teksilo_widgets/primitives/

grid.rs

// SPDX-License-Identifier: MPL-2.0
// SPDX-FileCopyrightText: 2026 FernTech

//! Grid — a 2D layout container with explicit row and column tracks.
//!
//! Columns and rows are declared as [`TrackSize`] slices supporting three
//! sizing modes: `Fixed(px)` (exact logical pixels), `Auto` (sized to the
//! largest child in that track), and `Fractional(fr)` (share of the remaining
//! space after fixed and auto tracks are allocated — the CSS `fr` unit).
//! Children are placed in **row-major order**: child 0 occupies cell
//! `(row=0, col=0)`, child 1 `(row=0, col=1)`, and so on. Dormant children
//! are excluded from placement while keeping their siblings at their original
//! cell positions, so toggling a cell visible/dormant does not shift other
//! cells.
//!
//! Fractional columns fall back to the child's natural width when the parent
//! provides no width constraint (intrinsic-measurement pass), preventing
//! wrap-aware children from reporting inflated heights.
//!
//! ```rust
//! # use teksilo_widgets::primitives::{Grid, TrackSize, RectWidget};
//! // Two equal columns with a fixed 40 dp row, separated by an 8 dp gap
//! let _grid = Grid::new()
//!     .columns(vec![TrackSize::Fractional(1.0), TrackSize::Fractional(1.0)])
//!     .rows(vec![TrackSize::Fixed(40.0)])
//!     .column_gap(8.0)
//!     .child(RectWidget::new())
//!     .child(RectWidget::new());
//! ```

use teksilo_canvas::{Point, Rect, Size, SizeProposal};
use teksilo_core::accessibility::AccessNodeBuilder;
use teksilo_core::signal::Prop;
use teksilo_core::widget::{LayoutContext, PaintContext, PendingChild, Widget, WidgetPlacement};
use teksilo_core::widget_id::WidgetId;

/// Sizing mode for a single row or column track in a [`Grid`].
#[derive(Debug, Clone, Copy, PartialEq)]
pub enum TrackSize {
    /// Fixed size in logical pixels regardless of available space.
    Fixed(f32),
    /// Share of the remaining space after `Fixed` and `Auto` tracks are
    /// resolved; equivalent to the CSS `fr` unit.  Multiple `Fractional`
    /// tracks divide the remainder proportionally to their weights.
    Fractional(f32),
    /// Sized to the largest intrinsic dimension among all children in the
    /// track; expands to fill content, never clips.
    Auto,
}

/// A 2D grid layout container with explicit track declarations.
#[derive(Debug)]
pub struct Grid {
    columns: Vec<TrackSize>,
    rows: Vec<TrackSize>,
    column_gap: Prop<f32>,
    row_gap: Prop<f32>,
    child_ids: Vec<WidgetId>,
    pending: Vec<PendingChild>,
}

impl Grid {
    /// Create a new `Grid` with a single `Auto` column and a single `Auto`
    /// row; configure track definitions with [`columns`](Self::columns) and
    /// [`rows`](Self::rows).
    pub fn new() -> Self {
        Self {
            columns: vec![TrackSize::Auto],
            rows: vec![TrackSize::Auto],
            column_gap: Prop::Static(0.0),
            row_gap: Prop::Static(0.0),
            child_ids: Vec::new(),
            pending: Vec::new(),
        }
    }

    /// Set the column track definitions; each entry describes one column's
    /// sizing mode.
    pub fn columns(mut self, columns: Vec<TrackSize>) -> Self {
        self.columns = columns;
        self
    }

    /// Set the row track definitions; each entry describes one row's sizing
    /// mode.
    pub fn rows(mut self, rows: Vec<TrackSize>) -> Self {
        self.rows = rows;
        self
    }

    /// Set the inter-column gap. Accepts static `f32` or `Signal<f32>`.
    pub fn column_gap(mut self, gap: impl Into<Prop<f32>>) -> Self {
        self.column_gap = gap.into();
        self
    }

    /// Set the inter-row gap. Accepts static `f32` or `Signal<f32>`.
    pub fn row_gap(mut self, gap: impl Into<Prop<f32>>) -> Self {
        self.row_gap = gap.into();
        self
    }

    /// Append an inline child widget in the next cell (row-major order).
    pub fn child(mut self, widget: impl teksilo_core::IntoTeksiChild) -> Self {
        self.pending
            .push(teksilo_core::IntoTeksiChild::into_pending(widget));
        self
    }

    /// Append multiple inline children from an iterator, each occupying the
    /// next cell in row-major order.
    pub fn children(
        mut self,
        iter: impl IntoIterator<Item = impl teksilo_core::IntoTeksiChild>,
    ) -> Self {
        for widget in iter {
            self.pending
                .push(teksilo_core::IntoTeksiChild::into_pending(widget));
        }
        self
    }

    /// Append an optional inline child; a `None` value is a no-op, keeping
    /// subsequent children at their original cell positions.
    pub fn child_opt(mut self, widget: Option<impl teksilo_core::IntoTeksiChild>) -> Self {
        if let Some(w) = widget {
            self.pending
                .push(teksilo_core::IntoTeksiChild::into_pending(w));
        }
        self
    }

    /// Resolve track sizes given available space, child sizes, and track definitions.
    fn resolve_tracks(
        tracks: &[TrackSize],
        gap: f32,
        available: Option<f32>,
        child_sizes: &[f32], // max child size per track
        child_mins: &[f32],  // compression floor per track (0 if no shrinkable child)
    ) -> Vec<f32> {
        let n = tracks.len();
        let total_gap = gap * (n as f32 - 1.0).max(0.0);

        // Phase 1: resolve Fixed and Auto tracks
        let mut resolved = vec![0.0_f32; n];
        let mut used = 0.0_f32;
        let mut total_fr = 0.0_f32;

        for (i, track) in tracks.iter().enumerate() {
            match *track {
                TrackSize::Fixed(px) => {
                    resolved[i] = px;
                    used += px;
                }
                TrackSize::Auto => {
                    let size = if i < child_sizes.len() {
                        child_sizes[i]
                    } else {
                        0.0
                    };
                    resolved[i] = size;
                    used += size;
                }
                TrackSize::Fractional(fr) => {
                    total_fr += fr;
                }
            }
        }

        // Phase 2: distribute remaining space to Fractional tracks.
        //
        // When the parent gives us an explicit `available` constraint
        // we share the remainder by flex weight. When it's `None`
        // (intrinsic-measurement pass — Switcher / ZStack / ScrollArea
        // ask their children with an unspecified proposal), there *is*
        // no remainder to share. We can't return zero for every
        // Fractional track: any child that re-measures against that
        // zero width will report a wildly inflated height (a TextWidget
        // with `proposal.width = Some(0)` wraps one glyph per line),
        // and that height bubbles up as the Grid's intrinsic height —
        // which is what callers like ScrollArea use to size their
        // scrollable content.
        //
        // Fall back to the child's natural width per track instead, so
        // an unconstrained Fractional column behaves like Auto. The
        // shared-remainder behavior still applies whenever a parent
        // *does* offer a width.
        if let Some(a) = available {
            let remaining = (a - total_gap - used).max(0.0);
            if total_fr > 0.0 {
                for (i, track) in tracks.iter().enumerate() {
                    if let TrackSize::Fractional(fr) = *track {
                        // Share the remainder by flex weight, but never below a
                        // track's compression floor. The floor is non-zero only
                        // when a child in that track opted into shrink with a
                        // `min < size`; rigid children contribute no floor, so
                        // their fractional columns still shrink (CSS-`fr`
                        // semantics). A track held at its floor may push the row
                        // past `available` — the intended residual overflow.
                        let share = remaining * fr / total_fr;
                        resolved[i] = share.max(child_mins.get(i).copied().unwrap_or(0.0));
                    }
                }
            }
        } else if total_fr > 0.0 {
            for (i, track) in tracks.iter().enumerate() {
                if matches!(*track, TrackSize::Fractional(_)) {
                    resolved[i] = child_sizes.get(i).copied().unwrap_or(0.0);
                }
            }
        }

        resolved
    }

    /// Get the (row, col) cell index for a given child index.
    fn cell_for(&self, child_index: usize) -> (usize, usize) {
        let cols = self.columns.len().max(1);
        (child_index / cols, child_index % cols)
    }
}

impl Default for Grid {
    fn default() -> Self {
        Self::new()
    }
}

impl Widget for Grid {
    fn layout_response(
        &self,
        proposal: SizeProposal,
        ctx: &LayoutContext,
    ) -> teksilo_core::widget::LayoutResponse {
        let num_cols = self.columns.len().max(1);
        let num_rows = self.rows.len().max(1);

        // Pass 1: measure children with unspecified width so Auto
        // columns can size to content. Wrapping children (TextWidget
        // with markup, etc.) report their single-line width — that's
        // fine for column resolution because Auto picks the largest
        // single-line width, and Fractional gets re-measured in pass 2.
        let intrinsic_proposal = SizeProposal::unspecified();
        let mut col_max = vec![0.0_f32; num_cols];
        let mut row_max = vec![0.0_f32; num_rows];
        // Per-column compression floor: the max `min.width` of *shrinkable*
        // children in the column. Rigid children contribute nothing, so their
        // fractional columns still shrink freely.
        let mut col_min = vec![0.0_f32; num_cols];

        for (i, &child_id) in self.child_ids.iter().enumerate() {
            let (row, col) = self.cell_for(i);
            if row >= num_rows || col >= num_cols {
                continue;
            }
            if let Some(r) = ctx.child_layout_response(child_id, intrinsic_proposal) {
                col_max[col] = col_max[col].max(r.size.width);
                row_max[row] = row_max[row].max(r.size.height);
                if r.shrink > 0.0 {
                    col_min[col] = col_min[col].max(r.min.width);
                }
            }
        }

        // Resolve column tracks against the parent's width proposal.
        let col_gap = self.column_gap.get();
        let row_gap = self.row_gap.get();
        let col_sizes =
            Self::resolve_tracks(&self.columns, col_gap, proposal.width, &col_max, &col_min);

        // Pass 2: for every child whose column is *narrower* than its
        // intrinsic width (typical of Fractional columns receiving the
        // remainder after Auto columns claim their natural sizes),
        // re-measure with the resolved column width as the proposal so
        // wrapping content reports its real wrapped height. Without
        // this, a TextWidget-with-markup inside a Fractional column
        // reports a 1-line height in pass 1 but actually paints
        // multi-line, bleeding outside its assigned cell.
        for (i, &child_id) in self.child_ids.iter().enumerate() {
            let (row, col) = self.cell_for(i);
            if row >= num_rows || col >= num_cols {
                continue;
            }
            let needs_remeasure = matches!(self.columns[col], TrackSize::Fractional(_))
                && col_sizes[col] + 0.5 < col_max[col];
            if needs_remeasure {
                let constrained = SizeProposal {
                    width: Some(col_sizes[col]),
                    height: None,
                };
                if let Some(s) = ctx.child_size(child_id, constrained) {
                    row_max[row] = row_max[row].max(s.height);
                }
            }
        }

        // Rows are not floored by `min` (the compression model targets the
        // main/horizontal axis); pass zeros.
        let row_sizes = Self::resolve_tracks(
            &self.rows,
            row_gap,
            proposal.height,
            &row_max,
            &vec![0.0_f32; num_rows],
        );

        let total_col_gap = col_gap * (num_cols as f32 - 1.0).max(0.0);
        let total_row_gap = row_gap * (num_rows as f32 - 1.0).max(0.0);
        let width = col_sizes.iter().sum::<f32>() + total_col_gap;
        let height = row_sizes.iter().sum::<f32>() + total_row_gap;

        Size::new(width, height).into()
    }

    fn place_children(
        &self,
        bounds: Rect,
        _proposal: SizeProposal,
        children: &mut [WidgetPlacement],
        ctx: &LayoutContext,
    ) {
        let num_cols = self.columns.len().max(1);
        let num_rows = self.rows.len().max(1);

        // Map each active child to its original cell index. The
        // framework filters dormant children out of `children`, so we
        // look up each child's position in `self.child_ids` (which
        // retains all children) to keep cell assignments stable.
        // `None` for a child not found in `child_ids` (an orphan from a
        // transient desync). Such a child is skipped entirely below rather than
        // mapped to cell 0 — placing it at (0,0) would overlap the real
        // cell-0 child and pollute that track's measured size.
        let original_indices: Vec<Option<usize>> = children
            .iter()
            .map(|c| self.child_ids.iter().position(|&id| id == c.id))
            .collect();

        // Pass 1: intrinsic sizes — same as size_that_fits.
        let intrinsic_proposal = SizeProposal::unspecified();
        let mut col_max = vec![0.0_f32; num_cols];
        let mut row_max = vec![0.0_f32; num_rows];
        let mut col_min = vec![0.0_f32; num_cols];

        for (i, child) in children.iter().enumerate() {
            let Some(orig) = original_indices[i] else {
                continue;
            };
            let (row, col) = self.cell_for(orig);
            if row >= num_rows || col >= num_cols {
                continue;
            }
            if let Some(r) = ctx.child_layout_response(child.id, intrinsic_proposal) {
                col_max[col] = col_max[col].max(r.size.width);
                row_max[row] = row_max[row].max(r.size.height);
                if r.shrink > 0.0 {
                    col_min[col] = col_min[col].max(r.min.width);
                }
            }
        }

        let col_gap = self.column_gap.get();
        let row_gap = self.row_gap.get();
        let col_sizes = Self::resolve_tracks(
            &self.columns,
            col_gap,
            Some(bounds.width),
            &col_max,
            &col_min,
        );

        // Pass 2: re-measure Fractional cells whose column shrank, so
        // their row height reflects wrap-induced growth.
        for (i, child) in children.iter().enumerate() {
            let Some(orig) = original_indices[i] else {
                continue;
            };
            let (row, col) = self.cell_for(orig);
            if row >= num_rows || col >= num_cols {
                continue;
            }
            let needs_remeasure = matches!(self.columns[col], TrackSize::Fractional(_))
                && col_sizes[col] + 0.5 < col_max[col];
            if needs_remeasure {
                let constrained = SizeProposal {
                    width: Some(col_sizes[col]),
                    height: None,
                };
                if let Some(s) = ctx.child_size(child.id, constrained) {
                    row_max[row] = row_max[row].max(s.height);
                }
            }
        }

        let row_sizes = Self::resolve_tracks(
            &self.rows,
            row_gap,
            Some(bounds.height),
            &row_max,
            &vec![0.0_f32; num_rows],
        );

        // Compute cell origins
        let mut col_origins = Vec::with_capacity(num_cols);
        let mut x = bounds.x;
        for (i, &w) in col_sizes.iter().enumerate() {
            col_origins.push(x);
            x += w;
            if i < num_cols - 1 {
                x += col_gap;
            }
        }

        let mut row_origins = Vec::with_capacity(num_rows);
        let mut y = bounds.y;
        for (i, &h) in row_sizes.iter().enumerate() {
            row_origins.push(y);
            y += h;
            if i < num_rows - 1 {
                y += row_gap;
            }
        }

        // Place each child in its cell
        for (i, child) in children.iter_mut().enumerate() {
            let Some(orig) = original_indices[i] else {
                child.size = Size::ZERO;
                continue;
            };
            let (row, col) = self.cell_for(orig);
            if row >= num_rows || col >= num_cols {
                child.size = Size::ZERO;
                continue;
            }
            child.origin = Point::new(col_origins[col], row_origins[row]);
            child.size = Size::new(col_sizes[col], row_sizes[row]);
        }
    }

    fn paint(&self, _bounds: Rect, _canvas: &mut teksilo_canvas::Canvas, _ctx: &PaintContext) {}

    fn accessibility(&self, builder: &mut AccessNodeBuilder) {
        builder.set_role(teksilo_core::accesskit::Role::GenericContainer);
    }

    fn children(&self) -> Vec<WidgetId> {
        self.child_ids.clone()
    }

    fn build(&mut self, ctx: &mut teksilo_core::build_context::BuildContext) -> Vec<WidgetId> {
        let pending = std::mem::take(&mut self.pending);
        if !pending.is_empty() {
            self.child_ids = pending
                .into_iter()
                .map(|child| match child {
                    PendingChild::Id(id) => id,
                    PendingChild::Deferred(w) => ctx.add_boxed(w),
                })
                .collect();
        }
        let self_id = ctx.self_id();
        let registry = ctx.binding_registry();
        self.column_gap.register_if_bound(
            self_id,
            registry,
            teksilo_core::binding::BindingLevel::Relayout,
        );
        self.row_gap.register_if_bound(
            self_id,
            registry,
            teksilo_core::binding::BindingLevel::Relayout,
        );
        self.child_ids.clone()
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use teksilo_core::widget_tree::WidgetTree;

    #[derive(Debug)]
    struct FixedLeaf(f32, f32);
    impl Widget for FixedLeaf {
        fn layout_response(
            &self,
            _proposal: SizeProposal,
            _ctx: &LayoutContext,
        ) -> teksilo_core::widget::LayoutResponse {
            Size::new(self.0, self.1).into()
        }
    }

    /// A shrinkable leaf with an explicit `min` width floor.
    #[derive(Debug)]
    struct ShrinkLeaf {
        w: f32,
        h: f32,
        min: f32,
    }
    impl Widget for ShrinkLeaf {
        fn layout_response(
            &self,
            _proposal: SizeProposal,
            _ctx: &LayoutContext,
        ) -> teksilo_core::widget::LayoutResponse {
            teksilo_core::widget::LayoutResponse::shrinkable(
                Size::new(self.w, self.h),
                Size::new(self.min, self.h),
                1.0,
            )
        }
    }

    #[test]
    fn fractional_column_floors_shrinkable_child_at_min() {
        // A Fractional column whose only child is shrinkable (min 80) must not
        // shrink the column below 80, even when the fr share would be smaller.
        let mut tree = WidgetTree::new();
        let a = tree.add(FixedLeaf(50.0, 20.0));
        let b = tree.add(ShrinkLeaf {
            w: 200.0,
            h: 20.0,
            min: 80.0,
        });
        let _grid = tree.add(
            Grid::new()
                .columns(vec![TrackSize::Fixed(50.0), TrackSize::Fractional(1.0)])
                .rows(vec![TrackSize::Fixed(40.0)])
                .child(a)
                .child(b),
        );
        // Total 100: col0 Fixed 50 leaves 50 for the fr column — below the 80
        // floor, so it clamps to 80 (residual overflow).
        tree.layout(SizeProposal::exact(100.0, 60.0));
        assert!(
            (tree.bounds(b).width - 80.0).abs() < 0.01,
            "fractional column should floor at min 80, got {}",
            tree.bounds(b).width
        );
    }

    #[test]
    fn fractional_column_still_shrinks_rigid_child_below_intrinsic() {
        // Regression guard: a *rigid* child (min == size) must NOT floor the
        // fractional column — CSS-`fr` semantics keep shrinking it.
        let mut tree = WidgetTree::new();
        let a = tree.add(FixedLeaf(50.0, 20.0));
        let b = tree.add(FixedLeaf(200.0, 20.0)); // rigid, intrinsic 200
        let _grid = tree.add(
            Grid::new()
                .columns(vec![TrackSize::Fixed(50.0), TrackSize::Fractional(1.0)])
                .rows(vec![TrackSize::Fixed(40.0)])
                .child(a)
                .child(b),
        );
        tree.layout(SizeProposal::exact(100.0, 60.0));
        // fr column gets 100 - 50 = 50, below the rigid child's 200 intrinsic.
        assert!(
            (tree.bounds(b).width - 50.0).abs() < 0.01,
            "rigid child's fractional column should shrink to 50, got {}",
            tree.bounds(b).width
        );
    }

    #[test]
    fn fixed_tracks_place_children_correctly() {
        let mut tree = WidgetTree::new();
        let a = tree.add(FixedLeaf(50.0, 30.0));
        let b = tree.add(FixedLeaf(50.0, 30.0));
        let c = tree.add(FixedLeaf(50.0, 30.0));
        let d = tree.add(FixedLeaf(50.0, 30.0));
        let _grid = tree.add(
            Grid::new()
                .columns(vec![TrackSize::Fixed(100.0), TrackSize::Fixed(100.0)])
                .rows(vec![TrackSize::Fixed(50.0), TrackSize::Fixed(50.0)])
                .child(a)
                .child(b)
                .child(c)
                .child(d),
        );
        tree.layout(SizeProposal::exact(300.0, 200.0));

        // a at (0,0), b at (0,1), c at (1,0), d at (1,1)
        assert!((tree.bounds(a).x - 0.0).abs() < 0.01);
        assert!((tree.bounds(a).y - 0.0).abs() < 0.01);
        assert!((tree.bounds(b).x - 100.0).abs() < 0.01);
        assert!((tree.bounds(b).y - 0.0).abs() < 0.01);
        assert!((tree.bounds(c).x - 0.0).abs() < 0.01);
        assert!((tree.bounds(c).y - 50.0).abs() < 0.01);
        assert!((tree.bounds(d).x - 100.0).abs() < 0.01);
        assert!((tree.bounds(d).y - 50.0).abs() < 0.01);
    }

    #[test]
    fn auto_tracks_size_to_content() {
        let mut tree = WidgetTree::new();
        let a = tree.add(FixedLeaf(60.0, 25.0));
        let b = tree.add(FixedLeaf(40.0, 35.0));
        let _grid = tree.add(
            Grid::new()
                .columns(vec![TrackSize::Auto, TrackSize::Auto])
                .rows(vec![TrackSize::Auto])
                .child(a)
                .child(b),
        );
        tree.layout(SizeProposal::exact(300.0, 200.0));

        // Column 0 sized to 60 (widest child), column 1 to 40
        assert!((tree.bounds(a).width - 60.0).abs() < 0.01);
        assert!((tree.bounds(b).width - 40.0).abs() < 0.01);
        assert!((tree.bounds(b).x - 60.0).abs() < 0.01);
    }

    #[test]
    fn fractional_tracks_distribute_remaining_space() {
        let mut tree = WidgetTree::new();
        let a = tree.add(FixedLeaf(10.0, 10.0));
        let b = tree.add(FixedLeaf(10.0, 10.0));
        let c = tree.add(FixedLeaf(10.0, 10.0));
        let _grid = tree.add(
            Grid::new()
                .columns(vec![
                    TrackSize::Fixed(50.0),
                    TrackSize::Fractional(1.0),
                    TrackSize::Fractional(2.0),
                ])
                .rows(vec![TrackSize::Fixed(40.0)])
                .child(a)
                .child(b)
                .child(c),
        );
        tree.layout(SizeProposal::exact(350.0, 100.0));

        // Remaining: 350 - 50 = 300, split 1:2 → 100, 200
        assert!((tree.bounds(a).width - 50.0).abs() < 0.01);
        assert!((tree.bounds(b).width - 100.0).abs() < 0.01);
        assert!((tree.bounds(c).width - 200.0).abs() < 0.01);
    }

    #[test]
    fn gaps_between_tracks() {
        let mut tree = WidgetTree::new();
        let a = tree.add(FixedLeaf(10.0, 10.0));
        let b = tree.add(FixedLeaf(10.0, 10.0));
        let c = tree.add(FixedLeaf(10.0, 10.0));
        let d = tree.add(FixedLeaf(10.0, 10.0));
        let _grid = tree.add(
            Grid::new()
                .columns(vec![TrackSize::Fixed(50.0), TrackSize::Fixed(50.0)])
                .rows(vec![TrackSize::Fixed(30.0), TrackSize::Fixed(30.0)])
                .column_gap(10.0)
                .row_gap(5.0)
                .child(a)
                .child(b)
                .child(c)
                .child(d),
        );
        tree.layout(SizeProposal::exact(200.0, 100.0));

        assert!((tree.bounds(b).x - 60.0).abs() < 0.01); // 50 + 10 gap
        assert!((tree.bounds(c).y - 35.0).abs() < 0.01); // 30 + 5 gap
    }

    #[test]
    fn grid_intrinsic_size() {
        let mut tree = WidgetTree::new();
        let a = tree.add(FixedLeaf(40.0, 20.0));
        let b = tree.add(FixedLeaf(60.0, 30.0));
        let grid = tree.add(
            Grid::new()
                .columns(vec![TrackSize::Auto, TrackSize::Auto])
                .rows(vec![TrackSize::Auto])
                .column_gap(10.0)
                .child(a)
                .child(b),
        );
        tree.layout(SizeProposal::unspecified());

        let gb = tree.bounds(grid);
        // Width: 40 + 10 + 60 = 110
        assert!((gb.width - 110.0).abs() < 0.01);
        // Height: max(20, 30) = 30
        assert!((gb.height - 30.0).abs() < 0.01);
    }

    /// Regression: under an unspecified-width proposal (intrinsic
    /// measurement — Switcher / ZStack / ScrollArea ask their children
    /// like this), a Fractional column has no parent constraint to
    /// share. Returning zero for those tracks would force a re-measure
    /// of every child against `width = 0`, which makes wrap-aware
    /// widgets like TextWidget report inflated heights (one glyph per
    /// line). The Grid must fall back to the child's natural width
    /// instead, so the measurement matches what the paint pass would
    /// produce at a normal width.
    #[test]
    fn fractional_tracks_under_unspecified_use_intrinsic() {
        let mut tree = WidgetTree::new();
        // Two Fractional columns with one child each. With the bug,
        // both columns would resolve to 0 width; with the fix, each
        // resolves to its child's natural width (50).
        let a = tree.add(FixedLeaf(50.0, 20.0));
        let b = tree.add(FixedLeaf(50.0, 20.0));
        let grid = tree.add(
            Grid::new()
                .columns(vec![TrackSize::Fractional(1.0), TrackSize::Fractional(1.0)])
                .rows(vec![TrackSize::Auto])
                .column_gap(8.0)
                .child(a)
                .child(b),
        );
        tree.layout(SizeProposal::unspecified());

        let gb = tree.bounds(grid);
        // Width: 50 + 8 + 50 = 108. With the bug this would be 8 (just the gap).
        assert!(
            (gb.width - 108.0).abs() < 0.01,
            "expected 108, got {}",
            gb.width,
        );
        // Height should be the natural row height, not inflated.
        assert!(
            (gb.height - 20.0).abs() < 0.01,
            "expected 20, got {}",
            gb.height,
        );
    }

    /// Fractional tracks still distribute parent-offered slack when a
    /// width *is* provided — the unspecified-fallback must not change
    /// the constrained behavior.
    #[test]
    fn fractional_tracks_constrained_still_share_remainder() {
        let mut tree = WidgetTree::new();
        let a = tree.add(FixedLeaf(10.0, 10.0));
        let b = tree.add(FixedLeaf(10.0, 10.0));
        let _grid = tree.add(
            Grid::new()
                .columns(vec![TrackSize::Fractional(1.0), TrackSize::Fractional(3.0)])
                .rows(vec![TrackSize::Fixed(20.0)])
                .child(a)
                .child(b),
        );
        tree.layout(SizeProposal::exact(400.0, 100.0));

        // 1:3 split of 400 → a=100, b=300.
        assert!((tree.bounds(a).width - 100.0).abs() < 0.01);
        assert!((tree.bounds(b).width - 300.0).abs() < 0.01);
    }

    #[test]
    fn dormant_child_preserves_cell_positions() {
        // 2x2 grid: a(0,0) b(0,1) c(1,0) d(1,1)
        // Making a dormant should leave b at (0,1), c at (1,0), d at (1,1).
        let mut tree = WidgetTree::new();
        let a = tree.add(FixedLeaf(50.0, 30.0));
        let b = tree.add(FixedLeaf(50.0, 30.0));
        let c = tree.add(FixedLeaf(50.0, 30.0));
        let d = tree.add(FixedLeaf(50.0, 30.0));
        let _grid = tree.add(
            Grid::new()
                .columns(vec![TrackSize::Fixed(80.0), TrackSize::Fixed(80.0)])
                .rows(vec![TrackSize::Fixed(40.0), TrackSize::Fixed(40.0)])
                .column_gap(10.0)
                .row_gap(5.0)
                .child(a)
                .child(b)
                .child(c)
                .child(d),
        );
        tree.layout(SizeProposal::exact(300.0, 200.0));

        // Before dormant: b at col 1 → x=90 (80+10), c at row 1 → y=45 (40+5)
        assert!((tree.bounds(b).x - 90.0).abs() < 0.01);
        assert!((tree.bounds(c).y - 45.0).abs() < 0.01);
        assert!((tree.bounds(d).x - 90.0).abs() < 0.01);
        assert!((tree.bounds(d).y - 45.0).abs() < 0.01);

        // Make a dormant — b, c, d keep their original cells
        tree.set_dormant(a);
        tree.layout(SizeProposal::exact(300.0, 200.0));

        // b stays at cell (0,1)
        assert!(
            (tree.bounds(b).x - 90.0).abs() < 0.01,
            "b.x should stay at 90, got {}",
            tree.bounds(b).x
        );
        assert!(
            (tree.bounds(b).y - 0.0).abs() < 0.01,
            "b.y should stay at 0, got {}",
            tree.bounds(b).y
        );
        // c stays at cell (1,0)
        assert!(
            (tree.bounds(c).x - 0.0).abs() < 0.01,
            "c.x should stay at 0, got {}",
            tree.bounds(c).x
        );
        assert!(
            (tree.bounds(c).y - 45.0).abs() < 0.01,
            "c.y should stay at 45, got {}",
            tree.bounds(c).y
        );
        // d stays at cell (1,1)
        assert!(
            (tree.bounds(d).x - 90.0).abs() < 0.01,
            "d.x should stay at 90, got {}",
            tree.bounds(d).x
        );
        assert!(
            (tree.bounds(d).y - 45.0).abs() < 0.01,
            "d.y should stay at 45, got {}",
            tree.bounds(d).y
        );
    }
}