ai-app/docs/HANDOFF.md

Handoff

Where the Iris retained-layout work stands for a worker picking it up cold. This file contains current decisions, the implementation plan and its checks. The durable layout design and measurement method are in docs/LAYOUT.md. The temporary investigation record is in docs/LAYOUT_LOG.md; delete that log when the one-ask protocol lands, after moving any fact that must survive.

Where things stand

Canonical upstream Iris main is ca2b4b2 (#17, the headless rig). PR #18's pushed branch is split/18-position-chain at e44dea3. Its detached comparison checkout is /home/bob/repos/iris-layout-baseline. It is the reviewed baseline this work must preserve or improve.

The continuation is /home/bob/repos/iris-layout-experiment, now on branch wip/hint-first at f6242aa, one commit over wip/one-ask at a888717 (nine commits over 4328eac, the head of wip/transparent-frames, which is unchanged). Both branches are pushed to origin. wip/one-ask replaced the old step 3 plan with the one-ask protocol below, 1512d84 and 23523ee made the frame a length of the window, and e8a5792 is what the step 1 review found. f6242aa is the step 5 result: a span takes a child's length from its hint or rule and asks it once, in its slot. It passes every check:

check at f6242aa	result
cargo fmt --all --check, clippy -D warnings, with and without layout-diagnostics	clean
cargo test --workspace (debug)	123 suite, 20 core, 11 generated, all green
cargo test --release --test generated	11/11
shrinker, 400 seeds, depth 5, all sixteen cases	agree, 63 s (34,488 widgets)
1000 seeds at depth 6	agree, 277 s
2000-seed depth-4 scan, all sixteen cases	agree, 283 s (82,203 widgets)
cold layout of 400 trees at depth 5, dumped and diffed against a888717	byte-identical (34,488 widgets)
renders, the tabs replay and the random resize	not re-run since a888717; step 3 below

The cold-layout dump is tests/layout_dump.rs, new at f6242aa: it prints every widget's box for many grown trees so two commits can be diffed on cold layout, which the warm/cold oracle cannot see a change to.

What implementing it corrected in the plan is in docs/LAYOUT_LOG.md; the four that would have shipped as wrong layout are a share inside padding losing the padding twice, a root resolving its own rule twice, a rule changed over two pads relocating the column under them instead of dividing it again, and a resize leaving a short scroll's window-tall content where it was. Each is pinned as a named test.

The renders and the replay are done and recorded in docs/LAYOUT_LOG.md: against #18, minimal and tabs are byte-identical (before and after the reference gesture), random differs in two pixels of glyph antialiasing, text moves one padded block one pixel, and a live resize of random matches a cold render at that size byte for byte. Re-run at a888717, all five are byte-identical to the same renders at a30971e, so the scroll fix below changed none of them.

Not done: the pre-submit review of f6242aa, the render set at it, the doc pruning and the landing (steps 1, 3 and 6 below). Step 5's measurements and the two ideas it tested are in docs/LAYOUT_LOG.md under "What the step 5 measurements found".

The worker's older step 3/4 experiment is preserved as branch wip/step3-experiment (one commit over 4328eac) and as ~/repos/iris-step3-experiment.patch. It is evidence, not the protocol. The app's Iris pin is unchanged.

The two rules to protect

These outrank the accumulated machinery:

1. A changed tree lays out exactly as if it had been drawn that way from the start. The warm/cold oracle and shrinker test this.
2. Lengths are predictable. px is that many pixels; rel(0.5) is half of the frame decided for the widget, wherever it sits; leftover is a share of the room left after every sibling's px and rel lengths are resolved.

Do not fix a failure with a tolerance, another measurement flag, a special case in Span, or another layout method.

What the previous plan got wrong

The full account is in docs/LAYOUT_LOG.md. The short version, because it is the third plan for this repair and the next one should not repeat it:

• Every plan kept the second draw. The old protocol drew a widget in the box it was asked in, then drew it again in the box its own answer placed it in whenever the first drawing's Holds did not cover that box. All the offer machinery -- offer_place, offer_part, at_offer, measured(), the local-redraw deferral -- existed to remember which of the two draws was the question. The plans tried to define that bit better; the defect was that there were two draws at all.
• The step 3 plan then over-corrected. It said "every drawing must hold for the answer box it supplies", and the worker implemented exactly that as an assertion in place. A wrapped Text asked at 45 px whose longest word is 89.5 px cannot satisfy it, and neither can any widget that reads its box and reports something other than it. The answer box is not a question, so no contract about it can be demanded of the widget.
• It also let a caller narrow a frame by position. A frame narrowed to a region (the worker's share frames) does not move when the part it sits in moves; only a frame narrowed to a length, put back into the part on every placement, does.

The protocol now in the experiment

A widget draws once, in the box it is asked in. Its answer is placed inside that box by re-expressing the drawing. Nothing is drawn again in a box an answer chose. Holds is a contract about the ask box alone, consulted only to decide whether a re-ask can be skipped. This is draw_inner at 1512d84:

let reused = (!stale)
    .then(|| self.retained_answer(id, part, info))
    .flatten()
    .and_then(|answer| {
        let extent = placed_extent(part, answer.0, declared, info.fill(), align);
        self.try_reuse(id, part, extent, info, rsc).map(|()| answer)
    });
let answer = reused.unwrap_or_else(|| {
    if old.is_none() {
        old = self.remove(id, false, rsc);
    }
    let answer = self.draw_at(id, part, info, old.take(), rsc);
    let extent = placed_extent(part, answer.0, declared, info.fill(), align);
    if extent != part {
        self.relocate(id, extent, info, rsc);
    }
    answer
});

try_reuse checks the drawing against part and relocates it to extent; the old place (redraw in the answer box) is gone, and with it every offer field's purpose. ActiveData keeps part as the ask box, asked as where it was asked and placed as where it was put (renamed in step 4).

A container that puts an answer somewhere other than where it asked says so with a new call that never runs the body:

/// Puts a child asked about in this draw somewhere else in this
/// widget's box: its answer, placed in this part instead. The drawing
/// is re-expressed there rather than made again -- what a row does once
/// it knows every slot, having measured each child from its cursor.
pub fn place_at<W: ?Sized>(&mut self, id: &StrongWidget<W>, place: [Place; 2])

A frame is narrowed by a length of the window, never a region and never a fraction of the parent's frame -- a row's slot cannot be written as a fraction of the row. The box stays whatever place names; only a declaration places the box inside it, by the child's alignment, on every placement:

pub fn widget_at<'s, W: ?Sized>(
    &'s mut self,
    id: &'s StrongWidget<W>,
    narrow: [Option<Len>; 2],
    place: [Place; 2],
) -> DrawResult<'s, 'a, W>

Span takes each child's length along itself from its hint where one says -- a rule, or a widget that always reports the whole of its box -- and otherwise asks the child once from its cursor (Within(From(cursor..far))). Then it moves a drawn fixed child to its slot, and asks every other child once in its decided slot, a share with the frame narrowed to it. This is Span::draw at f6242aa:

let size = match painter.size_hint(child, axis) {
    Some(len) => {
        measured.push(None);
        len
    }
    None => {
        let room = Place::Within(Part::From(along(cursor, far)));
        let size = painter
            .widget_at(child, [None; 2], axis.pair(room, across))
            .size();
        measured.push(Some(size));
        size.axis(axis)
    }
};
...
let slot = along(from, start);
let place = axis.pair(Place::Fill(Part::From(slot)), across);
let mut narrow = [None; 2];
if len.leftover > Weight::ZERO && shares {
    narrow[axis as usize] = Some(slot.len());
}
let used = match (measured, narrow[axis as usize]) {
    (Some(size), None) => {
        painter.place_at(child, place);
        size.axis(!axis)
    }
    _ => painter.widget_at(child, narrow, place).len(!axis),
};

A hint is the length the child's draw would report: Painter::size_hint resolves a fraction in it against the asking widget's frame, which is the frame a child asked with nothing narrowed gets, and pins that frame where it did. Scroll, Masked, a Stack without a sizing child and the fixture's Branch hint LEFTOVER, since each always reports it; Rect and () already did. A widget that reports a share its children gave it -- a span with a Rect in it -- has no hint and is still asked twice, which is the whole of what step 5 left.

Stack asks non-sizing children with Part::Sized(len) of what its sizing child decided, on every axis that is not a share -- a box of that length where their own alignment puts it, and that length as their frame -- and Scroll asks its content once in the viewport and place_ats it to the scrolled offset.

A local redraw asks the retained question again -- the same place of the box the parent was asked in -- and, if the answer stands, puts the fresh drawing back at the retained place of the box the parent's answer chose. Both halves are needed: seed 2 at depth 4 (a stack sized by its text) fails without the second.

A widget its parent asked more than once in one draw -- a share child, asked in the room and then in its slot -- has two questions and one record, so it cannot settle locally: redraw defers it to the parent the way it defers a widget whose declared length changed (ActiveData::re_asked, set by widget_at when the child is already in children). Seed 946 at depth 6 found the case: a fixed-height column that is a share while its rect fits and a fixed width once it does not, so emptying it changes the room answer and not the slot answer.

A symbolic length a child pinned now composes through Part::Of where the part is the whole box less pixels, and pins the parent's own length otherwise (in_parent). Dropping it let a zero Pad reuse a drawing across a narrowed frame of the same pixel length; the shrinker found six such seeds at depth 5.

Decisions

Decided with Bryan on 2026-09-17 and 2026-09-18, kept where still true.

One draw method, in a box decided from above

Widget::draw remains the only layout method. A container's body runs only in a box its parent offered or decided, never in a box derived from the container's own answer. The experiment extends this to every widget: a leaf is not drawn in its answer box either. Its drawing is re-expressed there, which for a text means the block it shaped at the asked width is positioned inside the box its reported size chose, and its lines do not change. examples/text.rs and random have not been rendered since; do that before landing and inspect any change.

Frames are narrowed by every length decided from above

A declared px or rel, a resolved share, and the box a stack's sizing child decided (Bryan, 2026-09-18: the sizing child, if any, determines how the rest are laid out) all narrow the frame. declared_lens still excludes leftover, which is right: a share has no length until the span divides its room, and it narrows the frame at the placing ask instead.

Padding is an inset, and the frame is a length while the box is a region

Bryan, 2026-09-18: padding is an inset. It subtracts from both the child's frame and its box and adds itself to the reported size, so rel(1.0) inside padding fills the parent without overflowing. A span's frame never subtracts siblings; only the padding subtracts from it. No outset kind and no mixed kind for now; the name stays Pad. Worked example, 900 px row:

let row = (rect(Color::RED).width(24), wtext(PARAGRAPH).wrap(true).pad(16)).span(Dir::RIGHT);

The text is asked in 900 − 24 − 32 = 844 px and wraps there; a rel(1.0) inside the same padding is 900 − 32 = 868 px and overflows the row by exactly the icon's width. With the pad in a share instead, both are the share less 32. An icon after the padded text overflows; a user who wanted otherwise meant leftover.

Implemented at 1512d84. Pad reads its own frame (Painter::frame_len, which pins it), takes the padding off, and hands that down as the child's frame, while the box it gives is the inset part of its own box. The two are different lengths whenever the box is narrower than the frame -- which is exactly the wrapping case above.

A share never adds room beyond the deciding box

Scroll resolves its content length from the fixed part of the answer and makes it at least the viewport (4328eac). Unchanged.

Existing fixed-point and box-chain design stays

Unchanged; see docs/LAYOUT.md.

Implementation plan

Work in /home/bob/repos/iris-layout-experiment on wip/hint-first from f6242aa. Make each step a warning-clean commit, push it, and run its named checks before the next. If a step exposes a different mechanism, stop and update this handoff rather than papering over it.

The order is 1, 3, 6: review the step 5 commit, render at it, then land. Steps 2, 4 and 5 are done and kept below for what they decided.

Anything a fuzzer finds is shrunk first (SHRINK_SEED=<seed> SHRINK_DEPTH=<depth> SHRINK_CASE=<case>, which now prints each level's frame, ask, box and size warm against cold), pinned as a named test in tests/cases/unsettled.rs, then fixed under the two rules above. Do not add a second draw back.

1. Review the step 5 commit

The six commits 4328eac..a888717 were reviewed as one diff (done at a888717; it found one wrong layout, fixed at e8a5792 and pinned by scroll::content_that_fits_is_placed_in_the_viewport_and_not_in_the_window, and three stale comments). Still to do: the pre-submit review of a888717..f6242aa, which the planner wrote and measured but did not review as a separate pass. It touches Span::draw, Painter::size_hint, four size_hint implementations and the dump rig. Things to look at:

• Painter::size_hint's pin: it sets frame_own_len where the hint has a fraction. A child asked in its slot afterwards pins the same frame through in_parent anyway, so the pin only matters for a caller that reads a hint and never asks the child. Confirm that reading is right, or delete the pin and say why.
• A hinted share child with no room is never asked and Span calls painter.undraw on it, which is what takes its last frame's drawing down. A drawn-in-the-room child got the same call before; check nothing else relied on the room draw having happened first (size_deps records the hint read, and draw_at's asked handles a dep that is not a child).
• The two comments in Span::draw above the passes, and the Stack hint's comment.

Check: the ordinary suite, then cargo test --release --test generated. Anything the review changes in Span or Painter re-runs the three long fuzzers and the cold dump (commands under step 6).

2. Make the frame a length and the box a region

Done at 1512d84 and 23523ee. The settled rule is in docs/LAYOUT.md under "Frames, decided boxes and padding"; what implementing it corrected in the plan -- four of them wrong layout that would have shipped -- is in docs/LAYOUT_LOG.md.

3. Render and replay

Done at adbedaf and re-run at a888717, against e44dea3 (#18); what each render showed is in docs/LAYOUT_LOG.md. Not yet run at f6242aa. Render the five examples, the tabs replay and the random resize at the reviewed head and compare with the same renders at a888717 (a git worktree add ../iris-a888717 a888717 with its own target directory, one process at a time). Expected: byte-identical, every one -- the cold dump already says so for 400 random trees, but the fuzzer grows no rel rules and the examples do, so text is the render that can still differ. Any difference is a finding: read the records before touching anything. view has no counterpart in #18 and is compared with a888717 only. The commands are under step 6.

4. Rename and delete

Done at a30971e. ActiveData and DrawInfo now say part for the box a widget was asked in, asked for the place it was asked at and placed for where its drawing was put; LayoutHolds::frame is window, since those ranges are window pixels and the frame's own entry is the frame_len pin beside them, and Painter::frame_own is window_own. answers_at had one caller and is inlined there. ActiveData::measured is kept: place_in reads it, and what it says -- the answer rather than the last drawing's report -- is worth a name.

5. Restore the expected retained cost

Done at f6242aa, by one change: a span takes a child's length from its hint or rule and asks it once, in its slot, instead of drawing it in the measuring room first. Work counters, seeds 1 and 13, depth 8, widget draws / distinct widgets, beside e44dea3 (#18) and a888717 (before it):

seed 1	e44dea3	a888717	f6242aa
cold	369/261	342/288	264/232
many	157/95	118/95	41/41
size	16/12	3/3	3/3
scroll	2	1	1
resize	13/13	44/13	36/13

seed 13	e44dea3	a888717	f6242aa
cold	1330/707	1278/982	758/627
many	524/159	429/366	16/16
resize	nothing	nothing	nothing

The command is

IRIS_SEED=1 IRIS_DEPTH=8 cargo test --release --features layout-diagnostics \
  --test layout_diagnostics -- --ignored --nocapture layout_cost

reading widget draws and distinct widgets from each phase's block, and hottest widget draws with IRIS_PHASE=resize IRIS_FRAMES=2 for who is drawn how often.

many, size and scroll are at their floor: every draw is a marked widget, or the parent a marked widget deferred to. cold is under #18 at both seeds. What is left is resize at seed 1, 36 draws of 13 widgets where #18 drew 13, and the cold draws over the distinct count (32 at seed 1, 131 at seed 13). Both are one mechanism, understood and not yet worth a change: a reported share -- a span whose children report leftover, a stack sized by a child, a wrapper round either -- has no hint, so its parent still asks it in the room and again in its slot, and each of those asks draws, since the room drawing divided the room and does not hold for the slot. Under the root's resize redraw that multiplies down the tree: the hottest widget at seed 1 is a Span drawn 8 times in one frame, two levels of reported shares under it drawn 4. The design that would remove it is in docs/LAYOUT_LOG.md ("two answers per record"); do it only once an app screen shows the cost, and measure that screen first.

Two ideas from the previous version of this step were tested and are not to be done; the measurements are in docs/LAYOUT_LOG.md:

• Not reading the span's own length where no slot depends on it (wip/inset at 5b181bc, pushed): identical counters at every phase. The symbolic pin survives a resize because every ask box is symbolically stable across one; the earlier "44 to 20" came from share spans reusing their room drawing in their slot, which is wrong layout rather than saved work.
• Resolving a rule that is a fraction of the frame from the record instead of redrawing: reuse outside: a frame length is 3 draws of 264 cold at seed 1 and 1 of 758 at seed 13. Nothing to recover.

Report every phase at both seeds, work counters first, medians only when the work agrees.

6. Full verification and landing

Run, in the experiment checkout:

cargo fmt --all --check
cargo clippy --workspace --all-targets -- -D warnings
cargo clippy --workspace --all-targets --features layout-diagnostics -- -D warnings
cargo test --workspace
cargo test --release --test generated
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
SHRINK_CASE=all SHRINK_SEEDS=2000 SHRINK_DEPTH=4 \
  cargo test --release --test shrink -- --ignored --nocapture

The last line is the 2000-seed depth-4 scan over all sixteen cases; the shrinker runs the same cases as the scan and reduces anything it finds, so no temporary test body is needed any more. Rng::new uses seed | 1, so adjacent even/odd seed pairs describe the same tree. All of these passed at f6242aa; re-run whatever a later commit could affect.

A change that could move cold layout also gets the dump diff, which the oracle cannot replace:

IRIS_DUMP_SEEDS=400 IRIS_DUMP_DEPTH=5 cargo test --release --test layout_dump \
  -- --ignored --nocapture | grep -E '^[0-9]+ [0-9]+ ' > /tmp/after.txt

once at the commit before and once after (check the earlier commit out in the same checkout so the build is incremental; the rig file is untracked there, copy it in with git show wip/hint-first:tests/layout_dump.rs), then diff the two. Zero differing lines is the expectation for a cost change.

Render view, minimal, random, tabs and text at 1920x1200 and inspect every intentional change. Also replay tabs and compare a live resize of random with a cold render at the same size. Read the installed graphics skill before rendering and confirm the renderer; an llvmpipe fallback can produce a plausible PNG. The headless rig reuses one compositor, so run one process at a time and give comparison worktrees separate target directories:

./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 reference replay is:

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

Before submitting, run the pre-submit review. Then land: move any surviving fact from docs/LAYOUT_LOG.md into docs/LAYOUT.md (the settled protocol, the measurement method including the dump rig, and the "not to be done" results under step 5), delete the log, update this handoff to the next actual task, and push every coherent commit. How the branch reaches upstream is Bryan's call and has not been asked: wip/hint-first is some sixty commits over #18's split/18-position-chain, which is still open, so either #18 lands first and this follows as one PR, or this replaces #18. Ask before opening anything. Update the app's Iris pin only when the Iris change is ready.

Follow-on work, not part of this repair

• CPU round-to-nearest and shader nearest-pixel snapping are approved as one separately verified change. Neither has landed. Re-derive Holds::through for the new rounding and run both long fuzzers plus the render set.
• Smaller layout items remain in docs/LAYOUT_LOG.md: an undrawn share's gap, nested share weights, inconsistent zero-divisor fallbacks, and the stale f32 identity comment.
• LazySpan, then SizeRule::{Min, Max, Clamp}. A cap may not contain leftover; whether Max narrows the child's drawing box is still a real product decision.
• Scroll taking a direction rather than one axis.

Other product work remains in docs/PLAN.md and the focused documents it links. Do not mix it into the Iris layout branch.