makeover-webview 0.5.0

//! The webview renderer for [`makeover_layout`].
//!
//! <!-- wiki: makeover-webview -->
//!
//! # The renderer that needs no palette
//!
//! `makeover-immediate` and `makeover-tui` both take a `Palette`, because egui
//! and a terminal need an actual colour before they can put anything on
//! screen. A webview does not: `var(--surface-raised)` *is* the late binding,
//! and the browser resolves it against whatever `themes.js` last wrote onto
//! `:root`.
//!
//! So this crate emits text naming intents, and never learns a colour. It is
//! the deferral rule with no adapter in the way, and it is why the webview was
//! always the wrong renderer to derive a vocabulary from: it can express
//! anything, so it never pushes back.
//!
//! # Phase A: the stylesheet only
//!
//! This emits component CSS and no markup, deliberately. GoingsOn has 145
//! `innerHTML` sites and Balanced Breakfast 175 `createElement` sites; moving
//! markup is a migration, while adopting a generated stylesheet is a deletion.
//! The apps keep every line of their markup and gain the classes.
//!
//! The bevel properties are byte-identical to what both apps already
//! hand-write, which is asserted below.
//!
//! # What phase A settled, and what it costs
//!
//! Decided 2026-07-29 against goingson's `styles.css` rather than against a
//! component list. The useful finding there was that `.btn` (line 644),
//! `.card` (768) and `.tag, .badge` (882) each hand-write the same
//! composition, so three quarters of phase A is one rule with several names.
//!
//! Two of the four decisions change how goingson looks, and adoption should
//! not be described as a pure deletion:
//!
//! - **Pressed carries its fill.** [`interactive_rules`] emits
//!   [`Depth::pressed`] whole. goingson presses to `--surface-sunken` today and
//!   will press to `--surface-well`, and hovers to `--surface-overlay` today
//!   and will hover to `--hover-surface`. Since `surface-well` inverts by theme
//!   where `surface-sunken` does not, a dark theme presses *lighter* than it
//!   hovers. That falls out of `makeover`'s own derivation, which says outright
//!   that `surface-sunken` cannot serve as a well, so if it reads wrong the
//!   answer is there and not here.
//! - **Badges go flat.** See [`token_rules`].
//!
//! The other two: the progress trough is renderer-local and the scrollbar
//! track was dropped ([`component_rules`]), and no class prefix ships by
//! default, so adoption means deleting the app's hand-written rule in the same
//! commit that adds the generated one. `.card`, `.badge` and the tab classes
//! all already exist in goingson, and while both rules exist the cascade order
//! decides which wins. That is the one real risk in adopting this, and it is
//! why the migration lands per component rather than in one commit.
//!
//! # Substitution, three ways
//!
//! `Fill::Well` has no colour on makeover before 2.3.0, and each renderer
//! answers that differently, which is the evidence that dropping
//! `Fill::fallback` from the description was right:
//!
//! - `makeover-immediate` substitutes the page in Rust.
//! - `makeover-tui` refuses to substitute and draws an edge instead, because a
//!   terminal would quantise the two together.
//! - here, CSS already has the mechanism: `var(--surface-well,
//!   var(--surface-page))` falls back in the browser, and nothing in Rust
//!   decides anything.

#![forbid(unsafe_code)]

use makeover_layout::{Bevel, Depth, Fill, Intent, RowPart, Selector, Token, Tone};
use std::fmt::Write as _;

/// How the emitted CSS is shaped.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct Emit {
    /// Bevel thickness, as a CSS length.
    ///
    /// A value, so it arrives from the caller: border widths belong to
    /// `makeover-geometry` and will come from there once it carries them.
    pub border_width: &'static str,
    /// Prefix for emitted class names, without the leading dot.
    pub class_prefix: &'static str,
}

impl Default for Emit {
    fn default() -> Self {
        Self {
            border_width: "1px",
            class_prefix: "",
        }
    }
}

/// The CSS custom property holding a bevel's composition.
#[must_use]
pub fn bevel_var(bevel: Bevel) -> &'static str {
    match bevel {
        Bevel::Raised => "--bevel-raised",
        Bevel::Inset => "--bevel-inset",
    }
}

/// A `var()` reference to a fill intent, with the browser's own fallback where
/// the intent may be absent.
///
/// The fallback is CSS syntax, not a decision made here. That is the whole
/// difference between this renderer and the other two.
#[must_use]
pub fn fill_var(fill: Fill) -> String {
    match fill {
        Fill::Well => format!("var(--{}, var(--{}))", fill.token(), Fill::Page.token()),
        other => format!("var(--{})", other.token()),
    }
}

/// The two-tone edge as a `box-shadow` value.
///
/// Two inset shadows, one per corner pair: the light one offset down and
/// right so it lands on the top and left edges, the dark one the other way.
/// The same assignment `makeover-immediate` draws with polylines and
/// `makeover-tui` draws with box-drawing characters.
#[must_use]
pub fn bevel_shadow(bevel: Bevel, opts: &Emit) -> String {
    let (top_left, bottom_right) = bevel.edges();
    let w = opts.border_width;
    format!(
        "inset {w} {w} 0 var(--{}), inset -{w} -{w} 0 var(--{})",
        top_left.token(),
        bottom_right.token()
    )
}

/// The custom properties both bevels resolve through.
///
/// Emitted as properties rather than inlined into every rule because that is
/// what the apps already do, and because a consumer that wants the edge
/// without the fill reads the property directly.
#[must_use]
pub fn bevel_properties(opts: &Emit) -> String {
    let mut css = String::new();
    for bevel in [Bevel::Raised, Bevel::Inset] {
        let _ = writeln!(
            css,
            "    {}: {};",
            bevel_var(bevel),
            bevel_shadow(bevel, opts)
        );
    }
    css
}

/// The class name for a depth.
#[must_use]
pub fn depth_class(depth: Depth, opts: &Emit) -> Option<String> {
    let name = match depth {
        Depth::Flat => return None,
        Depth::Raised => "raised",
        Depth::Well => "well",
        Depth::Sunken => "sunken",
    };
    Some(format!("{}{name}", opts.class_prefix))
}

/// A prefixed class name.
fn class(name: &str, opts: &Emit) -> String {
    format!("{}{name}", opts.class_prefix)
}

/// The fill and edge declarations for a depth, as a rule body.
///
/// Empty for [`Depth::Flat`], which has neither and inherits what it sits on.
/// Callers lean on the emptiness to skip the rule rather than emit a class that
/// sets nothing: a class that sets no properties is a class that means "I
/// thought about this", which is what comments are for.
///
/// The two halves are emitted independently because [`Depth::Sunken`] has a
/// fill and no bevel. Requiring both, which this did before makeover-layout
/// 0.3.0, silently dropped the fill for exactly that case. Independent does not
/// mean unpaired: both halves still come off one `Depth`, so they cannot
/// disagree about what the region is.
#[must_use]
pub fn depth_declarations(depth: Depth) -> String {
    let mut css = String::new();
    if let Some(fill) = depth.fill() {
        let _ = writeln!(css, "    background: {};", fill_var(fill));
    }
    if let Some(bevel) = depth.bevel() {
        let _ = writeln!(css, "    box-shadow: var({});", bevel_var(bevel));
    }
    css
}

/// One rule giving a selector a depth, or nothing when the depth declares
/// nothing.
#[must_use]
pub fn depth_rule(selector: &str, depth: Depth) -> String {
    let body = depth_declarations(depth);
    if body.is_empty() {
        return String::new();
    }
    format!(".{selector} {{\n{body}}}\n")
}

/// Hover and pressed, for a selector that answers a click.
///
/// Pressed emits [`Depth::pressed`] in full, fill and edge together. Emitting
/// only the edge is what left goingson hand-writing `background:
/// var(--surface-sunken)` on three separate rules, and a fill that does not
/// travel with its edge is precisely the disagreement `Depth` exists to make
/// unrepresentable. So the pressed fill comes from the description
/// (`--surface-well`) rather than from whatever each app reached for.
///
/// Hover has no member in the description and is renderer policy: a terminal
/// and an immediate-mode painter have no hover to express. It resolves against
/// `--hover-surface`, which `makeover` already derives and which nothing
/// consumed until now.
#[must_use]
pub fn interactive_rules(selector: &str) -> String {
    format!(
        ".{selector}:hover {{\n    background: var(--hover-surface);\n}}\n{}",
        depth_rule(&format!("{selector}:active"), Depth::Raised.pressed())
    )
}

/// One rule per depth: its fill and its edge, together.
///
/// A pressed rule rides along with the raised one, because the cascade can
/// carry a state that an immediate-mode renderer has to resolve per call site.
/// That is the one thing this renderer gets for free and the others do not.
#[must_use]
pub fn depth_rules(opts: &Emit) -> String {
    let mut css = String::new();
    for depth in [Depth::Raised, Depth::Well] {
        let Some(class) = depth_class(depth, opts) else {
            continue;
        };
        css.push_str(&depth_rule(&class, depth));
    }
    if let Some(raised) = depth_class(Depth::Raised, opts) {
        css.push_str(&interactive_rules(&raised));
    }
    css
}

/// The three surfaces that are a depth with a name.
///
/// `button` and `card` are both [`Depth::Raised`], and `field` is a
/// [`Depth::Well`] because that is the reading `Depth`'s own documentation
/// gives a text field. Their bodies come out identical by construction rather
/// than by hand: three hand-written copies in goingson's stylesheet is what
/// phase A deletes, and generating them from one call is what stops them
/// drifting apart again.
fn surface_rules(opts: &Emit) -> String {
    let mut css = String::new();
    for name in ["button", "card"] {
        let c = class(name, opts);
        css.push_str(&depth_rule(&c, Depth::Raised));
        css.push_str(&interactive_rules(&c));
    }

    let field = class("field", opts);
    css.push_str(&depth_rule(&field, Depth::Well));

    // Keyed on the ARIA attribute rather than on a class, so the visual state
    // and the accessible state cannot drift apart: there is one fact and both
    // read it. goingson already drove its invalid styling this way and was
    // right to; the `.invalid` class this emitted before 0.5.0 was a second
    // place to forget.
    //
    // The ring composes *after* the bevel rather than replacing it. box-shadow
    // is not additive, so a lone ring silently dropped the well out from under
    // an invalid field. Flat and unlit: this edge is saying "wrong", and
    // lighting one side would have it say "raised" at the same time.
    let _ = writeln!(
        css,
        ".{field}[aria-invalid=\"true\"] {{\n    box-shadow: var({}), 0 0 0 {} var(--danger);\n}}",
        bevel_var(Bevel::Inset),
        opts.border_width
    );
    css
}

/// Badges and chips.
///
/// The one place phase A changes how goingson looks rather than only where its
/// rules live. [`Token::Badge`] is [`Depth::Flat`], so a badge emits no fill
/// and no edge at all, where goingson ships `.tag, .badge` as a single rule
/// carrying the raised bevel. Splitting that means reading every call site to
/// decide which of the two it always was.
///
/// What a badge does carry is a [`Tone`], the intent family it shares with
/// notices and nothing else. Neutral is the bare class rather than a variant,
/// because it is the absence of a status and not a status called "none".
fn token_rules(opts: &Emit) -> String {
    let mut css = String::new();

    // No `depth_rule` call here, deliberately: `Token::Badge.depth(_)` is Flat,
    // and a label with an edge says it can be pressed.
    let badge = class("badge", opts);
    let _ = writeln!(
        css,
        ".{badge} {{\n    color: var(--{});\n}}",
        Tone::Neutral.token()
    );
    for tone in [Tone::Info, Tone::Success, Tone::Warning, Tone::Danger] {
        let _ = writeln!(
            css,
            ".{badge}[data-tone=\"{0}\"] {{\n    color: var(--{0});\n}}",
            tone.token()
        );
    }

    // A chip holds itself down, which is `Depth::pressed` arrived at
    // independently by two apps. `removable` is a remove affordance, so it is
    // markup and waits for phase B.
    let chip = class("chip", opts);
    let unlatched = Token::Chip { removable: false };
    css.push_str(&depth_rule(&chip, unlatched.depth(false)));
    css.push_str(&interactive_rules(&chip));
    css.push_str(&depth_rule(
        &format!("{chip}.latched"),
        unlatched.depth(true),
    ));
    css
}

/// The three selectors, each named by what it picks.
///
/// A tab comes *forward* to join the pane it opens, which is why
/// [`Selector::Tabs`] chooses [`Depth::Raised`] where a segment and a toggle
/// are held in. That is the folder semantic, and it is the whole reason the
/// three are not one member with a flag.
///
/// [`Selector::abutting`] is not emitted: whether the options touch is
/// spacing, and spacing is `makeover-geometry`'s question to answer.
///
/// Both states emit as of makeover-layout 0.3.0. Before it the description
/// named only the chosen option, so an unchosen one fell through to
/// [`Depth::Flat`] and nothing was drawn for it, which left goingson's tab
/// strip hand-writing the recess that makes its chosen tab read as forward.
fn selector_rules(opts: &Emit) -> String {
    let mut css = String::new();
    for (selector, name) in [
        (Selector::Tabs, "tab"),
        (Selector::Segmented, "segment"),
        (Selector::Toggle, "toggle"),
    ] {
        let c = class(name, opts);
        css.push_str(&depth_rule(&c, selector.unchosen()));
        css.push_str(&interactive_rules(&c));
        css.push_str(&depth_rule(&format!("{c}.chosen"), selector.chosen()));
    }
    css
}

/// The four parts of a list row.
fn row_rules(opts: &Emit) -> String {
    let mut css = String::new();
    let row = class("row", opts);
    for part in [
        RowPart::Primary,
        RowPart::Secondary,
        RowPart::Meta,
        RowPart::Actions,
    ] {
        let name = match part {
            RowPart::Primary => "row-primary",
            RowPart::Secondary => "row-secondary",
            RowPart::Meta => "row-meta",
            RowPart::Actions => "row-actions",
        };
        let c = class(name, opts);

        // Actions carry controls rather than text, and `RowPart::intent` says
        // so by returning the same intent inheriting already gives. Pinning it
        // would be louder than saying nothing.
        if !matches!(part, RowPart::Actions) {
            let _ = writeln!(css, ".{c} {{\n    color: var(--{});\n}}", part.intent());
        }

        if part.revealed_on_hover() {
            // Hidden rather than absent: the row must not change height when
            // the pointer arrives. `focus-within` carries the keyboard, which
            // hover on its own would lock out.
            let _ = writeln!(css, ".{c} {{\n    visibility: hidden;\n}}");
            let _ = writeln!(
                css,
                ".{row}:hover .{c},\n.{row}:focus-within .{c} {{\n    visibility: visible;\n}}"
            );
        }
    }
    css
}

/// The progress trough, which has nothing behind it in the description.
///
/// Renderer-local chrome, on the same licence the skeletons hold as the
/// webview's expression of `Readiness::Pending`: a determinate bar is a shape
/// CSS draws readily and a terminal would rather not be told about. It earns
/// the place empirically, goingson having grown four independent progress bars
/// before anything named one.
///
/// The trough is a [`Depth::Well`], the same reading a text field gets:
/// something with its content down inside it.
fn progress_rules(opts: &Emit) -> String {
    let progress = class("progress", opts);
    // `progress-fill` rather than a bare `fill`: an unprefixed build claims
    // these names in the app's own stylesheet, and `.fill` is grabby enough to
    // catch things that have nothing to do with progress. goingson already
    // calls it `.progress-fill`, so this is also the name that deletes.
    let fill = class("progress-fill", opts);
    let mut css = depth_rule(&progress, Depth::Well);

    // The untoned bar is `--action`, not [`Tone::Neutral`]. That is the one
    // place this differs from the badge rules, and deliberately: a badge with
    // no status is a muted label, while a bar with no status is still
    // reporting progress, and `content-muted` would read as disabled.
    let _ = writeln!(
        css,
        ".{progress} > .{fill} {{\n    background: var(--action);\n}}"
    );

    // A bar can be saying something, same as a badge: goingson colours subtask
    // progress as success and an over-estimate as danger, which is real
    // information rather than decoration. Emitting the tones is what lets that
    // survive adoption instead of staying hand-written.
    for tone in [Tone::Info, Tone::Success, Tone::Warning, Tone::Danger] {
        let _ = writeln!(
            css,
            ".{progress} > .{fill}[data-tone=\"{0}\"] {{\n    background: var(--{0});\n}}",
            tone.token()
        );
    }
    css
}

/// The component layer: every named thing phase A emits.
///
/// No scrollbar track. It was on the phase A list and came off: eight lines of
/// `::-webkit-scrollbar` with no shape a terminal or an immediate-mode painter
/// would want handed to it, so it stays with the apps.
#[must_use]
pub fn component_rules(opts: &Emit) -> String {
    let mut css = String::new();
    css.push_str(&surface_rules(opts));
    css.push_str(&token_rules(opts));
    css.push_str(&selector_rules(opts));
    css.push_str(&row_rules(opts));
    css.push_str(&progress_rules(opts));
    css
}

/// The whole phase-A stylesheet: properties, depth rules and components, with
/// a generated-file banner.
#[must_use]
pub fn stylesheet(opts: &Emit) -> String {
    format!(
        "/* Generated by makeover-webview from makeover-layout. Do not edit.\n   \
         Depth is a fill and an edge together; naming them apart is what let\n   \
         them disagree. See the crate's README and wiki note makeover-layout. */\n\
         :root {{\n{}}}\n\n{}\n{}",
        bevel_properties(opts),
        depth_rules(opts),
        component_rules(opts)
    )
}

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

    #[test]
    fn the_emitted_bevel_matches_what_the_apps_already_hand_write() {
        // Balanced Breakfast's styles.css, verbatim. Adoption has to be a
        // deletion, not a redesign, or nobody will take it.
        let opts = Emit::default();
        assert_eq!(
            bevel_shadow(Bevel::Raised, &opts),
            "inset 1px 1px 0 var(--bevel-light), inset -1px -1px 0 var(--bevel-dark)"
        );
        assert_eq!(
            bevel_shadow(Bevel::Inset, &opts),
            "inset 1px 1px 0 var(--bevel-dark), inset -1px -1px 0 var(--bevel-light)"
        );
    }

    #[test]
    fn no_colour_ever_reaches_the_output() {
        let css = stylesheet(&Emit::default());
        assert!(!css.contains('#'), "a hex literal escaped into the CSS");
        assert!(
            !css.contains("rgb"),
            "a colour function escaped into the CSS"
        );
        // Every colour is named, never resolved.
        assert!(css.contains("var(--surface-raised)"));
        assert!(css.contains("var(--bevel-light)"));
    }

    #[test]
    fn a_well_falls_back_through_css_rather_than_through_rust() {
        assert_eq!(
            fill_var(Fill::Well),
            "var(--surface-well, var(--surface-page))"
        );
        // Nothing else needs one.
        assert_eq!(fill_var(Fill::Raised), "var(--surface-raised)");
        assert_eq!(fill_var(Fill::Page), "var(--surface-page)");
    }

    #[test]
    fn raised_and_well_do_not_collapse_onto_each_other() {
        let css = depth_rules(&Emit::default());
        assert!(css.contains(".raised {"));
        assert!(css.contains(".well {"));
        assert!(css.contains("var(--bevel-raised)"));
        assert!(css.contains("var(--bevel-inset)"));
    }

    #[test]
    fn the_cascade_carries_the_pressed_state() {
        let css = depth_rules(&Emit::default());
        // The one thing this renderer gets free that the other two resolve by
        // hand, eighteen call sites deep in audiofiles' case.
        assert!(css.contains(".raised:active {"));
    }

    #[test]
    fn pressing_moves_the_fill_and_not_only_the_edge() {
        // The decision-1 guard, and the regression that mattered: emitting the
        // bevel flip alone is what left goingson hand-writing `background:
        // var(--surface-sunken)` on .btn, .card and .tag/.badge alike, so none
        // of the three could be deleted.
        let pressed = interactive_rules("button");
        assert!(pressed.contains(".button:active {"));
        assert!(
            pressed.contains("background: var(--surface-well, var(--surface-page))"),
            "pressed dropped its fill: {pressed}"
        );
        assert!(pressed.contains("box-shadow: var(--bevel-inset)"));
    }

    #[test]
    fn pressed_takes_its_fill_from_the_description_not_from_the_app() {
        // goingson presses to --surface-sunken. The description says a pressed
        // raised region reads as a well, and makeover says outright that
        // surface-sunken cannot serve as one, so the app is the thing that
        // moves.
        //
        // Scoped to the pressed rules rather than to the whole sheet: since
        // makeover-layout 0.3.0 an unchosen tab is legitimately
        // --surface-sunken, so the token appearing somewhere in the output no
        // longer means the app's choice leaked in.
        let css = stylesheet(&Emit::default());
        let mut checked = 0;
        for rule in css.split("}\n") {
            if !rule.contains(":active") {
                continue;
            }
            checked += 1;
            assert!(
                !rule.contains("surface-sunken"),
                "a pressed rule took the app's fill: {rule}"
            );
        }
        assert!(checked > 0, "no pressed rules found to check");
        assert_eq!(
            Depth::Raised.pressed().fill(),
            Some(Fill::Well),
            "the description changed under us"
        );
    }

    #[test]
    fn hover_resolves_against_the_token_makeover_already_derives() {
        let css = interactive_rules("card");
        assert!(css.contains(".card:hover {"));
        assert!(css.contains("background: var(--hover-surface)"));
        // Not the app's choice, which was --surface-overlay.
        assert!(!css.contains("surface-overlay"));
    }

    #[test]
    fn a_badge_gets_no_edge_and_no_fill() {
        // Decision 2, and the one visible redesign in phase A. Token::Badge is
        // Flat: an edge on a label says it can be pressed.
        let css = token_rules(&Emit::default());
        let badge = css
            .lines()
            .skip_while(|l| !l.starts_with(".badge {"))
            .take_while(|l| !l.starts_with('}'))
            .collect::<Vec<_>>()
            .join("\n");
        assert!(!badge.contains("box-shadow"), "badge kept an edge: {badge}");
        assert!(!badge.contains("background"), "badge kept a fill: {badge}");
        assert_eq!(Token::Badge.depth(false), Depth::Flat);
        assert_eq!(Token::Badge.depth(true), Depth::Flat);
    }

    #[test]
    fn a_badge_carries_a_tone_and_neutral_is_the_bare_class() {
        let css = token_rules(&Emit::default());
        // Neutral is the absence of a status, not a status named "none".
        assert!(css.contains(".badge {\n    color: var(--content-muted);"));
        assert!(!css.contains("data-tone=\"content-muted\""));
        for tone in ["info", "success", "warning", "danger"] {
            assert!(
                css.contains(&format!(".badge[data-tone=\"{tone}\"]")),
                "missing tone {tone}"
            );
            assert!(css.contains(&format!("color: var(--{tone})")));
        }
    }

    #[test]
    fn a_chip_is_raised_and_latches_into_a_well() {
        let css = token_rules(&Emit::default());
        assert!(css.contains(".chip {"));
        assert!(css.contains(".chip.latched {"));
        assert!(css.contains(".chip:active {"));
        // The whole difference from a badge: it answers a click.
        assert!(Token::Chip { removable: false }.interactive());
        assert!(!Token::Badge.interactive());
    }

    #[test]
    fn only_a_tab_comes_forward_when_chosen() {
        // The folder semantic. Collapsing the three selectors would lose it.
        let css = selector_rules(&Emit::default());
        assert!(css.contains(".tab.chosen {"));
        assert!(css.contains(".segment.chosen {"));
        assert!(css.contains(".toggle.chosen {"));
        assert_eq!(Selector::Tabs.chosen(), Depth::Raised);
        assert_eq!(Selector::Segmented.chosen(), Depth::Well);
        assert_eq!(Selector::Toggle.chosen(), Depth::Well);

        let tab = css
            .lines()
            .skip_while(|l| !l.starts_with(".tab.chosen {"))
            .take_while(|l| !l.starts_with('}'))
            .collect::<Vec<_>>()
            .join("\n");
        assert!(tab.contains("var(--bevel-raised)"), "tab was held in: {tab}");
    }

    #[test]
    fn an_unchosen_tab_recedes_without_looking_picked() {
        let css = selector_rules(&Emit::default());
        // Recessed by colour and given no edge. An edge would make every option
        // look picked; flat would leave the chosen one nothing to come forward
        // from, which is the gap makeover-layout 0.3.0 closed.
        assert!(
            css.contains(".tab {\n    background: var(--surface-sunken);\n}"),
            "unchosen tab is not recessed: {css}"
        );
        assert_eq!(Selector::Tabs.unchosen(), Depth::Sunken);
        assert!(css.contains(".tab:hover {"));
    }

    #[test]
    fn a_segment_stands_up_so_the_chosen_one_can_be_held_in() {
        // The inverse of the tab, and why the three selectors are not one
        // member with a flag.
        let css = selector_rules(&Emit::default());
        assert!(css.contains(".segment {\n    background: var(--surface-raised);"));
        assert_eq!(Selector::Segmented.unchosen(), Depth::Raised);
        assert_eq!(Selector::Segmented.chosen(), Depth::Well);
    }

    #[test]
    fn row_actions_are_revealed_without_moving_the_row() {
        let css = row_rules(&Emit::default());
        assert!(css.contains(".row-actions {\n    visibility: hidden;"));
        // Not display:none, which would reflow the row under the pointer.
        assert!(!css.contains("display: none"));
        // Hover alone would lock the keyboard out.
        assert!(css.contains(".row:focus-within .row-actions"));
        assert!(RowPart::Actions.revealed_on_hover());
    }

    #[test]
    fn the_three_text_parts_take_their_intents_and_actions_inherits() {
        let css = row_rules(&Emit::default());
        assert!(css.contains(".row-primary {\n    color: var(--content);"));
        assert!(css.contains(".row-secondary {\n    color: var(--content-secondary);"));
        assert!(css.contains(".row-meta {\n    color: var(--content-muted);"));
        // Actions carry controls, not text. Pinning the colour it would inherit
        // anyway is louder than saying nothing.
        assert!(!css.contains(".row-actions {\n    color:"));
    }

    #[test]
    fn the_progress_trough_is_a_well() {
        let css = progress_rules(&Emit::default());
        assert!(css.contains(".progress {"));
        assert!(css.contains("box-shadow: var(--bevel-inset)"));
        assert!(css.contains(".progress > .progress-fill {"));
        assert!(css.contains("background: var(--action)"));
        // A bare `.fill` would catch things that have nothing to do with
        // progress once the sheet lands unprefixed.
        assert!(!css.contains("> .fill "));
    }

    #[test]
    fn a_progress_bar_can_carry_a_tone_and_defaults_to_action() {
        let css = progress_rules(&Emit::default());
        // Untoned is --action, not Tone::Neutral's content-muted: a bar with no
        // status is still reporting progress, and muted would read as disabled.
        assert!(css.contains(".progress > .progress-fill {\n    background: var(--action);"));
        assert!(!css.contains("progress-fill {\n    color: var(--content-muted)"));
        for tone in ["info", "success", "warning", "danger"] {
            assert!(
                css.contains(&format!(
                    ".progress > .progress-fill[data-tone=\"{tone}\"]"
                )),
                "missing progress tone {tone}"
            );
        }
        // goingson's two live cases, which is why the tones are emitted at all.
        assert!(css.contains("[data-tone=\"success\"] {\n    background: var(--success);"));
        assert!(css.contains("[data-tone=\"danger\"] {\n    background: var(--danger);"));
    }

    #[test]
    fn no_scrollbar_track_is_emitted() {
        // Decision 3's negative half. It was on the phase A list and came off;
        // this is what stops it drifting back in.
        let css = stylesheet(&Emit::default());
        assert!(!css.contains("scrollbar"));
        assert!(!css.contains("::-webkit"));
    }

    #[test]
    fn an_invalid_field_is_ringed_without_being_lit() {
        let css = surface_rules(&Emit::default());
        assert!(css.contains(".field {"));
        // The ARIA attribute, not a class: one fact, read by both the visual
        // and the accessible state, so they cannot drift.
        assert!(css.contains(".field[aria-invalid=\"true\"] {"));
        assert!(!css.contains(".field.invalid"));
        // A flat ring: this edge says "wrong", and a two-tone bevel would have
        // it say "raised" at the same time.
        assert!(css.contains("0 0 0 1px var(--danger)"));
    }

    #[test]
    fn an_invalid_field_keeps_the_well_underneath_it() {
        // box-shadow is not additive. A lone ring replaces the bevel and drops
        // the well out from under the field, which is what this emitted before
        // 0.5.0 and is the whole reason the rule composes.
        let css = surface_rules(&Emit::default());
        let invalid = css
            .lines()
            .skip_while(|l| !l.starts_with(".field[aria-invalid"))
            .take_while(|l| !l.starts_with('}'))
            .collect::<Vec<_>>()
            .join("\n");
        assert!(
            invalid.contains("var(--bevel-inset)"),
            "the well was dropped: {invalid}"
        );
        assert!(invalid.contains("var(--danger)"));
    }

    #[test]
    fn button_and_card_come_out_identical_by_construction() {
        // The duplication phase A deletes. They are the same composition, so
        // the only honest way to emit both is from one call.
        let opts = Emit::default();
        let css = surface_rules(&opts);
        assert_eq!(
            depth_declarations(Depth::Raised),
            depth_declarations(Depth::Raised)
        );
        assert!(css.contains(".button {"));
        assert!(css.contains(".card {"));
        assert_eq!(
            interactive_rules("button").replace("button", "card"),
            interactive_rules("card")
        );
    }

    #[test]
    fn a_prefix_reaches_the_component_classes_too() {
        let opts = Emit {
            class_prefix: "mo-",
            ..Emit::default()
        };
        let css = stylesheet(&opts);
        for name in [
            "mo-button",
            "mo-card",
            "mo-field",
            "mo-badge",
            "mo-chip",
            "mo-tab",
            "mo-row-primary",
            "mo-progress",
            "mo-progress-fill",
        ] {
            assert!(css.contains(&format!(".{name}")), "unprefixed: {name}");
        }
        // The bare names must be gone entirely, or a prefixed build still
        // collides with the app's own stylesheet.
        assert!(!css.contains(".button {"));
        assert!(!css.contains(".card {"));
        assert!(!css.contains(".badge {"));
    }

    #[test]
    fn the_whole_sheet_still_names_every_colour() {
        // The crate's founding property, asserted over the component layer and
        // not only the primitives.
        let css = stylesheet(&Emit::default());
        assert!(!css.contains('#'));
        assert!(!css.contains("rgb"));
        for line in css.lines() {
            // Declarations only: a selector or an at-rule can carry a colon of
            // its own (`:root`, `:hover`) and declares nothing.
            let declaration = line.strip_prefix("    ").map(str::trim);
            let Some(Some((_, value))) = declaration.map(|d| d.split_once(": ")) else {
                continue;
            };
            if value.contains("var(--") {
                continue;
            }
            // Everything left has to be a keyword or a caller-supplied length,
            // never a colour.
            assert!(
                value.contains("hidden")
                    || value.contains("visible")
                    || value.contains("inset"),
                "unrecognised literal value: {line}"
            );
        }
    }

    #[test]
    fn flat_emits_nothing_at_all() {
        assert_eq!(depth_class(Depth::Flat, &Emit::default()), None);
        assert!(!depth_rules(&Emit::default()).contains("flat"));
    }

    #[test]
    fn a_prefix_namespaces_every_class() {
        let opts = Emit {
            class_prefix: "mo-",
            ..Emit::default()
        };
        let css = depth_rules(&opts);
        assert!(css.contains(".mo-raised {"));
        assert!(css.contains(".mo-well {"));
        assert!(!css.contains(".raised {"));
    }

    #[test]
    fn the_border_width_is_the_callers() {
        let opts = Emit {
            border_width: "2px",
            ..Emit::default()
        };
        assert!(bevel_shadow(Bevel::Raised, &opts).contains("inset 2px 2px 0"));
    }

    #[test]
    fn edges_agree_with_the_description() {
        // Not a tautology: it is the guard that a CSS-shaped convenience never
        // quietly reverses which side is lit.
        let (tl, br) = Bevel::Raised.edges();
        assert_eq!(tl.token(), Edge::Light.token());
        assert_eq!(br.token(), Edge::Dark.token());
    }
}