# 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 Pruned on 2026-09-17: the chronicle of closed defects, superseded verification lists and cross-fixture tables went, the rules and the lessons stayed.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 **`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. **Widen one fuzzer axis at a time, and record which.** Seeds **1121** and **1839** at **depth 4** failed on `ea6dbae` and on every commit before it, and nothing in the routine verification reached them: the fast oracle takes ten seeds, the shrinker 400 at depth 5 and the long oracle 1000 at depth 6, so a defect past seed 400 at depth 4 had nowhere to show. Both are fixed by `4bd8607`. The scan that found them, worth running again after any change to layout: ```rust // tests/scan.rs, deleted once it had done its job over_seeds((1..=2000).collect(), |seed| { let grown = plan(seed, 4, &Edits::default()); for &case in ALL.iter() { if let Some(how) = diverges(&grown, case, seed) { println!("HIT seed {seed} case {} {how}", case.name()); } } }); ``` 261 s for 2000 seeds at depth 4 over all fifteen cases, and clean at `4bd8607`. `Rng::new` is `seed | 1`, so an even seed and the odd one above it are one tree: 1120 and 1121 are the same counterexample, as are 1838 and 1839. **Two ideas outrank everything else in this document** (Bryan, 2026-09-17). First, a changed tree lays out exactly as if it had been drawn that way from the start; that is what the retained machinery is for and what the oracle and shrinker check. Second, widgets are predictable: `px` is that many pixels, `rel(0.5)` is half of the containing widget's area however many siblings there are and wherever it sits among them, and `leftover` is a share of the room left once every sibling's `px` and `rel` are resolved. The invariants listed further down were accumulated by agents chasing single failures; any of them may be simplified or deleted if those two ideas still hold. ## Review of 2026-09-17 A fresh read of `core/src/fixed.rs`, `orientation/`, `ui/holds.rs`, `ui/painter.rs`, `ui/render_state.rs` and the position widgets, outside of doing work on them, with each finding checked by a scratch test. **Verdict.** The concepts are sound and stay. Fixed point on a `1/1024` grid is the right base for a layout that decides "same box or not" by equality. Threading the pixel box down the draw, with `Holds::through` the exact preimage of that one multiply, is the strongest idea in the code: layout has one route to every length and the reuse test is its exact inverse. Offer, given and placed is the ordinary measure-then-arrange model. What needed work was the bookkeeping around the second ask, one boundary in `Span` computed by an expression other than the drawing it guards, and a `rel` that meant two things. The two-step residual is structural and no grid width fixes it; where it becomes visible is the shader's snap. ### Decided by Bryan - **`rel` is a fraction of the containing widget's whole area.** In a span, `rel(0.5)` is half the span whatever else is in it and wherever it sits. It is never a fraction of what was left after earlier children. This supersedes the 2026-09-16 reading that a report is a fraction of the box the widget was given, wherever that box is a remainder rather than the child's whole area. - **`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; the extent reaches the widget through the painter. See "Frame and extent" below. ### A report is a fraction of the containing widget (landed, `ffd79f3`) **Superseded in part.** Bryan refined the rule twice on 2026-09-17: a fraction means whatever the *parent* says it means, and it need not even mean the same thing for two siblings -- "Main point is truly that rel is decided by parent". The span conclusion below is unchanged and the reason for it stands (a span's offer differs per child and the row does not), but `reports_of` is the wrong shape for saying it; see the parked padding branch. `rel(0.5)` is half the span whatever else is in it and wherever the child sits. A report used to come back composed through the box it was offered, and a span offers each child the room from its cursor, so a nested span taking half of what it was given took a quarter of a row whose first half was spoken for -- where the same half written as a rule on the child took half the row. The offer is still the remainder, because a text has to wrap at the width actually there. What separated from it is the base a report's fractions are of, which the ask now carries as `reports_of`: ```rust pub fn widget_at<'s, W: ?Sized>( &'s mut self, id: &'s StrongWidget, region: UiRegion, reports_of: UiVec2, decided: [bool; 2], ) -> DrawResult<'s, 'a, W> ``` It is `region.size()` wherever the box offered is the child's whole area -- `Pad`'s inset, a `Stack` child, `Scroll`'s content -- and `Span` passes `UiVec2::FULL_SIZE` along its row. `widget_decided` is gone; `widget_at` says both things about an ask rather than one of them. **A span can now overflow itself without bound**, which is the consequence Bryan's rule asks for: two children reporting half each take the whole row and a third starts past the end. Under the old reading `total.rel` could not exceed one, so `Span`'s `fixed <= 0` branches were only reachable through declared fractions; they are ordinary now, and with them boxes of negative length passed down to children. ### An answer is not an answer while anything under it is dirty (`0e0d4af`, 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, both real, neither ready **Superseded on 2026-09-17: the two branches are one defect, and it is in the protocol rather than in either widget.** See "Frame and extent" below. Do not chase seed 1091's three steps or the Inset's 47.5 px; both are the placing ask resolving a fraction in the answer box. What each branch got right is kept: the `Inset`/`Outset` pair and its tests, and the stack test asserting that half stays half. Their painter changes go. **`wip/stack-fraction-twice`.** A stack sized by a child that reports a *fraction* applies that fraction twice: its parent places the stack at the reported length, and `box_of(size)` then takes the same fraction of that box. Half a row becomes a quarter. Pixels are idempotent under a second application, so only a share ever shrank -- and warm and cold shrink alike, so **no oracle can see it**. Bryan: "I believe this exact thing has come up multiple times for some reason." The fix is that every child gets the whole box, plus `widget_at` not resolving a rule into a box already chosen from it. `Painter::box_of` is deleted rather than guarded: a first attempt made it answer the whole box on a `decided` axis, which fails because `place` can reuse the stack's *measuring* drawing by remapping it, so `Stack::draw` never re-runs. **A drawing has to be a function of its box alone** -- if `decided` is in it, a reused drawing is wrong. What stops it landing: seed 1091 at depth 4, `shuffle-swap-for-three`, disagrees by three steps where the oracle tolerates two (warm 1053.9971 against cold 1054). `box_of` was also making placement *exact*, by handing a child a box of exactly the length it asked for, and the whole box puts a rounding back at each nesting level. Find that composition; do not widen `AGREE_STEPS`. **`wip/padding-outset-and-inset`.** Padding goes outside what it pads and never insets the child (Bryan, 2026-09-17): otherwise a child's `rel` and `leftover` would mean the inner box while its `px` meant the outer one. `Padding::region` moves the child's box in rather than shrinking it, a new `Inset` widget with `.inset()` is the old behaviour, and `Pad` is to be renamed **`Outset`** with `.outset()` to match. `in_parent_frame`'s composition and the `reports_of` argument go with it -- a report comes up raw and the parent says what it is a fraction of, which `Inset` does for itself. What stops it landing: a child declaring `rel(0.5)` under an `Inset` comes out 47.5 px wide of the 190 inside rather than 95, and the second halving is unaccounted for. The `Pad` half is green on its own; three tests moved to `.inset()` because they were using padding as scaffolding rather than testing it. ### `Span`'s leftover boundary is a third expression for the room The decision uses a rounded division, `total.px.div(fixed)`, while the room the children get is a floored multiply, so the two disagree at the boundary. Measured with 300 px, `rel(2/3)` and a `leftover` child: | row width | leftover child | its threaded length | | --- | --- | --- | | 900.000 | undrawn | | | 900.001 | drawn | 0 steps | | 900.002 | drawn | 0 steps | Harmless at two steps, but the three-branch block collapses into the inverse that already exists. `room` is computed a few lines below the block as `Len::rel_max() - Len::from_parts(total.rel, total.px)`, and its `to_px` is exactly the threaded length the leftover children share: ```rust let room = Len::rel_max() - Len::from_parts(total.rel, total.px); let mut shares = false; if total.leftover > Weight::ZERO { shares = room.to_px(painter.px_len(axis)) > Px::ZERO; let holds = match shares { true => Holds::from(Px::STEP..=Px::MAX), false => Holds::from(Px::MIN..=Px::ZERO), }; painter.holds(axis, holds.through(room)); } ``` `through` already handles a negative fraction and a zero one, so the `fixed < 0` and `fixed == 0` branches go with it. The general lesson: `mul` floors while `div`, `div_int` and `ratio` round to nearest, so a boundary derived with a division guards a drawing made with a multiply. Derive boundaries through `through`, or make the grid floor everywhere. ### Where the residual comes from, and the snap Both remaining steps are a symbolic region re-expressed by division rather than recomputed the way a cold draw computes it: `AxisRemap::Scale` divides to find a part's fraction of the old box, and `placed_box` scales a mixed `Len` by the alignment. Neither touches the threaded pixel chain, so every layout decision already agrees warm against cold; what differs is the composed position the shader and hit testing see, by up to 0.002 px. Closing `Scale` exactly is possible: keep each primitive's region in its widget's own coordinates and recompose on a move with `within`, eight multiplies and no division against the current four divisions and twelve multiplies, exact by construction, sixteen bytes more per primitive. Closing the alignment one means resolving alignment in pixels, which costs the retained resize path. Neither is worth a thousandth of a pixel on its own. Where it does matter is `snap_floor` in `prelude.wgsl`, which adds half a layout step before flooring: that absorbs float error and not a layout step, and truncation makes "one step under an integer" the common residue. A third of 900 px is 299.999 on the grid and lands at 299 on screen, which is the pixel `08c9d5a` moved `tabs`'s arcs by. Rounding to the nearest pixel absorbs both the truncation and the two-step residual everywhere except within two steps of a half pixel, where layout never lands on purpose, and keeps integer widths for equal fractional parts: ```wgsl fn snap_floor(v: vec2) -> vec2 { 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. ### Frame and extent Read on 2026-09-17, against Bryan's question of whether the approach is fundamentally wrong or needs adjusting. The pieces the earlier review approved are sound and the fuzzers agree: fixed point, the pixel chain threaded down, `Holds::through` as the exact preimage, and the bottom-up settle, with every residual they leave one or two steps in a position. One thing is wrong, and it is the protocol rather than any widget: **the placing ask replaces the box a widget's children resolve fractions against with the widget's own answer.** Every "fraction twice" item is that defect, and it has lasted because it was fixed per widget four times (`decided`, `reports_of`, `box_of`, the stack branch's `resolve`, `Inset` against `Pad`), each of which stops the re-resolution at one level while it happens again inside the child's second draw, which no flag on the parent's ask can reach. The repository asserts both answers. `tests/cases/layout.rs` pins a nested span's `rel(0.5)` child at a quarter of the row and calls it correct ("half of that final box is what its own child takes"); the parked stack branch's test pins the same shape as a half. Both are consequences of the one line where a child's region composes within the widget's current box, which the placing ask has made the answer: ```rust // painter.rs, widget_at let within = match local == UiRegion::FULL { true => self.region, false => local.within(&self.region), }; ``` The reuse path gives the same result, because `AxisRemap::Scale` re-expresses the child as a fraction of the new box. `Len::within` itself is correct geometry and was never the problem. **The protocol is one draw body evaluated twice, and that is right.** The first evaluation measures and the second places, and the retained machinery skips the second wherever it can. Only what the second is told its box is has to change. The widget stays in its **frame**, the box it was offered, and is handed its **extent** as a region in the frame's coordinates: ```rust pub struct Painter<'a> { /// This widget's frame: the box it was offered, in `move_idx` /// coordinates. Every region this draw writes is in its coordinates /// and every fraction in one is of it. It does not change between the /// ask that measures and the ask that places. pub(super) frame: UiRegion, /// What of the frame this widget's answer took, in the frame's own /// coordinates: `FULL` while the answer is not yet known, and the /// placed answer once its parent has chosen where it sits. extent: UiRegion, /// The frame in pixels; the extent is one `Len` of it. pub(super) px: PxVec2, /// Whether this draw read its extent, which makes the drawing one that /// holds only for that extent, the way `px_len` does for a length. reads_extent: bool, } impl Painter<'_> { pub fn extent(&mut self) -> UiRegion { self.reads_extent = true; self.extent } } ``` `px_len` stays the frame's length, which is what a text wraps at and what `Span`'s leftover decision reads. `widget_at` composes the child's region within `self.frame` and resolves the child's fractions, declared or reported, against the child's own offer, which `ActiveData::offer_len` already records. `draw_inner` places the answer as a length of the offer rather than of the region passed this time: ```rust let lens = placed_lens(answer.0, declared, info.decided); let extent = lens.within_len(info.offer_len); let placed = placed_box(region, extent, align); ``` Where the parent hands back exactly the answer the slack is zero and the multiply exact, so `decided` stops being load-bearing. `try_reuse` compares frames, not placed boxes: a nested span's frame is the row on both asks, so nothing remaps and its rect stays at half the row. Remapping is for a frame that moved, a translation in the common case and a scale on a resize, the two cases `AxisRemap` has. **For a widget author the rule is one sentence: regions are written in the frame, and `painter.extent()` is "my box".** ```rust // span.rs: children sit across the row inside the span's extent. let across = painter.extent().axis(!axis); let region = UiRegion::from_axis(axis, span, across); // pad.rs: the inner sits in the pad's extent less the padding. painter.widget_within(&self.inner, self.padding.region_of(painter.extent())) // stack.rs: every child gets the stack's extent; `box_of` is deleted. let region = painter.extent(); ``` `Span`'s cursor arithmetic does not change: it is already a sum of the children's answers, which are fractions of the row. Checked by hand against the failing shapes: - Nested span: the inner's frame is the row on both asks; its rect is 200 px. `a_span_reads_a_child_report_as_a_fraction_of_the_row` flips to assert 200..400 for the inner. - Stack sized by a `rel(0.5)` child: the child's extent and the stack's are the same half of the frame, zero slack, exact. Seed 1091's three steps were `placed_box` scaling a nonzero slack per level. - Pad and Inset: the inner's fraction is of its offer and its extent sits in `extent().inset(padding)`. The 47.5 px was the offer changing between asks. - Pad around a 40 px rect, bottom aligned: on the second ask the pad's extent is 60 px, the inner region 40 px, slack zero, rect at 20..60. This is the case that needs the second evaluation at all, and why a frame alone is not enough. - A rule: a stack declared `width(rel(0.5))` holding a `rel(0.5)` rect gives a quarter of the row, correctly. A rule sets the frame; a report does not. Reading the extent narrows reuse the way reading a pixel length does. A widget that never calls `extent()` has a first drawing that holds for any extent and keeps it: `Stack` without a sizing child, `Scroll`, every leaf, `Span` along its axis. One that reads it is redrawn on the second ask only where the extent differs from the frame, and its children reuse through their own `Holds` since their frames did not move. Suppressing primitives on the first evaluation would be an optimization over this, not a requirement. The one place it costs more than today is a stack whose sizing child reports less than the frame: `box_of` narrowed the other children's first draw so they landed right at once, and with `extent()` they draw at `FULL` and again at the extent. They are usually a background rect; let the rig's counters say whether it matters. Fixed point, the pixel chain and `Holds::through` are untouched. The chain already threads `offer_len` beside `given_len` and `offered_px` beside `px`; the change is that the offer chain becomes the coordinate base and the placed chain is derived from it, rather than the other way round. ## 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, and that ordering is what makes an answer trustworthy; `dirty_size_under` is to be deleted once a resize goes through the same walk (see "A frame settles strictly bottom-up"). - 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 `e44dea3`: - `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 \ --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. ~~A resize marks the root and goes through the walk.~~ Landed as `a0693ac` and `e44dea3`; see the two sections above. The re-parenting half turned out to be two defects rather than the predicted one, and neither was the `depth()` assertion. 2. **Frame and extent**, as written above. This is the fundamental change and comes before anything built on the placing ask. It lands the two parked branches' tests (the stack test unchanged, `Inset`/`Outset` with the rename and the `.pad()` audit), flips `a_span_reads_a_child_report_as_a_fraction_of_the_row`, and deletes `box_of`, `reports_of`'s composition in `in_parent_frame`, and the `through(lens)` translation in `draw_inner`. Run the long fuzzers and the render set once for it. 3. Write `ActiveData::answer` in one place. 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>`; 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.