iris/tests/cases/retained.rs

//! 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(Len::px(150.0), Len::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);
    // Asked once, from the cursor; its slot is its answer and the drawing is
    // moved there.
    assert_eq!(asked_draws.get(), 1);
}

#[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 top = UiSpan::new(Len::ZERO, Len::from_parts(Rel::ZERO, len.px));
        painter.widget_at(
            &self.inner,
            [None; 2],
            [Place::Within(Part::WHOLE), Place::Within(Part::From(top))],
        );
        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. The quarter-sized box
/// the answer places them in is not a question: the drawing is moved there,
/// so each length they are asked at costs one draw.
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_span_ruled_across_itself_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).height(rel(1.0)).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, LayoutLen::rel(1.0));
}

/// A row places its children as lengths from where its own box starts, so a
/// child that grew moves the ones after it and nothing else: each of them is
/// the same box in a new place, which the retained drawing follows without
/// being made again. Both kinds of length: one the row resolves from a rule,
/// and one it takes from what the child reported.
#[test]
fn a_row_moves_what_follows_a_child_that_grew_rather_than_drawing_it() {
    for declared in [false, true] {
        let mut h = Harness::new((400, 200));
        let first = rect(Color::RED).width(50).add(&mut h.rsc);
        let ruled = Rc::new(Cell::new(0));
        let second = Counted {
            draws: ruled.clone(),
            size: Size::LEFTOVER,
            reads_box: false,
        };
        let second = match declared {
            true => second.width(rel(0.25)).add(&mut h.rsc),
            false => second.width(60).add(&mut h.rsc),
        };
        let (third, reported) = counted(&mut h, Size::from((70, 20)), false);
        h.set_root((first, second, third).span(Dir::RIGHT).width(rel(1.0)));
        let (was_ruled, was_reported) = (ruled.get(), reported.get());
        // A quarter of the row is a quarter of the row, wherever it sits in
        // it and whatever the first child takes.
        let width = match declared {
            true => 100,
            false => 60,
        };
        assert_corners!(h, second, (50, 0), (50 + width, 200));

        h.set_len(first, Axis::X, 80);
        h.frame();

        assert_eq!(ruled.get(), was_ruled, "the ruled child was drawn again");
        assert_eq!(
            reported.get(),
            was_reported,
            "the reported child was drawn again"
        );
        assert_corners!(h, second, (80, 0), (80 + width, 200));
        assert_corners!(h, third, (80 + width, 90), (150 + width, 110));
    }
}

/// 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 + 1);
}

#[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 + 1, "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 + 1);
}

/// 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 = LayoutLen::px(100.0 + step * part);
        h.frame();
        assert_eq!(draws.get(), settled);
    }

    h.rsc[first].size.x = LayoutLen::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));
}

/// A retained drawing belongs to the layer it was made on: asked for again
/// on another one it has to be drawn there, since nothing about its geometry
/// says it is in a list that paints at a different moment.
#[test]
fn a_widget_asked_again_on_another_layer_is_drawn_there() {
    /// Draws its child on its own layer, then again one layer in -- which is
    /// what a container measuring a child by drawing it used to do.
    struct Twice(StrongWidget);

    impl Widget for Twice {
        fn draw(&mut self, painter: &mut Painter) -> Size {
            let size = painter.widget(&self.0).size();
            painter.child_layer();
            painter.widget(&self.0);
            size
        }
    }

    let mut h = Harness::new((400, 200));
    let (front, draws) = counted(&mut h, Size::from((100, 50)), false);
    let outer = Twice(front.add_strong(&mut h.rsc)).add(&mut h.rsc);
    h.set_root(outer);
    h.frame();

    assert_ne!(
        h.render.active[&front.id()].layer,
        h.render.active[&outer.id()].layer,
        "the first drawing was kept, on the layer it was measured on"
    );
    assert_eq!(draws.get(), 2, "the second ask could not reuse the first");
}

/// Which is why `Stack` measures the child that sizes it on the layer that
/// child draws on: one drawing, above the background it stacks over, rather
/// than one on each layer and the wrong one kept.
#[test]
fn a_stacks_sizing_child_is_drawn_once_where_it_belongs() {
    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()));
    assert_ne!(layer(front.id()), layer(background.id()));
    assert_eq!(draws.get(), 1);
}

/// A widget's own mask is not the one it inherited, and a redraw of it
/// inherits the second: handing back the first is handing it its own mask to
/// set a second time, which `set_mask` asserts against.
#[test]
fn a_masked_widget_redrawn_on_its_own_sets_its_mask_again() {
    let mut h = Harness::new((400, 200));
    let inner = rect(Color::BLUE).add(&mut h.rsc);
    let masked = inner.masked().add(&mut h.rsc);
    let other = rect(Color::RED).width(100).add(&mut h.rsc);
    h.set_root((other, masked).span(Dir::RIGHT));
    h.rsc.widgets_mut().get_dyn_mut(masked.id());
    h.frame();
    assert_corners!(h, inner, (100, 0), (400, 200));
}

/// The two spans a subtree changes hands between, and the branch that is not
/// in the tree yet -- kept alive by the test until it is.
struct Handover {
    leaf: WidgetId,
    first: WeakWidget<Span>,
    second: WeakWidget<Span>,
    root: WeakWidget<Span>,
    spare: StrongWidget,
}

/// A subtree that changes hands while its box does not move, so nothing about
/// reusing its drawing says it changed parents. `deeper` puts a span between
/// the root and `second`, so it changes depth by changing hands as well.
fn plant_handover(h: &mut Harness, moved: bool, deeper: bool, width: f32) -> Handover {
    let leaf = rect(Color::RED).add(&mut h.rsc);
    let sized = leaf.width(width).add(&mut h.rsc);
    let holder = (sized,).span(Dir::RIGHT).add(&mut h.rsc);
    let first = Span {
        children: match moved {
            true => Vec::new(),
            false => vec![holder.add_strong(&mut h.rsc)],
        },
        dir: Dir::RIGHT,
        gap: Px::ZERO,
    }
    .add(&mut h.rsc);
    let second = Span {
        children: match moved {
            true => vec![holder.add_strong(&mut h.rsc)],
            false => Vec::new(),
        },
        dir: Dir::RIGHT,
        gap: Px::ZERO,
    }
    .add(&mut h.rsc);
    let branch = match deeper {
        true => (second,).span(Dir::RIGHT).add_strong(&mut h.rsc),
        false => second.add_strong(&mut h.rsc),
    };
    let (in_tree, spare) = match moved {
        true => (branch, first.add_strong(&mut h.rsc)),
        false => (first.add_strong(&mut h.rsc), branch),
    };
    let root = Span {
        children: vec![in_tree],
        dir: Dir::RIGHT,
        gap: Px::ZERO,
    }
    .add(&mut h.rsc);
    h.state.root = Some(root.add_strong(&mut h.rsc));
    Handover {
        leaf: sized.id(),
        first,
        second,
        root,
        spare,
    }
}

/// Moves the subtree and swaps the branch it sits in for the one it left.
fn hand_over(h: &mut Harness, tree: Handover) -> WidgetId {
    let holder = h.rsc[tree.first].children.remove(0);
    h.rsc[tree.second].children.push(holder);
    h.rsc[tree.root].children.clear();
    h.rsc[tree.root].children.push(tree.spare);
    h.frame();
    tree.leaf
}

#[test]
fn a_subtree_that_changed_parents_is_not_undrawn_by_the_one_it_left() {
    let mut warm = Harness::new((400, 200));
    let tree = plant_handover(&mut warm, false, false, 40.0);
    warm.frame();
    let leaf = hand_over(&mut warm, tree);

    let mut cold = Harness::new((400, 200));
    let grown = plant_handover(&mut cold, true, false, 40.0);
    cold.frame();

    assert_eq!(
        warm.region(&leaf),
        cold.region(&grown.leaf),
        "the span it left still listed it and undrew it"
    );
}

#[test]
fn a_subtree_that_changed_parents_settles_at_the_depth_it_moved_to() {
    let mut warm = Harness::new((400, 200));
    let tree = plant_handover(&mut warm, false, true, 40.0);
    warm.frame();
    let leaf = hand_over(&mut warm, tree);
    // After it has changed hands, so what has to reach the new parent is a
    // change made under the subtree it now holds.
    warm.set_len(leaf, Axis::X, LayoutLen::px(90.0));
    warm.frame();

    let mut cold = Harness::new((400, 200));
    let grown = plant_handover(&mut cold, true, true, 90.0);
    cold.frame();

    assert_eq!(
        warm.region(&leaf),
        cold.region(&grown.leaf),
        "the span it moved to is the one the change has to reach"
    );
}

fn primitive_bounds(h: &Harness, id: WidgetId) -> Vec<PixelRegion> {
    h.render.active[&id]
        .primitives
        .iter()
        .map(|primitive| {
            let handle = &primitive.handle;
            let instance = &h.render.layers[handle.layer].primitives()[handle.kind as usize]
                .as_ref()
                .unwrap()
                .instances()[handle.inst_idx];
            h.render
                .moves
                .resolve(instance.move_idx, instance.region)
                .to_px(h.render.output_size())
        })
        .collect()
}

#[test]
fn changing_an_inherited_region_keeps_the_original_measurement_offer() {
    fn build(h: &mut Harness, width: i32, text: &str) -> (WeakWidget<Text>, WeakWidget<Rect>) {
        let first = rect(Color::RED).width(width).add(&mut h.rsc);
        let words = wtext(text).size(20).wrap(true).add(&mut h.rsc);
        let through = Stretchy {
            inner: words.add_strong(&mut h.rsc),
            draws: Rc::new(Cell::new(0)),
        }
        .add(&mut h.rsc);
        h.set_root((first, through).span(Dir::RIGHT));
        (words, first)
    }
    let short = "one two";
    let long = "one two three four five six seven eight nine ten eleven twelve";
    let mut warm = Harness::new((400, 200));
    let (words, first) = build(&mut warm, 50, short);
    warm.set_len(first, Axis::X, 200);
    warm.frame();
    *warm.rsc[words].content = long.to_string();
    warm.frame();
    let mut cold = Harness::new((400, 200));
    let (other, _) = build(&mut cold, 200, long);
    assert_eq!(warm.region(&words), cold.region(&other));
    assert_eq!(
        primitive_bounds(&warm, words.id()),
        primitive_bounds(&cold, other.id())
    );
}

#[test]
fn widening_text_without_soft_breaks_reuses_its_drawing() {
    struct CountedText {
        text: Text,
        draws: Rc<Cell<usize>>,
    }
    impl Widget for CountedText {
        fn draw(&mut self, painter: &mut Painter) -> Size {
            self.draws.set(self.draws.get() + 1);
            self.text.draw(painter)
        }
    }
    for content in ["Short text", "Two hard\nline breaks\nhere", ""] {
        let plant = |h: &mut Harness| {
            let mut text = Text::new(content);
            text.wrap = true;
            let draws = Rc::new(Cell::new(0));
            let root = CountedText {
                text,
                draws: draws.clone(),
            }
            .add(&mut h.rsc);
            h.set_root(root);
            (root, draws)
        };
        let mut warm = Harness::new((300, 200));
        let (root, draws) = plant(&mut warm);
        let before = draws.get();
        warm.resize((500, 200));
        warm.frame();
        assert_eq!(draws.get(), before, "{content:?}");
        let mut cold = Harness::new((500, 200));
        let (other, _) = plant(&mut cold);
        assert_eq!(warm.region(&root), cold.region(&other));
        assert_eq!(
            primitive_bounds(&warm, root.id()),
            primitive_bounds(&cold, other.id())
        );
    }
}

#[test]
fn resizing_a_fixed_frame_recomposes_its_contents_without_drawing_them() {
    struct Frame {
        child: StrongWidget,
        region: UiRegion,
    }
    impl Widget for Frame {
        fn draw(&mut self, painter: &mut Painter) -> Size {
            painter.widget_at(
                &self.child,
                [None; 2],
                [
                    Place::Within(Part::From(self.region.x)),
                    Place::Within(Part::From(self.region.y)),
                ],
            );
            Size::LEFTOVER
        }
    }
    struct Painted(Rc<Cell<usize>>);
    impl Widget for Painted {
        fn draw(&mut self, painter: &mut Painter) -> Size {
            self.0.set(self.0.get() + 1);
            painter.set_mask(UiRegion::FULL);
            painter.primitive(RectPrimitive::color(Color::BLUE));
            Size::LEFTOVER
        }
    }
    let fixed = |start, end| UiRegion::new(UiSpan::new(Len::px(start), Len::px(end)), UiSpan::FULL);
    for node in [false, true] {
        let plant = |h: &mut Harness, region| {
            let draws = Rc::new(Cell::new(0));
            let leaf = Painted(draws.clone()).add(&mut h.rsc);
            h.rsc.widgets_mut().set_region_node(leaf, node);
            let inner = Frame {
                child: leaf.add_strong(&mut h.rsc),
                region: UiRegion::new(UiSpan::new(Len::rel(0.23), Len::rel(0.83)), UiSpan::FULL),
            }
            .add_strong(&mut h.rsc);
            let root = Frame {
                child: inner,
                region,
            }
            .add(&mut h.rsc);
            h.set_root(root);
            (root, leaf, draws)
        };
        let mut warm = Harness::new((400, 200));
        let (root, leaf, draws) = plant(&mut warm, fixed(7.0, 104.0));
        let before = draws.get();
        warm.rsc[root].region = fixed(19.0, 180.0);
        warm.frame();
        assert_eq!(draws.get(), before);
        let mut cold = Harness::new((400, 200));
        let (_, other, _) = plant(&mut cold, fixed(19.0, 180.0));
        assert_eq!(warm.region(&leaf), cold.region(&other));
        assert_eq!(
            primitive_bounds(&warm, leaf.id()),
            primitive_bounds(&cold, other.id())
        );
        let mask = |h: &Harness, id: WidgetId| {
            let active = &h.render.active[&id];
            let mask = &h.rsc.ui().masks[active.mask.idx()];
            h.render
                .moves
                .resolve(mask.move_idx, mask.region)
                .to_px(h.render.output_size())
        };
        assert_eq!(mask(&warm, leaf.id()), mask(&cold, other.id()));
    }
}

#[test]
fn glyph_origins_compose_identically_when_drawn_and_when_retained() {
    struct Glyphs {
        buffer: TextBuffer,
        draws: Rc<Cell<usize>>,
    }
    impl Widget for Glyphs {
        fn draw(&mut self, painter: &mut Painter) -> Size {
            self.draws.set(self.draws.get() + 1);
            let text = painter.render_text(&mut self.buffer, &TextAttrs::default(), None);
            let origin = UiRegion::new(
                UiSpan::new(Len::rel(0.23) + Len::px(-7.125), Len::FULL),
                UiSpan::new(Len::rel(0.37) + Len::px(3.25), Len::FULL),
            );
            painter.glyphs(text, origin);
            Size::LEFTOVER
        }
    }
    struct Frame {
        child: StrongWidget,
        frame: UiRegion,
        region: UiRegion,
    }
    impl Widget for Frame {
        fn draw(&mut self, painter: &mut Painter) -> Size {
            painter.widget_at(
                &self.child,
                [Some(self.frame.x.len()), None],
                [
                    Place::Fill(Part::From(self.region.x)),
                    Place::Fill(Part::From(self.region.y)),
                ],
            );
            Size::LEFTOVER
        }
    }
    for node in [false, true] {
        let mut h = Harness::new((403, 211));
        let draws = Rc::new(Cell::new(0));
        let text = Glyphs {
            buffer: TextBuffer::new("Glyphs: gj AV\nsecond line"),
            draws: draws.clone(),
        }
        .add(&mut h.rsc);
        h.rsc.widgets_mut().set_region_node(text, node);
        let root = Frame {
            child: text.add_strong(&mut h.rsc),
            frame: UiRegion::FULL,
            region: UiRegion::FULL,
        }
        .add(&mut h.rsc);
        h.set_root(root);
        for (start, end) in [(0.13, 0.83), (-0.17, 1.23), (0.31, 0.67)] {
            let before = draws.get();
            h.rsc[root].frame.x = UiSpan::new(Len::px(13.125), Len::px(287.375));
            h.rsc[root].region = UiRegion::new(
                UiSpan::new(Len::rel(start), Len::rel(end)),
                UiSpan::new(Len::px(7.25), Len::rel(end)),
            );
            h.frame();
            assert_eq!(draws.get(), before);
            let retained = primitive_bounds(&h, text.id());
            assert!(!retained.is_empty());
            let _ = h.rsc.widgets_mut().get_dyn_mut(text.id());
            h.frame();
            assert!(draws.get() > before);
            assert_eq!(retained, primitive_bounds(&h, text.id()));
        }
    }
}

#[test]
fn resizing_does_not_remeasure_a_fixed_stack_for_its_unmeasured_overlay() {
    let mut h = Harness::new((400, 200));
    let (sizing, _) = counted(&mut h, Size::from((100, 80)), false);
    let (overlay, draws) = counted(&mut h, Size::LEFTOVER, true);
    h.set_root((sizing, overlay).stack().size(StackSize::Child(0)));
    let settled = draws.get();

    h.resize((800, 300));
    h.frame();

    assert_eq!(draws.get(), settled);
    assert_corners!(h, overlay, (350, 110), (450, 190));
}

struct Unmeasured {
    child: StrongWidget,
    draws: Rc<Cell<usize>>,
}

impl Widget for Unmeasured {
    fn draw(&mut self, painter: &mut Painter) -> Size {
        self.draws.set(self.draws.get() + 1);
        painter.widget(&self.child);
        Size::LEFTOVER
    }
}

#[test]
fn a_declared_size_change_stops_at_an_independent_parent() {
    let mut h = Harness::new((400, 200));
    let leaf = rect(Color::RED).width(100).add(&mut h.rsc);
    let parent = Unmeasured {
        child: leaf.add_strong(&mut h.rsc),
        draws: Rc::new(Cell::new(0)),
    }
    .add_strong(&mut h.rsc);
    let draws = Rc::new(Cell::new(0));
    h.set_root(Unmeasured {
        child: parent,
        draws: draws.clone(),
    });
    let settled = draws.get();

    h.set_len(leaf, Axis::X, 150);
    h.frame();

    assert_corners!(h, leaf, (125, 0), (275, 200));
    assert_eq!(draws.get(), settled);
}

#[test]
fn an_unmeasured_child_still_invalidates_its_parents_drawing_on_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,).stack());
    let settled = draws.get();

    h.resize((800, 200));
    h.frame();

    assert!(draws.get() > settled);
    assert_corners!(h, leaf, (300, 90), (500, 110));
}

#[test]
fn changed_drawing_dependencies_reach_ancestors_without_a_size_change() {
    let mut h = Harness::new((400, 200));
    let (leaf, draws) = counted(&mut h, Size::LEFTOVER, false);
    h.set_root(((leaf,).stack(),).stack());

    h.rsc[leaf].reads_box = true;
    h.frame();
    let settled = draws.get();
    h.resize((800, 200));
    h.frame();

    assert_eq!(draws.get(), settled + 1);
    assert_corners!(h, leaf, (0, 0), (800, 200));
}

#[test]
fn widening_and_restoring_a_contract_does_not_invalidate_its_reader() {
    let mut h = Harness::new((400, 200));
    let (leaf, leaf_draws) = counted(&mut h, Size::LEFTOVER, true);
    let draws = Rc::new(Cell::new(0));
    let child = leaf.add_strong(&mut h.rsc);
    h.set_root(Unmeasured {
        child,
        draws: draws.clone(),
    });
    let settled = draws.get();
    for reads_box in [false, true, false, true] {
        h.rsc[leaf].reads_box = reads_box;
        h.frame();
        assert_eq!(draws.get(), settled);
    }
    let settled = leaf_draws.get();
    h.resize((800, 200));
    h.frame();
    assert_eq!(leaf_draws.get(), settled + 1);
}
#[test]
fn padding_and_stack_boxes_follow_the_region_without_drawing_again() {
    struct Observed<W> {
        widget: W,
        draws: Rc<Cell<usize>>,
    }
    impl<W: Widget> Widget for Observed<W> {
        fn draw(&mut self, painter: &mut Painter) -> Size {
            self.draws.set(self.draws.get() + 1);
            self.widget.draw(painter)
        }
    }
    struct Frame {
        child: StrongWidget,
        region: UiRegion,
    }
    impl Widget for Frame {
        fn draw(&mut self, painter: &mut Painter) -> Size {
            painter.widget_at(
                &self.child,
                [None; 2],
                [
                    Place::Fill(Part::From(self.region.x)),
                    Place::Fill(Part::From(self.region.y)),
                ],
            );
            Size::LEFTOVER
        }
    }
    for node in [false, true] {
        let plant = |h: &mut Harness, region| {
            let draws = Rc::new(Cell::new(0));
            let leaf = rect(Color::BLUE).masked().add(&mut h.rsc);
            h.rsc.widgets_mut().set_region_node(leaf, node);
            let fixed = rect(Color::RED).width(31).height(19).add(&mut h.rsc);
            let stack = Observed {
                widget: Stack {
                    children: vec![leaf.add_strong(&mut h.rsc), fixed.add_strong(&mut h.rsc)],
                    size: StackSize::Default,
                },
                draws: draws.clone(),
            }
            .add_strong(&mut h.rsc);
            let pad = Observed {
                widget: Pad {
                    inner: stack,
                    padding: Padding::uniform(7).with_left(13),
                },
                draws: draws.clone(),
            }
            .add_strong(&mut h.rsc);
            let root = Frame { child: pad, region }.add(&mut h.rsc);
            h.set_root(root);
            (root, leaf, fixed, draws)
        };
        // The same box in three places. A pad places its child as lengths of
        // its own box measured from where that box starts, so moving it is
        // nothing to the pad -- where changing its length is a different
        // question, and does draw it again.
        let at = |start: f32| {
            let span = |start: Len| UiSpan::new(start, start + Len::rel(0.4));
            UiRegion::new(span(Len::rel(start) + Len::px(3.125)), span(Len::px(11.25)))
        };
        let mut warm = Harness::new((403, 211));
        let (root, leaf, fixed, draws) = plant(&mut warm, at(0.13));
        for start in [0.13, -0.17, 0.31] {
            let region = at(start);
            let before = draws.get();
            warm.rsc[root].region = region;
            warm.frame();
            assert_eq!(draws.get(), before);
            let mut cold = Harness::new((403, 211));
            let (_, other, other_fixed, _) = plant(&mut cold, region);
            for (a, b) in [(leaf.id(), other.id()), (fixed.id(), other_fixed.id())] {
                assert_eq!(warm.region(&a), cold.region(&b));
                assert_eq!(primitive_bounds(&warm, a), primitive_bounds(&cold, b));
            }
            let mask = |h: &Harness, id: WidgetId| {
                let active = &h.render.active[&id];
                let mask = &h.rsc.ui().masks[active.mask.idx()];
                h.render
                    .moves
                    .resolve(mask.move_idx, mask.region)
                    .to_px(h.render.output_size())
            };
            assert_eq!(mask(&warm, leaf.id()), mask(&cold, other.id()));
        }
    }
}

#[test]
fn moving_a_childs_region_preserves_the_slot_chosen_from_its_measurement() {
    struct Measured;
    impl Widget for Measured {
        fn draw(&mut self, painter: &mut Painter) -> Size {
            let width = painter.px_len(Axis::X);
            painter.primitive(RectPrimitive::color(Color::BLUE));
            Size::from((80, if width > Px::from_int(100) { 40 } else { 60 }))
        }
    }
    struct Frame {
        child: StrongWidget,
        start: f32,
    }
    impl Widget for Frame {
        fn draw(&mut self, painter: &mut Painter) -> Size {
            painter.widget_at(
                &self.child,
                [None; 2],
                [
                    Place::Fill(Part::From(UiSpan::new(
                        Len::px(self.start),
                        Len::px(self.start + 200.0),
                    ))),
                    Place::Fill(Part::From(UiSpan::FULL)),
                ],
            );
            Size::LEFTOVER
        }
    }
    let mut h = Harness::new((400, 200));
    let leaf = Measured.add(&mut h.rsc);
    let stack = (leaf,).stack().add_strong(&mut h.rsc);
    let root = Frame {
        child: stack,
        start: 0.0,
    }
    .add(&mut h.rsc);
    h.set_root(root);
    assert_corners!(h, leaf, (60, 80), (140, 120));
    h.rsc[root].start = 30.0;
    h.frame();
    assert_corners!(h, leaf, (90, 80), (170, 120));
    assert_eq!(
        primitive_bounds(&h, leaf.id()),
        vec![h.region(&leaf).unwrap()]
    );
}

#[test]
fn changing_regions_keep_fractional_reports_and_numeric_dependencies_valid() {
    struct Container {
        child: StrongWidget,
        region: UiRegion,
    }
    impl Widget for Container {
        fn draw(&mut self, painter: &mut Painter) -> Size {
            painter
                .widget_at(
                    &self.child,
                    [None; 2],
                    [
                        Place::Within(Part::From(self.region.x)),
                        Place::Within(Part::From(self.region.y)),
                    ],
                )
                .size()
        }
    }
    struct Frame {
        child: StrongWidget,
        region: UiRegion,
        answer: Rc<Cell<Size>>,
    }
    impl Widget for Frame {
        fn draw(&mut self, painter: &mut Painter) -> Size {
            self.answer.set(
                painter
                    .widget_at(
                        &self.child,
                        [None; 2],
                        [
                            Place::Fill(Part::From(self.region.x)),
                            Place::Fill(Part::From(self.region.y)),
                        ],
                    )
                    .size(),
            );
            Size::LEFTOVER
        }
    }
    for fractional in [false, true] {
        for region in [
            UiRegion::FULL,
            UiRegion::new(UiSpan::new(Len::rel(0.13), Len::rel(0.79)), UiSpan::FULL),
        ] {
            let plant = |h: &mut Harness, outer| {
                let size = if fractional {
                    Size {
                        x: rel(0.5),
                        y: LayoutLen::px(27),
                    }
                } else {
                    Size::from((80, 27))
                };
                let (leaf, _) = counted(h, size, !fractional);
                let child = Container {
                    child: leaf.add_strong(&mut h.rsc),
                    region,
                }
                .add_strong(&mut h.rsc);
                let answer = Rc::new(Cell::new(Size::ZERO));
                let root = Frame {
                    child,
                    region: outer,
                    answer: answer.clone(),
                }
                .add(&mut h.rsc);
                h.set_root(root);
                (root, leaf, answer)
            };
            let mut warm = Harness::new((403, 211));
            let (root, leaf, answer) = plant(&mut warm, UiRegion::FULL);
            for width in [191.125, 297.25, 83.75] {
                let region =
                    UiRegion::new(UiSpan::new(Len::px(13.125), Len::px(width)), UiSpan::FULL);
                warm.rsc[root].region = region;
                warm.frame();
                let mut cold = Harness::new((403, 211));
                let (_, other, other_answer) = plant(&mut cold, region);
                assert_eq!(answer.get(), other_answer.get());
                assert_eq!(warm.region(&leaf), cold.region(&other));
            }
        }
    }
}

struct OptionalMask {
    inner: StrongWidget,
    enabled: bool,
}

impl Widget for OptionalMask {
    fn draw(&mut self, painter: &mut Painter) -> Size {
        if self.enabled {
            painter.set_mask(UiRegion::FULL);
        }
        painter.widget(&self.inner);
        Size::LEFTOVER
    }
}

fn primitive_masks(h: &Harness, id: WidgetId) -> Vec<MaskIdx> {
    h.render.active[&id]
        .primitives
        .iter()
        .map(|primitive| {
            let handle = &primitive.handle;
            h.render.layers[handle.layer].primitives()[handle.kind as usize]
                .as_ref()
                .unwrap()
                .instances()[handle.inst_idx]
                .mask_idx
        })
        .collect()
}

#[test]
fn a_redrawn_mask_keeps_reused_primitives_clipped_when_it_moves() {
    for node in [false, true] {
        let mut h = Harness::new((400, 200));
        let first = rect(Color::RED).height(50).add(&mut h.rsc);
        let inner = rect(Color::BLUE).add(&mut h.rsc);
        let draws = Rc::new(Cell::new(0));
        let child = Stretchy {
            inner: inner.add_strong(&mut h.rsc),
            draws: draws.clone(),
        }
        .add(&mut h.rsc);
        let masked = child.masked().add(&mut h.rsc);
        h.rsc.widgets_mut().set_region_node(masked, node);
        h.set_root((first, masked).span(Dir::DOWN));
        let mask = h.render.active[&masked.id()].mask;
        let settled = draws.get();
        h.rsc.widgets_mut().get_dyn_mut(masked.id());
        h.frame();
        assert_eq!(primitive_masks(&h, inner.id()), vec![mask]);
        assert_eq!(draws.get(), settled, "a mask repaint must reuse its child");
        assert_eq!(h.render.active[&masked.id()].mask, mask);
        h.set_len(first, Axis::Y, 10);
        h.frame();
        let clip = h.rsc.ui().masks[mask.idx()];
        let clip = h
            .render
            .moves
            .resolve(clip.move_idx, clip.region)
            .to_px(h.render.output_size());
        assert_eq!(clip, h.region(&masked).unwrap());
        assert_corners!(h, inner, (0, 10), (400, 200));
    }
}

#[test]
fn adding_and_removing_a_mask_updates_existing_primitives() {
    let mut h = Harness::new((400, 200));
    let inner = rect(Color::BLUE).add(&mut h.rsc);
    let masked = OptionalMask {
        inner: inner.add_strong(&mut h.rsc),
        enabled: false,
    }
    .add(&mut h.rsc);
    h.set_root(masked);
    for enabled in [true, false, true, false] {
        h.rsc[masked].enabled = enabled;
        h.frame();
        let mask = h.render.active[&masked.id()].mask;
        assert_eq!(mask == MaskIdx::NONE, !enabled);
        assert_eq!(primitive_masks(&h, inner.id()), vec![mask]);
    }
    assert_eq!(h.rsc.ui().masks.len(), 1, "retired slots must be reusable");
}

#[test]
fn an_empty_masks_slot_is_released_when_the_mask_is_removed_or_undrawn() {
    let mut h = Harness::new((400, 200));
    let (inner, _) = counted(&mut h, Size::LEFTOVER, false);
    let masked = OptionalMask {
        inner: inner.add_strong(&mut h.rsc),
        enabled: true,
    }
    .add(&mut h.rsc);
    let row = (masked,).span(Dir::DOWN).add(&mut h.rsc);
    h.set_root(row);
    for _ in 0..3 {
        h.rsc[masked].enabled = false;
        h.frame();
        h.rsc[masked].enabled = true;
        h.frame();
        let child = h.rsc[row].pop().unwrap();
        h.frame();
        h.rsc[row].push(child);
        h.frame();
    }
    assert_eq!(h.rsc.ui().masks.len(), 1);
}

struct SharedChild(Rc<StrongWidget>);

impl Widget for SharedChild {
    fn draw(&mut self, painter: &mut Painter) -> Size {
        painter.widget(self.0.as_ref()).size()
    }
}

struct SwitchParent {
    choices: [StrongWidget; 2],
    choice: usize,
}

impl Widget for SwitchParent {
    fn draw(&mut self, painter: &mut Painter) -> Size {
        painter.widget(&self.choices[self.choice]).size()
    }
}

#[test]
fn a_redrawn_subtree_is_not_undrawn_by_the_parent_it_left() {
    for node in [false, true] {
        let mut h = Harness::new((400, 200));
        let leaf = rect(Color::RED).width(40).add(&mut h.rsc);
        let held: StrongWidget = leaf.add_strong(&mut h.rsc);
        let shared = Rc::new(held);
        let first = SharedChild(shared.clone()).add_strong(&mut h.rsc);
        let second = SharedChild(shared).add_strong(&mut h.rsc);
        h.rsc.widgets_mut().set_region_node(&second, node);
        let root = SwitchParent {
            choices: [first, second],
            choice: 0,
        }
        .add(&mut h.rsc);
        h.set_root(root);
        let before = h.region(&leaf);
        h.rsc[root].choice = 1;
        h.frame();
        assert_eq!(h.region(&leaf), before);
    }
}