strict layout decision, not a
  rounding tolerance: its `Holds` range must use the same exact boundary as
  drawing. A tolerant endpoint retained zero-height children in seed 16; a
  boundary moved off where boxes land was needed in floats and is not on
  the grid. **A structural decision may not be taken on a hair's breadth**
  that two routes can disagree about. Pinned by
  `unsettled::a_box_that_only_rounds_past_its_fixed_children_leaves_nothing_over`.
- A pixel comparison is equality. A length given in pixels is that many
  pixels wherever it ends up, structurally: `Len::within` adds a part's own
  pixels rather than scaling them. Pinned by
  `a_length_in_pixels_is_that_many_pixels_however_it_is_nested`. A length
  given as a share is not: equal shares come out one or two steps apart
  because positions, not lengths, are what gets rounded, so the row fills
  and no two children leave a seam
  (`equal_shares_differ_by_at_most_two_steps_and_fill_the_row`).
- **A move that keeps a box's length is a translation, and exact.** A box
  that changed length re-expresses each part as a fraction of the new one,
  which rounds. This inverts the float-era rule; `tests/cases/drift.rs`
  pins that the grid does not drift either way.
- `Scroll` must return the answer from the first box it asked about,
  whether retained or fresh; returning the final placed answer advanced one
  fixed-point iteration (seed 86). Content that fills the viewport unscrolled
  is handed back as it came, because the same box written as its own
  length in pixels does not round alike.
- An asked-but-undrawn size dependency names the widget that asked as its
  parent (seed 10). `Painter` records size-dependency edges only when a
  parent reads a child's size or hint; an undrawn measured child stays
  recorded so a later change reaches whoever decided not to draw it.
- Dirty widgets settle deepest-first. `dirty_size_under` stops a reader
  taking a retained answer while something below it is dirty; it is an
  optimization against laying out twice, not a validity mechanism.
- Declared non-`leftover` lengths are resolved by the parent where the
  widget is drawn, so a declared-length change redraws the parent. A rule
  wins on the axis it names and the widget under it never learns of it.
  **A cap may not contain `leftover`**: a cap must read the report, so rule
  and report are one equation, and a share puts the row's division into it
  -- the multiple-fixed-point failure again. A cap is pixels and a fraction,
  which is what `Len` is.
- Text shaping is retained separately from line breaking; a greedy break
  holds from its longest produced line through the width it was made at,
  reported through `Painter::holds`.
- Region nodes: a node holds a whole `UiRegion` in its parent node's
  coordinates, `FULL` is the identity, widgets opt in with `.region_node()`
  or `Widgets::set_region_node`, and changing it redraws the subtree once.
  `.scrollable()` sets it once; raw `Scroll::new` does not. A removed node's
  move entry stays alive until every descendant has migrated. `Span` and
  `Align` add no nodes.
- Alignment is one `f32` per axis (Bryan, 2026-09-15), default the middle
  on both because the edges assume a direction. One widget keeps one length
  per axis; a second length needs a second widget, `Wrapper` via
  `.wrapper()` (Bryan, 2026-09-16, `d21a215`).

## Verification at the current head

At `25e456e`:

- `cargo fmt --all --check`, `cargo clippy --workspace --all-targets --
  -D warnings`, `cargo test --workspace`: green, 90 suite tests, 19 core
  unit tests, 11 generated cases. Only the long runs and the profiling rigs
  are ignored; no known defect is.
- The release oracle at 100 seeds in 14.3 s, and **120 seeds in debug** in
  59 s -- the debug run exercises the `Holds` assertion in `draw_at`.
- All fifteen shrinker cases at 400 seeds of depth 5 in 56 s, the oracle at
  1000 seeds of depth 6 in 142 s, and **2000 seeds at depth 4 over all
  fifteen cases** in 262 s. The last is not routine and should be: it is
  the only run that has ever found anything past seed 400.
- `view`, `minimal`, `random` and `tabs` byte-identical at 1920x1200
  against `ea6dbae`. `random` live-resized from 1920x1200 to 1280x800 is
  byte-identical to a cold 1280x800 render. **`text` is a new picture**:
  `4bd8607` moved its lower paragraph one pixel, the box being a step wider
  and its left edge crossing the shader's snap, and `c8beca5` rewrote the
  alignment panel, which had all three labels in the middle of a box the
  width of the widest of them.
- Twenty-five rig work counters identical on `cold`, `repaint`, `scroll`,
  `resize` and `size` across `0e0d4af`; `many` differs only in reuse
  attempts. Across `a92c6ac` drawn widgets, widget draws and primitive
  writes are identical on every phase, and only the scheduling counters
  move.

**None of that reaches seeds 1121 and 1839 at depth 4**, which fail on this
head and on `ea6dbae` alike. Everything below is verification of what was
changed, not a claim that the branch is correct.

**A claim about a render holds for the commit it was checked at and no
further.** `tabs` changed twice across `d3b0ebf` with nobody looking; take
the oracle as the reference and the five renders as a spot check.

**Run the long two before believing a rounding change**, and run the
ordinary suite in debug:

```sh
cargo test --release --test generated -- --ignored a_long_run_of_seeds_agrees
SHRINK_CASE=all SHRINK_SEEDS=400 SHRINK_DEPTH=5 \
  cargo test --release --test shrink -- --ignored --nocapture
IRIS_GENERATED_SEEDS=1000 IRIS_GENERATED_DEPTH=6 \
  cargo test --release --test generated -- --ignored a_long_run_of_seeds_agrees
```

Depth is what finds things, but so is breadth: every late defect before
2026-09-17 surfaced at depth 5 or 6, and the two open ones were found by
running 2000 seeds at depth 4, which nothing routine does. Widen one axis
at a time and record which.

## Performance

**Threading a box in pixels down the draw is free on cold layout and 9-13%
off the retained paths** (2026-09-17). Instructions:u, medians of 21 runs of
binaries built in one worktree, seed 1 at depth 8, against `5b78002`:

| phase | before | after | |
| --- | --- | --- | --- |
| `cold`, 200 frames | 313.1M | 312.9M | -0.04% |
| `resize` | 408.1M | 405.6M | -0.61% |
| `many` | 1,924M | 1,756M | -8.75% |
| `scroll` | 357.3M | 323.4M | -9.49% |
| `repaint` | 363.3M | 315.4M | -13.18% |

`cold` and `resize` compare directly: all twenty-five work counters are
identical. The other three do less work: `repaint` goes from 23 draw
requests and 13 widget draws to 1 and 1, because `redraw` composes nothing
and a widget whose box moved without changing length settles itself instead
of escalating.

### How to measure here

- **Check the work counters before comparing two commits' times.** The rig
  prints drawn widgets, widget draws and primitive writes; a comparison is
  only worth reading when they match. `random.rs`'s `Branch` picks a
  subtree by a measured pixel length, so the fixture's shape moves with the
  thing measured; `Edits::fixed_branches` pins it for timing and the oracle
  keeps measured branches on purpose. A 3x this section once reported was
  that artifact.
- **`perf stat` in this VM returns garbage readings** for both
  `instructions:u` and `cycles:u`, roughly a quarter of the time, off by a
  factor of five to fifteen. Take medians of nine or more and report how
  many readings a filter kept. Cycles spread 1-3% between sets of one
  unchanged binary and 6.7% in the worst; instruction counts hold to 0.02%
  within a binary and move 0.5% across a rebuild, so build the baseline
  beside the thing measured and quote a delta. `ex_div_busy` held to 0.1%.
- **What moves cycles is whether `UiSpan::within` inlines.** It is the
  hottest line in layout; `nm` shows it as a symbol when it does not.
  Shrinking its body until the inliner takes it won; `#[inline]` on the
  body it had lost 1.5% cycles. Shrink it, do not annotate it.
- `Holds::through` divides twice per call and accounts for essentially all
  of a run's `i64` divisions: 21.3M cycles of a 500-frame `many`, 2.8%.
  The float head divided twice there too.

### Tried and rejected, with numbers

- A float reciprocal for `AxisRemap::apply_scalar`'s division: +6% cycles.
  `Holds::through`'s division has not been tried.
- Branchless `shift_round`: +6.7% cycles alone, and worse again with the
  short-circuits removed. Size, not the branch, is what keeps `within` out
  of line.
- Removing the per-child hash lookup in `remap_subtree`: 0.0%.
- Short-circuiting `apply_scalar` where the fraction is nought or one: +17%.
- Short-circuits guarding a saturating multiply stopped paying once the
  multiply wrapped. Re-price a short-circuit before keeping it.
- Rust does not contract `a + b * c`; the float head never had an FMA to
  compare the grid's multiply against.

Wrapping (`4febabf`) was -8.6% instructions and -6.6% cycles. Truncating
(`08c9d5a`, with `Fixed::scaled`'s zero test and `within`'s `is_full` tests
removed as one commit, since they are worth 61M instructions apart and 115M
together) costs a share a thousandth of a pixel of its row, makes a flipped
span sit a step from its mirror, and moved an antialiased edge in `tabs` by
one pixel. See "Where the residual comes from" for what that last one is.

## Rigs and reproduction

Ordinary framework verification:

```sh
cd /home/bob/repos/iris-pr18
cargo fmt --all --check
cargo clippy --workspace --all-targets -- -D warnings
cargo test --workspace
```

The ordinary tests are modules of one `tests/suite.rs` target; pick a module
with `cargo test --test suite layout::`. `profile.test` uses
`debug = "line-tables-only"`, which halved the test-target rebuild.

`tests/generated.rs` compares a warm incremental tree with a cold tree of
the same state; `IRIS_GENERATED_SEED`, `IRIS_GENERATED_SEEDS` and
`IRIS_GENERATED_DEPTH` select what it covers. `tests/shrink.rs` reduces a
failing tree over the same fifteen cases and the same trees --
`iris::random::plan(seed, depth, &edits)` and `build(rsc, &plan)`, so a
failing seed reduces directly and the oracle prints the command:

```sh
SHRINK_SEED=18 SHRINK_DEPTH=6 SHRINK_CASE=repaint-some \
  cargo test --release --test shrink -- --ignored --nocapture
```

The cases live in `tests/scenario/mod.rs`, included by both targets by
`#[path]`; a case only one rig knows is how the two drifted apart once. Turn
what the shrinker finds into a test of its own rather than leaving a seed as
the record. Both fuzzers take a thread per core but one. A `git bisect`
once named a commit that could not be the cause; read the tree rather than
the bisect when that happens.

`tests/layout_diagnostics.rs` is the retained CPU rig: `IRIS_PHASE` selects
`cold`, `many`, `repaint`, `size`, `scroll` or `resize`, the
`layout-diagnostics` feature gives the explanatory counters, and an
uninstrumented release binary under `perf` gives totals. Dump the counters
with

```sh
IRIS_SEED=1 IRIS_DEPTH=8 IRIS_FRAMES=500 IRIS_PHASE=many \
  <instrumented binary> --ignored --nocapture \
  | grep -E '^ +[a-z].*[0-9.]+$' | grep -v ' ms$' | sort
```

and `diff` two runs; identical output is what says a change is free.

The float head is checked out at `/home/bob/repos/iris-float-cmp`, at
`5ed9e87` with `Edits::fixed_branches` applied uncommitted. Its counters do
not match the grid's and will not, so a comparison against it is a bound
rather than a measurement.

The headless reference set runs one process at a time because the rig
reuses one compositor; comparison worktrees need separate target
directories.

```sh
./scripts/run-headless.sh tabs --mode 1920x1200@60Hz --shot /tmp/tabs.png
./scripts/run-headless.sh tabs --mode 1920x1200@60Hz \
  --resize 900x1200@60Hz --shot /tmp/resized.png
./scripts/run-headless.sh tabs --mode 1920x1200@60Hz \
  --replay /tmp/tabs.touch --shot /tmp/replay.png
```

The replay used for the reference check:

```text
0 down 1728 24
80 up 1728 24
400 down 1836 1116
480 up 1836 1116
800 down 1836 1116
880 up 1836 1116
```

## Next

In order, from the review above and Bryan's steer (2026-09-17):

1. **`wip/stack-fraction-twice`**: find the composition that went from
   exact to rounded when `box_of` stopped narrowing, and land it. This is
   the one outright wrong layout known on the branch.
2. **`wip/padding-outset-and-inset`**: account for the second halving under
   `Inset`, rename `Pad` to `Outset` and `.pad()` to `.outset()`, and audit
   every `.pad()` in the examples for which of the two it meant.
3. **A fast test for `0e0d4af`**, the stale-answer guard, which went in
   with only a fuzz seed behind it. The shape wanted is a widget drawing
   while a size dependency two levels under it is dirty, where its
   drawing is reusable at the measuring box and not at the placing one.
   Check at the same time whether `a92c6ac`'s ordering has made the guard
   dead outside the resize path.
4. Write `ActiveData::answer` in one place.
5. Keep the `DrawInfo` on `ActiveData`; delete the copied fields and the
   reconstruction in `redraw`.
6. **Round on the CPU and snap to the nearest pixel in the shader**, as
   one change with one verification. Bryan approved the snap on 2026-09-17
   (rendering may change wherever it brings the screen closer to what the
   user's code says: three equal sections of 1000 px need one of them
   rounded up), and to CPU rounding the same day. The reason for that one
   is different: a
   `Rel` is off by at most `2^-25` of its box, so with round-to-nearest
   every product whose true value is a whole number of steps is exact for
   boxes under about 8,000 px, where truncation leaves half of them one
   step short and layout then decides "does not fit" on a container the
   user meant to fit exactly. Use the branchless round-half-up form,
   `(a * b + (1 << (BY - 1))) >> BY`, not the sign-branching
   `shift_round`; re-derive `Holds::through` for `round` (its two shifted
   bounds move by half a `Rel` step); check with `nm` that `UiSpan::within`
   still inlines; expect a couple of percent of instructions and re-run the
   long fuzzers and the render set once for both.
7. The smaller items: the stale `f32` comment, the gap of an undrawn child,
   confirm nested `leftover` weights, one zero-divisor fallback. Add to
   them: a span that overflows itself hands a child a box of negative
   length, which is ordinary now rather than a corner, and nothing states
   what a widget may assume about one.
8. `LazySpan`, the next LAYOUT.md §2 item. Region nodes cover the movable
   subtree case; do not restore a separate child-placement API.
9. `SizeRule::{Min, Max, Clamp}`, restoring the `max_width`/`max_height`
   builders `8220a78` deleted. The clamp boundary is a hard layout decision
   with an exact `Holds` split at the crossover, both sides in `Px`. Still
   awaiting Bryan: whether a `Max` narrows the box the child draws in, or
   only what the parent reports for it.
10. `Scroll` taking a direction rather than one axis.
11. The measure/draw split, once the above is in. It deletes the two-ask
    protocol, which is what `0e0d4af` had to put a guard around.

`docs/LAYOUT.md` §4, §5 and the density section are stale: they name
`Painter::place`, `SetSize`, `desired_width`, `apply_rest`, `Len::dp`,
`Aligned` and `MaxSize`, none of which exist. Do not restore
`OnResize::Translate` or `OrthoSize`.

Other queued work, in dependency order: `UiRenderState` behind
`Rc<RefCell<_>>`; density-independent pixels; input restructuring (pointer
capture, drag slop and axis, cancellation, mask-aware hit testing,
timestamps); retained paints, selection, overlays and shared runtime state;
generic desktop/Android hosts and reusable example/APK tooling;
application-owned fonts and replaceable glyph-atlas buckets; positioned
text overflow and cluster-safe ellipsis.

The archive is a reference, not a patch: it predates returned `Size`, the
current box chain and the current length types. Recreate changes on current
types and keep app/session concepts out of Iris.