# Handoff Where the work in flight stands for a session picking it up cold. Keep current invariants, measurements, and failed hypotheses here; this is not a decisions log. Pruned on 2026-09-17: the chronicle of closed defects, superseded verification lists and cross-fixture tables went, the rules and the lessons stayed. ## Where things stand Canonical Iris `main` is **`ca2b4b2`** (#17, the headless rig). **#18 `split/18-position-chain`** is open in `/home/bob/repos/iris-pr18`, head **`ea6dbae`**, 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, and a box in pixels threaded down the draw. No PR review was present when checked on 2026-09-15. **Two ideas outrank everything else in this document** (Bryan, 2026-09-17). First, a changed tree lays out exactly as if it had been drawn that way from the start; that is what the retained machinery is for and what the oracle and shrinker check. Second, widgets are predictable: `px` is that many pixels, `rel(0.5)` is half of the containing widget's area however many siblings there are and wherever it sits among them, and `leftover` is a share of the room left once every sibling's `px` and `rel` are resolved. The invariants listed further down were accumulated by agents chasing single failures; any of them may be simplified or deleted if those two ideas still hold. ## Review of 2026-09-17 A fresh read of `core/src/fixed.rs`, `orientation/`, `ui/holds.rs`, `ui/painter.rs`, `ui/render_state.rs` and the position widgets, outside of doing work on them, with each finding checked by a scratch test. **Verdict.** The concepts are sound and stay. Fixed point on a `1/1024` grid is the right base for a layout that decides "same box or not" by equality. Threading the pixel box down the draw, with `Holds::through` the exact preimage of that one multiply, is the strongest idea in the code: layout has one route to every length and the reuse test is its exact inverse. Offer, given and placed is the ordinary measure-then-arrange model. What needs work is the bookkeeping around the second ask, one boundary in `Span` computed by an expression other than the drawing it guards, and a `rel` that means two things. The two-step residual is structural and no grid width fixes it; where it becomes visible is the shader's snap. ### Decided by Bryan - **`rel` is a fraction of the containing widget's whole area.** In a span, `rel(0.5)` is half the span whatever else is in it and wherever it sits. It is never a fraction of what was left after earlier children. This supersedes the 2026-09-16 reading that a report is a fraction of the box the widget was given, wherever that box is a remainder rather than the child's whole area. - **Splitting `Widget::draw` into a measure and a draw is acceptable.** His original reason against it: a span whose children have fixed sizes can place them exactly where they were measured and so draw once, and that must survive. The smaller items below come first. ### `rel` means two things today Measured in a 400 px row whose first child is 100 px wide: | second child | box | meaning | | --- | --- | --- | | `rect.width(rel(0.5))`, declared | 100 to 300 | half the row | | nested span reporting `rel(0.5)` | 100 to 250 | half the remainder | The declared case is the rule; the reported case is wrong, and `tests/cases/layout.rs::a_span_reads_a_child_report_as_a_fraction_of_what_it_offered` pins the wrong behaviour and has to change. The two placement functions in `core/src/ui/painter.rs` differ by one line: ```rust // placed_box: a reported length is composed through the offered box let len = lens.axis(axis).within_len(span.len()); // declared_box: a declared length is a fraction of the parent's own box let len = Len::from_parts(len.rel, len.px); ``` Making `declared_box` compose the same way would be the one-line fix in the *wrong* direction. The composition `in_parent_frame` does is right where the offer is the child's whole area -- `Pad`'s inset, a `Stack` child, `Scroll`'s content -- and wrong where the offer is a positional remainder, which is `Span` along its axis. `Span` offers each child the room from the cursor to the end because a text has to wrap at the width actually left, so the pixel width of the offer and the base its fractions are taken of have to be separated: the remainder for one, the row for the other. Candidate: the ask carries what a report's fractions are of, defaulting to the offered box, and `Span` passes its own extent along the row. A child that drew at half of its remainder is then placed at half the row and redrawn there by the placing ask, which is one draw more, and exactly what `place` already does for any child whose placed box differs from its offer. A nested span whose child asks for half of it reports half of the row, gets it, and its child gets half of that: no circularity, because the base is the row and not the nested span's own answer. Not designed yet; it is first in "Next". ### The offer's answer is overwritten by every ask `ActiveData::answer` is documented as what the widget answered at its offer. `Painter::widget_at` guards its write with `answers_offer`, but `UiRenderState::draw_inner` writes the field unconditionally and returns the same value, so the guard is dead and the placing second ask overwrites the offer's answer with one about the placed box. The guard is from `29c7881`; the unconditional write arrived with `d3b0ebf`. ```rust // painter.rs, widget_at let answers_offer = self.at_offer && px == offered_px; let (size, holds) = self.state.draw_inner(...); if answers_offer { self.state.active.get_mut(&id.id()).unwrap().answer = (size, holds); } // render_state.rs, draw_inner, on every ask active.answer = settled; ``` `known_len`'s first-ask write and `update`'s resize path also write the field as though `draw_inner` did not; `known_len`'s stores the value it just read from the same field. The oracle passes, so layout is not wrong. What it can cost is churn: an answer about the placed box can miss `retained_answer` on the next offer ask and fall through to a remap and back. Not confirmed against the counters. Write the field in one place, and keep the guarded one. ### `redraw` reassembles `DrawInfo` by hand `ActiveData` copies eight fields of `DrawInfo` and `redraw` rebuilds the struct field by field. This is where the mask defect fixed in `ea6dbae` lived for as long as there was a local-redraw path. ```rust let info = DrawInfo { layer: active.layer, parent: active.parent, depth: active.depth, parent_move: active.parent_move, region_node: rsc.widgets().is_region_node(id), mask: active.parent_mask, given_len: active.given_len, offer_len: active.offer_len, px: given_px, offered_px, decided: active.decided, }; ``` Store the `DrawInfo` on `ActiveData` and write `DrawInfo { px, offered_px, ..active.info }`: the duplicated fields and the reconstruction go, and a new field cannot be forgotten. The pixel pair stays out on purpose; see "px is not stored" below. ### `Span`'s leftover boundary is a third expression for the room The decision uses a rounded division, `total.px.div(fixed)`, while the room the children get is a floored multiply, so the two disagree at the boundary. Measured with 300 px, `rel(2/3)` and a `leftover` child: | row width | leftover child | its threaded length | | --- | --- | --- | | 900.000 | undrawn | | | 900.001 | drawn | 0 steps | | 900.002 | drawn | 0 steps | Harmless at two steps, but the three-branch block collapses into the inverse that already exists. `room` is computed a few lines below the block as `Len::rel_max() - Len::from_parts(total.rel, total.px)`, and its `to_px` is exactly the threaded length the leftover children share: ```rust let room = Len::rel_max() - Len::from_parts(total.rel, total.px); let mut shares = false; if total.leftover > Weight::ZERO { shares = room.to_px(painter.px_len(axis)) > Px::ZERO; let holds = match shares { true => Holds::from(Px::STEP..=Px::MAX), false => Holds::from(Px::MIN..=Px::ZERO), }; painter.holds(axis, holds.through(room)); } ``` `through` already handles a negative fraction and a zero one, so the `fixed < 0` and `fixed == 0` branches go with it. The general lesson: `mul` floors while `div`, `div_int` and `ratio` round to nearest, so a boundary derived with a division guards a drawing made with a multiply. Derive boundaries through `through`, or make the grid floor everywhere. ### Where the residual comes from, and the snap Both remaining steps are a symbolic region re-expressed by division rather than recomputed the way a cold draw computes it: `AxisRemap::Scale` divides to find a part's fraction of the old box, and `placed_box` scales a mixed `Len` by the alignment. Neither touches the threaded pixel chain, so every layout decision already agrees warm against cold; what differs is the composed position the shader and hit testing see, by up to 0.002 px. Closing `Scale` exactly is possible: keep each primitive's region in its widget's own coordinates and recompose on a move with `within`, eight multiplies and no division against the current four divisions and twelve multiplies, exact by construction, sixteen bytes more per primitive. Closing the alignment one means resolving alignment in pixels, which costs the retained resize path. Neither is worth a thousandth of a pixel on its own. Where it does matter is `snap_floor` in `prelude.wgsl`, which adds half a layout step before flooring: that absorbs float error and not a layout step, and truncation makes "one step under an integer" the common residue. A third of 900 px is 299.999 on the grid and lands at 299 on screen, which is the pixel `08c9d5a` moved `tabs`'s arcs by. Rounding to the nearest pixel absorbs both the truncation and the two-step residual everywhere except within two steps of a half pixel, where layout never lands on purpose, and keeps integer widths for equal fractional parts: ```wgsl fn snap_floor(v: vec2) -> vec2 { return floor(v + 0.5); } ``` A rendering decision, so proposed rather than made. The check is the reference render set plus the oracle; expect `tabs` to move its arcs back. ### Smaller items - The comment on the `local == UiRegion::FULL` shortcut in `widget_at` says composing through `FULL` "is not quite the identity in f32". On the grid it is exact; the shortcut is performance only now. - An undrawn `leftover` child still contributes its gap, so a vanished child leaves a double gap. - Nested spans pass `leftover` weight up, so three leftover children in one inner span beside one in another get three quarters to one quarter. No other layout system does that, and the doc's old example of two and two did not distinguish it from per-span division. Confirm it is wanted. - `Fixed::div` by zero answers `MIN`/`MAX` while `ratio` answers `ZERO`; both are caller bugs under `debug_assert`, but the fallbacks differ. ### Measure and draw, later Splitting `Widget::draw` into a measure and a draw would delete `at_offer`, `offered`, `answers_offer`, the placing second ask and the reuse dance after it. It costs two methods on every widget and is a rewrite of a core that passes a thousand seeds at depth six. The one-draw property Bryan wants kept holds either way: a child with a fixed size needs no measure, and a measured child costs a measure plus a draw where it now costs a draw plus a reuse or a redraw. Last in "Next". ## How layout is decided ### Fixed point Decided with Bryan on 2026-09-15. Layout decides on a grid rather than in floats. - **`Fixed` is an `i32` counting `1 / 2^SHIFT`.** Adding and subtracting are exact; `mul` drops to the step below (Bryan, 2026-09-16: truncation is preferable); `div`, `div_int` and `ratio` round to nearest; `to_scale` takes the nearest step. Two routes to one place that land on one number are the same place, so everything downstream compares for equality. - **`Px` is `1/1024` px, `Rel` is `1/2^24` of a box, `Weight` is `1/65536` of a share.** `PX_SHIFT` and `REL_SHIFT` are the only statement of the first two; the shader's copy is prepended from them by `render::module_source`. `Px` was `1/64` first, where one rounding's residue was 0.016 px and enough to move a box. Range is +/-2.1M px and conversion to `f32` is exact to 16,384 px. - A weight is not a fraction: a list divides its room by the total of its weights, and `Rel::ratio` turns two weights into a share on the finer grid. - **Arithmetic wraps** (`4febabf`, Bryan: a coordinate past the range will not draw reasonably anyway, so wrap and break clearly). Saturating cost a twelfth of layout's instructions. `MIN` and `MAX` stand in for an unbounded end and are only ever compared against; `from_f32` is the one operation that clamps, and `Holds` keeps a saturating `narrow`. - A pointer, a wheel notch, a shaped glyph advance and a window size arrive as floats and go on the grid where they arrive. `Vec2` is what the GPU and the platform speak; `PxVec2` is what layout decides in. - **Do not widen the grid to chase a residue.** Every failure this branch saw was one value reached by two expressions, sitting on a boundary defined by the same value coming back the other way. No precision shrinks a residue that is the whole distance. ### A box in pixels is one multiply from its parent's `ActiveData` keeps a widget's box as lengths of its parent's box -- `given_len`, and `offer_len` for the box it was first asked about -- `DrawInfo` carries the pixel lengths themselves (`px`, `offered_px`), and a draw threads them down one `Len::to_px` at a time: the box its parent gave it, then the part of that box its own answer placed its drawing in, which `placed_lens` states once for both `placed_box` and the walk. `Painter::px_size` and `px_len` read that value, and `UiRenderState::asked_px` takes the same steps back up the parent chain when a local redraw starts part-way down the tree. Neither chain has a coordinate frame in it, so a region node cannot break either, and warm and cold reach every length by the same expression. - **`Holds::through` is the exact preimage of `px + floor(rel * box)`**: `floor(rel * B) >= lo - px` is `rel * B >= (lo - px) << REL` and `floor(rel * B) <= hi - px` is `rel * B < (hi - px + 1) << REL`, two `div_toward`s once the sign of `rel` has said which bound is which. The answer is an interval even for a single length, because a floor is not invertible. The range has to contain the box a drawing was made in (the `Holds` assertion in `draw_at`, debug only) and must not contain a box the drawing does not hold for (the oracle); being the preimage makes those one statement rather than a trade-off. - **Symbolic regions are for the GPU, hit testing and remaps alone.** `Moves::resolve` is the only walk left and it is the vertex shader's. Nothing layout decides is composed back up the move chain. - **`px` is not stored on `ActiveData`, deliberately.** A resize every widget's `Holds` admits redraws nothing, so a stored pixel length would be stale on every widget in the tree with nothing to say so. `asked_px` walks up only where a widget is already being redrawn; the mean chain is 2.8 levels. - **The window is not a move entry** (`5b78002`). A chain bottoms out in `MoveIdx::NONE`; the window is applied where a fraction becomes pixels, `to_px(output_size)` on the CPU and the uniform in the shader. A resize rewrites no retained entry and re-uploads nothing but the uniform; its cost is whatever `Holds` redraws. - Failed hypothesis, kept as the shape of the mistake: an offer composed back up the chain fell back to `FULL` under a region node and was resolved against that node's *placed* box, so everything under a `Scroll` was re-asked at the content's width and confirmed its own answer. Pinned by `unsettled::a_widget_under_a_region_node_is_asked_in_the_box_that_node_was_offered`. The old chain with an allowance in `through` passed that case and the old chain with the exact `through` failed it; both halves had to land at once. ### What the fuzzers tolerate `AGREE_STEPS` in `tests/scenario/mod.rs` is **2**, and both steps are positions. One is a box centred in a fraction of its parent against the same box centred in its own pixels, 0.001 px on a handful of seeds. The other is `AxisRemap::Scale` re-expressing a part as a fraction of a box that changed length; one step fails the 400-seed shrinker on `resize-size`, seeds 384 and 162, by 0.002 px while passing the 100-seed oracle. Two of the earlier sources were fixed rather than tolerated (`bdab558`): `Scroll` wrote a box it had been given back out as its own length in pixels, and `Span` placed each child a step from where the last ended rather than as the fixed parts before it plus one rounded share. See "Where the residual comes from" above for what closing the rest would cost. ## Retained-layout invariants - `Holds` is the interval of box lengths for which a widget's drawing and reported size stay valid. Reading `Painter::px_len` or `px_size` narrows it to the length read; `Painter::holds` widens it. Parent validity is the intersection of what its children induce. The contract is trusted: a widget declaring a wrong range is a defective widget, and Iris adds no defensive work to recover from one. - A retained drawing can be reused only when its `Holds` contains the new pixel box on both axes, its parent node is unchanged, its region-node choice matches the retained structure, it is on the layer it is asked for, and the widget is clean. A valid ordinary subtree moves without redrawing by recursive remap; a region node moves by one entry. - **A retained drawing belongs to the layer it was made on.** A container that measures a child by drawing it measures on the layer that child will draw on -- `Painter::child_layer_at` -- or it pays two draws a frame. - The first box a parent asks about is the offer; a later box chosen from the answer is the final box, not another answer. A dirty widget is re-asked in the box its parent gave it, and only where that box is as long as the offer; anything else is its parent's question, with the mark left on. Lengths and not whole boxes: what a drawing depends on is its lengths, so the same lengths elsewhere is the same question. - An answer is reusable only where both its measurement and the drawing in its final placed box remain valid; the drawing's `Holds` is translated back through `placed_lens` and intersected with the answer's. - `Painter::widget_decided(child, region, [bool; 2])` says the parent chose this box from the child's own answer along those axes, so the answer is not placed inside it again. A report of "half of what you give me" has no fixed point but zero, so the framework asks exactly twice: at the offer, and in the box chosen from the answer, final on the decided axes. `Span` decides the row axis, `Scroll` both, `Stack` both for its sizing child. `Pad` overrides nothing: its inset is exactly the inner where the box is its answer, and the slack is the inner's to sit in otherwise. - **Placement cannot be applied after the fact.** Three attempts at "draw the widget, then move its drawing to where its alignment says" failed, because the move is a change of frame and no split of the stored state carries it: moving `ActiveData::region` with the drawing made a later local redraw ask a differently rounded question, and leaving it made `placed` accumulate without bound because `try_reuse` returns a clean subtree's size without walking into it. Alignment is applied where the size is known -- `declared_box` for a rule, the placing second ask otherwise. - A widget that clips to its box reports its box: `Scroll` and `Masked` report `LEFTOVER` on both axes, and a `debug_assert` holds any widget that set a mask this draw to it. Overflowing is otherwise ordinary, which is why the assertion is narrowed to mask-setters. Where content shorter than a `Scroll`'s viewport sits is the scroll's own alignment, and its "fits at the start of any box" widening is gated on near alignment. - **A widget's own mask is not the one it inherited.** `ActiveData` keeps both; they differ exactly where the widget called `set_mask`, which says whose mask a move rewrites, and a local redraw is handed the inherited one. Pinned by `retained::a_masked_widget_redrawn_on_its_own_sets_its_mask_again`. - A span is as long across itself as its longest fixed child, unless a rule gives that length outright (`Painter::has_exact_size`), in which case it does not read its children there at all. Any relative or `leftover` child makes it report `leftover`. **Do not choose between a fixed and a relative child in pixels at the span's current width**: that admits multiple self-sizing fixed points, and generated seed 13 settled differently warm and cold under it. The same circularity is what a cap containing `leftover` would put into `SizeRule::Max`. - `Span`'s leftover/no-leftover split is a strict layout decision, not a rounding tolerance: its `Holds` range must use the same exact boundary as drawing. A tolerant endpoint retained zero-height children in seed 16; a boundary moved off where boxes land was needed in floats and is not on the grid. **A structural decision may not be taken on a hair's breadth** that two routes can disagree about. Pinned by `unsettled::a_box_that_only_rounds_past_its_fixed_children_leaves_nothing_over`. - A pixel comparison is equality. A length given in pixels is that many pixels wherever it ends up, structurally: `Len::within` adds a part's own pixels rather than scaling them. Pinned by `a_length_in_pixels_is_that_many_pixels_however_it_is_nested`. A length given as a share is not: equal shares come out one or two steps apart because positions, not lengths, are what gets rounded, so the row fills and no two children leave a seam (`equal_shares_differ_by_at_most_two_steps_and_fill_the_row`). - **A move that keeps a box's length is a translation, and exact.** A box that changed length re-expresses each part as a fraction of the new one, which rounds. This inverts the float-era rule; `tests/cases/drift.rs` pins that the grid does not drift either way. - `Scroll` must return the answer from the first box it asked about, whether retained or fresh; returning the final placed answer advanced one fixed-point iteration (seed 86). Content that fills the viewport unscrolled is handed back as it came, because the same box written as its own length in pixels does not round alike. - An asked-but-undrawn size dependency names the widget that asked as its parent (seed 10). `Painter` records size-dependency edges only when a parent reads a child's size or hint; an undrawn measured child stays recorded so a later change reaches whoever decided not to draw it. - Dirty widgets settle deepest-first. `dirty_size_under` stops a reader taking a retained answer while something below it is dirty; it is an optimization against laying out twice, not a validity mechanism. - Declared non-`leftover` lengths are resolved by the parent where the widget is drawn, so a declared-length change redraws the parent. A rule wins on the axis it names and the widget under it never learns of it. **A cap may not contain `leftover`**: a cap must read the report, so rule and report are one equation, and a share puts the row's division into it -- the multiple-fixed-point failure again. A cap is pixels and a fraction, which is what `Len` is. - Text shaping is retained separately from line breaking; a greedy break holds from its longest produced line through the width it was made at, reported through `Painter::holds`. - Region nodes: a node holds a whole `UiRegion` in its parent node's coordinates, `FULL` is the identity, widgets opt in with `.region_node()` or `Widgets::set_region_node`, and changing it redraws the subtree once. `.scrollable()` sets it once; raw `Scroll::new` does not. A removed node's move entry stays alive until every descendant has migrated. `Span` and `Align` add no nodes. - Alignment is one `f32` per axis (Bryan, 2026-09-15), default the middle on both because the edges assume a direction. One widget keeps one length per axis; a second length needs a second widget, `Wrapper` via `.wrapper()` (Bryan, 2026-09-16, `d21a215`). ## Verification at the current head At `32542d0`, then `ea6dbae` on top: - `cargo fmt --all --check`, `cargo clippy --workspace --all-targets -- -D warnings`, `cargo test --workspace`: green, 87 suite tests, 18 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 14.4 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 58 s, and at 1000 seeds of depth 6 in 147 s. - `view`, `minimal`, `random`, `tabs` and `text` byte-identical at 1920x1200 against `5b78002`, and the `tabs` touch replay before and after the gesture. `random` live-resized from 1920x1200 to 1280x800 is byte-identical to a cold 1280x800 render. - Twenty-five rig work counters identical on the `cold` and `resize` phases, which is what makes those rows under "Performance" a measurement. **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: nothing failed at 100 seeds of depth 4, which was all the oracle ever routinely ran, and every late defect surfaced at depth 5 or 6. ## 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 \ --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. **`rel` as a fraction of the containing widget's whole area**, on `Span`'s row axis, per the decision above. Fix the test that pins the remainder reading, and add the two-child case from the table. 2. Write `ActiveData::answer` in one place. 3. Keep the `DrawInfo` on `ActiveData`; delete the copied fields and the reconstruction in `redraw`. 4. `Span`'s leftover boundary through `Holds::through`. 5. The shader snap, if Bryan takes it: change, then the render set and the oracle. 6. The smaller items: the stale `f32` comment, the gap of an undrawn child, confirm nested `leftover` weights, one zero-divisor fallback. 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. 10. The measure/draw split, once the above is in. `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>`; 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.