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 2026-09-18: the frame/extent prototype's chronicle went, its
four surviving findings and two failed hypotheses stayed, and the
transparent-frames plan became a record of what it landed as.

## 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
**`e44dea3`**, 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.

The separate frame/extent experiment lives in
`/home/bob/repos/iris-layout-experiment`. **The transparent-frames protocol
is implemented** on `wip/transparent-frames`, head **`0954770`**, two
commits over `34cafb6` (`wip/region-and-placement`, the experiment as it
was). The app's framework pin is unchanged, and **none of this is ready to
replace #18**: two fuzzer seeds still settle differently warm than cold, and
`many` and `resize` cost more than #18 on a deep tree. What it does, what it
cost and what it found are in **Transparent frames: what landed** below;
read that before anything else here about `Span`, `Pad`, `Stack`, the
placement pin or `measure_len`, all of which it supersedes.

## Transparent frames: what landed (2026-09-18)

Decided with Bryan on 2026-09-18 and implemented the same day. His rules are
kept below because they are the design; the protocol section says what the
code now does, and **What it cost** and **What is not done, and why** say
where it stands. The step list the plan carried is gone: steps 1 to 5 are in
`1956be3`, the measurements that follow are in `0954770`, and steps 6 to 9
are the open questions at the end.

### Decided by Bryan

- **Containers are transparent.** The frame a widget's fractions are of is
  forwarded from its parent through a span, a stack and a scroll unchanged,
  and through an inset narrowed by its margins. Any number of nested spans
  lay out against one frame. The reason: `px` already passes through, `rel`
  should behave the same way, and `leftover` is already the way to say
  "fill the containing widget", so `rel(1.0)` meaning that too would be two
  ways to say one thing.
- A frame is narrowed only by what is decided from above: a declared length
  on the widget (`.width(rel(0.5))` on a span narrows its children's frame
  too), an inset's margins, the root. A box that is an *answer* (a row's
  height, a stack sized by a child) is never anything's frame.
- `rel` overflows on purpose when it sums past one or has pixels beside it.
- `Inset` and `Outset` take `px`, `rel` and `leftover` margins. Outset adds
  its margins to the child's report and moves the child in; Inset draws the
  child in a frame with the margins subtracted and reports the child's size
  plus what it subtracted. An inset resolves what it subtracts, so `px` and
  `rel` margins narrow the frame symbolically and a `leftover` margin is
  resolved in pixels from the extent after the child answers, like a span's
  shares. A general `Pad` would outset pixels and inset `rel` and
  `leftover`. **Do not build these yet**; make them a few lines each to
  write.
- Along its own axis a span places a child whose length is known in pixels
  at its final slot while measuring, so it is drawn once, and moves a child
  that is in the wrong place, `px` or `rel`, by translation rather than
  drawing it again. Across itself it may still re-place by the answer.
- Layers may later be tied to another widget (a popup near an anchor). A
  position is never defined by two widgets; positions compose up the tree.

### The protocol

Two boxes per widget, both in the *parent's frame coordinates*:

- **frame** -- what a declared or reported fraction is a fraction of.
  `UiRegion::FULL` for a transparent parent; narrowed by a declared length,
  and that is the only narrowing in the code today. Its *length* is the same
  on every ask of the widget, which is the property the whole retained model
  rests on.
- **extent** -- where the drawing goes, as a part of the parent's own box.

```rust
/// What of a widget's own box a child is given, along one axis.
pub enum Part {
    /// The whole of it.
    All,
    /// Frame lengths from where the box starts, which is what a container
    /// dividing room among its children speaks: a child's report is a length
    /// of the frame, so the cursor that sums those reports is one too. A
    /// moved box re-places every child by re-adding its start, exactly.
    From(UiSpan),
    /// A part of the box in its own coordinates, which is what a container
    /// that insets one speaks: taking eleven pixels off the end needs no
    /// length, where saying the same thing in frame lengths would make the
    /// container read its own box -- and a box chosen from its own answer
    /// then feeds back into the answer.
    Of(UiSpan),
}

/// Where a child goes along one axis, as a part of this widget's box.
pub enum Place {
    /// The child's answer, aligned inside the part by the child's alignment.
    Within(Part),
    /// Exactly the part; the answer is not placed inside it again.
    Fill(Part),
}
```

`Part::Of` is not in the plan; **it is what the measurements asked for**, and
the reasoning is in its doc comment above. `Part::From` is the plan's
extent-relative span.

The widget's `draw` sees only its own box: `px_len`/`px_size` are that box's
pixels and narrow the extent range, `holds` widens it. Primitives and masks
are written in its coordinates; there is no `DrawRegion` and no
frame-coordinate primitive. A container reads `extent_len(axis)`, the box's
*symbolic* length along one axis in frame units, which pins its drawing to
that length and to nothing about the start -- one axis at a time, because a
span dividing one of them holds for any length of the other. A span that
divides room also reads `frame_px_len(axis)` (was `region_px_len`) and widens
through `frame_holds` (was `region_holds`). `placement()`, `region()`,
`box_of`, `measure_len`, `widget_within`, `DrawRegion` and `ExtentPlacement`
are gone.

The parent side is one call, with a shorthand:

```rust
pub fn widget_at<'s, W: ?Sized>(
    &'s mut self,
    id: &'s StrongWidget<W>,
    frame: UiRegion,     // in this widget's frame coordinates; FULL forwards it
    place: [Place; 2],
) -> DrawResult<'s, 'a, W>;

/// The transparent default: the frame as given, the answer aligned in the box.
pub fn widget<'s, W: ?Sized>(&'s mut self, id: &'s StrongWidget<W>) -> DrawResult<'s, 'a, W> {
    self.widget_at(id, UiRegion::FULL, [Place::Within(Part::All); 2])
}
```

`Span` is the plan's, without the known-length shortcut (step 7, not done):
it reads `far = painter.extent_len(axis)`, measures each child at
`Place::Within(Part::From(along(cursor, far)))`, and places it at
`Place::Fill(Part::From(along(from, start)))`, with `Place::Within(Part::All)`
across itself. `Stack` gives its sizing child `Fill(All)` and the rest
`Within(All)`; `Scroll` measures at `Fill(All)` and places at
`Fill(From(px content box))`; `Pad` is transparent and insets by
`Within(Of(..))`; `Masked` sets its mask over its own box.

A report is still `LayoutLen { px, rel, leftover }` per axis, a fraction of
the reporting widget's frame, so it passes a transparent parent unchanged and
is composed by `within_len(frame.len())` through a narrowing one. **A stack
sized by a child that reports `rel(0.5)` therefore reports `rel(0.5)` and the
fraction is applied once** -- the parked `wip/stack-fraction-twice` defect is
closed by the protocol rather than by a guard, and `placed_extent` is where:
it takes the answer's length *from* the part rather than composing it into
the part.

### Retained state and reuse

Per widget (`ActiveData`), replacing `region`/`placement`/`given_region`/
`offer_len`/`offer_placement`:

- `frame: UiRegion` in the parent's frame coordinates and `frame_abs` in
  `parent_move`'s; `extent: UiRegion` in the frame's own coordinates, which
  `window_region` and recomposition compose as `extent.within(&frame_abs)`.
  There is no `offer_frame`: the frame's length is the same on every ask.
- `place: [Place; 2]` as last given, `offer_place` from the ask its answer
  came from, and `offer_part: UiRegion` -- the box that ask gave it. The box
  is kept and not worked out again from where the parent's own box is now: a
  parent drawn again in the box its answer chose gives its children boxes it
  never measured anything in.
- `answer: Option<(Size, LayoutHolds)>` from that ask; `holds: LayoutHolds`
  for the drawing, where

```rust
pub struct LayoutHolds {
    pub frame: [Holds; 2],            // frame pixel lengths
    pub extent: [Holds; 2],           // pixel lengths of the widget's own box
    pub extent_len: [Option<Len>; 2], // its symbolic length, where read
}
```

  The `placement: Option<UiRegion>` pin is gone. Nothing may depend on where
  a box starts.

- primitives and the mask retained in the widget's own box's coordinates.

**Reuse** of a drawing at an ask: same layer, parent move and region-node
choice; frame pixels inside `frame`; the box resolved from `place` (`Fill`
is the part; `Within` is the answer aligned in the part, or the part where
the answer fills) has pixels inside `extent` and, where pinned, the same
symbolic length. A frame that moved recomposes the subtree from retained
local coordinates (`recompose_subtree`). A box that moved **re-places every
child** through its retained `place` (`reposition`, generalised from the old
`extent_children` to all children because every child is now a part of it).

**Dependencies composed into the parent** (`in_parent`): a child's `frame`
range goes through the child's frame length into the parent's frame range. A
child's own-box range goes into the parent's *frame* range through the
part's length where the part is `From` (a frame length), into the parent's
*own-box* range through the part where it is `Of`, and straight into it
where the part is `All`. A pin composes only for `All`. Answer dependencies
come from children whose answer was read, drawing dependencies from every
child drawn.

**Local redraw** (`redraw`) still defers where the box it is given is not as
long as the box it was measured in -- see **What is not done, and why**.

### What it cost

Widget draws / distinct widgets / update, from
`tests/layout_diagnostics.rs`, at depth 8. Draw counts are deterministic, so
these are single runs rather than medians; `e44dea3` is #18's head and
`34cafb6` the commit this branch starts from.

| seed 1  | e44dea3      | 34cafb6      | `0954770`     |
| ---     | ---          | ---          | ---           |
| cold    | 369/261/10.6 | 463/274/13.3 | 516/288/12.0  |
| repaint | 1            | 1            | 1             |
| many    | 157/95/0.33  | 263/108/0.59 | 187/119/0.52  |
| size    | 16/12/0.018  | 3/3          | 3/3/0.010     |
| scroll  | 2/0.002      | 1            | 1/0.004       |
| resize  | 13/13/0.019  | 22/15/0.032  | 24/76/0.090   |

| seed 13 | e44dea3       | `0954770`      |
| ---     | ---           | ---            |
| cold    | 1330/707/20.3 | 2940/982/28.3  |
| many    | 524/159/1.09  | 1091/423/2.39  |
| resize  | nothing drawn | 2215/510/6.56  |

So `size` and `scroll` keep the experiment's wins, `many` is better than the
commit it starts from and still well short of #18, and **`resize` on a deep
tree is where it loses badly**: #18 draws nothing at all at seed 13, because
every fraction scales and every `Holds` admits the new window.

Three measured findings, each already applied:

- **Lazy `Within` placement costs more than it saves.** The plan's step 5 --
  leave a child's answer to be placed at the end of the parent's draw --
  puts the drawing in the part first and in the answer's box after, and
  where it does not hold for both that is two drawings rather than one.
  Seed 1's resize went from 391 widget draws to 29 with it removed. The test
  that pinned three draws for a numeric leaf in a span went with it.
- **An inset said in frame lengths makes a container read its own box.**
  "Less eleven pixels at the end" needs the length, and a container whose box
  is its own answer then depends on its own answer: `Pad` drew sixty-four
  times in one resize frame at seed 13, chasing its own width. `Part::Of`
  says it as a part of the box instead and composes without a length.
- **Pin one axis at a time.** `extent_len` pinning both made a span
  dividing one axis hold for one length of the other, and a resize broke
  every span whose cross-axis answer moved.

### What is not done, and why

**Step 6, `redraw` without the deferral, does not hold.** Removing it --
asking the measuring question locally and placing the answer afterwards --
makes seeds 104 (`align`) and 210 (`reorder`) at depth 5 settle differently
warm than cold. The plan said to stop and report rather than restore it; it
is restored, in the form the protocol allows (`info.part.size() !=
offered.part.size()`), and this is the report.

**What is underneath it.** A `Place` is relative to the box the container is
being drawn in, so the same ask expression resolves to a different box
depending on which of its own boxes the container is drawing in -- and a
container is routinely drawn twice, once in the box its parent measured it
in and once in the box its own answer chose. So "the box this widget's
answer was measured in" cannot be recovered from the ask, which is why
`offer_part` is retained; and whether a given draw is a measurement cannot
be recovered either, which is where the two open seeds live:

- **seed 2, `repaint`, depth 5**: `Stack > Pad{6,14,16,15} > Span{DOWN}`
  with a wrapping text under the span. The span answers 880 px measured in
  the pad's first box and 874.24 px measured in the box the pad's answer
  then chose, and both are fixed points of "measure in the box, place at the
  answer, measure again". Cold reuses the drawing and keeps the first; a
  repaint redraws and lands on the second.
- **seed 1 at depth 4 and 108 at depth 5, `reorder`**: the same shape
  through `Branch`, which chooses a subtree from a measured length, so the
  two fixed points are two different trees.

Four bookkeeping rules were tried for "which draw is a measurement": the
ask's first-ness alone, the place matching the retained offer place, the box
matching the box the answer was measured in, and a retained flag on the
drawing. Each fixes some seeds and breaks others, which is the signal that
the question is the protocol's rather than the bookkeeping's. What the
baseline does instead is never act on it: a local redraw whose box is not
the offer's defers to the parent, and the parent re-asks from a geometry
that a cold layout also reaches.

**Also not done**: step 7 (`Span`'s known-length shortcut and the cross-axis
report in frame pixels), step 8 (`Inset`/`Outset` as test widgets) and the
`LazySpan`/`SizeRule` work that sits on top of this.

**`Pad` is an outset and that is visible.** `examples/text.rs` has
`wtext(..).width(rel(1.0))` inside a `.pad(16)`: under the old reading that
was the padded box, and under transparent frames it is the window, so the
labels now sit at the window's edges and the left one is clipped. `view` and
`minimal` are byte-identical to `34cafb6`; `tabs` moves its red square 3 px;
`random` is a generated tree and moves where nested spans do. The plan's
"a general `Pad` would outset pixels and inset `rel` and `leftover`" is the
answer to this and is **not expressible in the protocol as it stands**: an
inset that narrows the frame *and* places the child in a part of its own box
needs the child's box expressed in the child's narrowed frame, and
`(0.5·B - 20)/(B - 20)` is not `rel + px`. Either the frame narrows and the
box goes with it (what the code does, so an inset inside a row draws its
child across the whole row), or the box is a part and the frame does not
narrow (`Part::Of`, what `Pad` does), or a third thing Bryan decides.

## 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.
- **`draw` stays the only layout method on `Widget`** (Bryan, 2026-09-17,
  reversing the same day's acceptance of a measure/draw split). Ease of
  writing a widget is half the reason. The real one is that a second method
  holding the same layout drifts from the first, which a span makes
  extremely easy, and the shared logic then gets pulled into helpers both
  call that still have to be applied carefully in each. Where the framework
  needs a widget's layout twice it runs the same body again with the
  painter in a different state, or hands it more through the painter.
- **A widget's frame does not change between the ask that measures and the
  ask that places.** Fractions are of the frame; its own box reaches the
  widget through the painter. This is what transparent frames implements.

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

`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 ask carried the base separately (`reports_of`) for a day; transparent
frames replaced it with the frame, which is the same statement made once per
widget rather than once per ask. The offer is still the remainder, because a
text has to wrap at the width actually there.

**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`, superseded by `a0693ac`)

**Superseded: `dirty_size_under` is deleted.** Once a resize goes through
the settling walk the ordering makes the whole category unreachable rather
than checked, which is Bryan's steer and the better answer. The defect and
its reasoning are kept below because they say what the ordering is buying.


`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` was not the optimization its comment claimed: it was
what made an answer an answer, until the walk made the state it guarded
against impossible to be in.

Found at seed 564, depth 6, `shuffle-every-other`, reachable only once a
span could overflow itself. No hand-built tree ever reproduced it, 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".

**The walk is sound on its own, and the second entry point is closed**
(`a0693ac`). By induction on depth: when a widget at depth d draws fresh,
every dirty widget deeper has been popped, so each is settled or deferred,
and a deferred one has marked its parent. A clean child asked by that draw
therefore has a clean subtree, because anything dirty under it would have a
dirty parent, and so on up to the child itself.

The entry point that was left was `update` drawing the root for a resize
before the walk ran, top-down over a tree with dirty widgets still in it.
It is closed by marking the root instead, so layout is one walk a frame and
`dirty_size_under` is gone at both call sites. **The root is marked only
where the new output falls outside what its answer holds for**: that range
is the intersection of everything under it, so admitting the new output
says the whole tree stands, and nothing above the root moved. Marking it
unconditionally cost the root its own `Holds` -- a leaf root that scales
with its box was drawn again on every resize, which two tests caught.

The rig says the cost is scheduling only: on `resize`, one queue pop, one
local redraw and one depth read appear and one failed reuse attempt goes,
and every other counter on `cold`, `repaint`, `many`, `size`, `scroll` and
`resize` is identical.

### A subtree that changes hands is recorded on both sides (landed, `e44dea3`)

A subtree can be reused whole under a different parent -- same box, same
layer, same region node, clean -- and nothing in the drawing says it moved.
Two things read who its parent is, and both were wrong after one of these.

- The **old parent still listed it**, and a parent's next draw undraws
  whatever is missing from that list. Two spans under one root, with the
  root swapping which of them it holds, drew the subtree under the new span
  and then erased it when the old one drew.
- Its **depth** was the one it had under the old parent, which is what the
  settling walk orders by, so a change made under it afterwards settled at
  the wrong point in the frame.

Both are written where `draw_inner` already records what the ask decided:
`active.parent` is replaced and the old parent's `children` repaired, and
`try_reuse` re-walks the subtree's depths -- only where the top of it
moved, which is what makes that free in the ordinary case. Pinned by
`retained::a_subtree_that_changed_parents_is_not_undrawn_by_the_one_it_left`
and `..._settles_at_the_depth_it_moved_to`; each fails without one half.

The fuzzer never re-parents (`reshuffle` only trades children between a
span and its own spares), which is why nothing generated reached either.

### 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, one of them now closed

**`wip/stack-fraction-twice` is closed by transparent frames.** A stack
sized by a child that reported a fraction applied that fraction twice --
half a row became a quarter -- because the placing ask resolved the fraction
in the box the answer had already chosen. Under the protocol above a report
is a fraction of the frame and `placed_extent` takes it *from* the part
rather than composing it into the part, so it is resolved once. No oracle
could see it (warm and cold shrank alike), so the test that came with the
branch is what pins it.

**`wip/padding-outset-and-inset` is superseded** by the `Pad` question at the
end of "What is not done, and why". What it got right is kept: padding goes
outside what it pads, and an `Inset` is a separate widget. What stopped it --
a child declaring `rel(0.5)` under an `Inset` coming out 47.5 px of the 190
inside -- is the same second application of a fraction, and the protocol
removes 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

The `e44dea3` baseline's two-step allowance came from inverse remapping and
alignment composed by different routes. The frame/extent continuation below
retains local widget frames as well as local primitive coordinates, recomposes
in the same order as a cold draw, and removes inverse remapping. Its oracle
requires exact pixel-region equality. That experiment is not yet the pinned
framework; the baseline's snap decision below still applies there.

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.

### Before transparent frames, and what survives from it

The frame/extent prototype that `34cafb6` is the head of separated a
widget's fraction reference from where its drawing sits, and the
transparent-frames protocol above is that idea finished. Four of its
findings still hold and are why the code is shaped this way:

- **An answer and a drawing each retain their dependencies.** A child's size
  answer constrains its parent only when the parent reads that answer; the
  child's drawing constrains the parent's retained drawing whether its size
  was read or not. `Painter` collects the two separately (`answer_under` and
  `under`), which is what stopped a stack sized by one child from
  remeasuring because an unmeasured overlay wrapped at another width.
- **A wider contract does not invalidate an existing guarantee.** When a
  local redraw comes back with the same size under a contract covering the
  old one, the old one is kept; comparing whole contracts by equality
  doubled `scroll` cycles.
- **No measurement is different from a measured zero**: `ActiveData::answer`
  is optional, and a previously undrawn pure share must not answer with the
  placeholder zero it never gave.
- **The settling walk takes the deepest mark from a `BTreeSet` keyed by
  depth**, and what ends it is the mark set rather than the queue.

Two failed hypotheses from it, kept because they are cheap to repeat:

- **The placement pin was blamed for the `many` gap and is not the cause.**
  Disabling it (unsound, a bound) still redrew 487 distinct widgets a frame
  at seed 13 against `e44dea3`'s 159. What the per-widget trace showed
  instead was local redraws deferring to their parents and the deferrals
  chaining to the root -- 43 deferrals in one frame, in chains such as
  424 → 425 → 437 → 449 → 483 → 487 → 491, because a span handed its
  children its own placement as their frame and that placement changed
  between the measuring ask and the placed one. Transparent frames is the
  answer to that and it works: the frame no longer changes under a widget.
- **`wip/local-reask`** re-asked a dirty widget at its offer instead of
  deferring, under the *old* protocol, and diverged at seeds 532 and 398 of
  depth 6. The same shape is what "What is not done, and why" is about; the
  branch is superseded and can be deleted.

## 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

The frame/extent continuation removes `AGREE_STEPS`: warm and cold pixel
regions must compare exactly. This is distinct from the equal-share test:
when a row's grid-step count is not divisible by the number of children,
individual share widths can differ while every rerun of that layout must
still agree exactly. The PR #18 baseline still allows two position steps;
its earlier failure at one step (`resize-size`, seeds 384 and 162 at depth 5)
is a useful regression target for the experimental recomposition.

## 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: measured children for an answer,
  all painted children for a drawing. 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 where its measurement contract holds. Its final
  drawing is checked independently and may need redrawing even while the answer
  stands. Drawing validity is translated back through the chosen placement for
  the parent's drawing contract, not intersected into the retained answer.
- **A widget's frame passes through every container that only divides room,
  and its length is the same on every ask.** What narrows it is decided
  above the widget: a declared length, and an inset once there is one. A box
  that is an answer -- a row's height, a stack sized by a child -- is never
  anything's frame. `Painter::widget_at(child, frame, [Place; 2])` says both
  things about an ask: what the child's fractions are of, and what of this
  widget's own box the drawing takes.
- **A fraction is resolved once, against the frame.** A report comes back
  raw and is composed into the parent's frame by `within_len` only where the
  parent narrowed the frame; `placed_extent` takes the reported length
  *from* the part rather than composing it into the part.
- **Nothing may depend on where a box starts.** A container reads
  `extent_len(axis)` for the length it divides, which pins that length
  symbolically, and places children as parts of its own box, so moving the
  box re-places them without drawing anything again.
- 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`. (The plan's step 7 changes this to the longest
  `px`-or-`rel` child compared in frame pixels; not done.) **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, including during resize. A deferred
  child leaves its parent marked, so no clean answer can hide an unsettled
  size dependency. `dirty_size_under` has been deleted.
- 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 `0954770` on `wip/transparent-frames`: `cargo fmt --all --check`, clippy
with `-D warnings`, 108 suite tests and 20 core tests in debug, the 11
generated cases, and the six rig phases in the table above. The shrinker at
400 trees of depth 5 **fails**, at seeds 2 (`repaint`) and 108 (`reorder`);
the long oracle at 1000/6 and the 2000/4 scan have not been run since they
would only find more of the same. The five reference renders were taken and
compared with `34cafb6`: `view` and `minimal` byte-identical, `tabs` 2,332
pixels, `text` and `random` as described above. Venus on the RX 7900 XT,
confirmed by `vulkaninfo --summary` in the same session.

At `e44dea3`, which is what #18 would merge:

- `cargo fmt --all --check`, `cargo clippy --workspace --all-targets --
  -D warnings`, `cargo test --workspace`: green, 92 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 14.2 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 57 s, the oracle at
  1000 seeds of depth 6 in 143 s, and **2000 seeds at depth 4 over all
  fifteen cases** in 260 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`, `tabs` and `text` byte-identical at
  1920x1200 against `25e456e`, as is `tabs` under the recorded replay.
  `random` live-resized from 1920x1200 to 1280x800 is byte-identical to a
  cold 1280x800 render. Rendered through Venus on the host's RX 7900 XT,
  confirmed against `vulkaninfo --summary` in the same session -- an
  llvmpipe fallback makes the same PNG and nothing in it says so.
- All rig counters identical on `cold`, `repaint`, `many`, `size` and
  `scroll` across `25e456e`. `resize` gains one queue pop, one local
  redraw and one depth read and loses one failed reuse attempt, which is
  the root going through the walk; drawn widgets, widget draws, draw
  requests and primitive writes do not move.

Everything above 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. **Two questions for Bryan, both from "What is not done, and why".** What
   a widget's answer is the answer *to*, when a container is drawn in two of
   its own boxes in one frame -- which is what the two open fuzzer seeds
   turn on and what step 6 needs before the deferral can go. And what `Pad`
   should be now that a frame passes through: an outset (what the code does,
   and `examples/text.rs` shows what it looks like), an inset that takes the
   child's box with it, or the plan's "outset pixels, inset `rel` and
   `leftover`", which the protocol cannot express as it stands.
2. **The rest of transparent frames**: step 7 (`Span`'s known-length
   shortcut and the cross-axis report in frame pixels), then step 8
   (`Inset`/`Outset` as test widgets, once the question above is answered).
   `wip/local-reask` is superseded and can be deleted.
3. Write `ActiveData::answer` in one place -- `try_reuse` hands back what
   the last drawing reported, which is a measurement only where that drawing
   was one.
4. Keep the `DrawInfo` on `ActiveData`; delete the copied fields and the
   reconstruction in `redraw`.
5. **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.
6. 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.
7. `LazySpan`, the next LAYOUT.md §2 item. Region nodes cover the movable
   subtree case; do not restore a separate child-placement API.
8. `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.
9. `Scroll` taking a direction rather than one axis.

`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.