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.

## 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`; its local
head is **`4febabf`**, seventy-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.

The current head completes LAYOUT.md §2's position chain and the requested
`leftover` behavior. A child whose length is only `leftover` is not drawn when
nothing is left. A child that also asks for pixels or a relative fraction keeps
that part and overflows as before.

`29c7881` replaces the parallel resize rules with one retained-layout contract,
`Holds`: the interval of box lengths for which a widget's drawing and reported
size stay valid. Reading `Painter::px_len` or `px_size` narrows the interval to
the length read; `Painter::holds` lets the widget widen it. Parent validity is
the intersection of the ranges its children induce. This contract is trusted.
A widget that declares an incorrect range is a defective widget; Iris does not
add defensive work to recover optimizations from a false declaration.

`9d8415d` deletes `OrthoSize`, which `f437495` had restored as an explicit
`Span::ortho(OrthoSize::{Children, Full})`. It was the rule beside a widget
written a second time: a span across itself is as long as its longest child
*unless a rule already says how long it is*, and `Painter::ruled(axis)` is how
it asks which case it is in. Under a rule it does not read its children across
that axis at all -- the answer is not wanted, and reading one is what would
make its size depend on theirs. Not under one, the largest fixed orthogonal
length is reported and any relative or `leftover` child makes it report
`leftover`, which is the old conservative behavior. Either way the children's
`Holds` ranges propagate through final-box drawing, so a resize repositions
them without redrawing when their own contracts permit it.

`a8898aa` gives a length with no share in it its own type again, which is the
separation canonical `main` had and this branch had collapsed: `Len` is pixels
plus a fraction of a box -- a position being the length from the box's start,
which is why a span is two of them -- and `LayoutLen` is a `Len` plus a claim
only a container dividing its room can answer. What was `UiScalar` is `Len`,
what was `Len` is `LayoutLen`, and `From<Len> for LayoutLen` is the one-way
step. `490918b` renames `Painter::ruled` to `has_exact_size`, over
`SizeRule::exact`: "is there a rule here" and "is my report moot" are the same
question only while `Exact` is the only rule, and `Min`/`Max`/`Clamp` are
queued.

`71c9c39` replaces the public `Painter::place` distinction with an opt-in
widget property. `.region_node()` gives a widget one independently movable
retained region; `Widgets::set_region_node` can change that choice at runtime
and causes one structural redraw. Widgets without the property remain at the
default shallow chain depth: Iris recursively remaps their retained primitive
and mask regions when they move. `.scrollable()` enables a region node on its
content once as its convenient default; raw `Scroll::new` respects the
caller's choice, and the property can be disabled later without breaking
scrolling. `Span` and `Align` do not add nodes to their children.

Do not make a span choose its orthogonal size by comparing children in pixels
at its current width. A fixed child and a relative child can create multiple
self-sizing fixed points; generated seed 13 settled differently warm and cold
under that attempted implementation. A `Holds` interval says where an already
chosen answer stays valid, but cannot make that circular choice unique. The
same circularity is what a `leftover` cap would put into `SizeRule::Max` --
see the clamp item under "Next".

The implementation also fixes three counterexamples found while finishing the
rewrite:

- An asked-but-undrawn size dependency must name the widget that asked as its
  parent. Using the asker's parent skipped a reader and made generated seed 10
  settle differently warm and cold.
- `Scroll` must return the answer from the first box it asked about, whether
  that answer came from a retained length or a fresh measurement. Returning
  the final placed answer only on the retained path advanced one fixed-point
  iteration and broke seed 86.
- A widget retains the layer it was entered on, not the last child layer its
  painter visited. The old value drifted on local redraw and put a redrawn tab
  background above its retained text.
- `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. `5ed9e87` moved that boundary rather
  than softening it, and `39e4ca2` removed the move: on the grid the box and
  the sum are the same count. See "Fixed point" 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 `394d514`

`4febabf` on top of it -- `Fixed` wrapping rather than saturating -- passed
the workspace tests with all features (105), the release 100-seed oracle in
9.5 s, and rendered `random`, `tabs` and `text` byte-identical at 1920x1200
against `394d514`. The measurements are under "Performance".

- `cargo fmt --all --check`
- `cargo build --workspace --all-features`
- `cargo clippy --workspace --all-targets --all-features -- -D warnings`
- `cargo test --workspace --all-features`: 105 passed, 10 ignored
- The release generated cold-layout oracle passed 100 seeds in 9.4 s, and a
  shrinker case at 300 seeds in 3.5 s: both run a thread per core but one.
- The release shrinker passed **all five** cases -- `resize`, `repaint`,
  `resize-repaint`, `reorder`, `size-change` -- at 300 seeds of depth 5:
  26,001 widgets per case, largest tree 331. It passed 1000 seeds of depth 6
  (159,024 widgets per case, largest tree 587) at `5ed9e87`, before fixed
  point; re-run that before quoting it again.
- `tabs`, `view`, `minimal`, `text` and `random` render byte-identical at
  1920x1200 across the whole fixed-point sequence, and across this session's
  renames and arithmetic changes on top of it.
- `tests/cases/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
  `1d397c57b9914a2e596fa907029bc6aab4629e4d74deb0715e81325c703bcdb3`.
  The run used the Venus adapter backed by the host RX 7900 XT.
- `minimal`, `text` and `view` render byte-identical at 1920x1200 across
  `8220a78`. `tabs` differs only in the widget count it prints about itself,
  which is two wrapper types smaller -- so it is no longer a byte-identical
  reference and the generated oracle is the check that matters.
- At preceding head `29c7881`, reference renders against
  `/home/bob/repos/iris-main-cmp` at `ca2b4b2` covered:
  `tabs`, `view`, `minimal`, and `text` at 1920x1200; `tabs` cold at
  900x1200; live resize from 1920x1200 to 900x1200; and the tab interaction
  before and after replay. Every comparison had zero differing pixels. The
  live-resize image is also byte-identical to the cold 900x1200 image.

The built-in-alignment pre-submit review was run in four passes. It caught
three distinctions the smaller tests had missed: an answer's validity must
include the drawing made in its final placed box; a region-node box is resolved
through its parent move rather than through its own move twice; and boxes with
equal dimensions but different positions still need the parent that placed
them. The 100-seed oracle and both 300-seed shrink cases pass after those
fixes. The earlier region-node review
caught an index-reuse hazard when removing a node; its move entry now remains
alive until every descendant has migrated. The generated oracle then exposed
the exact `Span` threshold described above. The earlier retained-layout
review caught the layer defect above, corrected validity-range inversion for
negative relative extents, and removed an impossible-state `unwrap` from
`Scroll`. The orthogonal-sizing review caught both the circular longest-child
choice and the incompatible visual effect of making `Full` the default.

### Performance

**The 3x this section used to report was a fixture artifact, withdrawn
2026-09-16.** `random.rs`'s `Branch` picks which of two subtrees to draw by
comparing a measured pixel length with a threshold, so the fixture's shape
moves with the thing being measured. Seed 1 at depth 8 draws 88 widgets and
writes 2,298 primitives a frame at `5ed9e87`, and 115 and 8,209 at `bd6de71`
-- three and a half times the work behind a number read as three and a half
times the cost. `394d514` gives the rig `Edits::fixed_branches`; the oracle
keeps measured branches, which is the whole point of them.

**Check the work counters before comparing two commits' times.** The rig
prints drawn widgets, widget draws and primitive writes. An undrawn
`leftover` child still moves them and no flag removes that, so the trees are
near rather than equal -- and `9d8415d` changed the generator itself, since
an X span's `OrthoSize::Full` became a size rule.

Measured on the fixed-shape fixture, seed 1, depth 8, 500 frames of `many`:

| | drawn widgets | primitive writes | instructions | cycles |
| --- | ---: | ---: | ---: | ---: |
| `5ed9e87`, before fixed point | 100 | 4,272 | 1,763M | 693M |
| `4cbb242` | 97 | 3,951 | 2,093M | ~819M |
| `4febabf`, wrapping | 97 | 3,951 | 1,912M | 771M |

The two ends were re-measured 2026-09-16 as medians of nine runs of two
freshly built binaries and are the numbers above; `4cbb242`'s row is still
the single reading it was taken from. Raw, the head is 1.085x the float head
on instructions and 1.113x on cycles. Normalised by primitive writes it is
1.17x and 1.20x; per drawn widget 1.12x. **Take none of those for the grid's
cost.** The two heads do different work rather than the same work at
different speeds -- `4febabf` remaps 53 subtrees a frame and redraws 27
where `5ed9e87` remaps 38 and redraws 43 -- and dividing by primitive writes
does not correct for a different mix of remapping and redrawing.

**The fused multiply-add this section used to name does not exist,
withdrawn 2026-09-16.** `5ed9e87`'s binary holds no FMA instruction at all,
and none appears when it is rebuilt with `-C target-feature=+fma,+avx2`
either. Rust does not contract `a + b * c`, so `LerpUtil::lerp`'s
`from + (to - from) * self` is `subss`, `mulss`, `addss` on every target
there is. The float side never had the single instruction the grid's
multiply was being compared against, here or anywhere.

**Nor is the multiply the cost.** Each row below is `4febabf` with one
thing taken out, in one worktree so the rows compare to each other, 500
frames of `many` at seed 1 depth 8, medians of nine to eleven runs. **Every
row leaves the work counters where they were**, so unlike the cross-commit
numbers above these compare one tree against itself: all twenty-seven
counters diffed identical for the all-three row that the conclusion rests
on, and six of them checked for the others.

| | instructions | cycles |
| --- | ---: | ---: |
| `4febabf` | 1,921M | 772M |
| `shift_round` out of `Fixed::mul`, truncating instead | 1,878M | 774M |
| `is_full` short-circuits out of `UiSpan::within` | 1,910M | 765M |
| zero short-circuit out of `Fixed::scaled` | 1,914M | 768M |
| all three | **1,803M** | **715M** |
| branchless `shift_round`, both short-circuits out | 1,964M | 773M |

Taking the rounding out alone removes 43M instructions and moves cycles by
+2M, which is inside the noise: the grid's arithmetic is instructions the
machine has spare issue width for, not time.

**IPC has reversed since this section recorded it**, and that is the same
finding from the other side. It was 2.58 before fixed point and 2.59 after,
which is what "the cost is instruction count at unchanged throughput" was
read from. On the re-measured medians it is **2.54 before and 2.48 after**:
the head both executes more instructions and retires them more slowly, so
there is something to find beyond counting them. The all-three build
recovers half of it against its own worktree's baseline, 2.49 to 2.52.

**What moves cycles is whether `UiSpan::within` inlines, and the three cuts
are superadditive because of it.** Separately they are worth 61M
instructions and 8M cycles between them; together, 118M and 57M -- four
fifths of the instruction gap and two thirds to three quarters of the cycle
gap, which is the looser number. The mechanism is visible in `nm`:
`<UiSpan>::within` is a symbol in `4febabf` and in neither `5ed9e87` nor the
all-three build. Shortening its body past the inliner's threshold is what
pays, and only the whole set of cuts reaches it. None of them is shippable
-- truncating breaks the one rounding the grid is for -- but that is where
to push, and a cheaper `Holds` or a smaller `Len::within` is worth trying
for this reason rather than for its own instruction count.

**Cycle noise is larger than this section used to claim.** Across ten sets
of nine to eleven runs, each of one unchanged binary, cycles spread 1-3%
usually and 6.7% in the worst set -- not the 0.23% recorded from three
runs, which was a lucky set read as the machine's precision. Worse,
`instructions:u` in this VM intermittently returns a garbage value
-- 11.0e9, 25.8e9 and 28.4e9 against a true 1.9e9, in roughly a quarter of
readings. Take medians of nine or more, drop anything off by a factor, and
do not trust a single `perf stat`. `ex_div_busy` was the one counter that
held to 0.1% across runs.

**Instruction counts hold to 0.02% within a binary and move 0.5% across a
rebuild of the same source**, which is why the two tables above disagree
about `4febabf` by 9M: their baselines were built in different worktrees.
The *difference* against `5ed9e87` was 149M in both builds. So compare
against a baseline built beside the thing being measured, quote a delta
rather than an absolute, and re-measure the baseline whenever anything is
rebuilt.

**"The division is 0.00% of cycles" was read off the wrong function.** It
is true of `AxisRemap::apply_scalar`, where most moves are translations and
the multiply is 1.5%. It is not true of the run: `Holds::through` divides
twice on every call and accounts for essentially all of the 2.23M `i64`
divisions a 500-frame `many` executes, which hold the integer divider busy
**21.30M cycles** -- the steadiest counter on this machine, 0.1% across
runs, and 2.8% of the run. A controlled extra `div_toward` in `through`,
against a de-const control so the inlining does not move, costs 6.3 cycles
each, so the two real ones are about 14M. `5ed9e87` divides twice there too,
in floats, so this is a real cost but a smaller *difference* than the
source-line profile makes it look, where `div_toward`'s `num / den` is the
largest single grid line in the head at 106M of 5,361M over 4,000 frames,
against `shift_round`'s 71M.

Recovered this session, each measured before and after on one commit with
byte-identical renders proving the layout unchanged -- on the old fixture,
689.6M cycles and 1,886M instructions down to 638.9M and 1,657M:

- `cb1bba4` works a move out once per subtree rather than per scalar: -3.8%
  instructions, -0.8% cycles. The gap between those two is the finding.
- `1940e85` translates a whole region at once, since that is what a
  translation is: -4% instructions, -3.4% cycles.
- `d75a1e2` and `4cbb242` skip multiplies that cannot change anything --
  composing through a full box, and `lerp` where the ends are equal:
  together -2.7% instructions, -1.8% cycles when they landed. **That no
  longer reproduces at `4febabf`**: removing either one now *saves* a
  little, and removing both is part of the 118M/57M above. Wrapping changed
  what a multiply costs, and a guard priced against a saturating multiply is
  priced against something that is gone. Re-price a short-circuit before
  keeping it.

Tried and rejected, with numbers, so they are not tried again:

- A float reciprocal for the division: **+6% cycles**. The conversions cost
  more than the `idiv` did there. That was `apply_scalar`'s division; the
  one in `Holds::through` is a different question and has not been tried.
- Branchless `shift_round`: **+6.7% cycles** when tried alone, and
  re-tested 2026-09-16 with both short-circuits also removed, where the
  branchy form reaches 1,803M/715M: 1,964M instructions and 773M cycles,
  worse than the head on both. It is the *size* of the rounding that keeps
  `within` out of line, not its branch, so making it branchless makes it
  bigger and loses twice.
- Removing the per-child hash lookup in `remap_subtree` with `mem::take`:
  **0.0%**. `FxHashMap` on a hot line is not the cost.
- Short-circuiting `apply_scalar` where the fraction is nought or one: +17%.
  (From the previous session.)

- `#[inline]` on `UiSpan::within`, whose prologue and epilogue were a sixth
  of its own cycles: -0.2% instructions, **+1.5% cycles**. That is not in
  tension with the inlining finding above: forcing the body it has now into
  every caller loses, and shrinking the body until the inliner takes it of
  its own accord wins. Shrink it; do not annotate it.

**`4febabf` makes `Fixed` wrap rather than saturate** (Bryan, 2026-09-16: a
coordinate past the range is not going to draw reasonably anyway, so wrap
and break clearly): -8.6% instructions, -6.6% cycles, more on cycles than
the 2.7% the instruction share predicted. `from_f32` still clamps and
`Holds` keeps its saturating `narrow`, since a range of box lengths past
`i32` really is unbounded.

**One posture question is still open, with its number.** `shift_round`
rounds halves away from zero so a value and its negation round to the same
distance, which is a sign branch on every multiply. Rounding halves up
instead measured **-1.6% instructions, -2% cycles** (1,888M, ~757M), and
would let a flipped span land a step from its mirror image at exact halves.
Not applied; Bryan's call.

Earlier, against #18's own history: the retained rewrite took `many` from
25.17M instructions a frame at `691e3eb` to 6.14M at `29c7881`, and `resize`
from 16.08M to 8.39M. That fixture has since changed twice; do not compare
across it.

## Retained-layout invariants

- **A retained drawing belongs to the layer it was made on.** Asked for again
  on another layer it is redrawn, since nothing about its geometry says it is
  in a list that paints at a different moment. A container that measures a
  child by drawing it therefore measures on the layer that child will draw on
  -- `Painter::child_layer_at` addresses one -- or it pays two draws a frame
  forever and keeps whichever the second ask left.
- A widget that clips its contents to its box reports its box: `Scroll` and
  `Masked` both report `LEFTOVER`, and a `debug_assert` holds any widget that
  **set a mask this draw** to it. Overflowing is otherwise ordinary and a text
  too tall for its box says so -- which is why the assertion is narrowed to
  mask-setters rather than to every widget.
  `Masked` is the only one that sets a mask; `Scroll` reports its box because
  a drawing placed in a longer box is *scaled* by recomposition, not because
  it masks. So a `LazySpan` is unaffected: it clips by not drawing, and may
  report its whole content length, which is what the `Scroll` above it
  measures. One that set its own mask would have to report its box and hand
  the content length up another way.
- A span is as long across itself as its longest child, unless a rule beside
  it says how long it is -- and then it does not read its children there at
  all, since the answer is not wanted and reading one is what makes its size
  depend on it. `OrthoSize` was that second case written twice and is gone
  (`9d8415d`); `Painter::ruled` is how a container asks which it is in, and
  the only thing a widget may learn about a rule over it.

- A region node holds a whole `UiRegion` in its parent node's coordinates.
  `UiRegion::FULL` is the identity. Widgets opt in with `.region_node()` or
  `Widgets::set_region_node`; ordinary widgets share the nearest ancestor
  node. Region nodes therefore add chain depth only where moving a whole
  subtree through one entry is useful.
- A widget's `ActiveData::region` is its box in its parent node. A region-node
  widget draws in `FULL`; its box lives in its node. Moving an ordinary
  retained subtree instead remaps its primitive, mask, and active regions.
  Remapping stops at a descendant region node after rewriting that one entry.
- Changing `region_node` redraws the subtree once to rebuild the coordinate
  boundary. The property belongs to widget identity, which is safe because a
  widget has one parent. `.scrollable()` sets it once; raw `Scroll::new` does
  not, and `Scroll` never reasserts it while drawing.
- The first box a parent asks about is the offer. A later box chosen from the
  child's answer is the final box, not another independent answer. Dirty
  widgets are re-asked at the offer and only then drawn in the final box. An
  offer composes through its ancestors' offers, not through their current
  placed boxes.
- An answer is reusable only where both its measurement and the drawing made
  in its final placed box remain valid. The final drawing's `Holds` interval
  is translated back into lengths of the offered box and intersected with the
  answer's interval.
- Equal box lengths do not imply equal placement. An ordinary widget whose
  offered and current boxes differ in position must involve its parent again;
  a region node can settle itself only when its own alignment, rather than a
  container override, determines the final box.
- A retained drawing can be reused only when its `Holds` interval contains the
  new pixel box on both axes, its parent node is unchanged, its region-node
  choice matches the retained structure, and the widget is clean. A valid
  ordinary subtree may move without redrawing because its regions are
  recursively remapped.
- `Painter` records size-dependency edges only when a parent reads a child's
  size or hint. An undrawn measured child remains recorded so a later change
  reaches the parent that decided whether to draw it.
- Dirty widgets settle deepest-first. `dirty_size_under` prevents a reader
  from taking a retained answer while something below that answer is still
  dirty; the walk is an optimization against laying out twice, not a second
  validity mechanism.
- Declared non-`leftover` lengths are resolved by the widget's parent where the
  widget is drawn. A declared-length change therefore redraws the parent.
- A pixel comparison is equality: lengths are whole counts of `1/1024` px,
  so a change too small to reach the next step is not a change and one that
  reaches it is, however little of a pixel it is worth.
- 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. The room to
  divide is `len * fixed - total.px`; pure `leftover` children are undrawn
  where there is none. The box a parent hands back and the sum of what the
  children asked for are counts of the same step, so the boundary needs no
  margin and the validity interval is split exactly at it.
- `Scroll` reports `Size::LEFTOVER`; what it takes from the first box it asked
  about is its *content's* length, which it clamps and holds by rather than
  reports. Its drawing can survive container-length changes only over the
  interval in which clamping and its current offset do not change.
- **A move that keeps a box's length is a translation, and an offset is
  exact on the grid.** A box that also changed length has to re-express each
  part as a fraction of the new one, and that division and multiplication
  round: `39e4ca2` translates where `from.len() == to.len()` and scales only
  where it must, which is what made the shrinker's `resize` case agree
  exactly. This inverts the float-era rule, and the measurements behind that
  rule are why `tests/cases/drift.rs` exists: in floats, offsetting both ends
  of a span shortened that fixture's row by 0.071 px over 20,000 moves and
  0.712 over 200,000, while re-expressing fractions stayed exact. 20,000
  moves is five minutes of scrolling at 60Hz. On the grid the drift is gone
  either way, and `tests/cases/drift.rs` pins that it stays gone.

## Built-in alignment

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
fuzzer covers size rules, alignment and region nodes and changes all three at
runtime. Region nodes had **no generated coverage at all** before that.

### Alignment is two fractions, not four directions

Decided 2026-09-15 (Bryan). A widget's alignment is one `f32` per axis, so a
quarter of the way along an axis is expressible. `AxisAlign`'s three familiar
positions are named constants over that number, which is what every expression
already uses: the layout math only ever reads `AxisAlign::rel()`, so nothing
downstream changes shape.

The default is **the middle on both axes**, because the two edges are the ones
that assume a direction -- which edge is the near one depends on the writing
system and on which way a container runs. "Near edge" throughout this document
means the *start of the box in the box's own orientation*, which for a
reversed span is its visually far end, not "top left".

### Placement cannot be applied after the fact

**Do not re-attempt "draw the widget, then move its drawing to where its
alignment says".** Three attempts failed, and the reason is structural: the
move is a change of coordinate frame, and no consistent split of the stored
state carries it.

- Move `ActiveData::region` with the drawing, and a later local redraw asks a
  differently rounded question. Measured: `(0,0)..(0,305.936)` re-expressed as
  `(0.5,-81.5)..(0.5,224.436)` reads its length back as `305.93604`, which
  crosses `Span`'s leftover/no-leftover boundary -- the one that must be exact
  -- and draws a child a cold layout leaves undrawn.
- Leave `region` alone, and `placed` accumulates without bound, because
  `try_reuse` returns a clean subtree's size **without walking into it**: only
  the top widget's `placed` is recomputed while an ancestor's shift carries the
  whole subtree. Measured 7,048,813 where a cold layout says 456.

The replacement applies alignment in the two places that
already exist and are exact. Where the size is known before drawing, from a
rule, `declared_box` hands the child its aligned box directly -- one draw, no
move. Where the size is only known after drawing, the widget is **re-asked in
its placed box**, with its alignment forced to the near edge on the second ask
so it terminates; that ask goes through `try_reuse`, which moves by
recomposing, which `tests/cases/drift.rs` pins as exact. That deletes
`ActiveData::placed` and `shift_subtree`. The cost is a second ask for a
measured widget that is not near-aligned, which is exactly what `Aligned` cost
before this work.

### Placement compounds, which is what the align override is for

A container that reports a child's size while handing that child a **bigger
box** gets its content placed twice: once by the child, once by the box around
it. Found three times before the class was fixed rather than the instances --
`Stack::size(Child(i))`, `Scroll`'s orthogonal axis, and `Pad`.

`Painter::widget_aligned(child, region, align)` is the override: an
`Option<RegionAlign>` carried in `DrawInfo` and resolved once in `draw_inner`,
so the root resolves like anything else. `Pad`, `Stack` and `Scroll` pass the
near edge for children whose size they report. `ActiveData` keeps both the
resolved alignment, so a local redraw asks the question its parent asked, and
the widget's own, which is what a change is compared against -- an override
means the answer is the parent's to give again.

### A widget occupies its box, and must not report more than it draws

`Scroll` reports `Size::LEFTOVER` on **both** axes: it clips its content to its
box, so it can neither take less of one nor honestly ask for more. The
content's length is what it scrolls through, not what it is. Reporting the
content length instead made the framework place a 400-long drawing in a
200-long box, and placement by recomposition **scales** in that case, because a
part stored at fraction 2 of its box stays at fraction 2 of a box twice as
long. Reporting the content's *cross* length had the box around it place
content already placed.

Where content shorter than the viewport sits is now `Scroll`'s own alignment.
Its `Holds` widening -- "content of a fixed length that fits sits at the start
of any box it fits in" -- is therefore gated on near alignment: anchored
anywhere else it is a part of the room left over, so it moves with every
length the box takes and the drawing holds for that length alone.

`Stack` gives every child the box its sizing child defines, through the new
`Painter::box_of`, for the same reason.

`95fb4f9` adds the `debug_assert` this asked for, and narrows it to what is
actually true: a widget that **set a mask** this draw must report inside the
box it drew in. As "a reported size does not exceed its box" it fires on
ordinary overflow instead -- measured, a hundred fuzzer trees produce
thousands, every one a text too tall for the box it was offered, which is
what a text is meant to say. The same commit fixed `Masked`, which was
passing its inner's size up; `tests/cases/scroll.rs` has a clipping widget
that reports its content, so the assertion is itself covered.

## Fixed point

Layout decides on a grid rather than in floats, in four commits: `7548139`
the number, `4e28f10` positions, `bd6de71` lengths, `39e4ca2` the last of the
pixels and `Holds`. Decided with Bryan on 2026-09-15.

- **`Fixed<SHIFT>` is an `i32` counting `1 / 2^SHIFT`.** Adding and
  subtracting are exact; a multiply or a conversion rounds once, back onto
  the same steps. Two routes to one place that come within half a step land
  on the same number, 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, and the shader's copy is prepended from them by
  `render::module_source` rather than written again in WGSL.
- A weight is not a fraction: a list divides its room by the total of its
  weights, so `Weight` trades precision for the range to hold a whole list,
  and `Rel::ratio` turns two weights into a share on the finer grid.
- **Arithmetic wraps, as of `4febabf`.** It saturated first, so that a
  clamped coordinate kept the ordering a wrapped one inverts, and that cost
  a twelfth of layout's instructions to keep order two million pixels out
  where nothing draws. `MIN` and `MAX` still stand in for an unbounded end,
  which is safe only because every use compares against them and none adds
  to them; `from_f32` is the one operation that clamps.
- `Px` was `1/64` first. The residue of a length reached two ways is one
  rounding, so it scales with the step: at `1/64` that was 0.016 px, enough
  to move a box, and at `1/1024` it is a thousandth of a pixel. Range is
  +/-2.1M px and conversion to `f32` is exact to 16,384 px.
- **What the fuzzers ask for is a step per level of nesting**, which is two
  for these trees. Traced on 2026-09-16 to the same box reached two ways,
  each rounding where the other does not -- not accumulation, and not one
  place. Two of them are fixed in `bdab558`:
  - `Scroll` wrote a box it had been given back out as its own length in
    pixels. Centring a part in `rel 1` lands a step from centring it in
    `px 900`, because `a(x - y)` and `ax - ay` do not round alike. Content
    that fills the viewport unscrolled is handed back as it came, and the
    `repaint` and `resize-repaint` cases became exact.
  - `Span` placed each child a step from where the last ended, carrying every
    share's rounding along the row. A position is now the fixed parts before
    it, exact, plus one rounded share. Two hundred equal shares of a 1000 px
    row ended at 999.999 and now end at 1000.
  What is left is a box centred in a fraction of its parent against the same
  box centred in its own pixels, one step per level between them. Closing it
  means alignment resolved in pixels everywhere -- which costs the retained
  resize path, since it is the fractional form that re-centres a subtree
  without redrawing it. Not worth it at a thousandth of a pixel.
- `Holds::through` inverts `px + rel * box`, which rounds, so the answer is
  an interval even for a single length. It maps the half step either side,
  plus one more for the two ways above; inverting the length alone gives a
  point that need not contain the box the part was drawn in.
- A pointer, a wheel notch, a shaped glyph advance and a window size arrive
  as floats and are put on the grid where they arrive. `Vec2` stays what the
  GPU and the platform speak; `PxVec2` is what layout decides in.

## 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 was 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. **The move and the constant are gone
since `39e4ca2`**: on the grid the box a parent hands back and the sum of what
the children asked for are whole counts of the same step, and both routes land
on the same count, so the boundary needs no margin. What the section still
records is why a decision may not be taken on a hair's breadth, and the
regression that pins it. `tests/cases/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:

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

`cb955f1` put the ordinary tests in `tests/cases/`, as modules of one
`tests/suite.rs` target -- eleven links became one, and with
`profile.test`'s `debug = "line-tables-only"` a rebuild of `iris`'s test
targets went from 14.3 s to 7.7 s and `target/` from 45 GB to 13 GB. Pick a
module out with `cargo test --test suite layout::`. The fuzzers and the
`*_cost` measurements are still their own targets.

The fuzzers take a thread per core but one (`9d8415d`), since a seed grows,
lays out and drops its tree alone: the oracle's hundred seeds went from 68 s
to 9.4 s and a shrinker case at 300 seeds from 18 s to 3.5 s. A failing seed
still shrinks and panics on its own thread.

Run the long generated oracle only after ordinary tests pass:

```sh
cargo test --release --test generated -- --ignored a_long_run_of_seeds_agrees
```

`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 failures. `tests/shrink.rs` reduces a failing
tree; use it to turn a seed into a readable regression rather than leaving the
seed as the only record.

**The float head is checked out at `/home/bob/repos/iris-float-cmp`**, at
`5ed9e87` with `394d514`'s `Edits::fixed_branches` cherry-applied to
`src/random.rs` and `tests/layout_diagnostics.rs` but not committed, since
the flag is the only way to measure the two on one tree. Build it and
`iris-pr18` with `--test layout_diagnostics` and run the binaries directly
rather than through cargo. A comparison is only worth reading when the
`--features layout-diagnostics` counters match on both sides; check
`primitive writes`, `widget draws`, `reuse remapped` and `placed by
redrawing` before any timing.

The headless reference set must be run one process at a time because the rig
reuses one compositor. Comparison worktrees need separate target directories.
Useful commands:

```sh
./scripts/run-headless.sh tabs --mode 1920x1200@60Hz --shot /tmp/tabs.png
./scripts/run-headless.sh tabs --mode 900x1200@60Hz --shot /tmp/cold.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 final check was:

```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
```

`tests/layout_diagnostics.rs` is the retained CPU rig. Select `cold`, `many`,
`repaint`, `size`, `scroll`, or `resize` with `IRIS_PHASE`; use the feature for
explanatory counters and an uninstrumented release binary under `perf` for
instruction totals.

## Next

The next small LAYOUT.md §2 item is `LazySpan`. Region nodes now cover the
independently movable-subtree use case; do not restore a separate
child-placement API. `2d86058` brought `docs/LAYOUT.md` §2 and §3 to what
shipped; **§4, §5 and the density section are still stale** -- they name
`Painter::place`, `SetSize`, `desired_width`, `apply_rest`, `Len::dp`,
`Aligned` and `MaxSize`, none of which exist.

**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 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:

- `Scroll` should take a direction rather than one axis: vertical, horizontal,
  or both. Reporting `LEFTOVER` on both axes is already the right shape for it.
- Restore `max_width`/`max_height` as `SizeRule::{Min, Max, Clamp}`. `8220a78`
  deleted `MaxSize` **and its builders**, so that is public API owed back. A
  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: its `Holds` range must be exact and split at the crossover, the
  way generated seed 16 taught for `Span`. On the grid the crossover no
  longer needs moving off where boxes land -- see "Fixed point" -- but it
  does need both sides of the comparison to be `Px`.

  **A cap may not contain `leftover`** (found 2026-09-16, answering whether
  the parent could resolve one). `Exact` works with a share in it because it
  makes the widget's report moot -- `ruled()` is true, nothing reads the
  report, and the parent divides a weight that does not depend on what it
  divides. A cap must read the report, so rule and report are in one
  equation, and a share puts the division into it too: a child capped at one
  share contributes its drawn pixels to the row's total while it fits and a
  weight once it does not, which moves the room, which moves the share. The
  two assignments are each self-consistent, which is the multiple-fixed-point
  failure that generated seed 13 punished for orthogonal sizing -- and
  resolving it the way flexbox does, by freezing violated children and
  dividing again, costs an ask per round where the retained contract allows
  one offer per child. `min(report, cap)` is also not a `Len`: `Len` is a sum
  of the three parts, and the smaller of two of them is not.

  So a cap takes pixels and a fraction and no share, which is what `Len` is
  since `a8898aa`. Still awaiting Bryan: whether a `Max` narrows the box the
  child draws in, or only what the parent reports for it.

Other queued work, in dependency order:

- `UiRenderState` behind `Rc<RefCell<_>>`.
- Split `Len`/layout length and add density-independent pixels.
- Input restructuring: pointer capture, drag slop and axis, cancellation,
  mask-aware hit testing, and 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.