Link the ordinary tests once, and keep their debug info to line tables

Eleven `tests/*.rs` were eleven binaries, each linking the whole graph --
`wgpu` and all -- to run a handful of cases. They are modules of one target
now, under `tests/cases/`, and `cargo test --test suite layout::` still picks
one out. The fuzzers and the `*_cost` measurements stay their own targets:
they are run on their own and want to be selectable without building the
rest.

`profile.test` takes `debug = "line-tables-only"`, which is what a backtrace
here actually reads; the type and variable information was the bulk of what
the linker was writing.

Measured on this machine, rebuilding `iris`'s test targets after a change to
the crate: 14.3 s before, 9.8 s with one target, 7.7 s with both. `target/`
went from 45 GB to 13 GB. The suite still passes 102 tests, and the binary
still carries `.debug_line`.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/cases/determinism.rs

@@ -0,0 +1,101 @@
+//! A measurement that decides control flow.
+//!
+//! Comparing boxes catches a widget that moved. It does not catch a widget
+//! that measured a child, believed a different answer from the one a cold
+//! start would give, and took the other branch -- which is the same defect
+//! arriving somewhere it cannot be ignored. A widget here branches on what it
+//! measured, so a disagreement shows up as a different tree.
+
+use iris::harness::Harness;
+use iris::prelude::*;
+
+/// Measures `probe` across `axis` and draws one of two children on the
+/// answer. Its own configuration never changes, so which child is drawn is a
+/// property of the layout alone.
+struct BranchesOnMeasurement {
+    probe: StrongWidget,
+    wide: StrongWidget,
+    narrow: StrongWidget,
+    threshold: f32,
+}
+
+impl Widget for BranchesOnMeasurement {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        let mut top = UiRegion::FULL;
+        top.y.end = top.y.start.offset(Px::from_int(40));
+        let measured = painter.widget_within(&self.probe, top).len(Axis::X);
+        let px = measured.apply_leftover().to_px(painter.px_len(Axis::X));
+
+        let mut below = UiRegion::FULL;
+        below.y.start = below.y.start.offset(Px::from_int(40));
+        match px > Px::from_f32(self.threshold) {
+            true => painter.widget_within(&self.wide, below),
+            false => painter.widget_within(&self.narrow, below),
+        };
+        Size::LEFTOVER
+    }
+}
+
+fn plant(h: &mut Harness, threshold: f32) -> (WidgetId, WidgetId) {
+    let words = "the quick brown fox jumps over the lazy dog and keeps running";
+    let probe = wtext(words).size(16).wrap(true).add(&mut h.rsc);
+    let wide = rect(Color::RED).add(&mut h.rsc);
+    let narrow = rect(Color::BLUE).add(&mut h.rsc);
+    let branch = BranchesOnMeasurement {
+        probe: probe.add_strong(&mut h.rsc),
+        wide: wide.add_strong(&mut h.rsc),
+        narrow: narrow.add_strong(&mut h.rsc),
+        threshold,
+    }
+    .add(&mut h.rsc);
+    let side = rect(Color::GREEN).width(120).add(&mut h.rsc);
+    h.set_root((side, branch).span(Dir::RIGHT));
+    (wide.id(), narrow.id())
+}
+
+/// Which of the two branches drew, as a pair a test can compare.
+fn taken(h: &Harness, wide: WidgetId, narrow: WidgetId) -> (bool, bool) {
+    (h.region(&wide).is_some(), h.region(&narrow).is_some())
+}
+
+#[test]
+fn a_branch_taken_on_a_measurement_holds_across_repaints() {
+    for threshold in [0.0, 200.0, 400.0, 600.0, 779.0, 780.0, 781.0, 2000.0] {
+        let mut h = Harness::new((900, 600));
+        let (wide, narrow) = plant(&mut h, threshold);
+        let first = taken(&h, wide, narrow);
+        assert_ne!(first, (false, false), "threshold {threshold}: neither drew");
+
+        for frame in 0..4 {
+            h.rsc.widgets_mut().get_dyn_mut(wide);
+            h.rsc.widgets_mut().get_dyn_mut(narrow);
+            h.frame();
+            assert_eq!(
+                taken(&h, wide, narrow),
+                first,
+                "threshold {threshold}, repaint {frame}: the branch moved when nothing did"
+            );
+        }
+    }
+}
+
+#[test]
+fn a_branch_taken_on_a_measurement_is_the_one_a_cold_start_takes() {
+    for threshold in [0.0, 200.0, 400.0, 600.0, 779.0, 780.0, 781.0, 2000.0] {
+        let mut warm = Harness::new((900, 600));
+        let (wide, narrow) = plant(&mut warm, threshold);
+        warm.resize((640, 480));
+        warm.frame();
+        warm.rsc.widgets_mut().get_dyn_mut(wide);
+        warm.frame();
+
+        let mut cold = Harness::new((640, 480));
+        let (cwide, cnarrow) = plant(&mut cold, threshold);
+
+        assert_eq!(
+            taken(&warm, wide, narrow),
+            taken(&cold, cwide, cnarrow),
+            "threshold {threshold}: warm and cold took different branches"
+        );
+    }
+}

tests/cases/drift.rs

@@ -0,0 +1,45 @@
+//! What a retained drawing costs in accuracy when it is moved instead of made
+//! again. A subtree's stored regions are the only record of where it is, so a
+//! move that works from the last answer rather than from the box it is now in
+//! integrates its own rounding, and nothing later recomputes it. Re-expressing
+//! each part as the same fraction of the new box is what keeps a long-lived
+//! layout on the one a cold start produces.
+
+use iris::harness::Harness;
+use iris::prelude::*;
+
+/// A row of a fixed height under a bar, so changing the bar's height moves the
+/// row without changing the box it is given: the move path, repeatedly.
+fn plant(h: &mut Harness, bar_height: f32) -> (WeakWidget<Rect>, WeakWidget<Rect>) {
+    let bar = rect(Color::RED).height(bar_height).add(&mut h.rsc);
+    let inner = rect(Color::BLUE).add(&mut h.rsc);
+    let row = (inner, rect(Color::GREEN)).span(Dir::RIGHT).height(100);
+    h.set_root((bar, row).span(Dir::DOWN));
+    (bar, inner)
+}
+
+/// Enough moves to pass the 0.05 physical pixels layout treats as the same
+/// place, for a move that adds an offset to the last answer. Measured on this
+/// fixture on 2026-09-15: adding the offset to both ends of a span shortened
+/// the row by 0.071 over this many moves and by 0.712 over ten times as many,
+/// growing with the count rather than settling. Placing the far end from the
+/// near one instead left 0.069, because the length is re-derived either way.
+const MOVES: usize = 20_000;
+
+#[test]
+fn a_subtree_moved_many_times_stays_where_a_cold_layout_puts_it() {
+    let mut warm = Harness::new((640, 900));
+    let (bar, inner) = plant(&mut warm, 40.0);
+    let mut height = 40.0;
+    for step in 0..MOVES {
+        height = 40.0 + (step % 300) as f32 * 0.37;
+        warm.set_len(bar, Axis::Y, height);
+        warm.frame();
+    }
+
+    let mut cold = Harness::new((640, 900));
+    let (_, cold_inner) = plant(&mut cold, height);
+    cold.frame();
+
+    assert_eq!(warm.region(&inner), cold.region(&cold_inner));
+}

tests/cases/idempotence.rs

@@ -0,0 +1,29 @@
+//! Whether measuring a widget and then giving it the length it reported is a
+//! fixed point, which is what a span that sizes to its children needs.
+
+use iris::harness::Harness;
+use iris::prelude::*;
+
+#[test]
+fn a_wrapping_text_in_a_span_settles_on_one_width() {
+    let mut h = Harness::new((900, 600));
+    let words = "the quick brown fox jumps over the lazy dog and keeps on running \
+                 until it reaches the end of a rather long line of text";
+    let t = wtext(words).size(16).wrap(true).add(&mut h.rsc);
+    let filler = rect(Color::BLUE).add(&mut h.rsc);
+    h.set_root((t, filler).span(Dir::RIGHT));
+
+    let mut widths = Vec::new();
+    for _ in 0..6 {
+        let r = h.region(&t.id()).unwrap();
+        widths.push(r.bot_right.x - r.top_left.x);
+        // Redrawing it changes nothing about the state, so nothing may move.
+        h.rsc.widgets_mut().get_dyn_mut(t.id());
+        h.frame();
+    }
+    println!("widths over six frames: {widths:?}");
+    assert!(
+        widths.windows(2).all(|w| w[0] == w[1]),
+        "a repaint that changed nothing moved it: {widths:?}"
+    );
+}

tests/cases/layout.rs

@@ -0,0 +1,454 @@
+//! Where a frame puts things, with no window to put them in.
+
+use iris::harness::{Harness, assert_corners};
+use iris::prelude::*;
+
+/// A fixed 100 wide, and the rest of the 400 to its neighbour.
+fn two_rects(h: &mut Harness) -> (WidgetId, WidgetId) {
+    let left = rect(Color::RED).width(100).add(&mut h.rsc);
+    let right = rect(Color::BLUE).add(&mut h.rsc);
+    h.set_root((left, right).span(Dir::RIGHT));
+    (left.id(), right.id())
+}
+
+#[test]
+fn a_span_gives_each_child_the_width_it_asked_for() {
+    let mut h = Harness::new((400, 200));
+    let (left, right) = two_rects(&mut h);
+
+    assert_corners!(h, left, (0, 0), (100, 200));
+    assert_corners!(h, right, (100, 0), (400, 200));
+}
+
+#[test]
+fn a_full_ortho_span_reports_relative_full() {
+    let mut h = Harness::new((400, 200));
+    let child = rect(Color::RED).height(40).add(&mut h.rsc);
+    let span = (child,)
+        .span(Dir::RIGHT)
+        .ortho(OrthoSize::Full)
+        .add(&mut h.rsc);
+    h.set_root(span);
+
+    assert_eq!(h.render.active[&span.id()].size.y, Len::rel(1.0));
+}
+
+#[test]
+fn a_children_ortho_span_reports_its_tallest_fixed_child() {
+    let mut h = Harness::new((400, 200));
+    let short = rect(Color::RED).height(40).add(&mut h.rsc);
+    let tall = rect(Color::BLUE).height(70).add(&mut h.rsc);
+    let span = (short, tall)
+        .span(Dir::RIGHT)
+        .ortho(OrthoSize::Children)
+        .add(&mut h.rsc);
+    h.set_root(span);
+
+    assert_eq!(h.render.active[&span.id()].size.y, Len::px(70.0));
+}
+
+#[test]
+fn resizing_relays_out_against_the_new_output() {
+    let mut h = Harness::new((400, 200));
+    let (left, right) = two_rects(&mut h);
+
+    h.resize((800, 100));
+    assert!(h.needs_redraw());
+    h.frame();
+
+    assert_corners!(h, left, (0, 0), (100, 100));
+    assert_corners!(h, right, (100, 0), (800, 100));
+}
+
+#[test]
+fn an_empty_widget_takes_a_share_of_a_span() {
+    let mut h = Harness::new((400, 200));
+    let gap = ().add(&mut h.rsc);
+    let right = rect(Color::BLUE).width(100).add(&mut h.rsc);
+    h.set_root((gap, right).span(Dir::RIGHT));
+
+    assert_corners!(h, gap, (0, 0), (300, 200));
+    assert_corners!(h, right, (300, 0), (400, 200));
+}
+
+#[test]
+fn a_child_drawn_twice_moves_once() {
+    let mut h = Harness::new((400, 200));
+    // The span measures a child and then places it; listing it twice would
+    // move it twice. The span's own fixed total is shorter than the window,
+    // so the span is centred in it and everything under it carries that.
+    let inner = rect(Color::BLUE).add(&mut h.rsc);
+    let centered = inner.center().width(200).add(&mut h.rsc);
+    let left = rect(Color::RED).width(100).add(&mut h.rsc);
+    h.set_root((left, centered).span(Dir::RIGHT));
+    assert_corners!(h, inner, (150, 0), (350, 200));
+
+    h.set_len(left, Axis::X, 150);
+    h.frame();
+
+    assert_corners!(h, inner, (175, 0), (375, 200));
+}
+
+#[test]
+fn alignment_accepts_an_arbitrary_fraction_and_changes_at_runtime() {
+    let mut h = Harness::new((400, 200));
+    let fixed = rect(Color::BLUE).sized((100, 100)).add(&mut h.rsc);
+    h.rsc
+        .widgets_mut()
+        .set_alignment(fixed, Axis::X, AxisAlign::new(0.25));
+    h.rsc
+        .widgets_mut()
+        .set_alignment(fixed, Axis::Y, AxisAlign::NEG);
+    h.set_root(fixed);
+    assert_corners!(h, fixed, (75, 0), (175, 100));
+
+    h.rsc
+        .widgets_mut()
+        .set_alignment(fixed, Axis::X, AxisAlign::new(0.75));
+    h.frame();
+    assert_corners!(h, fixed, (225, 0), (325, 100));
+}
+
+#[test]
+fn a_resize_lands_where_a_cold_start_would() {
+    let build = |h: &mut Harness| {
+        let para = wtext(
+            "Wrapping shapes one source into as many lines as its container leaves room \
+             for, so the height of a paragraph is an answer rather than a setting.",
+        )
+        .size(20)
+        .wrap(true)
+        .pad(16)
+        .add(&mut h.rsc);
+        let below = rect(Color::RED).add(&mut h.rsc);
+        let root = (para, below).span(Dir::DOWN).pad(12);
+        h.set_root(root);
+        (para, below)
+    };
+
+    let mut cold = Harness::new((900, 1200));
+    let (cold_para, cold_below) = build(&mut cold);
+
+    let mut resized = Harness::new((1920, 1200));
+    let (para, below) = build(&mut resized);
+    resized.resize((900, 1200));
+    resized.frame();
+
+    assert_eq!(resized.region(&para), cold.region(&cold_para), "paragraph");
+    assert_eq!(resized.region(&below), cold.region(&cold_below), "below");
+}
+
+#[test]
+fn a_fixed_box_is_drawn_again_rather_than_stretched() {
+    let mut h = Harness::new((400, 400));
+    // The panel fills a stack sized by its sibling, so it is first asked in
+    // the whole box and then given the shorter one. Reusing it in that fixed
+    // box afterwards would leave it whatever height it happened to have.
+    let panel = rect(Color::BLUE).add(&mut h.rsc);
+    let leaf = rect(Color::RED).height(100).add(&mut h.rsc);
+    let stack = (panel, leaf)
+        .stack()
+        .size(StackSize::Child(1))
+        .add(&mut h.rsc);
+    h.set_root(stack.align(Align::TOP));
+    assert_corners!(h, panel, (0, 0), (400, 100));
+
+    h.set_len(leaf, Axis::Y, 250);
+    h.frame();
+
+    assert_corners!(h, panel, (0, 0), (400, 250));
+}
+
+#[test]
+fn a_moved_subtree_takes_its_children_with_it() {
+    let mut h = Harness::new((400, 400));
+    let first = rect(Color::RED).height(40).add(&mut h.rsc);
+    let inner = rect(Color::BLUE).add(&mut h.rsc);
+    let row = inner.pad(10).height(40).region_node().add(&mut h.rsc);
+    // 80 of fixed rows in a 400 window, so the span takes 80 and sits in the
+    // middle of what it was given.
+    h.set_root((first, row).span(Dir::DOWN));
+    assert_corners!(h, inner, (10, 210), (390, 230));
+
+    h.set_len(first, Axis::Y, 80);
+    h.frame();
+
+    // The row opted into one movable region, so its descendants follow one
+    // entry rather than having their primitive regions rewritten.
+    assert_corners!(h, inner, (10, 230), (390, 250));
+}
+
+#[test]
+fn a_fixed_length_child_keeps_it_when_the_box_around_it_grows() {
+    let mut h = Harness::new((400, 200));
+    let fixed = rect(Color::BLUE).width(50).add(&mut h.rsc);
+    let leftover = rect(Color::GREEN).add(&mut h.rsc);
+    let panel = (fixed, leftover).span(Dir::RIGHT).add(&mut h.rsc);
+    // Changing the bar's width is the only thing that changes the box the
+    // panel and everything under it was drawn for.
+    let bar = rect(Color::RED).width(100).add(&mut h.rsc);
+    h.set_root((bar, panel).span(Dir::RIGHT));
+    assert_corners!(h, fixed, (100, 0), (150, 200));
+    assert_corners!(h, leftover, (150, 0), (400, 200));
+
+    h.set_len(bar, Axis::X, 200);
+    h.frame();
+
+    // The panel's box is 100 shorter, so the fixed child is the same 50 wide
+    // against its new start and the one taking what is left absorbs the change.
+    assert_corners!(h, fixed, (200, 0), (250, 200));
+    assert_corners!(h, leftover, (250, 0), (400, 200));
+}
+
+#[test]
+fn a_box_with_a_fixed_length_can_be_stretched_on_its_other_axis() {
+    let mut h = Harness::new((400, 200));
+    // The row is 40 tall whatever happens, which used to make its drawing
+    // impossible to take out of: recovering a fraction of a box needs a
+    // relative extent, and it has none on that axis.
+    let inner = rect(Color::BLUE).add(&mut h.rsc);
+    let row = inner.pad(10).height(40).add(&mut h.rsc);
+    let filler = rect(Color::GREEN).add(&mut h.rsc);
+    // This column is an item in a row, so it takes the width left for it
+    // rather than asking for a full row-width in addition to the bar.
+    let column = (row, filler)
+        .span(Dir::DOWN)
+        .ortho(OrthoSize::Children)
+        .add(&mut h.rsc);
+    let bar = rect(Color::RED).width(100).add(&mut h.rsc);
+    h.set_root((bar, column).span(Dir::RIGHT));
+    assert_corners!(h, inner, (110, 10), (390, 30));
+
+    h.set_len(bar, Axis::X, 200);
+    h.frame();
+
+    assert_corners!(h, inner, (210, 10), (390, 30));
+}
+
+#[test]
+fn only_a_region_node_lengthens_the_chain_and_it_can_be_removed() {
+    let mut h = Harness::new((400, 200));
+    let leaf = rect(Color::BLUE).add(&mut h.rsc);
+    let buried = leaf.pad(4).pad(4).pad(4).pad(4).add(&mut h.rsc);
+    let bar = rect(Color::RED).width(100).add(&mut h.rsc);
+    h.set_root((bar, buried).span(Dir::RIGHT));
+
+    let move_idx = h.render.active[&leaf.id()].parent_move;
+    assert_eq!(h.render.moves.depth(move_idx), 1, "only the root region");
+
+    h.rsc.widgets_mut().set_region_node(buried, true);
+    h.frame();
+    let move_idx = h.render.active[&leaf.id()].parent_move;
+    assert_eq!(
+        h.render.moves.depth(move_idx),
+        2,
+        "the opted-in widget's region and the root region"
+    );
+
+    h.rsc.widgets_mut().set_region_node(buried, false);
+    h.frame();
+    let move_idx = h.render.active[&leaf.id()].parent_move;
+    assert_eq!(h.render.moves.depth(move_idx), 1);
+}
+
+/// A span that sizes from its children passes their `leftover` weight up
+/// than collapsing it to one share, so nesting divides the same space instead
+/// of re-dividing a share of it.
+#[test]
+fn nested_spans_divide_the_space_once_however_deep_the_nesting_is() {
+    let mut h = Harness::new((400, 200));
+    let (a, b, c, d) = (
+        rect(Color::RED).add(&mut h.rsc),
+        rect(Color::BLUE).add(&mut h.rsc),
+        rect(Color::GREEN).add(&mut h.rsc),
+        rect(Color::WHITE).add(&mut h.rsc),
+    );
+    let left = (a, b).span(Dir::RIGHT).add(&mut h.rsc);
+    let right = (c, d).span(Dir::RIGHT).add(&mut h.rsc);
+    h.set_root((left, right).span(Dir::RIGHT));
+
+    for (i, id) in [a, b, c, d].into_iter().enumerate() {
+        let x = i as f32 * 100.0;
+        assert_corners!(h, id, (x, 0), (x + 100.0, 200));
+    }
+}
+
+/// The same space, unevenly nested: weights carried up mean a share is a
+/// share of the whole, not of whatever branch a widget happens to sit in.
+#[test]
+fn an_uneven_nesting_still_gives_every_share_the_same_length() {
+    let mut h = Harness::new((400, 200));
+    let (a, b, c, d) = (
+        rect(Color::RED).add(&mut h.rsc),
+        rect(Color::BLUE).add(&mut h.rsc),
+        rect(Color::GREEN).add(&mut h.rsc),
+        rect(Color::WHITE).add(&mut h.rsc),
+    );
+    let one = (a,).span(Dir::RIGHT).add(&mut h.rsc);
+    let three = (b, c, d).span(Dir::RIGHT).add(&mut h.rsc);
+    h.set_root((one, three).span(Dir::RIGHT));
+
+    for (i, id) in [a, b, c, d].into_iter().enumerate() {
+        let x = i as f32 * 100.0;
+        assert_corners!(h, id, (x, 0), (x + 100.0, 200));
+    }
+}
+
+/// Where the shader puts an edge: the two parts of a scalar are floored
+/// apart, so a fraction and a pixel offset snap independently, and each is
+/// taken to the boundary it composes to within half a step of. Kept in step
+/// with `snap_floor` in `prelude.wgsl`.
+fn drawn_edges(h: &Harness, id: WidgetId, axis: Axis) -> (f32, f32) {
+    let active = &h.render.active[&id];
+    let region = h.render.moves.resolve(active.parent_move, active.region);
+    let dim = h.size().axis(axis);
+    let snap = |v: f32| (v + Px::STEP.to_f32() * 0.5).floor();
+    let edge = |s: UiScalar| snap(s.rel.to_f32() * dim) + snap(s.px.to_f32());
+    let span = region.axis(axis);
+    (edge(span.start), edge(span.end))
+}
+
+fn hairline(h: &mut Harness, marks: &mut Vec<WidgetId>) -> StrongWidget {
+    let mark = rect(Color::RED).width(1).add_strong(&mut h.rsc);
+    marks.push(mark.id());
+    mark
+}
+
+fn share(h: &mut Harness, inner: StrongWidget, ratio: f32) -> StrongWidget {
+    h.set_len(&inner, Axis::X, Len::leftover(ratio));
+    inner
+}
+
+/// Shares in weights no binary fraction lands on, a padding on one branch
+/// and not the other, so an edge falls near an integer as often as it can.
+fn hairlines(h: &mut Harness, depth: usize, marks: &mut Vec<WidgetId>) -> StrongWidget {
+    let mut span = Span::empty(Dir::RIGHT);
+    if depth == 0 {
+        let left = rect(Color::BLUE).add_strong(&mut h.rsc);
+        let left = share(h, left, 3.0);
+        span.push(left);
+        let mark = hairline(h, marks);
+        span.push(mark);
+        let right = rect(Color::BLUE).add_strong(&mut h.rsc);
+        let right = share(h, right, 7.0);
+        span.push(right);
+        return span.add_strong(&mut h.rsc);
+    }
+    let first = hairlines(h, depth - 1, marks);
+    let first = share(h, first, 3.0);
+    span.push(first);
+    let second = hairlines(h, depth - 1, marks);
+    let second = Pad {
+        padding: Padding {
+            left: Px::from_int(3),
+            right: Px::from_int(7),
+            top: Px::ZERO,
+            bottom: Px::ZERO,
+        },
+        inner: second,
+    }
+    .add_strong(&mut h.rsc);
+    let second = share(h, second, 5.0);
+    span.push(second);
+    span.add_strong(&mut h.rsc)
+}
+
+/// A one-pixel line is a pixel wherever it is drawn. Both edges of a fixed
+/// length share their box's fraction, so composing the chain moves them
+/// together and the shader's `floor` cannot round the pixel between them
+/// away -- only shift it. A separator that disappeared at one window size
+/// would be a defect no size comparison catches.
+#[test]
+fn a_one_pixel_line_keeps_its_pixel_through_a_chain() {
+    let mut h = Harness::new((1920, 1200));
+    let mut marks = Vec::new();
+    let root = hairlines(&mut h, 4, &mut marks);
+    h.state.set_root(root);
+    h.frame();
+    assert_eq!(marks.len(), 16);
+
+    for size in [(1920, 1200), (1919, 1201), (997, 1003), (1367, 733)] {
+        h.resize(size);
+        h.frame();
+        for mark in &marks {
+            let (start, end) = drawn_edges(&h, *mark, Axis::X);
+            assert_eq!(end - start, 1.0, "at {size:?}, mark {mark:?}");
+        }
+    }
+}
+
+/// A span short of room takes it from its shares, which go to nothing and
+/// then to nothing wider; the fixed lengths between them keep their pixels.
+/// Collapsing those to make room would delete a separator the caller asked
+/// for, which is worse than overflowing.
+#[test]
+fn a_span_out_of_room_shrinks_its_shares_and_not_its_fixed_lengths() {
+    let mut h = Harness::new((400, 20));
+    let mut marks = Vec::new();
+    let mut span = Span::empty(Dir::RIGHT);
+    for _ in 0..3 {
+        let share_of = rect(Color::BLUE).add_strong(&mut h.rsc);
+        let share_of = share(&mut h, share_of, 1.0);
+        span.push(share_of);
+        let mark = hairline(&mut h, &mut marks);
+        span.push(mark);
+    }
+    let root = span.add_strong(&mut h.rsc);
+    h.state.set_root(root);
+    h.frame();
+
+    for width in [400, 10, 3, 1] {
+        h.resize((width, 20));
+        h.frame();
+        for mark in &marks {
+            let (start, end) = drawn_edges(&h, *mark, Axis::X);
+            assert_eq!(end - start, 1.0, "at {width} wide, mark {mark:?}");
+        }
+    }
+}
+
+#[test]
+fn only_a_pure_leftover_child_disappears_when_nothing_is_left() {
+    let mut h = Harness::new((100, 20));
+    let fixed = rect(Color::RED).width(100).add(&mut h.rsc);
+    let leftover = rect(Color::BLUE).add(&mut h.rsc);
+    h.set_root((fixed, leftover).span(Dir::RIGHT));
+
+    assert_corners!(h, fixed, (0, 0), (100, 20));
+    assert_eq!(h.region(&leftover), None);
+
+    // An undrawn child remains a dependency of the span, so making room for
+    // it draws it without rebuilding the tree.
+    h.set_len(fixed, Axis::X, 60);
+    h.frame();
+    assert_corners!(h, leftover, (60, 0), (100, 20));
+
+    let mut h = Harness::new((100, 20));
+    let fixed = rect(Color::RED).width(100).add(&mut h.rsc);
+    let mixed = rect(Color::BLUE)
+        .width(Len::px(20) + Len::LEFTOVER)
+        .add(&mut h.rsc);
+    h.set_root((fixed, mixed).span(Dir::RIGHT));
+
+    // Pixels and fractions still overflow; only a child whose entire length
+    // is leftover is omitted.
+    assert_corners!(h, mixed, (100, 0), (120, 20));
+}
+
+#[test]
+fn leftover_children_disappear_at_the_exact_fixed_content_boundary() {
+    let mut h = Harness::new((100, 100));
+    let first = rect(Color::RED).height(90).add(&mut h.rsc);
+    let a = rect(Color::GREEN).add(&mut h.rsc);
+    let b = rect(Color::BLUE).add(&mut h.rsc);
+    let inner = (a, b).span(Dir::DOWN).gap(4).add(&mut h.rsc);
+    h.set_root((first, inner).span(Dir::DOWN));
+    assert!(h.region(&a).is_some());
+    assert!(h.region(&b).is_some());
+
+    h.set_len(first, Axis::Y, 96.0);
+    h.frame();
+
+    assert!(h.region(&a).is_none());
+    assert!(h.region(&b).is_none());
+}

tests/cases/pointer.rs

@@ -0,0 +1,85 @@
+//! Which widget an input reaches.
+
+use std::{cell::RefCell, rc::Rc};
+
+use iris::harness::{Harness, TouchScript};
+use iris::prelude::*;
+
+#[test]
+fn a_press_reaches_only_the_widget_under_the_cursor() {
+    let mut h = Harness::new((400, 200));
+    let clicks = Rc::new(RefCell::new(Vec::new()));
+
+    let (on_left, on_right) = (clicks.clone(), clicks.clone());
+    let left = rect(Color::RED)
+        .width(100)
+        .on(CursorSense::click(), move |_, _| {
+            on_left.borrow_mut().push("left")
+        })
+        .add(&mut h.rsc);
+    let right = rect(Color::BLUE)
+        .on(CursorSense::click(), move |_, _| {
+            on_right.borrow_mut().push("right")
+        })
+        .add(&mut h.rsc);
+    h.set_root((left, right).span(Dir::RIGHT));
+
+    h.click((50, 100));
+    assert_eq!(*clicks.borrow(), ["left"]);
+
+    h.click((300, 100));
+    assert_eq!(*clicks.borrow(), ["left", "right"]);
+}
+
+#[test]
+fn hover_ends_when_the_cursor_leaves_the_window() {
+    let mut h = Harness::new((400, 200));
+    let hovered = Rc::new(RefCell::new(0));
+    let ended = Rc::new(RefCell::new(0));
+
+    let (h_count, e_count) = (hovered.clone(), ended.clone());
+    let widget = rect(Color::RED)
+        .on(CursorSense::HoverStart, move |_, _| {
+            *h_count.borrow_mut() += 1
+        })
+        .on(CursorSense::HoverEnd, move |_, _| {
+            *e_count.borrow_mut() += 1
+        })
+        .add(&mut h.rsc);
+    h.set_root(widget);
+
+    h.move_to((200, 100));
+    assert_eq!((*hovered.borrow(), *ended.borrow()), (1, 0));
+
+    // A second sample inside the same widget is not a second hover.
+    h.move_to((210, 100));
+    assert_eq!((*hovered.borrow(), *ended.borrow()), (1, 0));
+
+    h.leave();
+    assert_eq!((*hovered.borrow(), *ended.borrow()), (1, 1));
+}
+
+#[test]
+fn a_recorded_gesture_presses_where_it_says() {
+    let mut h = Harness::new((400, 200));
+    let clicks = Rc::new(RefCell::new(Vec::new()));
+
+    let (on_left, on_right) = (clicks.clone(), clicks.clone());
+    let left = rect(Color::RED)
+        .width(100)
+        .on(CursorSense::click(), move |_, _| {
+            on_left.borrow_mut().push("left")
+        })
+        .add(&mut h.rsc);
+    let right = rect(Color::BLUE)
+        .on(CursorSense::click(), move |_, _| {
+            on_right.borrow_mut().push("right")
+        })
+        .add(&mut h.rsc);
+    h.set_root((left, right).span(Dir::RIGHT));
+
+    let script = TouchScript::parse("0 down 300 100\n80 up 300 100").unwrap();
+    h.replay(&script);
+
+    assert_eq!(*clicks.borrow(), ["right"]);
+}

tests/cases/pointer_routing.rs

@@ -0,0 +1,225 @@
+//! Input across layers: what stops at a layer, what passes through it, and
+//! where hovering stops.
+
+use std::{cell::RefCell, rc::Rc};
+
+use iris::harness::Harness;
+use iris::prelude::*;
+
+const WINDOW: f32 = 100.0;
+
+/// Every sense that has fired on one widget since it was last read.
+#[derive(Default, Clone)]
+struct Fired(Rc<RefCell<Vec<CursorSense>>>);
+
+impl Fired {
+    fn take(&self) -> Vec<CursorSense> {
+        std::mem::take(&mut self.0.borrow_mut())
+    }
+}
+
+/// A widget filling whatever it is given, recording the senses it is sent.
+fn listener(h: &mut Harness, senses: impl Into<CursorSenses>) -> (WeakWidget<Rect>, Fired) {
+    let fired = Fired::default();
+    let record = fired.clone();
+    let id = rect(Color::WHITE)
+        .on(senses.into(), move |ctx, _| {
+            record.0.borrow_mut().push(ctx.data.sense)
+        })
+        .add(&mut h.rsc);
+    (id, fired)
+}
+
+/// A widget with no senses of its own, to leave a gap beside one that has.
+fn blank(h: &mut Harness) -> WeakWidget<Rect> {
+    rect(Color::WHITE).add(&mut h.rsc)
+}
+
+fn harness() -> Harness {
+    Harness::new((WINDOW, WINDOW))
+}
+
+#[test]
+fn hover_stops_at_the_topmost_widget() {
+    let mut h = harness();
+    let (bottom, bottom_hover) = listener(&mut h, CursorSense::HoverStart);
+    let (middle, middle_hover) = listener(&mut h, CursorSense::HoverStart);
+    let (top, top_hover) = listener(&mut h, CursorSense::HoverStart);
+    h.set_root((bottom, middle, top).stack());
+
+    h.move_to((50, 50));
+
+    assert_eq!(top_hover.take(), [CursorSense::HoverStart]);
+    assert_eq!(
+        middle_hover.take(),
+        [],
+        "hover is not shared with a layer below"
+    );
+    assert_eq!(bottom_hover.take(), []);
+}
+
+#[test]
+fn a_scroll_passes_through_every_widget_that_does_not_want_it() {
+    let mut h = harness();
+    let (list, scrolled) = listener(&mut h, CursorSense::Scroll);
+    let (button, clicked) = listener(&mut h, CursorSense::click());
+    let (overlay, overlay_clicked) = listener(&mut h, CursorSense::click());
+    h.set_root((list, button, overlay).stack());
+
+    h.move_to((50, 50));
+    h.scroll((0, 10));
+
+    assert_eq!(
+        scrolled.take(),
+        [CursorSense::Scroll],
+        "two layers of click-only widgets do not stop a scroll"
+    );
+    assert_eq!(clicked.take(), []);
+    assert_eq!(overlay_clicked.take(), []);
+}
+
+#[test]
+fn hovering_a_button_above_does_not_stop_a_later_scroll() {
+    let mut h = harness();
+    let (list, scrolled) = listener(&mut h, CursorSense::Scroll);
+    let (button, _clicked) = listener(&mut h, CursorSense::click());
+    h.set_root((list, button).stack());
+
+    // The hover arrives in its own frame, as a window delivers it.
+    h.move_to((50, 50));
+    assert_eq!(scrolled.take(), []);
+
+    h.scroll((0, 10));
+    assert_eq!(
+        scrolled.take(),
+        [CursorSense::Scroll],
+        "a hover already resting on the button must not consume the wheel"
+    );
+}
+
+#[test]
+fn only_the_topmost_listener_takes_a_press() {
+    let mut h = harness();
+    let (below, below_clicked) = listener(&mut h, CursorSense::click());
+    let (above, above_clicked) = listener(&mut h, CursorSense::click());
+    h.set_root((below, above).stack());
+
+    h.click((50, 50));
+
+    assert_eq!(above_clicked.take(), [CursorSense::click()]);
+    assert_eq!(below_clicked.take(), [], "one press goes to one widget");
+}
+
+#[test]
+fn a_press_beside_the_button_reaches_the_layer_below() {
+    let mut h = harness();
+    let (list, list_clicked) = listener(&mut h, CursorSense::click());
+    // The row above the list covers it, but only its left half is the button.
+    let (button, button_clicked) = listener(&mut h, CursorSense::click());
+    let row = (button, blank(&mut h)).span(Dir::RIGHT).add(&mut h.rsc);
+    h.set_root((list, row).stack());
+
+    h.click((20, 50));
+    assert_eq!(button_clicked.take(), [CursorSense::click()]);
+    assert_eq!(list_clicked.take(), []);
+
+    h.click((80, 50));
+    assert_eq!(button_clicked.take(), [], "the cursor is not on the button");
+    assert_eq!(
+        list_clicked.take(),
+        [CursorSense::click()],
+        "a press beside the button belongs to what is under it"
+    );
+}
+
+#[test]
+fn leaving_a_widget_still_ends_its_hover() {
+    let mut h = harness();
+    let (widget, hover) = listener(&mut h, CursorSense::HoverStart | CursorSense::HoverEnd);
+    h.set_root(widget);
+
+    h.move_to((50, 50));
+    assert_eq!(hover.take(), [CursorSense::HoverStart]);
+
+    h.leave();
+    assert_eq!(hover.take(), [CursorSense::HoverEnd]);
+}
+
+#[test]
+fn leaving_a_widget_does_not_block_the_layer_below() {
+    let mut h = harness();
+    let (below, below_hover) = listener(&mut h, CursorSense::HoverStart);
+    // Only the left half of the layer above is a widget, so the cursor can
+    // leave it without leaving the one underneath.
+    let (above, above_hover) = listener(&mut h, CursorSense::HoverStart | CursorSense::HoverEnd);
+    let row = (above, blank(&mut h)).span(Dir::RIGHT).add(&mut h.rsc);
+    h.set_root((below, row).stack());
+
+    h.move_to((20, 50));
+    assert_eq!(above_hover.take(), [CursorSense::HoverStart]);
+    assert_eq!(below_hover.take(), [], "the layer above is over it");
+
+    h.move_to((80, 50));
+    assert_eq!(above_hover.take(), [CursorSense::HoverEnd]);
+    assert_eq!(
+        below_hover.take(),
+        [CursorSense::HoverStart],
+        "ending a hover above must not stop the hover below"
+    );
+}
+
+#[test]
+fn covering_a_widget_ends_its_hover() {
+    let mut h = harness();
+    let (below, below_hover) = listener(&mut h, CursorSense::HoverStart | CursorSense::HoverEnd);
+    let (above, above_hover) = listener(&mut h, CursorSense::HoverStart);
+    let row = (above, blank(&mut h)).span(Dir::RIGHT).add(&mut h.rsc);
+    h.set_root((below, row).stack());
+
+    h.move_to((80, 50));
+    assert_eq!(below_hover.take(), [CursorSense::HoverStart]);
+
+    h.move_to((20, 50));
+    assert_eq!(above_hover.take(), [CursorSense::HoverStart]);
+    assert_eq!(
+        below_hover.take(),
+        [CursorSense::HoverEnd],
+        "a widget covered by one that took the input is no longer hovered"
+    );
+
+    h.move_to((80, 50));
+    assert_eq!(
+        below_hover.take(),
+        [CursorSense::HoverStart],
+        "uncovering it hovers it again"
+    );
+}
+
+#[test]
+fn hover_starts_and_ends_once_each() {
+    let mut h = harness();
+    // Only the left half is the widget, so the cursor can leave it without
+    // leaving the window.
+    let (widget, hover) = listener(&mut h, CursorSense::HoverStart | CursorSense::HoverEnd);
+    let row = (widget, blank(&mut h)).span(Dir::RIGHT).add(&mut h.rsc);
+    h.set_root(row);
+
+    h.move_to((20, 50));
+    assert_eq!(hover.take(), [CursorSense::HoverStart]);
+
+    h.move_to((30, 50));
+    assert_eq!(hover.take(), [], "staying inside is not a second start");
+
+    h.move_to((80, 50));
+    assert_eq!(hover.take(), [CursorSense::HoverEnd]);
+
+    h.move_to((90, 50));
+    assert_eq!(hover.take(), [], "an ended hover does not end again");
+
+    h.move_to((20, 50));
+    assert_eq!(
+        hover.take(),
+        [CursorSense::HoverStart],
+        "re-entering starts it"
+    );
+}

tests/cases/retained.rs

@@ -0,0 +1,597 @@
+//! What a second frame draws again, and what it keeps.
+
+use std::{cell::Cell, rc::Rc};
+
+use iris::harness::{Harness, assert_corners};
+use iris::prelude::*;
+
+/// A leaf that counts its draws and reports whatever size it is given, so a
+/// test can see what the retained path skipped. One that reads its box in
+/// pixels has a drawing that holds for that box alone.
+struct Counted {
+    draws: Rc<Cell<usize>>,
+    size: Size,
+    reads_box: bool,
+}
+
+impl Widget for Counted {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        self.draws.set(self.draws.get() + 1);
+        if self.reads_box {
+            painter.px_size();
+        }
+        self.size
+    }
+}
+
+struct Counts(Rc<Cell<usize>>);
+
+impl Counts {
+    fn get(&self) -> usize {
+        self.0.get()
+    }
+}
+
+fn counted(h: &mut Harness, size: Size, reads_box: bool) -> (WeakWidget<Counted>, Counts) {
+    let draws = Rc::new(Cell::new(0));
+    let id = Counted {
+        draws: draws.clone(),
+        size,
+        reads_box,
+    }
+    .add(&mut h.rsc);
+    (id, Counts(draws))
+}
+
+struct Layered {
+    children: [StrongWidget<Rect>; 2],
+    _revision: usize,
+}
+
+impl Widget for Layered {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        painter.child_layer();
+        painter.widget(&self.children[0]);
+        painter.next_layer();
+        painter.widget(&self.children[1]);
+        Size::default()
+    }
+}
+
+#[test]
+fn a_redrawn_layered_widget_keeps_the_layer_it_was_entered_on() {
+    let mut h = Harness::new((400, 200));
+    let children = [
+        rect(Color::RED).add_strong(&mut h.rsc),
+        rect(Color::BLUE).add_strong(&mut h.rsc),
+    ];
+    let root = Layered {
+        children,
+        _revision: 0,
+    }
+    .add(&mut h.rsc);
+    h.set_root(root);
+
+    h.rsc[root]._revision += 1;
+    h.frame();
+
+    let label = h.rsc.widgets().label(root.id());
+    let active = h
+        .render
+        .debug(h.rsc.widgets(), label)
+        .find(|active| active.id == root.id())
+        .unwrap();
+    assert_eq!(active.layer, 0);
+}
+
+/// A fixed-width leaf beside one that takes what is left over, so changing
+/// the first hands the second a different box without the output changing.
+fn pair(h: &mut Harness, reads_box: bool) -> (WeakWidget<Counted>, Counts, WidgetId) {
+    let (first, _) = counted(h, Size::from((100, 200)), false);
+    let (second, draws) = counted(h, Size::LEFTOVER, reads_box);
+    h.set_root((first, second).span(Dir::RIGHT));
+    (first, draws, second.id())
+}
+
+#[test]
+fn a_leaf_that_ignores_its_box_is_not_drawn_again_when_the_box_changes() {
+    let mut h = Harness::new((400, 200));
+    let (first, draws, second) = pair(&mut h, false);
+    let settled = draws.get();
+    assert_corners!(h, second, (100, 0), (400, 200));
+
+    h.rsc[first].size = Size::from((150, 200));
+    h.frame();
+
+    assert_eq!(
+        draws.get(),
+        settled,
+        "its box is a field to write, not a reason to draw"
+    );
+    assert_corners!(h, second, (150, 0), (400, 200));
+}
+
+#[test]
+fn moving_an_ordinary_subtree_remaps_its_mask() {
+    let mut h = Harness::new((400, 200));
+    let (first, _) = counted(&mut h, Size::from((100, 200)), false);
+    let inner = rect(Color::BLUE).add(&mut h.rsc);
+    let masked = inner.masked().add(&mut h.rsc);
+    h.set_root((first, masked).span(Dir::RIGHT));
+
+    h.rsc[first].size = Size::from((150, 200));
+    h.frame();
+
+    let active = &h.render.active[&masked.id()];
+    assert_eq!(
+        h.rsc.ui().masks[active.mask.idx()].region,
+        UiRegion::new(
+            UiSpan::new(UiScalar::px(150.0), UiScalar::rel_max()),
+            UiSpan::FULL,
+        )
+    );
+    assert_corners!(h, inner, (150, 0), (400, 200));
+}
+
+#[test]
+fn a_leaf_that_depends_on_its_box_is_drawn_again_when_the_box_changes() {
+    let mut h = Harness::new((400, 200));
+    let (first, draws, second) = pair(&mut h, true);
+    let settled = draws.get();
+
+    h.rsc[first].size = Size::from((150, 200));
+    h.frame();
+
+    // The preceding fixed child makes the remaining box this child's real
+    // box, so measuring it also draws it in its final box.
+    assert_eq!(draws.get(), settled + 1);
+    assert_corners!(h, second, (150, 0), (400, 200));
+}
+
+#[test]
+fn a_span_child_that_declares_its_length_is_drawn_once() {
+    let mut h = Harness::new((400, 200));
+    let (told, told_draws) = counted(&mut h, Size::from((100, 200)), false);
+    let (asked, asked_draws) = counted(&mut h, Size::from((100, 200)), true);
+    // The span takes one child's length from its hint and has to draw the
+    // other to find out, so only the second is drawn before its final box.
+    let hinted = told.width(100).add(&mut h.rsc);
+    h.set_root((hinted, asked).span(Dir::RIGHT));
+
+    assert_eq!(told_draws.get(), 1);
+    // Reading its box makes its drawing hold for the measuring box alone,
+    // and it reports less than that box: so it is drawn again in the box its
+    // answer places it in, and once more in the final box the span chooses.
+    // A widget that says what it holds for, as text does, skips the middle
+    // one.
+    assert_eq!(
+        asked_draws.get(),
+        3,
+        "drawn to be measured, in its placed box, then in its final box"
+    );
+}
+
+#[test]
+fn a_span_relays_out_when_a_child_it_measured_changes() {
+    let mut h = Harness::new((400, 200));
+    let (first, _, second) = pair(&mut h, false);
+
+    h.rsc[first].size = Size::from((250, 200));
+    h.frame();
+
+    assert_corners!(h, first, (0, 0), (250, 200));
+    assert_corners!(h, second, (250, 0), (400, 200));
+}
+
+#[test]
+fn a_repaint_that_keeps_its_size_does_not_relay_out() {
+    let mut h = Harness::new((400, 200));
+    let (first, draws) = counted(&mut h, Size::from((100, 200)), false);
+    let (second, _) = counted(&mut h, Size::LEFTOVER, false);
+    h.set_root((first, second).span(Dir::RIGHT));
+    let settled = draws.get();
+
+    // Taking mutable access is the ordinary content-change signal. This
+    // widget returns the same size, so the parent has nothing to lay out.
+    let _ = h.rsc.widgets_mut().get_dyn_mut(first.id());
+    h.frame();
+
+    assert_eq!(draws.get(), settled + 1);
+}
+
+#[test]
+fn a_span_child_survives_the_next_frame() {
+    let mut h = Harness::new((400, 200));
+    // Both children declare a length, so the span chooses their boxes from
+    // hints rather than drawing them to find out.
+    let top = rect(Color::RED).height(80).add(&mut h.rsc);
+    let bottom = rect(Color::BLUE).height(120).add(&mut h.rsc);
+    h.set_root((top, bottom).span(Dir::DOWN));
+
+    h.rsc.widgets_mut().get_dyn_mut(top.id());
+    h.frame();
+
+    assert_corners!(h, top, (0, 0), (400, 80));
+    assert_corners!(h, bottom, (0, 80), (400, 200));
+}
+
+/// Lays its child out from the hint alone, never reading what it drew.
+struct FromHint {
+    inner: StrongWidget,
+}
+
+impl Widget for FromHint {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        let len = painter.size_hint(&self.inner, Axis::Y).unwrap();
+        let mut region = UiRegion::FULL;
+        region.y.end = region.y.start.offset(len.px);
+        painter.widget_within(&self.inner, region);
+        Size::LEFTOVER
+    }
+}
+
+#[test]
+fn a_parent_that_only_read_a_hint_relays_out_when_the_hint_changes() {
+    let mut h = Harness::new((400, 200));
+    let inner = rect(Color::RED).height(80).add(&mut h.rsc);
+    let parent = FromHint {
+        inner: inner.add_strong(&mut h.rsc),
+    }
+    .add(&mut h.rsc);
+    h.set_root(parent);
+    assert_corners!(h, inner, (0, 0), (400, 80));
+
+    h.set_len(inner, Axis::Y, 120);
+    h.frame();
+
+    assert_corners!(h, inner, (0, 0), (400, 120));
+}
+
+/// Reads its box's size, which nothing but its own draw can put right.
+struct ReadsBox {
+    draws: Rc<Cell<usize>>,
+}
+
+impl Widget for ReadsBox {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        self.draws.set(self.draws.get() + 1);
+        Size::from_px(painter.px_size().div_int(4))
+    }
+}
+
+/// Reads its box across one axis only, so its drawing holds for a taller
+/// box on its own and only a wider one is worth a draw.
+///
+/// Both of these report a quarter of what they read, without saying that the
+/// drawing holds there too, so each length they are asked at costs two draws:
+/// one to answer, and one in the quarter-sized box that answer places them
+/// in. The counts below are in those pairs.
+struct ReadsWidth {
+    draws: Rc<Cell<usize>>,
+}
+
+impl Widget for ReadsWidth {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        self.draws.set(self.draws.get() + 1);
+        Size::from_px(PxVec2::new(
+            painter.px_len(Axis::X).div_int(4),
+            Px::from_int(20),
+        ))
+    }
+}
+
+#[test]
+fn a_resize_does_not_redraw_what_the_shader_can_move() {
+    let mut h = Harness::new((400, 200));
+    let (leaf, draws) = counted(&mut h, Size::LEFTOVER, false);
+    h.set_root(leaf);
+    let settled = draws.get();
+
+    h.resize((800, 100));
+    assert!(h.needs_redraw());
+    h.frame();
+
+    assert_eq!(
+        draws.get(),
+        settled,
+        "a scaling drawing follows its box, and the output is one"
+    );
+    assert_corners!(h, leaf, (0, 0), (800, 100));
+}
+
+#[test]
+fn a_full_ortho_span_moves_its_child_without_redrawing_it() {
+    let mut h = Harness::new((400, 200));
+    let (leaf, draws) = counted(&mut h, Size::LEFTOVER, false);
+    let span = (leaf,)
+        .span(Dir::RIGHT)
+        .ortho(OrthoSize::Full)
+        .add(&mut h.rsc);
+    h.set_root(span);
+    let settled = draws.get();
+
+    h.resize((400, 100));
+    h.frame();
+
+    assert_eq!(draws.get(), settled);
+    assert_corners!(h, leaf, (0, 0), (400, 100));
+    assert_eq!(h.render.active[&span.id()].size.y, Len::rel(1.0));
+}
+
+/// The output is the root of the box chain, so a resize is a box that changed
+/// length like any other -- there is not a second rule for the window. A
+/// drawing that holds for one length is drawn again whichever box moved.
+#[test]
+fn a_resize_redraws_what_does_not_scale() {
+    let mut h = Harness::new((400, 200));
+    let (leaf, draws) = counted(&mut h, Size::LEFTOVER, true);
+    h.set_root(leaf);
+    let settled = draws.get();
+
+    h.resize((800, 100));
+    h.frame();
+
+    assert_eq!(draws.get(), settled + 1, "its box is a different length");
+    assert_corners!(h, leaf, (0, 0), (800, 100));
+}
+
+#[test]
+fn a_resize_redraws_what_read_its_box() {
+    let mut h = Harness::new((400, 200));
+    let draws = Rc::new(Cell::new(0));
+    let leaf = ReadsBox {
+        draws: draws.clone(),
+    }
+    .add(&mut h.rsc);
+    h.set_root(leaf);
+    let settled = draws.get();
+
+    h.resize((800, 100));
+    h.frame();
+
+    assert_eq!(draws.get(), settled + 2);
+}
+
+#[test]
+fn a_resize_only_redraws_read_axes() {
+    let mut h = Harness::new((400, 200));
+    let draws = Rc::new(Cell::new(0));
+    let leaf = ReadsWidth {
+        draws: draws.clone(),
+    }
+    .add(&mut h.rsc);
+    h.set_root(leaf);
+    let settled = draws.get();
+
+    h.resize((400, 300));
+    h.frame();
+    assert_eq!(draws.get(), settled, "height was never read");
+
+    h.resize((800, 300));
+    h.frame();
+    assert_eq!(draws.get(), settled + 2, "width changes its answer");
+}
+
+/// A window is measured onto the grid like everything else, so a resize too
+/// small to reach the next step is not a resize at all -- and one that does
+/// reach it is, however little of a pixel it is worth.
+#[test]
+fn a_resize_within_one_step_is_not_a_resize() {
+    let mut h = Harness::new((400, 200));
+    let draws = Rc::new(Cell::new(0));
+    let leaf = ReadsWidth {
+        draws: draws.clone(),
+    }
+    .add(&mut h.rsc);
+    h.set_root(leaf);
+    let settled = draws.get();
+
+    // All of these are 400 px to the nearest step.
+    let step = Px::STEP.to_f32();
+    for part in [0.1, 0.2, 0.3] {
+        h.resize((400.0 + step * part, 200.0));
+        h.frame();
+        assert_eq!(draws.get(), settled);
+    }
+
+    h.resize((400.0 + step, 200.0));
+    h.frame();
+    assert_eq!(draws.get(), settled + 2);
+}
+
+/// The same for a box that changes because a sibling did: what is compared
+/// is the length on the grid, and three lengths that land on one step are
+/// one length.
+#[test]
+fn a_box_change_within_one_step_is_not_a_change() {
+    let mut h = Harness::new((400, 200));
+    let (first, draws, _) = pair(&mut h, true);
+    let settled = draws.get();
+
+    let step = Px::STEP.to_f32();
+    for part in [0.1, 0.2, 0.3] {
+        h.rsc[first].size.x = Len::px(100.0 + step * part);
+        h.frame();
+        assert_eq!(draws.get(), settled);
+    }
+
+    h.rsc[first].size.x = Len::px(100.0 + step);
+    h.frame();
+    assert_eq!(draws.get(), settled + 1);
+}
+
+#[test]
+fn reporting_the_same_output_size_does_not_start_a_resize() {
+    let mut h = Harness::new((400, 200));
+    let draws = Rc::new(Cell::new(0));
+    let leaf = ReadsBox {
+        draws: draws.clone(),
+    }
+    .add(&mut h.rsc);
+    h.set_root(leaf);
+    let settled = draws.get();
+
+    h.resize((400, 200));
+
+    assert!(!h.needs_redraw());
+    h.frame();
+    assert_eq!(draws.get(), settled);
+}
+
+#[test]
+fn narrowing_the_output_reflows_text_and_relays_out_around_it() {
+    let mut h = Harness::new((600, 400));
+    let para = wtext(
+        "Wrapping shapes one source into as many lines as its container leaves \
+        room for, so the height of a paragraph is an answer rather than a setting.",
+    )
+    .size(20)
+    .wrap(true)
+    .add(&mut h.rsc);
+    let below = rect(Color::RED).add(&mut h.rsc);
+    h.set_root((para, below).span(Dir::DOWN));
+    let top = h.region(&below).expect("drew nothing").top_left.y;
+
+    h.resize((300, 400));
+    h.frame();
+
+    let lower = h.region(&below).expect("drew nothing").top_left.y;
+    assert!(lower > top, "same words, half the width: {top} -> {lower}");
+}
+
+#[test]
+fn a_change_two_levels_under_its_reader_still_reaches_it() {
+    let mut h = Harness::new((400, 400));
+    // Every wrapper up to the outer pad read the size below it, so the outer
+    // pad is what draws again -- and the span it hands the box to is the same
+    // size as before, which is what lets a draw reuse its way past the leaf.
+    let (leaf, _) = counted(&mut h, Size::px((100, 100).into()), true);
+    let padded = leaf.pad(10).add(&mut h.rsc);
+    let below = rect(Color::RED).add(&mut h.rsc);
+    h.set_root((padded, below).span(Dir::DOWN).pad(12));
+    assert_corners!(h, below, (12, 132), (388, 388));
+
+    h.rsc[leaf].size = Size::px((100, 200).into());
+    h.frame();
+
+    assert_corners!(h, below, (12, 232), (388, 388));
+}
+
+/// Reads nothing of its box, so its drawing holds for any length, and has a
+/// child so that whatever asks about the subtree has one to reach.
+struct Stretchy {
+    inner: StrongWidget,
+    draws: Rc<Cell<usize>>,
+}
+
+impl Widget for Stretchy {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        self.draws.set(self.draws.get() + 1);
+        painter.widget(&self.inner).size()
+    }
+}
+
+#[test]
+fn stretching_a_subtree_carries_the_children_in_it() {
+    let mut h = Harness::new((400, 400));
+    let first = rect(Color::RED).height(40).add(&mut h.rsc);
+    let inner = rect(Color::BLUE).add(&mut h.rsc);
+    let draws = Rc::new(Cell::new(0));
+    let outer = Stretchy {
+        inner: inner.add_strong(&mut h.rsc),
+        draws: draws.clone(),
+    }
+    .add(&mut h.rsc);
+    h.set_root((first, outer).span(Dir::DOWN));
+    let settled = draws.get();
+    assert_corners!(h, inner, (0, 40), (400, 400));
+
+    h.set_len(first, Axis::Y, 80);
+    h.frame();
+
+    assert_eq!(
+        draws.get(),
+        settled,
+        "its drawing follows its box, rather than being made again"
+    );
+    assert_corners!(h, outer, (0, 80), (400, 400));
+    assert_corners!(h, inner, (0, 80), (400, 400));
+}
+
+#[test]
+fn a_widened_row_redraws_what_reads_its_length_and_nothing_else() {
+    let mut h = Harness::new((400, 200));
+    // What a transcript row is: something whose shaping depends on the width
+    // it is given, beside something that only has to be the right shape.
+    let (wraps, wrap_draws) = counted(&mut h, Size::LEFTOVER, true);
+    let (backing, back_draws) = counted(&mut h, Size::LEFTOVER, false);
+    let row = (backing, wraps).span(Dir::RIGHT).add(&mut h.rsc);
+    let bar = rect(Color::RED).width(100).add(&mut h.rsc);
+    h.set_root((bar, row).span(Dir::RIGHT));
+    let (settled_wrap, settled_back) = (wrap_draws.get(), back_draws.get());
+
+    h.set_len(bar, Axis::X, 200);
+    h.frame();
+
+    // The span reads every child's size, so redrawing one takes the span
+    // with it -- and the span then measures and places the redrawn child.
+    assert!(wrap_draws.get() > settled_wrap, "reads the width it got");
+    assert_eq!(back_draws.get(), settled_back, "only has to be the shape");
+    assert_corners!(h, backing, (200, 0), (300, 200));
+    assert_corners!(h, wraps, (300, 0), (400, 200));
+}
+
+#[test]
+fn a_declared_length_child_is_not_redrawn_when_the_box_around_it_grows() {
+    let mut h = Harness::new((400, 200));
+    // Its box is a fixed 80 wherever the row's edges end up, so drawing it
+    // again would be for a width it does not have. The declared width is what
+    // lets the span say that without drawing it: a width the span learnt by
+    // drawing the child in its own box is only an answer for that box.
+    let (counter, draws) = counted(&mut h, Size::from((80, 200)), true);
+    let fixed = counter.width(80).add(&mut h.rsc);
+    let (leftover, _) = counted(&mut h, Size::LEFTOVER, false);
+    let row = (fixed, leftover).span(Dir::RIGHT).add(&mut h.rsc);
+    let bar = rect(Color::RED).width(100).add(&mut h.rsc);
+    h.set_root((bar, row).span(Dir::RIGHT));
+    let settled = draws.get();
+
+    h.set_len(bar, Axis::X, 200);
+    h.frame();
+
+    assert_eq!(draws.get(), settled, "its own length did not change");
+    assert_corners!(h, fixed, (200, 0), (280, 200));
+}
+
+/// `Stack` measures the child that sizes it by drawing it, then draws it
+/// again above the background it stacks over. The second ask is for the same
+/// box, so nothing geometric says the answer has gone stale, and reusing it
+/// leaves the drawing under the background. The same tree got away with it
+/// while the two asks differed by a rounding.
+#[test]
+fn a_widget_asked_again_on_another_layer_is_drawn_there() {
+    let mut h = Harness::new((400, 200));
+    let background = rect(Color::RED).add(&mut h.rsc);
+    let (front, draws) = counted(&mut h, Size::from((100, 50)), false);
+    let stack = Stack {
+        children: vec![
+            background.add_strong(&mut h.rsc),
+            front.add_strong(&mut h.rsc),
+        ],
+        size: StackSize::Child(1),
+    }
+    .add(&mut h.rsc);
+    h.set_root(stack);
+    h.frame();
+
+    let layer = |id| h.render.active[&id].layer;
+    assert_ne!(
+        layer(front.id()),
+        layer(stack.id()),
+        "the measured drawing was left on the stack's own layer"
+    );
+    assert_ne!(layer(front.id()), layer(background.id()));
+    // Measured once for the size and once where it goes, which is what the
+    // stack costs and not something this test is asserting a number for.
+    assert_eq!(draws.get(), 2);
+}

tests/cases/scroll.rs

@@ -0,0 +1,61 @@
+//! Scrolling moves content and stops at its ends.
+
+use iris::harness::{Harness, assert_corners};
+use iris::prelude::*;
+
+#[test]
+fn scrollable_enables_a_region_node_but_raw_scroll_does_not() {
+    let mut h = Harness::new((100, 100));
+    let default_child = ().add(&mut h.rsc);
+    let _default = default_child.scrollable().add(&mut h.rsc);
+    assert!(h.rsc.widgets().is_region_node(default_child));
+    h.rsc.widgets_mut().set_region_node(default_child, false);
+    assert!(!h.rsc.widgets().is_region_node(default_child));
+
+    let raw_child = ().add(&mut h.rsc);
+    let _raw = Scroll::new(raw_child.add_strong(&mut h.rsc), Axis::Y).add(&mut h.rsc);
+    assert!(!h.rsc.widgets().is_region_node(raw_child));
+
+    let explicit = ().region_node().add(&mut h.rsc);
+    assert!(h.rsc.widgets().is_region_node(explicit));
+}
+
+#[test]
+fn a_scrollable_child_can_drop_its_region_node() {
+    let mut h = Harness::new((400, 200));
+    let top = rect(Color::RED).height(200).add(&mut h.rsc);
+    let bottom = rect(Color::BLUE).height(200).add(&mut h.rsc);
+    let content = (top, bottom).span(Dir::DOWN).add(&mut h.rsc);
+    h.set_root(content.scrollable());
+    h.rsc.widgets_mut().set_region_node(content, false);
+    h.frame();
+
+    h.move_to((200, 100));
+    h.scroll((0, 1));
+    h.frame();
+
+    assert!(!h.rsc.widgets().is_region_node(content));
+    assert_corners!(h, top, (0, -150), (400, 50));
+}
+
+#[test]
+fn a_wheel_scrolls_the_content_and_stops_at_its_end() {
+    let mut h = Harness::new((400, 200));
+    // Twice the window's height, so there is 200 to scroll.
+    let top = rect(Color::RED).height(200).add(&mut h.rsc);
+    let bottom = rect(Color::BLUE).height(200).add(&mut h.rsc);
+    h.set_root((top, bottom).span(Dir::DOWN).scrollable());
+    h.move_to((200, 100));
+
+    // `Scroll` starts snapped to the end.
+    assert_corners!(h, top, (0, -200), (400, 0));
+
+    // The handler scales a wheel line by 50.
+    h.scroll((0, 1));
+    h.frame();
+    assert_corners!(h, top, (0, -150), (400, 50));
+
+    h.scroll((0, 10));
+    h.frame();
+    assert_corners!(h, top, (0, 0), (400, 200));
+}

tests/cases/tasks.rs

@@ -0,0 +1,23 @@
+//! What a background task can change, and how it gets back to the ui.
+
+use std::time::Duration;
+
+use iris::harness::Harness;
+use iris::prelude::*;
+
+#[test]
+fn a_task_update_reaches_the_tree() {
+    let mut h = Harness::new((400, 200));
+    let widget = rect(Color::RED).add(&mut h.rsc);
+    h.set_root(widget.task_on(CursorSense::click(), async move |mut ctx| {
+        ctx.update(move |_, rsc| widget(rsc).color = Color::BLUE);
+    }));
+
+    h.click((200, 100));
+
+    assert!(
+        h.await_update(Duration::from_secs(5)),
+        "the task sent no update"
+    );
+    assert_eq!(h.rsc[widget].color, Color::BLUE);
+}

tests/cases/text_edit.rs

@@ -0,0 +1,67 @@
+use iris::prelude::*;
+
+fn editor(text: &str, mode: EditMode) -> (TextEdit, TextData) {
+    let view = TextView::new(TextBuffer::new(text), TextAttrs::default(), None);
+    (TextEdit::new(view, mode), TextData::default())
+}
+
+fn press(edit: &mut TextEditCtx<'_>, x: f32) {
+    edit.select(vec2(x, 10.0), vec2(400.0, 200.0), false, false);
+}
+
+#[test]
+fn pressing_an_empty_field_places_input() {
+    let (mut text, mut data) = editor("", EditMode::SingleLine);
+    let mut edit = TextEditCtx {
+        text: &mut text,
+        data: &mut data,
+    };
+
+    press(&mut edit, 40.0);
+    edit.insert("hello");
+
+    assert_eq!(edit.text.content(), "hello");
+}
+
+#[test]
+fn preedit_replaces_the_previous_composition() {
+    let (mut text, mut data) = editor("", EditMode::SingleLine);
+    let mut edit = TextEditCtx {
+        text: &mut text,
+        data: &mut data,
+    };
+
+    press(&mut edit, 0.0);
+    edit.replace(0, "に");
+    edit.replace(1, "日本");
+
+    assert_eq!(edit.text.content(), "日本");
+}
+
+#[test]
+fn backspace_respects_utf8_boundaries() {
+    let (mut text, mut data) = editor("aé", EditMode::SingleLine);
+    let mut edit = TextEditCtx {
+        text: &mut text,
+        data: &mut data,
+    };
+
+    press(&mut edit, f32::MAX);
+    edit.backspace(false);
+
+    assert_eq!(edit.text.content(), "a");
+}
+
+#[test]
+fn typing_replaces_the_selection() {
+    let (mut text, mut data) = editor("hello", EditMode::SingleLine);
+    let mut edit = TextEditCtx {
+        text: &mut text,
+        data: &mut data,
+    };
+
+    edit.select_all();
+    edit.insert("goodbye");
+
+    assert_eq!(edit.text.content(), "goodbye");
+}

tests/cases/unsettled.rs

@@ -0,0 +1,405 @@
+//! The smallest trees that laid out differently warm than cold, each shrunk
+//! by `tests/shrink.rs` from hundreds of widgets. The first two are a cold
+//! frame that had not settled: a wrapping text shaped at a width it was
+//! measured in rather than the one it was given. The rest are a widget
+//! measured again in a box its own answer had decided, where the old answer
+//! is a fixed point whatever the content now says. The last is neither: one
+//! box length, composed two ways, landing either side of the boundary that
+//! decided whether a child was drawn at all.
+
+use iris::harness::Harness;
+use iris::prelude::*;
+
+/// Six widgets, shrunk from a 402-widget tree the fuzzer found. Nothing about
+/// the tree changes -- every widget is marked for redraw and the frame is
+/// taken again -- so no box may move, and a warm frame has to land where a
+/// cold one does.
+fn plant(h: &mut Harness) -> Vec<WidgetId> {
+    let plain = wtext("Wrapping").size(16).wrap(false).add(&mut h.rsc);
+    let wrapped = wtext("Wrapping shapes").size(16).wrap(true).add(&mut h.rsc);
+    let sized = wrapped.width(76).add(&mut h.rsc);
+    let aligned = sized;
+    h.rsc
+        .widgets_mut()
+        .set_alignment(sized, Axis::X, AxisAlign::POS);
+    h.rsc
+        .widgets_mut()
+        .set_alignment(sized, Axis::Y, AxisAlign::POS);
+    let stack = Stack {
+        children: vec![plain.add_strong(&mut h.rsc), aligned.add_strong(&mut h.rsc)],
+        size: StackSize::Child(0),
+    }
+    .add(&mut h.rsc);
+    let root = (stack,).span(Dir::RIGHT).add(&mut h.rsc);
+    h.set_root(root);
+    vec![
+        plain.id(),
+        wrapped.id(),
+        sized.id(),
+        aligned.id(),
+        stack.id(),
+        root.id(),
+    ]
+}
+
+/// The first frame does not reach the layout a second one does, so "cold" is
+/// not a fixed point and comparing against it compares against a tree that
+/// has not settled.
+#[test]
+fn one_frame_is_enough() {
+    let mut h = Harness::new((640, 900));
+    let ids = plant(&mut h);
+    let first = h.region(&ids[1]).unwrap();
+    for _ in 0..3 {
+        for &id in &ids {
+            h.rsc.widgets_mut().get_dyn_mut(id);
+        }
+        h.frame();
+    }
+    let settled = h.region(&ids[1]).unwrap();
+    println!(
+        "first frame {} tall, settled {} tall",
+        first.bot_right.y - first.top_left.y,
+        settled.bot_right.y - settled.top_left.y
+    );
+    assert_eq!(
+        first.bot_right.y - first.top_left.y,
+        settled.bot_right.y - settled.top_left.y,
+        "the first frame had not finished laying out"
+    );
+}
+
+#[test]
+fn repainting_everything_moves_nothing() {
+    let mut warm = Harness::new((640, 900));
+    let ids = plant(&mut warm);
+    for &id in &ids {
+        warm.rsc.widgets_mut().get_dyn_mut(id);
+    }
+    warm.frame();
+
+    let mut cold = Harness::new((640, 900));
+    let cold_ids = plant(&mut cold);
+
+    let mut wrong = Vec::new();
+    for (i, (&w, &c)) in ids.iter().zip(&cold_ids).enumerate() {
+        let (got, want) = (warm.region(&w), cold.region(&c));
+        if got != want {
+            wrong.push(format!("widget {i}: warm {got:?} cold {want:?}"));
+        }
+    }
+    assert!(wrong.is_empty(), "{}", wrong.join("\n"));
+}
+
+/// Six widgets, shrunk from 905. Everything inside the declared 189x176 box
+/// is the same size whatever the output is, so a resize may not change any of
+/// it -- but the text comes out 3.92px narrower warm than cold.
+fn plant_fixed(h: &mut Harness) -> Vec<WidgetId> {
+    let words = "Wrapping shapes one source into as many lines as the box leaves";
+    let text = wtext(words).size(16).wrap(true).add(&mut h.rsc);
+    let aligned = text;
+    h.rsc
+        .widgets_mut()
+        .set_alignment(text, Axis::X, AxisAlign::NEG);
+    let inner = (aligned,).span(Dir::RIGHT).add(&mut h.rsc);
+    let sized = inner.sized((189, 176)).add(&mut h.rsc);
+    let filler = rect(Color::RED).add(&mut h.rsc);
+    let root = (filler, sized).span(Dir::RIGHT).add(&mut h.rsc);
+    h.state.root = Some(root.add_strong(&mut h.rsc));
+    vec![
+        text.id(),
+        aligned.id(),
+        inner.id(),
+        sized.id(),
+        filler.id(),
+        root.id(),
+    ]
+}
+
+#[test]
+fn a_resize_does_not_reach_inside_a_box_of_declared_pixels() {
+    let mut warm = Harness::new((1920, 1200));
+    let ids = plant_fixed(&mut warm);
+    warm.frame();
+    warm.resize((640, 900));
+    warm.frame();
+
+    let mut cold = Harness::new((640, 900));
+    let cold_ids = plant_fixed(&mut cold);
+    cold.frame();
+
+    let mut wrong = Vec::new();
+    for (i, (&w, &c)) in ids.iter().zip(&cold_ids).enumerate() {
+        let (got, want) = (warm.region(&w), cold.region(&c));
+        if got != want {
+            wrong.push(format!("widget {i}: warm {got:?} cold {want:?}"));
+        }
+    }
+    assert!(wrong.is_empty(), "{}", wrong.join("\n"));
+}
+
+/// Four widgets, shrunk from 486. A span's two children are swapped: warm by
+/// moving them, cold by growing them that way. Same widgets, same sizes, one
+/// ends up 29.9px from where the other does.
+fn plant_pair(h: &mut Harness, swapped: bool) -> (Vec<WidgetId>, WeakWidget<Span>) {
+    let wrapped = wtext("Wrapping shapes one source into as many lines")
+        .size(16)
+        .wrap(true)
+        .add(&mut h.rsc);
+    let plain = wtext("one line, overflowing whatever it is given")
+        .size(16)
+        .wrap(false)
+        .add(&mut h.rsc);
+    let first: StrongWidget = wrapped.add_strong(&mut h.rsc);
+    let second: StrongWidget = plain.add_strong(&mut h.rsc);
+    let children = match swapped {
+        true => vec![second, first],
+        false => vec![first, second],
+    };
+    let span = Span {
+        children,
+        dir: Dir::RIGHT,
+        gap: Px::ZERO,
+        ortho: OrthoSize::Children,
+    }
+    .add(&mut h.rsc);
+    let span_handle = span;
+    let aligned = span;
+    h.rsc
+        .widgets_mut()
+        .set_alignment(span, Axis::X, AxisAlign::CENTER);
+    h.state.root = Some(aligned.add_strong(&mut h.rsc));
+    (
+        vec![wrapped.id(), plain.id(), span.id(), aligned.id()],
+        span_handle,
+    )
+}
+
+#[test]
+fn swapping_two_children_lands_where_growing_them_that_way_does() {
+    let mut warm = Harness::new((640, 900));
+    let (ids, span) = plant_pair(&mut warm, false);
+    warm.frame();
+    warm.rsc[span].children.rotate_left(1);
+    warm.frame();
+
+    let mut cold = Harness::new((640, 900));
+    let (cold_ids, _) = plant_pair(&mut cold, true);
+    cold.frame();
+
+    let mut wrong = Vec::new();
+    for (i, (&w, &c)) in ids.iter().zip(&cold_ids).enumerate() {
+        let (got, want) = (warm.region(&w), cold.region(&c));
+        if got != want {
+            wrong.push(format!("widget {i}: warm {got:?} cold {want:?}"));
+        }
+    }
+    assert!(wrong.is_empty(), "{}", wrong.join("\n"));
+}
+
+/// Eight widgets, shrunk from 80. The scroll decides how wide to make its
+/// content from what the content says, and hands that box down through a
+/// pass-through; the span under it was given that box once, so nothing at its
+/// own edge says the box was its own answer.
+fn plant_scrolled(h: &mut Harness, swapped: bool) -> (Vec<WidgetId>, [WeakWidget<Span>; 2]) {
+    let words = "Wrapping shapes one source into as many lines as the box leaves room for,";
+    let text = wtext(words).size(16).wrap(true).add(&mut h.rsc);
+    let filler = rect(Color::RED).add(&mut h.rsc);
+    let mut inner_children: Vec<StrongWidget> =
+        vec![text.add_strong(&mut h.rsc), filler.add_strong(&mut h.rsc)];
+    if swapped {
+        inner_children.rotate_left(1);
+    }
+    let inner = Span {
+        children: inner_children,
+        dir: Dir::RIGHT,
+        gap: Px::ZERO,
+        ortho: OrthoSize::Children,
+    }
+    .add(&mut h.rsc);
+    let block = rect(Color::RED).add(&mut h.rsc);
+    let fixed = block.width(87).add(&mut h.rsc);
+    let mut outer_children: Vec<StrongWidget> =
+        vec![fixed.add_strong(&mut h.rsc), inner.add_strong(&mut h.rsc)];
+    if swapped {
+        outer_children.rotate_left(1);
+    }
+    let outer = Span {
+        children: outer_children,
+        dir: Dir::RIGHT,
+        gap: Px::ZERO,
+        ortho: OrthoSize::Children,
+    }
+    .add(&mut h.rsc);
+    // Carried no rule even before rules were a property: it is here to be a
+    // widget between the span and the scroll, not to declare anything.
+    let through = (outer,).span(Dir::RIGHT).add(&mut h.rsc);
+    let scroll = Scroll::new(through.add_strong(&mut h.rsc), Axis::X).add(&mut h.rsc);
+    h.state.root = Some(scroll.add_strong(&mut h.rsc));
+    (
+        vec![
+            text.id(),
+            filler.id(),
+            inner.id(),
+            block.id(),
+            fixed.id(),
+            outer.id(),
+            through.id(),
+            scroll.id(),
+        ],
+        [inner, outer],
+    )
+}
+
+#[test]
+fn a_span_given_the_box_its_answer_decided_matches_a_cold_layout() {
+    let mut warm = Harness::new((640, 900));
+    let (ids, spans) = plant_scrolled(&mut warm, false);
+    warm.frame();
+    for span in spans {
+        warm.rsc[span].children.rotate_left(1);
+    }
+    warm.frame();
+
+    let mut cold = Harness::new((640, 900));
+    let (cold_ids, _) = plant_scrolled(&mut cold, true);
+    cold.frame();
+
+    let mut wrong = Vec::new();
+    for (i, (&w, &c)) in ids.iter().zip(&cold_ids).enumerate() {
+        let (got, want) = (warm.region(&w), cold.region(&c));
+        if got != want {
+            wrong.push(format!("widget {i}: warm {got:?} cold {want:?}"));
+        }
+    }
+    assert!(wrong.is_empty(), "{}", wrong.join("\n"));
+}
+
+/// Reports a width derived from the box it is asked in. Reading through the
+/// painter is its declaration that the answer holds for that width only.
+struct Wider {
+    extra: f32,
+}
+
+impl Widget for Wider {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        Size {
+            x: Len {
+                px: painter.px_len(Axis::X) + Px::from_f32(self.extra),
+                ..Len::ZERO
+            },
+            y: Len::LEFTOVER,
+        }
+    }
+}
+
+fn plant_wider(h: &mut Harness, extra: f32) -> (WeakWidget<Wider>, WidgetId) {
+    let content = Wider { extra }.add(&mut h.rsc);
+    let scroll = Scroll::new(content.add_strong(&mut h.rsc), Axis::X).add(&mut h.rsc);
+    let root = scroll;
+    h.rsc
+        .widgets_mut()
+        .set_alignment(scroll, Axis::X, AxisAlign::NEG);
+    h.set_root(root);
+    (content, scroll.id())
+}
+
+#[test]
+fn a_scrolls_retained_answer_is_the_one_a_cold_layout_asks_for() {
+    let mut warm = Harness::new((100, 100));
+    let (content, scroll) = plant_wider(&mut warm, 50.0);
+    warm.rsc[content].extra = 70.0;
+    warm.frame();
+
+    let mut cold = Harness::new((100, 100));
+    let (_, cold_scroll) = plant_wider(&mut cold, 70.0);
+
+    assert_eq!(warm.region(&scroll), cold.region(&cold_scroll));
+}
+
+/// Six widgets, shrunk from 266. `measured`'s box is exactly the height of its
+/// one fixed child, which is the box a parent sizing itself from that answer
+/// hands back -- so whether its leftover-only child was drawn at all came down
+/// to the 0.00003 px the composed length differs by, one way warm and the
+/// other cold.
+fn plant_boundary(h: &mut Harness, swapped: bool) -> (Vec<WidgetId>, [WeakWidget<Span>; 2]) {
+    let filler = rect(Color::RED).add(&mut h.rsc);
+    let plain = wtext("one line, overflowing whatever it is given")
+        .size(16)
+        .wrap(false)
+        .add(&mut h.rsc);
+    let mut pair: Vec<StrongWidget> =
+        vec![filler.add_strong(&mut h.rsc), plain.add_strong(&mut h.rsc)];
+    if swapped {
+        pair.rotate_left(1);
+    }
+    let measured = Span {
+        children: pair,
+        dir: Dir::DOWN,
+        gap: Px::ZERO,
+        ortho: OrthoSize::Children,
+    }
+    .add(&mut h.rsc);
+    // Takes the whole box on its own, so the span above has nothing left to
+    // divide and `measured` is given exactly the text's height.
+    let whole = rect(Color::RED).add(&mut h.rsc);
+    h.rsc
+        .widgets_mut()
+        .set_size_rules(whole, None, Some(Len::rel(1.0)));
+    let mut inner_children: Vec<StrongWidget> = vec![
+        measured.add_strong(&mut h.rsc),
+        whole.add_strong(&mut h.rsc),
+    ];
+    if swapped {
+        inner_children.rotate_left(1);
+    }
+    let inner = Span {
+        children: inner_children,
+        dir: Dir::DOWN,
+        gap: Px::ZERO,
+        ortho: OrthoSize::Children,
+    }
+    .add(&mut h.rsc);
+    h.rsc
+        .widgets_mut()
+        .set_size_rules(inner, None, Some(Len::px(198.0)));
+    // One more span above it: without a box composed through it, both trees
+    // round the same way and the boundary is never crossed.
+    let outer = (inner,).span(Dir::DOWN).add(&mut h.rsc);
+    h.set_root(outer);
+    (
+        vec![
+            filler.id(),
+            plain.id(),
+            measured.id(),
+            whole.id(),
+            inner.id(),
+            outer.id(),
+        ],
+        [measured, inner],
+    )
+}
+
+#[test]
+fn a_box_that_only_rounds_past_its_fixed_children_leaves_nothing_over() {
+    let mut warm = Harness::new((640, 900));
+    let (ids, spans) = plant_boundary(&mut warm, false);
+    warm.frame();
+    for span in spans {
+        warm.rsc[span].children.rotate_left(1);
+    }
+    warm.frame();
+
+    let mut cold = Harness::new((640, 900));
+    let (cold_ids, _) = plant_boundary(&mut cold, true);
+    cold.frame();
+
+    let mut wrong = Vec::new();
+    for (i, (&w, &c)) in ids.iter().zip(&cold_ids).enumerate() {
+        let (got, want) = (warm.region(&w), cold.region(&c));
+        if got != want {
+            wrong.push(format!("widget {i}: warm {got:?} cold {want:?}"));
+        }
+    }
+    assert!(wrong.is_empty(), "{}", wrong.join("\n"));
+}