makeover_webview/

list.rs

//! Column layout and row structure for lists and tables.
//!
//! The other half of phase B. [`form`](crate::form) renders a field; this
//! renders the frame a list of rows sits in: which columns exist, how wide they
//! are, which ones survive a narrow viewport, and the cell containers a row is
//! made of.
//!
//! # What this does not do
//!
//! It does not render a cell's contents. That is the crate's own limit, stated
//! in `makeover_layout`'s "Where the description stops": generate the boring
//! 80% so the bespoke 20% gets the attention. A goingson task row carries
//! delegated action hooks with argument substitution, four nested sub-renderers,
//! conditional state classes and aria labels built from data. A description
//! expressive enough to emit that is a templating language wearing a
//! description's name.
//!
//! So the split is the one [`Markup`] already draws for forms: this owns the
//! structure and the app owns what goes in it. What that removes from an app is
//! not small — cell order, cell classes, the grid tracks, and above all the
//! narrowing rules, which is where addressing columns by position goes wrong.
//!
//! # Why positions are the bug
//!
//! goingson hides its mobile columns with `nth-child(n+5)` against a
//! seven-column table, plus a separate `nth-child(3)`, plus two class-based
//! rules — the same fact said three ways, two of them positional. Insert a
//! column anywhere left of the cut and the wrong one disappears, silently,
//! because nothing in the stylesheet knows what column five *is*.
//! [`Priority`] is the fix: a renderer narrows by raising a cutoff, and never
//! by counting.

use crate::form::Markup;
use crate::{Emit, push_class};
use makeover_layout::{CellPart, Column, Flow, Priority, RowPart, Width};
use std::fmt::Write as _;

/// The lengths the description deferred.
///
/// [`Width`] says `Content`, `Fixed` or `Fill` and deliberately carries no
/// magnitude, because a magnitude is a CSS answer and the description is read
/// by renderers that have no pixels. So the numbers arrive here instead, the
/// way a field's value arrives in [`Filling`](crate::form::Filling) rather than
/// in `Field`.
///
/// Looked up by column name, because an app's columns are not all one size:
/// goingson's task table has six distinct fixed widths.
#[derive(Debug, Clone, Copy, Default)]
pub struct Sizing<'a> {
    /// `(column name, CSS length)`. The length is the track for a
    /// [`Width::Fixed`] column and the floor for a [`Width::Fill`] one.
    pub lengths: &'a [(&'a str, &'a str)],
    /// Used for a column with no entry above. Empty means `auto`.
    pub fallback: &'a str,
}

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

    /// The grid track for one column.
    fn track(&self, column: &Column<'_>) -> String {
        match column.width {
            Width::Content => "max-content".to_owned(),
            Width::Fixed => self.length_for(column.name).to_owned(),
            Width::Fill => format!("minmax({}, 1fr)", self.length_for(column.name)),
            // A width added to the description since this renderer was built.
            // `auto` is the track that makes no claim, which is the honest
            // answer to a claim this renderer cannot read.
            _ => "auto".to_owned(),
        }
    }
}

/// The class a cell of this column carries.
///
/// Derived from the column's own name, which is what makes the narrowing rules
/// addressable. `data-column` would do as well; a class is what both webview
/// apps already key their cell styling on.
///
/// The name is reduced to identifier characters first. See [`push_column_name`].
#[must_use]
pub fn column_class(column: &Column<'_>, opts: &Emit) -> String {
    let mut out = String::new();
    push_column_class(&mut out, column, opts);
    out
}

/// The class a cell of this column carries, written into a buffer the caller
/// already has.
///
/// [`column_class`]'s streaming form. It is the one that runs per cell per row,
/// where the allocating form pays twice: once for `col-<name>` and once for the
/// prefix in front of it.
pub fn push_column_class(out: &mut String, column: &Column<'_>, opts: &Emit) {
    out.push_str(opts.class_prefix);
    out.push_str("col-");
    push_column_name(out, column.name);
}

/// A column's name as the identifier half of its class.
///
/// # Why this is not escaping
///
/// The name is the one app-supplied string this crate puts in a class attribute
/// rather than in text or an `aria-label`, so it may never go in raw. A column
/// named `a" onclick="steal()` emitted
///
/// ```html
/// <div class="cell col-a" onclick="steal() cell-fill cell-keeps">
/// ```
///
/// which is a live event handler on every cell of that column. HTML escaping is
/// the reflex and it is the wrong tool here, because a class is read twice: once
/// by the HTML parser, which would decode `&quot;` back to a quote, and once by
/// a CSS selector, which [`narrowing_css`] writes from this same function. An
/// escaped name is safe in the attribute and unmatchable from the stylesheet,
/// so the two halves of the narrowing would stop meeting -- silently, the way
/// every other defect this module's comments record did.
///
/// Reducing the name to identifier characters answers both. What comes out is a
/// valid CSS identifier, so the selector matches, and it holds none of the five
/// characters an attribute value can be ended with, so there is nothing to
/// escape.
///
/// # What it changes for a name that was already fine
///
/// Nothing. Alphanumerics, `_` and `-` pass through, and every column name in
/// the tree is made of those. A name that is *not* was already broken rather
/// than merely unsafe: `Due date` emitted `col-Due date`, which the HTML parser
/// reads as the two classes `col-Due` and `date`, and which `narrowing_css`
/// wrote as a descendant selector that matched neither. Both now agree on
/// `col-Due-date`.
///
/// Alphanumeric in the Unicode sense, not the ASCII one. CSS identifiers admit
/// everything from U+00A0 up, so a column named `Größe` keeps its name; folding
/// it to `Gr--e` would collide with a neighbouring column for nothing.
pub fn push_column_name(out: &mut String, name: &str) {
    for ch in name.chars() {
        // Substituted rather than dropped. Two columns called `a b` and `ab`
        // are different columns, and dropping would give them one class and one
        // set of narrowing rules between them.
        if ch.is_alphanumeric() || ch == '_' || ch == '-' {
            out.push(ch);
        } else {
            out.push('-');
        }
    }
}

/// The class saying how wide a cell of this column asks to be.
///
/// A bounded vocabulary, unlike [`column_class`], which is why the stylesheet
/// can carry the rule. [`Width`] is `#[non_exhaustive]`, and a member added
/// upstream lands on the fill class: a column that takes the slack is the
/// behaviour that makes no claim, matching the `auto` track
/// [`Sizing::track`] falls back to for the same reason.
fn width_class(width: Width) -> &'static str {
    match width {
        Width::Content => "cell-content",
        Width::Fixed => "cell-fixed",
        _ => "cell-fill",
    }
}

/// The class saying when a cell of this column drops.
///
/// [`Priority`] said as a class rather than as a cutoff, so the hiding can live
/// in the stylesheet instead of being generated per table. That is what a
/// [`display: table`](crate::table_rules) frame needs and a grid one cannot use:
/// a grid also has to shorten its track list, which only the columns themselves
/// can say.
fn drop_class(priority: Priority) -> &'static str {
    match priority {
        Priority::Optional => "cell-drops-first",
        Priority::Secondary => "cell-drops-next",
        // A priority added upstream keeps its column. `Priority` is
        // `#[non_exhaustive]`, and of the two ways to be wrong about one this
        // renderer has not learned, showing a column that should have dropped
        // is the one the user can see and work around.
        _ => "cell-keeps",
    }
}

/// Every class a cell of this column carries.
///
/// The column's own name, how wide it asks to be, and when it drops. A header
/// cell has to carry the same three or the header and the body disagree about
/// which column just disappeared, and a renderer emitting its own header row
/// should call this rather than assemble the list a second time.
#[must_use]
pub fn column_classes(column: &Column<'_>, opts: &Emit) -> String {
    let mut out = String::new();
    push_column_classes(&mut out, column, opts);
    out
}

/// Every class a cell of this column carries, written into a buffer the caller
/// already has.
///
/// [`column_classes`]'s streaming form, and four allocations fewer per cell: the
/// three names and the string joining them.
pub fn push_column_classes(out: &mut String, column: &Column<'_>, opts: &Emit) {
    push_column_class(out, column, opts);
    out.push(' ');
    push_class(out, width_class(column.width), opts);
    out.push(' ');
    push_class(out, drop_class(column.priority), opts);
}

/// The `grid-template-columns` value for the columns kept at `cutoff`.
///
/// Emitting only the surviving tracks is what keeps the track list and the
/// hiding in agreement. An app that hides a cell with `display: none` but
/// leaves its track in place gets a column of empty space, which is the other
/// half of goingson's mobile bug: its narrow rule drops to four tracks by hand
/// and has to be edited in step with the `nth-child` cut.
#[must_use]
pub fn grid_template_columns(
    columns: &[Column<'_>],
    sizing: &Sizing<'_>,
    cutoff: Priority,
) -> String {
    columns
        .iter()
        .filter(|column| column.kept_at(cutoff))
        .map(|column| sizing.track(column))
        .collect::<Vec<_>>()
        .join(" ")
}

/// The rules that narrow `selector` to the columns kept at `cutoff`.
///
/// Both halves together: the shortened track list, and `display: none` on each
/// dropped column *by its own class*. Nothing counts positions, so inserting a
/// column changes what is emitted rather than changing which column vanishes.
///
/// `selector` may be a selector list. A descendant is appended to each part
/// rather than to the whole, because appending to the whole changes what the
/// earlier parts match: `.head, .row > .col-x` reads as "`.head`, or a `.col-x`
/// inside `.row`", so `.head` itself would be hidden.
#[must_use]
pub fn narrowing_css(
    columns: &[Column<'_>],
    selector: &str,
    sizing: &Sizing<'_>,
    cutoff: Priority,
    opts: &Emit,
) -> String {
    let parts: Vec<&str> = selector.split(',').map(str::trim).collect();

    let mut css = format!(
        "{} {{\n    grid-template-columns: {};\n}}\n",
        parts.join(", "),
        grid_template_columns(columns, sizing, cutoff)
    );

    for column in columns.iter().filter(|c| !c.kept_at(cutoff)) {
        let class = column_class(column, opts);
        let targets: Vec<String> = parts
            .iter()
            .map(|part| format!("{part} > .{class}"))
            .collect();
        let _ = write!(css, "{} {{\n    display: none;\n}}\n", targets.join(",\n"));
    }
    css
}

/// One cell of a row.
///
/// The contents are [`Markup`] rather than text, and that is the whole shape of
/// this module: a cell holds whatever the app builds, and the app says so by
/// naming it. Escaping a cell here would be wrong as well as impossible — a
/// task row's description cell is five nested spans and a badge.
#[derive(Debug, Clone, Copy)]
pub struct Cell<'a> {
    /// Which column this fills, by name.
    pub column: &'a str,
    /// What the cell holds, when the whole cell is one thing.
    ///
    /// Carries the cell-part class the stylesheet half emits, so a cell that is
    /// nothing but controls says so in the description's own words rather than
    /// in the app's.
    ///
    /// `Option<CellPart>` and never `Option<RowPart>`: a table cell borrowing
    /// the list row's vocabulary is drift. A row's parts answer a different
    /// question (which of six emphases this run of text takes) from a
    /// cell's (whether this is text, tokens, controls or a link).
    ///
    /// `None` for a cell mixing parts. A cell holding a value *and* a strip of
    /// tokens *and* a control is three parts in one container, and each one
    /// wears its own class inside — this field is for the single-part case,
    /// where a wrapper span would say nothing the cell has not already said.
    pub part: Option<CellPart>,
    /// The contents. Trusted app markup.
    pub content: Markup<'a>,
}

impl<'a> Cell<'a> {
    /// A cell with no cell part.
    #[must_use]
    pub const fn new(column: &'a str, content: Markup<'a>) -> Self {
        Self {
            column,
            part: None,
            content,
        }
    }
}

/// The class for a row part.
///
/// [`RowPart`] is `#[non_exhaustive]`, so a member added upstream does not stop
/// this compiling.
///
/// The fallback is what that costs. A member added upstream lands here as a
/// bare `row-part` with no rule of its own, which is a thing rendering plainly
/// rather than a build that stops. Grep this function when adopting a new
/// makeover-layout.
///
/// Public, because a row's parts are emitted by whoever builds the row element
/// and that is not always this crate: `cells_html` emits a table's cells, but a
/// list row carries the app's identity and hooks, so a screen renderer writes
/// it. A renderer that reimplements this list rather than calling it takes on
/// the obligation above without knowing it.
/// Every class [`part_class`] can return, including the fallback.
///
/// Beside the match rather than derived from it, because a `match` over a
/// `#[non_exhaustive]` enum cannot be enumerated from outside. It carries the
/// same obligation the match does and a test below holds the two together, so
/// a new arm added without a new entry fails rather than silently narrowing
/// what a checker believes this crate can emit.
pub const ROW_PART_CLASSES: &[&str] = &[
    "row-primary",
    "row-secondary",
    "row-meta",
    "row-actions",
    "row-tokens",
    "row-proportion",
    "row-part",
];

/// Every class [`flow_class`] can return.
///
/// `Flow::Tight` has no class: one line is what a run already does, so a rule
/// for it would restate the default on every part in every row. Only the tier
/// that departs from it is named, which is also why a renderer emitting nothing
/// for an unknown flow is correct rather than lossy.
pub const FLOW_CLASSES: &[&str] = &["row-relaxed"];

/// The classes a row inside a hierarchy carries.
///
/// Declared here because the generated sheet defines rules for them: the
/// vocabulary has to say so, or the check that every emitted name has a rule
/// cannot see them.
pub const NESTING_CLASSES: &[&str] = &["row-nested", "row-branch", "row-disclose"];

/// The class for a part's flow, if it needs one.
///
/// `None` for [`Flow::Tight`] and for any tier added upstream that this crate
/// has not been taught, which lands as one line: the same trade
/// [`part_class`]'s fallback makes, and the safe direction, since a part that
/// grows without bound breaks the rows around it while a part that stays on one
/// line only looks like the old rendering. Grep this when adopting a new
/// makeover-layout.
#[must_use]
pub fn flow_class(flow: Flow) -> Option<&'static str> {
    match flow {
        Flow::Relaxed => Some("row-relaxed"),
        _ => None,
    }
}

/// Every class [`width_class`](fn@width_class) can return, including the
/// fallback.
///
/// See [`ROW_PART_CLASSES`] for why it is written out. Only two of the three
/// carry a rule -- a fill is what a cell does when the sheet says nothing --
/// which is exactly why the list is here rather than being read off the
/// generated CSS: `cell-fill` reached every table in the tree and the
/// vocabulary named it nowhere.
pub const CELL_WIDTH_CLASSES: &[&str] = &["cell-content", "cell-fixed", "cell-fill"];

/// Every class [`drop_class`](fn@drop_class) can return, including the
/// fallback.
///
/// [`CELL_WIDTH_CLASSES`]' argument, one column property over: `cell-keeps` is
/// the tier the narrowing never hides, so the sheet writes no rule for it and
/// a scraped set cannot see it.
pub const CELL_DROP_CLASSES: &[&str] = &["cell-drops-first", "cell-drops-next", "cell-keeps"];

/// Every class [`cell_part_class`] can return, including the fallback.
///
/// See [`ROW_PART_CLASSES`] for why it is written out.
pub const CELL_PART_CLASSES: &[&str] = &[
    "cell-value",
    "cell-tokens",
    "cell-actions",
    "cell-link",
    "cell-part",
];

#[must_use]
pub fn part_class(part: RowPart) -> &'static str {
    match part {
        RowPart::Primary => "row-primary",
        RowPart::Secondary => "row-secondary",
        RowPart::Meta => "row-meta",
        RowPart::Actions => "row-actions",
        RowPart::Tokens => "row-tokens",
        RowPart::Proportion => "row-proportion",
        _ => "row-part",
    }
}

/// The class for a cell part.
///
/// [`part_class`]'s table half, for [`CellPart`]. The fallback is there for the
/// same reason and costs the same thing: a member added upstream lands as a
/// bare `cell-part` with no rule of its own, rather than as a build that stops.
/// Grep this function too when adopting a new makeover-layout. Public for the
/// reason [`part_class`] is.
#[must_use]
pub fn cell_part_class(part: CellPart) -> &'static str {
    match part {
        CellPart::Value => "cell-value",
        CellPart::Tokens => "cell-tokens",
        CellPart::Actions => "cell-actions",
        CellPart::Link => "cell-link",
        _ => "cell-part",
    }
}

/// A row's cells, in column order.
///
/// Ordered by the columns and not by the cells, so a row cannot silently
/// disagree with its table about what comes where. A column with no cell gets
/// an empty container, which keeps the grid aligned; a cell naming no column is
/// dropped, because there is nowhere to put it.
///
/// Emits the cells alone, not the row element. The row carries the app's
/// identity and hooks — `data-id`, a context-menu binding, a tabindex, its
/// state classes — and none of that is describable here.
///
/// # Not for a webview's scroll path
///
/// This has no consumer in either webview app, deliberately, and wiring one in
/// would be a mistake worth naming. goingson renders rows through a virtual
/// scroller whose `_render` calls its row builder **synchronously** while
/// scrolling; the code's own comment says scroll events fire at 60Hz+ and that
/// this is the hot path. Reaching Rust from there means an IPC round trip and
/// an `await` in that loop, per visible range, during a drag.
///
/// So this is for the hosts where rendering already happens in Rust: an axum
/// route, and the router when it lands. There the objection does not apply,
/// because nothing crosses a process boundary to reach it. A webview app should
/// take [`narrowing_css`] and [`column_class`] and keep building its own rows.
#[must_use]
pub fn cells_html(columns: &[Column<'_>], cells: &[Cell<'_>], opts: &Emit) -> String {
    let mut html = String::new();
    cells_html_into(columns, cells, opts, &mut html);
    html
}

/// A row's cells, written into a buffer the caller already has.
///
/// [`cells_html`]'s streaming form, byte-identical to it, and the one a host
/// rendering a table should call: a row is emitted once per row per render, so
/// this is where a `String` per cell class is paid for most often.
pub fn cells_html_into(columns: &[Column<'_>], cells: &[Cell<'_>], opts: &Emit, out: &mut String) {
    for column in columns {
        let found = cells.iter().find(|cell| cell.column == column.name);
        out.push_str("<div class=\"");
        push_class(out, "cell", opts);
        out.push(' ');
        push_column_classes(out, column, opts);
        if let Some(part) = found.and_then(|cell| cell.part) {
            out.push(' ');
            push_class(out, cell_part_class(part), opts);
        }
        out.push_str("\">");
        out.push_str(found.map_or("", |cell| cell.content.0));
        out.push_str("</div>");
    }
}

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

    #[test]
    fn every_width_and_drop_class_is_one_the_vocabulary_wrote_down() {
        // The obligation ROW_PART_CLASSES carries. Both matches have a wildcard
        // arm, so a member added upstream lands on a class that is already in
        // the list; what this catches is a new arm returning a new name, which
        // would otherwise narrow what a checker believes this crate emits
        // without narrowing what it writes.
        for width in [Width::Content, Width::Fixed, Width::Fill] {
            assert!(
                CELL_WIDTH_CLASSES.contains(&width_class(width)),
                "{width:?} is missing from CELL_WIDTH_CLASSES"
            );
        }
        for priority in [Priority::Optional, Priority::Secondary, Priority::Essential] {
            assert!(
                CELL_DROP_CLASSES.contains(&drop_class(priority)),
                "{priority:?} is missing from CELL_DROP_CLASSES"
            );
        }
        let names = crate::vocabulary::names(&Emit::default());
        for name in CELL_WIDTH_CLASSES.iter().chain(CELL_DROP_CLASSES) {
            assert!(names.contains(*name), "{name} is not in the vocabulary");
        }
    }

    #[test]
    fn a_column_name_cannot_break_out_of_the_class_attribute() {
        // Until 0.41.0 the name went in raw, so this emitted
        // `class="cell col-a" onclick="steal() cell-fill ...">` -- a live
        // handler on every cell of the column. The name is the one
        // app-supplied string this crate puts in a class rather than in text.
        let name = "a\" onclick=\"steal()";
        let columns = vec![Column::new(name)];
        let cells = vec![Cell {
            column: name,
            part: None,
            content: Markup("x"),
        }];
        let html = cells_html(&columns, &cells, &Emit::default());

        assert!(!html.contains("onclick=\"steal()"), "{html}");
        assert!(html.contains("col-a--onclick--steal--"), "{html}");
        // Two quotes in the whole cell, both this crate's: the ones opening and
        // closing the class attribute. A third would be the name ending it.
        assert_eq!(html.matches('"').count(), 2, "{html}");
    }

    #[test]
    fn the_class_and_the_selector_that_hides_it_agree_on_the_name() {
        // The reason the fix is a filter and not an escape. A class is read by
        // the HTML parser and again by a CSS selector; an escaped name would be
        // safe in the attribute and unmatchable from the stylesheet, so the
        // narrowing would stop hiding the column it names.
        let columns = vec![Column {
            priority: Priority::Optional,
            ..Column::new("Due date")
        }];
        let cells = vec![Cell {
            column: "Due date",
            part: None,
            content: Markup("x"),
        }];
        let opts = Emit::default();

        let html = cells_html(&columns, &cells, &opts);
        let css = narrowing_css(&columns, ".row", &sizing(), Priority::Essential, &opts);

        // One class, not the two `col-Due date` parsed as.
        assert!(html.contains("class=\"cell col-Due-date "), "{html}");
        assert!(css.contains(".row > .col-Due-date {"), "{css}");
    }

    #[test]
    fn a_name_already_made_of_identifier_characters_is_untouched() {
        // Every column name in the tree is one of these, which is what makes
        // 0.41.0 a fix rather than a rename.
        for name in ["description", "due", "progress", "Name", "col_2", "a-b"] {
            let mut out = String::new();
            push_column_name(&mut out, name);
            assert_eq!(out, name);
        }
    }

    #[test]
    fn a_name_outside_ascii_keeps_itself() {
        // CSS identifiers admit everything from U+00A0 up, so folding these to
        // dashes would collide two columns for nothing.
        let mut out = String::new();
        push_column_name(&mut out, "Größe");
        assert_eq!(out, "Größe");
    }

    fn columns() -> Vec<Column<'static>> {
        vec![
            Column {
                width: Width::Fill,
                priority: Priority::Essential,
                ..Column::new("description")
            },
            Column {
                width: Width::Fixed,
                priority: Priority::Secondary,
                ..Column::new("due")
            },
            Column {
                width: Width::Fixed,
                priority: Priority::Optional,
                ..Column::new("progress")
            },
        ]
    }

    fn sizing() -> Sizing<'static> {
        Sizing {
            lengths: &[
                ("description", "200px"),
                ("due", "110px"),
                ("progress", "100px"),
            ],
            fallback: "",
        }
    }

    #[test]
    fn a_fill_column_gets_a_floor_and_the_slack() {
        let tracks = grid_template_columns(&columns(), &sizing(), Priority::Optional);
        assert_eq!(tracks, "minmax(200px, 1fr) 110px 100px");
    }

    #[test]
    fn a_column_with_no_length_makes_no_claim() {
        let sizing = Sizing::default();
        let tracks = grid_template_columns(&columns(), &sizing, Priority::Optional);
        assert_eq!(tracks, "minmax(auto, 1fr) auto auto");
    }

    /// The point of the module. Raising the cutoff drops columns by what they
    /// are worth, and the track list shortens to match, so the two cannot
    /// disagree the way a hand-written `nth-child` cut and a hand-written
    /// track list can.
    #[test]
    fn raising_the_cutoff_drops_columns_and_their_tracks_together() {
        let columns = columns();

        let wide = grid_template_columns(&columns, &sizing(), Priority::Optional);
        assert_eq!(wide.split(' ').count(), 4); // minmax(200px, + 1fr) + 2

        let narrow = grid_template_columns(&columns, &sizing(), Priority::Secondary);
        assert_eq!(narrow, "minmax(200px, 1fr) 110px");

        let narrowest = grid_template_columns(&columns, &sizing(), Priority::Essential);
        assert_eq!(narrowest, "minmax(200px, 1fr)");
    }

    #[test]
    fn narrowing_hides_a_dropped_column_by_its_own_class_not_its_position() {
        let css = narrowing_css(
            &columns(),
            ".ui-mode-mobile .task-row",
            &sizing(),
            Priority::Secondary,
            &Emit::default(),
        );
        assert!(
            css.contains("grid-template-columns: minmax(200px, 1fr) 110px;"),
            "{css}"
        );
        assert!(
            css.contains(".ui-mode-mobile .task-row > .col-progress {"),
            "{css}"
        );
        assert!(!css.contains("nth-child"), "{css}");
        // The kept columns are not mentioned as hidden.
        assert!(!css.contains(".col-due {\n    display: none"), "{css}");
    }

    /// A selector list has to distribute, or the earlier parts of it get the
    /// child combinator appended to the whole and start matching things they
    /// never named. This hid an entire table header the first time it ran.
    #[test]
    fn a_selector_list_distributes_the_hidden_column() {
        let css = narrowing_css(
            &columns(),
            ".task-header-row, .task-row",
            &sizing(),
            Priority::Secondary,
            &Emit::default(),
        );
        assert!(
            css.contains(".task-header-row > .col-progress,\n.task-row > .col-progress {"),
            "{css}"
        );
        // The bare header selector must never appear as a hiding target.
        assert!(
            !css.contains(".task-header-row {\n    display: none"),
            "{css}"
        );
        assert!(
            css.contains(".task-header-row, .task-row {\n    grid-template-columns:"),
            "{css}"
        );
    }

    #[test]
    fn cells_follow_the_columns_and_carry_their_column_class() {
        let cells = [
            Cell {
                column: "due",
                part: Some(CellPart::Value),
                content: Markup("tomorrow"),
            },
            Cell::new("description", Markup("<span>Ship it</span>")),
        ];
        let html = cells_html(&columns(), &cells, &Emit::default());

        // Column order, not cell order: description was passed second.
        let description = html.find("Ship it").expect("description cell");
        let due = html.find("tomorrow").expect("due cell");
        assert!(description < due, "{html}");

        // Three classes, not one: the column's own name, how wide it asks to
        // be, and when it drops. The last two are what lets the stylesheet
        // carry rules a described table cannot generate per table.
        assert!(
            html.contains(r#"<div class="cell col-description cell-fill cell-keeps">"#),
            "{html}"
        );
        assert!(
            html.contains(r#"<div class="cell col-due cell-fixed cell-drops-next cell-value">"#),
            "{html}"
        );
        // progress had no cell, so it is present and empty rather than absent,
        // or the grid would shift left by one.
        assert!(
            html.contains(r#"<div class="cell col-progress cell-fixed cell-drops-first"></div>"#),
            "{html}"
        );
    }

    /// A row is emitted once per row per render, so the streaming form is the
    /// one a host should call and the two have to agree byte for byte.
    #[test]
    fn streamed_cells_are_the_cells_the_other_form_returns() {
        let opts = Emit {
            class_prefix: "mk-",
            ..Emit::default()
        };
        let cells = [
            Cell {
                column: "due",
                part: Some(CellPart::Value),
                content: Markup("tomorrow"),
            },
            Cell::new("description", Markup("<span>Ship it</span>")),
        ];
        for cells in [&cells[..], &[]] {
            let mut streamed = String::new();
            cells_html_into(&columns(), cells, &opts, &mut streamed);
            assert_eq!(streamed, cells_html(&columns(), cells, &opts));
        }
        for column in &columns() {
            let mut streamed = String::new();
            push_column_classes(&mut streamed, column, &opts);
            assert_eq!(streamed, column_classes(column, &opts));
        }
    }

    #[test]
    fn a_cell_naming_no_column_is_dropped() {
        let cells = [Cell::new("nonexistent", Markup("nowhere"))];
        let html = cells_html(&columns(), &cells, &Emit::default());
        assert!(!html.contains("nowhere"), "{html}");
    }

    #[test]
    fn the_class_prefix_reaches_the_cells_and_the_narrowing() {
        let opts = Emit {
            class_prefix: "mk-",
            ..Emit::default()
        };
        let cells = [Cell::new("due", Markup("x"))];
        assert!(
            cells_html(&columns(), &cells, &opts).contains("mk-cell mk-col-due"),
            "prefix missing"
        );
        assert!(
            narrowing_css(&columns(), ".t", &sizing(), Priority::Secondary, &opts)
                .contains(".mk-col-progress"),
            "prefix missing"
        );
    }
}