makeover_immediate/

table.rs

//! Columns, narrowing, cell parts and the sort caret, over `egui_extras`.
//!
//! `makeover-webview`'s `list` module and `makeover-tui`'s `table` in the shape
//! immediate mode allows. It owns the same four things: which columns exist, how
//! wide they are, which ones survive a narrow viewport, and what each part of a
//! cell is. It does not own what goes in a cell, which here is not a policy but
//! a fact of the mode: a cell's contents are drawn by the app's own closure, the
//! way [`group`](crate::group) already takes one per field.
//!
//! # Why `egui_extras` and not egui
//!
//! egui itself has no table. [`egui::Grid`] gives no per-column sizing, no
//! sticky header and no scroll sync, which is why audiofiles reached for
//! `egui_extras::TableBuilder` rather than building on `Grid`. Writing a third
//! answer here would be reimplementing that crate worse, so this is a mapping
//! layer over it.
//!
//! It is the first dependency this crate has taken beyond egui itself, and it
//! moves in lockstep with egui's own version, which is the cost worth naming.
//!
//! # What immediate mode costs the narrowing
//!
//! The terminal renderer measures a [`Width::Content`] column from its cells,
//! because it holds every cell before it draws any. Here the cells do not exist
//! until the app's closure runs, so nothing can be measured before the layout is
//! decided.
//!
//! That splits the answer in two, and both halves are honest:
//!
//! - **Sizing** hands a content column to
//!   [`egui_extras::Column::auto`], which measures it and holds the result
//!   between frames. This is better than the terminal gets, not worse.
//! - **Narrowing** cannot wait for that, so it budgets a content column at the
//!   floor the app declared in [`Sizing`]. A column that turns out wider than
//!   its floor is still drawn; it is the *decision to drop* that uses the
//!   declared number, and a floor is what the app already has to supply for its
//!   fill columns.
//!
//! # Why positions are the bug
//!
//! Carried from the other two renderers, because the mistake is not a CSS
//! mistake and not a terminal one. goingson hides its mobile columns with
//! `nth-child(n+5)` against a seven-column table; insert a column left of the
//! cut and the wrong one disappears, silently. A renderer narrows by raising a
//! cutoff and never by counting.

use crate::Palette;
use egui::{Response, RichText, Sense, Ui};
use egui_extras::{Column as Track, TableBuilder};
use makeover_layout::{CellPart, Column, Priority, Sort, Width};

/// The cutoffs, weakest first.
///
/// [`Priority`] is `#[non_exhaustive]` and a tier added upstream has to be added
/// here in its place in the sequence, or a table will never narrow to it. Grep
/// this when adopting a new `makeover-layout`; `makeover-tui` carries the same
/// list for the same reason, and the two have to agree or a description narrows
/// differently in a window than in a terminal.
const CUTOFFS: [Priority; 3] = [Priority::Optional, Priority::Secondary, Priority::Essential];

/// The lengths the description deferred, in points.
///
/// [`Width`] says `Content`, `Fixed` or `Fill` and carries no magnitude, because
/// a magnitude is an answer for one renderer and the description is read by
/// three. The other two renderers hold this same type over CSS lengths and over
/// terminal cells.
#[derive(Debug, Clone, Copy, Default)]
pub struct Sizing<'a> {
    /// `(column name, points)`. The track for a [`Width::Fixed`] column, the
    /// floor for a [`Width::Fill`] one, and the narrowing budget for a
    /// [`Width::Content`] one.
    pub lengths: &'a [(&'a str, f32)],
    /// Used for a column with no entry above.
    pub fallback: f32,
}

impl Sizing<'_> {
    /// The length for a named column.
    fn length_for(&self, name: &str) -> f32 {
        self.lengths
            .iter()
            .find(|(column, _)| *column == name)
            .map_or(self.fallback, |(_, length)| *length)
    }
}

/// The tones and metrics a table draws with.
///
/// Metrics only, and the tones come from [`Palette`]. That is the division this
/// crate already draws: [`FieldStyle`](crate::FieldStyle) carries gaps and a
/// marker while the colours stay in the palette, and a table's colours are the
/// palette's `content`, `content_muted` and `action` rather than six new ones.
/// `makeover-tui` splits it the other way round because its palette carries no
/// text tones at all.
#[derive(Debug, Clone, Copy, PartialEq)]
pub struct TableStyle {
    /// The height of the heading row.
    pub header_height: f32,
    /// The height of a body row.
    pub row_height: f32,
    /// Drawn after the heading of an ascending column.
    pub ascending: &'static str,
    /// Drawn after the heading of a descending column.
    pub descending: &'static str,
    /// Whether alternate rows take a different background.
    ///
    /// egui_extras' own striping, off by default: the description has no word
    /// for it, and a renderer that turned it on would be adding a claim the
    /// other two cannot make.
    ///
    /// Not every setting egui_extras has becomes a field here. A sticky heading
    /// is what `TableBuilder::header` does and there is no version that does
    /// not, so the knob 0.12.0 briefly carried for it offered a choice this
    /// renderer cannot make. This one and [`resizable`](Self::resizable) are the
    /// two that pass that test.
    pub striped: bool,
    /// Whether the user can drag the divider between two columns.
    ///
    /// The second knob that is not a metric, and it passes the same test
    /// `sticky_header` failed: egui_extras offers both settings and a renderer
    /// can honestly make either choice. Off by default for `striped`'s reason:
    /// the description has no word for it, so a default that turned it on would
    /// be this renderer adding a claim the other two cannot make.
    ///
    /// It does not fight the narrowing. A drag moves a track for the frames it
    /// is held; [`cutoff_for`] still decides which columns exist, off the widths
    /// the app declared in [`Sizing`], so a resize can never drop a column.
    pub resizable: bool,
}

impl Default for TableStyle {
    fn default() -> Self {
        Self {
            header_height: 20.0,
            row_height: 18.0,
            // The pair audiofiles already draws, so a sorted column points the
            // same way here as it does in a terminal.
            ascending: " \u{25B2}",
            descending: " \u{25BC}",
            striped: false,
            resizable: false,
        }
    }
}

/// The body's own facts for this frame: how many rows, which are selected, and
/// which one to bring into view.
///
/// Held apart from [`TableStyle`] because none of it is style and none of it
/// survives the frame: a row count changes when a folder does, a selection when
/// the user clicks, and a scroll request exists for exactly one frame. Held
/// apart from the [`Column`] slice because none of it is description either.
/// The description says what a table *is*, and this says what it holds right
/// now.
///
/// Both of the optional fields are here rather than left to the app because
/// egui_extras answers them on a handle the app never sees: `set_selected` is a
/// method on the row, and `scroll_to_row` a method on the builder, and this
/// crate owns both. That is the same reason [`cell`] exists.
#[derive(Default)]
pub struct Body<'a> {
    /// How many rows to draw.
    pub rows: usize,
    /// Whether a row is selected, by index.
    ///
    /// A predicate rather than a set, so an app whose selection is a range, a
    /// bitmap or a single index does not have to build a collection to be asked.
    /// `None` is a table no row of which is selected, which is not the same
    /// claim as a predicate that always answers false and costs nothing to make.
    pub selected: Option<&'a dyn Fn(usize) -> bool>,
    /// A row to bring into view this frame.
    ///
    /// Set it from a request the app then clears, the way a keyboard cursor
    /// moving off-screen raises one: held rather than taken, it would fight
    /// every scroll the user makes with the mouse.
    pub scroll_to: Option<usize>,
}

impl std::fmt::Debug for Body<'_> {
    // Hand-written because `selected` is a closure and `#[derive(Debug)]` will
    // not have it. What is worth printing is whether one was supplied.
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.debug_struct("Body")
            .field("rows", &self.rows)
            .field("selected", &self.selected.is_some())
            .field("scroll_to", &self.scroll_to)
            .finish()
    }
}

/// The colour a cell of this part takes.
///
/// [`CellPart`] is `#[non_exhaustive]`, and a member added upstream lands on
/// `content`: a part this renderer has not learned draws as text, which is a
/// cell rendering plainly rather than a build that stops. Grep this when
/// adopting a new `makeover-layout`.
#[must_use]
pub const fn part_color(part: Option<CellPart>, palette: &Palette) -> egui::Color32 {
    match part {
        // A token paints its own background and carries its own tone. What is
        // set here is what shows between them, not what paints them.
        Some(CellPart::Tokens) => palette.content_muted,
        // The drift `CellPart` exists to end: a control in a cell inheriting the
        // cell's text colour. Both of these take the action intent instead.
        Some(CellPart::Actions | CellPart::Link) => palette.action,
        _ => palette.content,
    }
}

/// Draw a cell's contents with the tone its part takes.
///
/// The app calls this inside its own cell closure, wrapping whatever it draws.
/// A scoping function rather than a parameter on [`table`], for the reason
/// [`frame`](crate::frame) is one: the part is a property of the cell, the cell
/// does not exist until the closure runs, and immediate mode has no cascade to
/// carry the answer down on its own. This is the cascade, for one scope.
///
/// ```no_run
/// # use makeover_layout::CellPart;
/// # let palette: makeover_immediate::Palette = unimplemented!();
/// # let ui: &mut egui::Ui = unimplemented!();
/// makeover_immediate::table::cell(ui, Some(CellPart::Link), &palette, |ui| {
///     ui.label("opens the item");
/// });
/// ```
pub fn cell<R>(
    ui: &mut Ui,
    part: Option<CellPart>,
    palette: &Palette,
    add_contents: impl FnOnce(&mut Ui) -> R,
) -> R {
    let restore = ui.visuals().override_text_color;
    ui.visuals_mut().override_text_color = Some(part_color(part, palette));
    let out = add_contents(ui);
    ui.visuals_mut().override_text_color = restore;
    out
}

/// The heading, with the caret if the table is ordered by this column.
///
/// A column [`sorted`](Column::sorted) but not [`sortable`](Column::sortable)
/// still gets its caret. Both combinations mean something, which is why the
/// description holds the two fields apart: a list ordered by a key the user
/// cannot change is a real thing, and the caret is how it says so.
#[must_use]
pub fn heading(column: &Column<'_>, style: &TableStyle) -> String {
    match column.sorted {
        Some(Sort::Ascending) => format!("{}{}", column.name, style.ascending),
        Some(Sort::Descending) => format!("{}{}", column.name, style.descending),
        None => column.name.to_owned(),
    }
}

/// How wide a column asks to be at its narrowest, in points.
fn min_width(column: &Column<'_>, sizing: &Sizing<'_>) -> f32 {
    // Every arm is the declared length, including `Content`: nothing can be
    // measured before the app's closure has drawn it. See the module header on
    // what immediate mode costs the narrowing.
    sizing.length_for(column.name)
}

/// Whether the columns kept at `cutoff` fit in `width`.
fn fits(columns: &[Column<'_>], sizing: &Sizing<'_>, cutoff: Priority, width: f32) -> bool {
    columns
        .iter()
        .filter(|c| c.kept_at(cutoff))
        .map(|c| min_width(c, sizing))
        .sum::<f32>()
        <= width
}

/// The weakest cutoff whose columns fit in `width`.
///
/// Raised until the layout fits, and never past [`Priority::Essential`]: the
/// essential columns are what makes a row identify itself, so a window too
/// narrow for them gets them squeezed rather than dropped. Nothing here counts
/// positions, so which column drops is a property of the column.
#[must_use]
pub fn cutoff_for(columns: &[Column<'_>], sizing: &Sizing<'_>, width: f32) -> Priority {
    for cutoff in CUTOFFS {
        if fits(columns, sizing, cutoff, width) {
            return cutoff;
        }
    }
    Priority::Essential
}

/// The track for one column.
fn track(column: &Column<'_>, sizing: &Sizing<'_>) -> Track {
    match column.width {
        // The one place immediate mode beats the terminal: egui_extras measures
        // this and remembers it between frames, where `makeover-tui` has to walk
        // the cells itself.
        Width::Content => Track::auto(),
        Width::Fixed => Track::exact(sizing.length_for(column.name)),
        // Includes a width added to the description since this renderer was
        // built. Taking the slack above a floor is the behaviour that makes no
        // claim, which is the same fallback the webview renderer's `auto` track
        // is chosen to be.
        _ => Track::remainder().at_least(sizing.length_for(column.name)),
    }
}

/// A described table, narrowed for the width available.
///
/// `draw` is called once per cell of each kept column, in column order, for each
/// of [`Body::rows`] rows. Taking a closure rather than a slice of contents is
/// what keeps the app's own data borrowed one cell at a time, which is
/// [`group`](crate::group)'s reasoning and immediate mode's habit.
///
/// `body` is borrowed immutably and `draw` is `FnMut`, which is the split a
/// caller has to plan for: a selection read by [`Body::selected`] cannot be the
/// same value `draw` mutates. Snapshot it before the call. That is not this
/// crate imposing anything. It is the borrow the app already takes when it
/// clones its row list to hand egui a closure.
///
/// Returns the sortable column whose heading was pressed this frame, if any. The
/// app owns the ordering, so this reports the press and changes nothing: what a
/// press *calls* is an address, and the description names none. That is
/// [`Column::sortable`]'s own documented split.
///
/// A heading is only pressable when its column says
/// [`sortable`](Column::sortable). A column sorted by a key the user cannot
/// change still draws its caret and does not answer.
pub fn table<'a>(
    ui: &mut Ui,
    columns: &'a [Column<'a>],
    body: &Body<'_>,
    sizing: &Sizing<'_>,
    palette: &Palette,
    style: &TableStyle,
    mut draw: impl FnMut(&mut Ui, &'a Column<'a>, usize),
) -> Option<&'a Column<'a>> {
    let cutoff = cutoff_for(columns, sizing, ui.available_width());
    let kept: Vec<&'a Column<'a>> = columns.iter().filter(|c| c.kept_at(cutoff)).collect();

    // egui_extras panics on a table with no tracks, and a description whose
    // every column dropped is reachable: `kept_at` keeps the essential ones, and
    // a table described with none at all has nothing to keep.
    if kept.is_empty() {
        return None;
    }

    let mut builder = TableBuilder::new(ui)
        .striped(style.striped)
        .resizable(style.resizable)
        // Not a knob, because there is no second honest answer: a cell's
        // contents sit on the row's centre line. CSS says `vertical-align:
        // middle` and a terminal row is one line tall, so a field offering the
        // choice would be offering one only this renderer could take. egui's own
        // default is top-aligned, which is why it has to be said at all.
        .cell_layout(egui::Layout::left_to_right(egui::Align::Center));
    for column in &kept {
        builder = builder.column(track(column, sizing));
    }
    if let Some(row) = body.scroll_to {
        builder = builder.scroll_to_row(row, None);
    }

    // Written through a Cell rather than returned, because egui_extras hands the
    // header and the body their own closures and neither can return a value past
    // the other.
    let pressed = std::cell::Cell::new(None::<&'a Column<'a>>);

    builder
        .header(style.header_height, |mut header| {
            for column in &kept {
                header.col(|ui| {
                    if press(ui, column, palette, style) {
                        pressed.set(Some(column));
                    }
                });
            }
        })
        .body(|table_body| {
            table_body.rows(style.row_height, body.rows, |mut row| {
                let index = row.index();
                if let Some(selected) = body.selected {
                    // Before the cells, and on the row rather than on any of
                    // them: a selection marks the whole row, and a renderer that
                    // tinted each cell would leave the gaps between them
                    // unpainted.
                    row.set_selected(selected(index));
                }
                for column in &kept {
                    row.col(|ui| draw(ui, column, index));
                }
            });
        });

    pressed.get()
}

/// One heading, and whether it was pressed.
fn press(ui: &mut Ui, column: &Column<'_>, palette: &Palette, style: &TableStyle) -> bool {
    let text = RichText::new(heading(column, style)).strong();
    if !column.sortable {
        // Muted, and not sensed. A heading a user cannot press must not look
        // like one they can, which is the affordance `Column::sortable` exists
        // to carry.
        ui.label(text.color(palette.content_muted));
        return false;
    }
    let tone = if column.sorted.is_some() {
        palette.content
    } else {
        palette.content_muted
    };
    let response: Response = ui
        .add(egui::Label::new(text.color(tone)).sense(Sense::click()))
        .on_hover_cursor(egui::CursorIcon::PointingHand);
    response.clicked()
}

#[cfg(test)]
mod tests {
    use super::*;
    use egui::Color32;

    fn palette() -> Palette {
        Palette {
            page: Color32::from_rgb(1, 1, 1),
            raised: Color32::from_rgb(2, 2, 2),
            overlay: Color32::from_rgb(3, 3, 3),
            well: Color32::from_rgb(4, 4, 4),
            sunken: Color32::from_rgb(5, 5, 5),
            bevel_light: Color32::WHITE,
            bevel_dark: Color32::BLACK,
            elevation: Color32::from_black_alpha(46),
            content: Color32::from_rgb(6, 6, 6),
            content_secondary: Color32::from_rgb(56, 56, 56),
            content_muted: Color32::from_rgb(7, 7, 7),
            action: Color32::from_rgb(8, 8, 8),
            danger: Color32::from_rgb(9, 9, 9),
            success: Color32::from_rgb(10, 10, 10),
            warning: Color32::from_rgb(11, 11, 11),
            info: Color32::from_rgb(12, 12, 12),
        }
    }

    fn columns() -> Vec<Column<'static>> {
        vec![
            Column {
                name: "name",
                width: Width::Fill,
                priority: Priority::Essential,
                sortable: true,
                sorted: Some(Sort::Ascending),
            },
            Column {
                name: "size",
                width: Width::Fixed,
                priority: Priority::Secondary,
                sortable: true,
                sorted: None,
            },
            Column {
                name: "note",
                width: Width::Content,
                priority: Priority::Optional,
                sortable: false,
                sorted: None,
            },
        ]
    }

    fn sizing() -> Sizing<'static> {
        Sizing {
            lengths: &[("name", 120.0), ("size", 60.0), ("note", 80.0)],
            fallback: 40.0,
        }
    }

    #[test]
    fn narrowing_drops_the_optional_column_first_and_the_essential_one_never() {
        let (cols, sz) = (columns(), sizing());
        assert_eq!(cutoff_for(&cols, &sz, 300.0), Priority::Optional);
        assert_eq!(cutoff_for(&cols, &sz, 200.0), Priority::Secondary);
        assert_eq!(cutoff_for(&cols, &sz, 150.0), Priority::Essential);
        // Narrower than the essential column, which stays anyway.
        assert_eq!(cutoff_for(&cols, &sz, 10.0), Priority::Essential);
    }

    #[test]
    fn a_column_inserted_left_of_the_cut_does_not_change_what_drops() {
        // The goingson bug, as a test. `nth-child(n+5)` against a seven-column
        // table hides whatever lands at position five, so inserting a column
        // moves the cut onto a different column with nothing edited.
        //
        // Asserted at a fixed cutoff, because that is where the two ways of
        // addressing a column disagree. A narrower budget SHOULD drop more; what
        // must not change is which ones, for a given cutoff.
        let dropped = |cols: &[Column<'_>], cutoff| -> Vec<String> {
            cols.iter()
                .filter(|c| !c.kept_at(cutoff))
                .map(|c| c.name.to_owned())
                .collect()
        };
        let before = columns();
        let mut after = vec![Column {
            name: "mark",
            width: Width::Fixed,
            priority: Priority::Essential,
            sortable: false,
            sorted: None,
        }];
        after.extend(columns());

        for cutoff in CUTOFFS {
            assert_eq!(dropped(&before, cutoff), dropped(&after, cutoff));
        }
        assert_eq!(dropped(&before, Priority::Secondary), vec!["note"]);
    }

    #[test]
    fn the_two_renderers_narrow_a_description_the_same_way() {
        // The cutoff ladder is duplicated in `makeover-tui` because neither
        // crate depends on the other, and duplication is what drifts. This is
        // the assertion that would catch it: the ladder is the description's
        // order, weakest first, and a tier added upstream belongs in both.
        assert_eq!(CUTOFFS.len(), 3);
        assert!(CUTOFFS.windows(2).all(|pair| pair[0] < pair[1]));
        assert_eq!(CUTOFFS[0], Priority::Optional);
        assert_eq!(CUTOFFS[2], Priority::Essential);
    }

    #[test]
    fn a_content_column_is_measured_by_egui_and_budgeted_by_its_floor() {
        // The split the module header names. The track defers to egui_extras,
        // which can measure; the narrowing cannot wait for that and uses the
        // declared floor. Both readings of the same column, and both honest.
        let cols = columns();
        let sz = sizing();
        let note = &cols[2];
        assert!(matches!(note.width, Width::Content));
        assert!((min_width(note, &sz) - 80.0).abs() < f32::EPSILON);
        // 120 + 60 + 80 is 260, so 300 fits and 250 does not.
        assert!(fits(&cols, &sz, Priority::Optional, 300.0));
        assert!(!fits(&cols, &sz, Priority::Optional, 250.0));
    }

    #[test]
    fn a_column_with_no_length_of_its_own_takes_the_fallback() {
        let column = Column {
            name: "unlisted",
            width: Width::Fixed,
            priority: Priority::Essential,
            sortable: false,
            sorted: None,
        };
        assert!((min_width(&column, &sizing()) - 40.0).abs() < f32::EPSILON);
    }

    #[test]
    fn the_parts_a_cell_can_be_are_coloured_apart() {
        // The drift `CellPart` exists to end: one colour for a whole cell paints
        // a control as though it were text.
        let p = palette();
        assert_eq!(part_color(Some(CellPart::Value), &p), p.content);
        assert_eq!(part_color(Some(CellPart::Tokens), &p), p.content_muted);
        assert_eq!(part_color(Some(CellPart::Actions), &p), p.action);
        assert_eq!(part_color(Some(CellPart::Link), &p), p.action);
        assert_ne!(part_color(Some(CellPart::Link), &p), p.content);
        // A cell mixing parts says nothing, and takes the text colour.
        assert_eq!(part_color(None, &p), p.content);
    }

    #[test]
    fn the_ordered_column_draws_a_caret_and_the_others_do_not() {
        let style = TableStyle::default();
        let cols = columns();
        assert_eq!(heading(&cols[0], &style), "name \u{25B2}");
        assert_eq!(heading(&cols[1], &style), "size");
        assert_eq!(heading(&cols[2], &style), "note");
    }

    #[test]
    fn a_column_sorted_without_being_sortable_still_draws_its_caret() {
        // A list ordered by a key the user cannot change is a real thing to
        // describe, which is why the description holds the two fields apart.
        let column = Column {
            name: "rank",
            width: Width::Content,
            priority: Priority::Essential,
            sortable: false,
            sorted: Some(Sort::Descending),
        };
        assert_eq!(heading(&column, &TableStyle::default()), "rank \u{25BC}");
    }

    #[test]
    fn the_carets_match_the_terminal_renderers() {
        // Two crates, one glyph pair, and no dependency between them to enforce
        // it. A description sorted ascending must not point up in a window and
        // down in a terminal.
        let style = TableStyle::default();
        assert_eq!(style.ascending, " \u{25B2}");
        assert_eq!(style.descending, " \u{25BC}");
    }

    #[test]
    fn striping_is_off_because_the_description_has_no_word_for_it() {
        // egui_extras offers it and the other two renderers cannot say it. A
        // default that turned it on would be this renderer adding a claim.
        assert!(!TableStyle::default().striped);
        // Same test, same answer, and the reason `sticky_header` failed it: that
        // one had no second setting to offer.
        assert!(!TableStyle::default().resizable);
    }

    #[test]
    fn a_body_claims_nothing_until_it_is_asked_to() {
        // The default is a table of no rows, no selection and no scroll
        // request. All three absences are the honest reading of an app that has
        // not said otherwise, which is why they are `Option` and not a
        // predicate that always answers false.
        let body = Body::default();
        assert_eq!(body.rows, 0);
        assert!(body.selected.is_none());
        assert!(body.scroll_to.is_none());
    }

    #[test]
    fn a_selection_is_asked_per_row_and_not_collected() {
        // A predicate, so an app whose selection is a range or a single index
        // does not build a set to be asked. Exercised the way `table` asks it:
        // once per row index, in order.
        let selected = |index: usize| index.is_multiple_of(2);
        let body = Body {
            rows: 4,
            selected: Some(&selected),
            scroll_to: None,
        };
        let f = body.selected.expect("a predicate was supplied");
        assert_eq!(
            (0..body.rows).map(f).collect::<Vec<_>>(),
            vec![true, false, true, false]
        );
    }

    #[test]
    fn narrowing_reads_the_declared_widths_and_not_a_dragged_track() {
        // `resizable` lets the user move a divider, and `cutoff_for` must not
        // hear about it: a drag that could drop a column would make the
        // narrowing a thing the user does by accident rather than a property of
        // the description. That `cutoff_for` takes no `TableStyle` at all is the
        // structural half of the guarantee; this is the behavioural half, and it
        // is what would fail if a measured width were ever threaded in beside
        // the declared one.
        let (cols, sz) = (columns(), sizing());
        assert_eq!(cutoff_for(&cols, &sz, 300.0), Priority::Optional);
        assert_eq!(cutoff_for(&cols, &sz, 200.0), Priority::Secondary);
    }
}