makeover-layout 0.9.0

//! The renderer-agnostic half of the make-family design system.
//!
//! <!-- wiki: makeover-layout -->
//!
//! `makeover` answers *what colour*, and varies by theme. `makeover-geometry`
//! answers *how much space*, and varies by density and surface. This crate
//! answers *what the thing is*, and varies by nothing.
//!
//! # The deferral rule
//!
//! A description names intents and relationships, never values. Say
//! [`Fill::Raised`], never `#D9DDF4`. Say `Gap::Peer`, never `6px`. What is
//! left once colour and spacing are deferred is **composition**: which edges
//! are lit, what inverts on press, what nests in what.
//!
//! The constraint that shapes all of it: a renderer that can only paint
//! rectangles has to be able to express the result. egui has no
//! `box-shadow: inset` and one stroke per widget with no per-side control; a
//! terminal has box-drawing characters and one cell of resolution, and cannot
//! draw a two-tone lit edge at all. A description that assumes per-side edges
//! is a CSS description wearing a neutral name. So this crate names the
//! *intent* — this region is a well — and each renderer chooses an expression
//! it can actually produce, including dropping half of one.
//!
//! # Scope
//!
//! Depth came first: the bevel and the surfaces it shapes. That much was
//! settled the hard way — the vocabulary here was read off audiofiles'
//! `ui::theme` and `ui::widgets`, which are the only implementation written
//! by a consumer with no CSS, then checked against both webview apps. All
//! three agreed once Balanced Breakfast's fills were corrected.
//!
//! 0.2.0 adds the rest of the description, each member drawn the same way,
//! from what the three apps already hand-write rather than from a taxonomy:
//!
//! - Components. [`Token`] (badge against chip), [`Notice`] (toast against
//!   banner), [`RowPart`], [`Heading`], [`Selector`], [`Readiness`], and
//!   [`Tone`], which is the one intent family they share.
//! - Schemas. [`Field`] for forms and [`Column`] for lists and tables.
//! - Structure. [`Region`] for the parts of a screen, [`Arrangement`] for how
//!   a screen is put together.
//!
//! **Validation** is absent on purpose rather than pending: neither app has a
//! shared story, and a schema describing fields but not constraints acquires a
//! constraint layer per app, which is how the divergence this crate exists to
//! end got started.
//!
//! 0.3.0 closes a gap the first real adoption found, which is what adopting
//! against goingson first was for. [`Selector`] described only the *chosen*
//! option, so an unchosen one fell through to [`Depth::Flat`] and no renderer
//! drew it; goingson's tab strip recesses its unchosen tabs by hand and could
//! not delete the line, because being recessed is *why* the chosen tab reads as
//! coming forward. So [`Selector::unchosen`] joins `chosen`, and saying it
//! needed [`Fill::Sunken`] and [`Depth::Sunken`]: a surface set back by colour
//! with no edge, which is neither a well nor level-with.
//!
//! 0.7.0 adds [`State`], the interaction axis, closing the gap that adopting
//! against three apps rather than one made visible. The description named
//! rest and, through [`Depth::pressed`], pressed. It named neither focus nor
//! disabled, so `makeover-webview` emitted a hover rule and stopped, and each
//! consumer completed the primitive from outside by out-specifying a rule it
//! did not own: 19 such rules in goingson, 21 in the MNW server, a further set
//! in Balanced Breakfast, and three focus rings that do not match. The axis is
//! deliberately two members wide, because hover and pressed belong where they
//! already are. [`State`]'s own docs carry that argument.
//!
//! 0.8.0 finishes [`Field`], which described a field well enough to label it and
//! not well enough to draw it. Writing `makeover-webview`'s form emitter found
//! three things missing and the renderer supplied all three from outside: the
//! current value, a select's options, and the placeholder. Two of those move
//! here and one does not.
//!
//! - [`Field::placeholder`] is user-facing text sitting beside `label` and
//!   `hint`. There was never a reading on which it was renderer state; it was
//!   outside only because adding a field to a published struct is breaking.
//! - [`Field::options`] moves because every renderer needs them and each was
//!   going to invent its own shape. [`Choice`] is the shape `makeover-webview`
//!   already arrived at, taken as-is rather than redesigned.
//! - The current value stays renderer-side and is not coming here. It is the
//!   one of the three that is genuinely state: a webview reads it out of the
//!   DOM, an immediate-mode renderer holds a `&mut` to the app's own field, and
//!   a description that carried it would be a form model.
//!
//! 0.9.0 opens [`RowPart`], which was the last closed enum in the vocabulary,
//! and adds [`RowPart::Tokens`]. Both halves come from the same finding, made
//! by the first two real screens described through the router rather than by
//! reading a stylesheet.
//!
//! A goingson project card carries two trailing badges, a type and a toned
//! status; a contact card carries a primary email *and* a strip of tags. `Meta`
//! is one slot and one string, so both ports joined their facts with a
//! separator and lost what the second one was: a status reads as text where it
//! used to read as colour. [`Token`] already says exactly the right thing — a
//! small labelled thing with a kind, a tone and an optional action — and could
//! only ever be a node in its own right, never inside a row.
//!
//! So the missing thing was permission rather than a concept. `Tokens` is that
//! permission, and `#[non_exhaustive]` arrives with it so the next member is not
//! a lockstep event across three renderers. The pairing is the point: this
//! enum's own consumer in `makeover-webview` carried a comment predicting it
//! would stop compiling one day, which is a lockstep break written down and
//! waited for rather than prevented.
//!
//! Balanced Breakfast was checked before the member was added, because one
//! consumer wanting something is not evidence. It packs a count and two icon
//! buttons into the same single `Meta` slot while leaving `Actions` empty, so
//! the slot was already straining under a second consumer for a different
//! reason.
//!
//! # Where the description stops
//!
//! The bespoke widgets, a day-plan timeline and a kanban board and a calendar,
//! are not describable here and will not become describable. A description
//! expressive enough to produce a timeline is a widget library wearing a
//! description's name. Generate the boring 80% so the bespoke 20% gets the
//! attention.
//!
//! [`Region::Bespoke`] is how that limit is stated rather than hidden. The
//! description names the *place* and the app owns the contents, so a screen
//! containing a timeline is still a whole screen and still routable. Without
//! it, the four goingson screens that make the app worth using would need a
//! second, undescribed path beside the router, and two paths is how a
//! vocabulary starts drifting from its app again.

#![forbid(unsafe_code)]

/// A colour intent this crate refers to but never resolves.
///
/// The string is the token name `makeover` publishes, so a renderer can look
/// it up without this crate knowing what colour came back.
pub trait Intent {
    /// The `makeover` intent token this resolves against.
    fn token(self) -> &'static str;
}

/// Which way the light falls across a two-tone edge.
///
/// The whole content of a bevel, once colour and thickness are deferred. The
/// light is always assumed to come from the top left: every consumer measured
/// agreed on that and none of them ever varied it, so it is an invariant here
/// rather than a parameter.
///
/// # The two corners that belong to both edges
///
/// Top-right and bottom-left are where the lit run meets the shaded one, and
/// the description's claim is that they belong to *both*. How a renderer says
/// that is its own business, because the answer is bounded by resolution and
/// not by taste:
///
/// - A terminal cell is roughly 8x17 device pixels, so giving the whole corner
///   to one tone thickens that edge by a cell and reads as one run overrunning
///   the other. A half-cell glyph divides the cell already, so `makeover-tui`
///   splits it and recovers real information. Its box-drawing fallback cannot:
///   a single stroke has no half to give, so there both corners go to dark.
/// - A pixel bevel is a one-point stroke by default, which makes the corner a
///   one-point square. There is nothing to divide — a diagonal seam across one
///   point is sub-pixel, and antialiasing renders it as the blend a mitred join
///   already produces. So `makeover-immediate` mitres and is *not* diverging;
///   it is the same rule at a resolution where the split degenerates.
///
/// Stated here so the difference reads as a decision rather than as drift. A
/// renderer with room to divide the corner should; one without should mitre or
/// pick the shaded tone, and neither is a bug.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub enum Bevel {
    /// Lit from the top left: light on top and left, dark on bottom and right.
    Raised,
    /// The same edge inverted, which is also the pressed state of anything
    /// that draws itself [`Bevel::Raised`].
    Inset,
}

impl Bevel {
    /// The edge intents, as `(top_left, bottom_right)`.
    ///
    /// Split out from any painting because the inversion *is* the idea, and
    /// it is the one part every renderer implements identically.
    #[must_use]
    pub const fn edges(self) -> (Edge, Edge) {
        match self {
            Self::Raised => (Edge::Light, Edge::Dark),
            Self::Inset => (Edge::Dark, Edge::Light),
        }
    }

    /// Pressing inverts. A raised control reads as inset while held.
    ///
    /// Stated here rather than left to each consumer because a cascade can
    /// carry a pressed state and an immediate-mode renderer cannot: audiofiles
    /// resolves this per call site, eighteen times.
    #[must_use]
    pub const fn pressed(self) -> Self {
        match self {
            Self::Raised => Self::Inset,
            Self::Inset => Self::Raised,
        }
    }
}

/// One side of a bevel, named by the intent it takes.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub enum Edge {
    /// The lit side.
    Light,
    /// The shadowed side.
    Dark,
}

impl Intent for Edge {
    fn token(self) -> &'static str {
        match self {
            Self::Light => "bevel-light",
            Self::Dark => "bevel-dark",
        }
    }
}

/// A surface intent a region is filled with.
///
/// `#[non_exhaustive]`, so a renderer must carry a wildcard arm and a new
/// member is additive rather than breaking. Added 0.4.0, after [`Sunken`]
/// (an additive member, 0.3.0) hard-broke `makeover-tui` and
/// `makeover-immediate` at compile time and left neither able to move until
/// both published. The vocabulary exists to grow and the renderers exist to
/// disagree about how much of it they answer, so growth must not be a
/// lockstep event. The renderer's wildcard is not a hole: [`Fill`] is
/// resolved through a fallible lookup, and a missing intent is answered with
/// structure rather than with a substituted colour.
///
/// [`Sunken`]: Fill::Sunken
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
#[non_exhaustive]
pub enum Fill {
    /// The page behind everything.
    Page,
    /// A surface lifted off the page: cards, controls, menus, toasts.
    Raised,
    /// A surface floating above the page rather than resting on it.
    Overlay,
    /// The inside of a well.
    Well,
    /// A surface set back from the one it sits on, by colour and nothing else.
    ///
    /// Not a well. A well is a hole with an edge, and the two are authored in
    /// opposite directions: `makeover` derives `surface-well` by inverting
    /// against the theme's own content colour, while `surface-sunken` is
    /// authored and free to sit darker than raised (goingson's does). Naming
    /// only the well left the recessed-with-no-edge surface unsayable, which is
    /// what an unchosen tab is: it recedes so the chosen one can come forward,
    /// and it carries no bevel of its own.
    ///
    /// Added 0.3.0, from goingson's tab strip, which hand-writes exactly this
    /// and could not delete the line because no member described it.
    Sunken,
}

// No `fallback` here, deliberately. An earlier cut had `Fill::Well` fall back
// to `Fill::Page` so a consumer on makeover 2.2.0, which has no `surface-well`,
// had something to paint. makeover-tui found that wrong within a day: page is
// the surface a well is usually cut into, so on a terminal that substitution
// produces exactly the invisibility it was meant to prevent, and the right
// answer there is a drawn edge rather than a different colour.
//
// Substituting one intent for another is renderer policy. The description says
// what the region is and stops.

impl Intent for Fill {
    fn token(self) -> &'static str {
        match self {
            Self::Page => "surface-page",
            Self::Raised => "surface-raised",
            Self::Overlay => "surface-overlay",
            Self::Well => "surface-well",
            Self::Sunken => "surface-sunken",
        }
    }
}

/// How a region sits relative to the surface behind it.
///
/// Fill and bevel are named together because naming them apart is what let
/// them disagree. Every consumer measured had at least one region carrying a
/// raised bevel over a recessed fill: audiofiles fixed it in `raised_frame`
/// and recorded the bug in its doc comment, and Balanced Breakfast still had
/// twelve of them a year later. A single name for the pair makes that
/// unrepresentable.
/// `#[non_exhaustive]` for the same reason as [`Fill`], and in the same
/// release: a depth this renderer has no drawing for should cost it a
/// wildcard arm, not a compile error and a wait on someone else's publish.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
#[non_exhaustive]
pub enum Depth {
    /// Level with its surroundings. No edge.
    Flat,
    /// A card laid on the panel it sits in.
    Raised,
    /// A hole in the panel, with content down inside it. For anything the
    /// user looks *into*: a table body, a tag tree, a text field.
    Well,
    /// Set back from what it sits on, by colour alone. No edge.
    ///
    /// The one member carrying a fill without a bevel, so a renderer cannot
    /// assume the two arrive together. That is deliberate and it is still the
    /// pairing rule: both halves come off the same `Depth`, so they cannot
    /// disagree, and here one half is legitimately absent.
    ///
    /// Distinct from [`Depth::Flat`], which has no fill either and inherits.
    /// Recessed and level-with are different claims, and only one of them
    /// needs a colour.
    Sunken,
}

impl Depth {
    /// The edge this depth is drawn with, if it has one.
    #[must_use]
    pub const fn bevel(self) -> Option<Bevel> {
        match self {
            // Sunken joins Flat here, for the opposite reason: Flat has no edge
            // because nothing separates it from its surroundings, and Sunken has
            // none because its colour is already doing the separating.
            Self::Flat | Self::Sunken => None,
            Self::Raised => Some(Bevel::Raised),
            Self::Well => Some(Bevel::Inset),
        }
    }

    /// The surface this depth is filled with.
    ///
    /// [`Depth::Flat`] has no fill of its own: it inherits whatever it sits on,
    /// which is the difference between level-with and painted-the-same-colour.
    #[must_use]
    pub const fn fill(self) -> Option<Fill> {
        match self {
            Self::Flat => None,
            Self::Raised => Some(Fill::Raised),
            Self::Well => Some(Fill::Well),
            Self::Sunken => Some(Fill::Sunken),
        }
    }

    /// Pressing a raised region reads as a well, and nothing else moves.
    #[must_use]
    pub const fn pressed(self) -> Self {
        match self {
            Self::Raised => Self::Well,
            other => other,
        }
    }
}

/// An interaction state a region can be in, beside whatever [`Depth`] it is.
///
/// Orthogonal to depth on purpose. A disabled button is still [`Depth::Raised`]
/// and a disabled field is still a [`Depth::Well`], so folding either member
/// into `Depth` would make [`Depth::bevel`] and [`Depth::fill`] answer for
/// something that is not a depth, and would leave disabled-button and
/// disabled-field sharing one variant that cannot tell them apart.
///
/// # Why hover and pressed are not members
///
/// The line is whether every renderer has the state to express, not whether CSS
/// does. Hover is renderer policy and `makeover-webview` says so in its own
/// header: a terminal and an immediate-mode painter have no pointer hovering
/// over anything, and pressed already arrives through [`Bevel::pressed`] and
/// [`Depth::pressed`], where it belongs, because pressing is a depth inversion
/// rather than a separate condition.
///
/// Focus and disabled are different in kind. A TUI has a focused widget and a
/// greyed-out one; so does egui. Both were unsayable here, so all three webview
/// consumers supplied them from outside the primitive by out-specifying rules
/// they did not own: goingson alone carries 19 of them, and the MNW server
/// another 21. That is the divergence this crate exists to end, arriving one
/// layer down.
///
/// # The principle this encodes
///
/// A primitive owns every state it implies. A renderer that emits a hover rule
/// for a thing owes disabled, focus and the capability answer for that same
/// thing, because anything less exports the completion work to N consumers who
/// will each do it differently.
///
/// `#[non_exhaustive]` for the reason [`Fill`] and [`Depth`] carry it: growth
/// must not be a lockstep event across the three renderers.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
#[non_exhaustive]
pub enum State {
    /// Keyboard focus, as distinct from the pointer having landed on something.
    ///
    /// One ring, not one per primitive. Where the ring sits is [`Depth`]'s
    /// question and not a per-component choice: a well takes it inside its own
    /// edge and a raised surface takes it outside. That is one decision with
    /// two renderings rather than one decision per component, which is how the
    /// three apps ended up with three rings.
    Focus,
    /// Present, visible, and not answering.
    ///
    /// Not the same as absent, and deliberately not a [`Fill`]: a disabled
    /// control keeps the surface it always had and stops responding, so what
    /// changes is its content and its interactivity rather than what it is.
    Disabled,
}

impl State {
    /// Whether a region in this state stops answering the pointer.
    ///
    /// Stated in the description rather than left to each renderer, on the same
    /// reasoning as [`Bevel::pressed`]: a cascade carries it for free and an
    /// immediate-mode renderer resolves it per call site, so leaving it unsaid
    /// means resolving it once per consumer and disagreeing.
    #[must_use]
    pub const fn suppresses_interaction(self) -> bool {
        match self {
            Self::Disabled => true,
            Self::Focus => false,
        }
    }
}

impl Intent for State {
    fn token(self) -> &'static str {
        match self {
            // Already derived by `makeover` from `action.primary`, and unused
            // until now for the same reason `hover-surface` was: nothing
            // emitted the rule that would consume it.
            Self::Focus => "focus-ring",
            // Reusing the muted content intent rather than minting a
            // `disabled` colour. Disabled is a reduction and not a status, and
            // `makeover-webview`'s progress rules already record the reading
            // that `content-muted` is what disabled looks like.
            Self::Disabled => "content-muted",
        }
    }
}

/// What a region is saying, when it is saying something.
///
/// The one intent family shared by badges, notices and nothing else. Kept
/// separate from [`Fill`] because a surface is where a thing sits and a tone is
/// what it means, and the three apps agree on the four statuses:
/// `info_banner` / `warning_banner` in audiofiles, `.toast-info` /
/// `.toast-success` / `.toast-error` in goingson, `.toast.success` /
/// `.toast.error` in Balanced Breakfast.
///
/// The per-tag palette (`category-one` through `category-six`) is deliberately
/// not here. Which colour a *particular* tag takes is app domain, and both
/// webview apps already carry it as a `data-color` attribute.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub enum Tone {
    /// No status. Reads as ordinary de-emphasised content.
    Neutral,
    /// Something worth knowing and nothing to do about it.
    Info,
    /// Something finished and it worked.
    Success,
    /// Something the user should look at before continuing.
    Warning,
    /// Something broken, or something about to be destroyed.
    Danger,
}

impl Intent for Tone {
    fn token(self) -> &'static str {
        match self {
            // Neutral has no status token of its own. It takes the muted
            // content intent, which is what both webview apps already spell as
            // `data-color="muted"`.
            Self::Neutral => "content-muted",
            Self::Info => "info",
            Self::Success => "success",
            Self::Warning => "warning",
            Self::Danger => "danger",
        }
    }
}

/// A small labelled thing that sits inside something else.
///
/// Two members, because the three apps drew three taxonomies and only one line
/// runs through all of them: does it answer a click. audiofiles has
/// `classification_badge` (a label) against `tag_chip`, `tag_chip_removable`
/// and `selectable_tag` (all of which do). Balanced Breakfast has `.tag` and
/// `.badge` against `.tag-chip`. goingson is the one that has to move: its
/// `.tag` and `.badge` are a single CSS rule, so every call site has to be read
/// to decide which of the two it always was.
///
/// The evidence that a chip is a real concept rather than a badge with a
/// cursor: audiofiles inverts its bevel on press and Balanced Breakfast latches
/// `.tag-chip.active` with the inset bevel. Two independent arrivals at "a chip
/// holds itself down", which is exactly what [`Depth::pressed`] already says.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub enum Token {
    /// Non-interactive status or count. Answers no click.
    Badge,
    /// An interactive or removable token. Answers a click, and latches if it
    /// stands for a filter that is either on or off.
    Chip {
        /// Whether it carries its own remove affordance.
        removable: bool,
    },
}

impl Token {
    /// Whether this answers a click.
    ///
    /// The whole difference between the two members, and the reason a renderer
    /// with no hover (a touch surface, a terminal) can still tell them apart.
    #[must_use]
    pub const fn interactive(self) -> bool {
        matches!(self, Self::Chip { .. })
    }

    /// How it sits, given whether it is currently latched down.
    ///
    /// A badge is flat: it is a label, and giving it an edge would say it can
    /// be pressed. A chip is raised, and inset while latched.
    #[must_use]
    pub const fn depth(self, latched: bool) -> Depth {
        match self {
            Self::Badge => Depth::Flat,
            Self::Chip { .. } if latched => Depth::Well,
            Self::Chip { .. } => Depth::Raised,
        }
    }
}

/// Something the app is telling the user, unprompted.
///
/// Two concepts, not one with a placement. They differ in more than where they
/// sit: a toast is transient, stacked and self-dismissing, and a banner is
/// persistent, in flow, one per region, and dismissed by fixing the condition
/// it reports. Folding them into one member with a placement parameter would
/// make lifetime, stacking and dismissal all placement-dependent, which is the
/// description leaking renderer policy.
///
/// All three apps have banners: `info_banner` and `warning_banner` in
/// audiofiles, five of them in goingson (sync, sync-result, vacation-day,
/// timer-active, past-review), `.update-banner` in Balanced Breakfast. The two
/// webview apps also have toasts. So neither member is speculative, and no app
/// gains a concept it lacks except audiofiles, whose renderer may legitimately
/// decline to draw a toast at all.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub enum Notice {
    /// Transient, stacked, dismisses itself.
    Toast,
    /// Persistent, in flow, one per region, dismissed by fixing the cause.
    Banner,
}

impl Notice {
    /// Whether it goes away on its own.
    #[must_use]
    pub const fn transient(self) -> bool {
        matches!(self, Self::Toast)
    }

    /// How it sits.
    ///
    /// A toast floats above the page rather than resting on it, which is
    /// [`Fill::Overlay`]'s whole reason to exist. A banner is a card in the
    /// flow. Both are raised, and they are raised off different things.
    #[must_use]
    pub const fn fill(self) -> Fill {
        match self {
            Self::Toast => Fill::Overlay,
            Self::Banner => Fill::Raised,
        }
    }
}

/// The parts of a list row.
///
/// Four to begin with, taken from Balanced Breakfast, which was the only
/// consumer that had all of them (`row-primary`, `row-secondary`, `row-meta`,
/// `row-actions`). audiofiles has two and no slot structure at all, so it gains
/// meta and actions as real work rather than a rename; goingson moves off
/// `task-row` / `task-cell`.
///
/// [`Tokens`](Self::Tokens) joined at 0.9.0, and `#[non_exhaustive]` with it.
/// See the crate header for why the two arrived together.
///
/// # Meta against Tokens
///
/// The line is whether the thing has its own standing. `Meta` is one short
/// trailing fact about the row, written as text: a count, a size, a date.
/// `Tokens` is a set of small labelled things, each of which can be toned and
/// can answer a click. "3 files" is meta. A status badge that is amber, and a
/// tag you can click to filter by, are tokens.
///
/// Keeping them apart is what a single widened slot would have foreclosed. A
/// renderer can right-align one string and cannot usefully do the same to a
/// strip of chips, and a fact that is not clickable should not be drawn as
/// though it were.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
#[non_exhaustive]
pub enum RowPart {
    /// The thing itself. What the row is called.
    Primary,
    /// Supporting text under the primary.
    Secondary,
    /// A short trailing fact: a count, a size, a date.
    Meta,
    /// Controls that act on this row.
    Actions,
    /// Small labelled things belonging to the row: badges, chips, tags.
    ///
    /// Each carries its own [`Token`] kind and [`Tone`], so a renderer with no
    /// colour still has the kind to work with, and one with no chips still has
    /// the label. That is the constrained-consumer test this vocabulary exists
    /// to pass, and it is why the tone lives on the token rather than on the
    /// part.
    Tokens,
}

impl RowPart {
    /// Whether the part stays hidden until the row is hovered or focused.
    ///
    /// Behaviour of the part, not app policy: Balanced Breakfast and goingson
    /// grew the same hover-reveal on their actions independently and neither
    /// applies it to anything else.
    ///
    /// A renderer with no hover shows it always. That is a renderer decision
    /// and this returning `true` does not forbid it.
    #[must_use]
    pub const fn revealed_on_hover(self) -> bool {
        matches!(self, Self::Actions)
    }

    /// The content intent the part takes.
    #[must_use]
    pub const fn intent(self) -> &'static str {
        match self {
            Self::Primary => "content",
            Self::Secondary => "content-secondary",
            Self::Meta => "content-muted",
            // Actions carry controls rather than text, so they inherit.
            Self::Actions => "content",
            // So do tokens: each one carries its own tone, and a part-level
            // intent underneath it would fight the token that sits on it.
            Self::Tokens => "content",
        }
    }
}

/// How far down the heading tree a title sits.
///
/// Three, and only the three that are actually headings. The bands those used
/// to be filed with (goingson's `.page-header`, Balanced Breakfast's `.header`
/// and `.detail-header`) are arrangement, not type, and live at
/// [`Region::Band`]. One of them contains no text at all.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub enum Heading {
    /// Names the whole screen. One per screen.
    Page,
    /// Names a block within the screen.
    Section,
    /// Names a sub-block inside an already-named section.
    Subsection,
}

impl Heading {
    /// Whether a rule follows the heading.
    ///
    /// audiofiles' `section_header` draws a separator and its
    /// `subsection_label` deliberately does not, which is the only thing
    /// distinguishing the two once weight and colour are deferred.
    #[must_use]
    pub const fn separated(self) -> bool {
        matches!(self, Self::Section)
    }
}

/// A control that picks between things.
///
/// Three, because three distinct behaviours are in play and collapsing any two
/// loses something. A segmented control picks a value; a tab picks a pane; a
/// toggle picks nothing and simply holds itself on or off.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub enum Selector {
    /// Exactly one of N, and the options abut.
    Segmented,
    /// Independent on or off, on its own.
    Toggle,
    /// Navigation between panes. The folder semantic.
    Tabs,
}

impl Selector {
    /// How the chosen option sits.
    ///
    /// Held in for a segmented control and a toggle, which is the same shape
    /// pressing produces and the whole economy of the idiom: one appearance,
    /// two reasons to wear it. A tab is the exception, because the selected
    /// folder tab comes *forward* to join the pane it opens.
    #[must_use]
    pub const fn chosen(self) -> Depth {
        match self {
            Self::Segmented | Self::Toggle => Depth::Well,
            Self::Tabs => Depth::Raised,
        }
    }

    /// How the options that were *not* picked sit.
    ///
    /// Added 0.3.0. Describing only [`Selector::chosen`] left the unchosen
    /// option falling through to [`Depth::Flat`], which says it is level with
    /// the strip it sits in, and no renderer emitted anything for it. That is
    /// wrong in both directions and goingson proved it: its unchosen tabs are
    /// recessed by hand, and being recessed is *why* the chosen one reads as
    /// coming forward. Against a flat strip, a raised chosen tab is a bevel
    /// drawn on the strip's own colour, which is a much weaker folder effect
    /// than the contrast the idiom is named after.
    ///
    /// Each member is the inverse of its chosen state, which is the whole
    /// content of "picked" once colour is deferred:
    ///
    /// - Tabs recede, so the chosen one comes forward.
    /// - A segment and a toggle stand up, so the chosen one is held in.
    #[must_use]
    pub const fn unchosen(self) -> Depth {
        match self {
            Self::Tabs => Depth::Sunken,
            Self::Segmented | Self::Toggle => Depth::Raised,
        }
    }

    /// Whether the options touch.
    ///
    /// The gap is the entire difference between a segmented control and a row
    /// of buttons that happen to sit near each other, which is what audiofiles'
    /// `segmented_control` says in its own comment and why it zeroes the
    /// spacing by hand.
    #[must_use]
    pub const fn abutting(self) -> bool {
        matches!(self, Self::Segmented | Self::Tabs)
    }
}

/// Whether the content of a region has arrived.
///
/// The state, not the shimmer. Whether pending paints a skeleton, a spinner or
/// nothing at all is renderer policy, the same class of decision that got
/// `Fill::fallback` deleted from this crate. goingson and Balanced Breakfast
/// each grew a skeleton with differently-named parts; both keep them, as the
/// webview renderer's expression of [`Readiness::Pending`]. audiofiles has none
/// and needs none, because an immediate-mode renderer simply repaints.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub enum Readiness {
    /// The content is here.
    Ready,
    /// The content is on its way.
    Pending,
}

/// A named part of a screen.
///
/// The thing `makeover-geometry` deliberately does not name: it names the space
/// *between* things by relationship, and nothing named the things. Six named
/// members, taken from what the two webview apps actually use, plus
/// [`Region::Bespoke`] for the parts no description should reach. Both apps'
/// `layout.css` currently names exactly two things, `.raised` and `.well`, so
/// this layer is absent rather than divergent, which makes it the cheapest of
/// the schemas to add and the easiest to over-build.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub enum Region<'a> {
    /// A full-width strip with a title slot and an actions cluster, either of
    /// which may be empty. goingson's `.page-header`, Balanced Breakfast's
    /// `.header` and `.detail-header` are all this, differing only in which
    /// slots they fill.
    Band,
    /// A persistent column beside the content, holding navigation.
    Sidebar,
    /// A region of content with its own scroll.
    Pane,
    /// Two panes side by side, where the left chooses what the right shows.
    Split,
    /// A set of panes, one visible at a time, with a [`Selector::Tabs`] above.
    TabGroup,
    /// Content over a scrim, taking input until dismissed.
    Modal,
    /// A region this crate names the *place* of and nothing else. The app owns
    /// what goes in it.
    ///
    /// The escape hatch, and the thing that keeps the description honest about
    /// its own limits. A day-plan timeline, a kanban board, a calendar and the
    /// paint interaction over the timeline are not describable here and are not
    /// going to become describable: a description expressive enough to produce
    /// a timeline is a widget library wearing a description's name.
    ///
    /// But a screen containing one still has to be a screen. Without this
    /// member the description covers only the boring screens, and the four that
    /// make goingson worth using would need a second, undescribed path beside
    /// the router. Two paths is how the vocabulary starts drifting from the app
    /// again, which is the exact failure this crate exists to end.
    ///
    /// So the description says "a thing called `day-plan` goes here" and stops.
    /// The name is opaque: this crate never interprets it, and no renderer is
    /// expected to know what it means beyond handing the space over.
    Bespoke {
        /// What the app calls it. Never interpreted here.
        name: &'a str,
    },
}

impl Region<'_> {
    /// How the region sits on what is behind it.
    #[must_use]
    pub const fn depth(self) -> Depth {
        match self {
            Self::Band | Self::Sidebar | Self::Split | Self::TabGroup => Depth::Flat,
            // A pane is looked into, the same as a table body or a tag tree.
            Self::Pane => Depth::Well,
            Self::Modal => Depth::Raised,
            // Flat because it inherits: a bespoke region takes the depth of
            // whatever frames it. An app that wants its timeline in a well puts
            // it in a `Pane`, which composes rather than adding a knob here.
            Self::Bespoke { .. } => Depth::Flat,
        }
    }

    /// Whether this crate can say anything about the region's contents.
    ///
    /// A renderer walks the description and hands every region it understands
    /// to the right drawing code. This is how it tells the two apart, and the
    /// reason it is a method rather than a `matches!` at each renderer: there
    /// is exactly one opaque member and there should stay exactly one.
    #[must_use]
    pub const fn described(self) -> bool {
        !matches!(self, Self::Bespoke { .. })
    }
}

/// How a screen is laid out.
///
/// Two, and the second is not a variant of the first. goingson is list-detail,
/// Balanced Breakfast is sidebar plus content, and neither app has a third.
/// The tab group is a modifier rather than a member, because goingson uses it
/// *inside* the same content region rather than instead of one.
///
/// This exists at all because the router has to be able to express a screen
/// rather than only a control. Discovering the arrangement layer missing after
/// the renderers exist is a redesign; naming two now is a morning.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub enum Arrangement {
    /// A list that chooses what the detail beside it shows.
    ListDetail {
        /// Whether the detail side is a [`Region::TabGroup`].
        tabbed: bool,
    },
    /// Navigation down the side, content filling the rest.
    SidebarContent,
}

/// What kind of value a form field takes.
///
/// The union of the two vocabularies that diverged, which is what triggered
/// this crate. They have since converged on their own: both apps now have a
/// `renderFormField` emitting the same anatomy, and what is left differing is
/// the kind set, the error shape, and whether the return is a string or a node.
///
/// Validation is deliberately absent. Neither app has a shared story (goingson
/// validates after collecting the form data, with per-field transform hooks;
/// Balanced Breakfast has `required` and nothing else), and a schema that
/// describes fields but not constraints acquires a constraint layer per app,
/// which is exactly how the current divergence started. Naming it absent is a
/// decision; leaving it unmentioned would not be.
/// `#[non_exhaustive]` for the reason [`Fill`] is: renderers match on this and
/// the set keeps growing, so growth must not be a lockstep event. Email, Url
/// and Tel arriving in 0.5.0 is the second growth in two releases.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
#[non_exhaustive]
pub enum FieldKind {
    /// A single line of text.
    Text,
    /// A single line of text that must never be echoed, logged or round-tripped
    /// through anything that might persist it.
    Secret,
    /// A number.
    Number,
    /// An email address.
    ///
    /// Distinct from [`Text`](Self::Text) because the distinction is not
    /// decoration: a webview renderer emits `type="email"`, which on a touch
    /// device changes the keyboard that appears and turns on the platform's own
    /// validation. goingson ships to iOS, so collapsing this into text costs a
    /// keyboard with no `@` on it.
    ///
    /// Added 0.5.0, from goingson's contact form.
    Email,
    /// A URL. Same reasoning as [`Email`](Self::Email).
    ///
    /// Added 0.5.0, from goingson's contact-social and contact-feed forms.
    Url,
    /// A telephone number. Same reasoning as [`Email`](Self::Email), and the
    /// clearest case of it: the keyboard is a numeric pad rather than letters.
    ///
    /// Added 0.5.0, from goingson's contact-phone form.
    Tel,
    /// Several lines of text.
    Textarea,
    /// One of a fixed set, offered behind a control that shows one at a time.
    Select,
    /// One of a fixed set, with every option on screen at once.
    ///
    /// Not a presentation of [`Select`](Self::Select), which is the reading to
    /// resist: what differs is a property of the *question*. A choice that is
    /// consequential or irreversible has to be readable without opening
    /// anything, because a closed control shows one option and hides the rest,
    /// and the one it shows is whichever was current before the user had read
    /// the alternatives. audiofiles asks whether a library copies samples into
    /// its store or references them where they lie — which cannot be changed
    /// afterwards — and had already promoted that out of a checkbox by hand,
    /// with a comment giving this reason, before the description could say it.
    ///
    /// It is also the one HTML input type this enum was missing. Everything
    /// else here is an `<input type=...>`, a `<select>` or a `<textarea>`, and
    /// the hole was `radio`.
    ///
    /// Added 0.8.1, from audiofiles' Add Library form.
    Radio,
    /// On or off.
    Checkbox,
    /// Carried through the form and never shown.
    Hidden,
}

impl FieldKind {
    /// Whether the field is drawn at all.
    #[must_use]
    pub const fn visible(self) -> bool {
        !matches!(self, Self::Hidden)
    }

    /// Whether the value must be kept out of logs and diagnostics.
    #[must_use]
    pub const fn confidential(self) -> bool {
        matches!(self, Self::Secret)
    }

    /// Where the field's own label sits.
    ///
    /// A checkbox labels itself on the right of the box; everything else takes
    /// a label above. Both webview apps already do this and both special-case
    /// it inline, which is the tell that it belongs in the description.
    ///
    /// A [`Radio`](Self::Radio) is not one of them, and the near-miss is worth
    /// naming: its *options* each label themselves, but the field still asks a
    /// question above them, so the group takes a label like everything else.
    #[must_use]
    pub const fn labels_itself(self) -> bool {
        matches!(self, Self::Checkbox)
    }

    /// Whether the kind reads [`Field::options`].
    ///
    /// Two kinds do, so the pair is named once here rather than spelled out at
    /// each renderer and again in [`Field::options`]' own doc, where "every
    /// kind but `Select`" was true for exactly one release. A third
    /// option-taking kind should land here and nowhere else.
    #[must_use]
    pub const fn offers_options(self) -> bool {
        matches!(self, Self::Select | Self::Radio)
    }
}

/// One option offered by a field [`FieldKind::offers_options`] accepts.
///
/// Two strings, because the submitted value and the read label are different
/// facts and every renderer that has tried to collapse them has had to
/// un-collapse them later. `makeover-webview` invented this shape writing its
/// form emitter and it is taken here unchanged; moving it down rather than
/// re-deriving it is the point, since the second and third renderers were each
/// going to arrive at a near-miss of it.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub struct Choice<'a> {
    /// What is submitted.
    pub value: &'a str,
    /// What is read.
    pub label: &'a str,
}

impl<'a> Choice<'a> {
    /// An option whose submitted value is also its label.
    #[must_use]
    pub const fn plain(value: &'a str) -> Self {
        Self {
            value,
            label: value,
        }
    }
}

/// One field of a form.
///
/// Borrowed rather than owned: a description is built, read once by a renderer,
/// and dropped. Nothing here outlives the screen it describes.
///
/// # What it carries, and what it does not
///
/// Stated here so the next renderer does not re-ask, which is what the first
/// two both did. It carries everything a renderer needs to *draw* the field:
/// its kind, what it is called, what it is asked for, its standing help, what
/// is wrong with it now, whether it is compulsory, whether it hides behind a
/// disclosure, its ghost text, and the options it offers.
///
/// It does not carry the **current value**, and it is not going to. That is the
/// one thing here that is genuinely renderer state: a webview reads it back out
/// of the DOM, an immediate-mode renderer holds a `&mut` to the app's own field
/// and writes through it, and a terminal keeps an edit buffer. A description
/// that carried the value would have to carry a way to write it back, at which
/// point it is a form model and no longer a description.
///
/// **Validation** is absent for the reason [`FieldKind`] records: [`error`] is
/// the *result* of validating, supplied by whoever validated. Nothing here
/// decides that a value is wrong.
///
/// [`error`]: Field::error
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub struct Field<'a> {
    /// What kind of value it takes.
    pub kind: FieldKind,
    /// The name the value is submitted under.
    pub name: &'a str,
    /// What the user is asked for.
    pub label: &'a str,
    /// Standing help, shown whether or not anything is wrong.
    pub hint: Option<&'a str>,
    /// What is currently wrong with the value.
    pub error: Option<&'a str>,
    /// Ghost text shown while the field is empty.
    ///
    /// User-facing text, and it sits with `label` and `hint` rather than with
    /// the value because it is a property of the *question* and not of the
    /// answer. It lived renderer-side in `makeover-webview` until 0.8.0 for one
    /// reason and it was not a reading on where it belonged: adding a field to
    /// a published struct is a breaking change.
    ///
    /// Not a substitute for a label. A field labelled only by its placeholder
    /// loses its label the moment anything is typed, and no renderer here can
    /// make that not happen, so the description keeps both.
    pub placeholder: Option<&'a str>,
    /// The options offered, in the order they are offered.
    ///
    /// Empty for every kind [`FieldKind::offers_options`] rejects. A field
    /// described with no options is sayable on purpose: it is what an app with
    /// an unfinished-loading option list actually has, and a renderer showing
    /// an empty control says so on screen rather than in a log.
    ///
    /// Which option is *current* is not here. That is the value, and the value
    /// is renderer state.
    pub options: &'a [Choice<'a>],
    /// Whether the form refuses to submit without it.
    pub required: bool,
    /// Whether the field lives behind a "more options" disclosure.
    pub extended: bool,
}

impl<'a> Field<'a> {
    /// A plain required-nothing field of the given kind.
    #[must_use]
    pub const fn new(kind: FieldKind, name: &'a str, label: &'a str) -> Self {
        Self {
            kind,
            name,
            label,
            hint: None,
            error: None,
            placeholder: None,
            options: &[],
            required: false,
            extended: false,
        }
    }

    /// A select offering the given options.
    ///
    /// One of the two kinds under-described by [`Field::new`], so it gets a
    /// constructor rather than leaving every call site to remember that a
    /// select with an empty `options` renders as an empty select.
    #[must_use]
    pub const fn select(name: &'a str, label: &'a str, options: &'a [Choice<'a>]) -> Self {
        Self::offering(FieldKind::Select, name, label, options)
    }

    /// A radio group offering the given options.
    ///
    /// The other. Same hazard as [`select`](Self::select) and a worse one: a
    /// radio group with no options draws nothing at all, so a call site that
    /// forgot them has an empty rectangle rather than a visibly empty control.
    #[must_use]
    pub const fn radio(name: &'a str, label: &'a str, options: &'a [Choice<'a>]) -> Self {
        Self::offering(FieldKind::Radio, name, label, options)
    }

    /// The shared body of the two constructors that take options.
    ///
    /// Private, and keyed on the kind rather than exposed, because the two
    /// public names are the point: a call site says which question it is
    /// asking, not which flag it is setting.
    const fn offering(
        kind: FieldKind,
        name: &'a str,
        label: &'a str,
        options: &'a [Choice<'a>],
    ) -> Self {
        Self {
            options,
            ..Self::new(kind, name, label)
        }
    }

    /// Whether the field is currently reporting a problem.
    ///
    /// Read this rather than testing `error.is_some()` at each renderer: the
    /// error state has to mark the field's whole group and not only the
    /// message, because a renderer with no descendant selectors (egui, a
    /// terminal) cannot find the group from the message. goingson already marks
    /// the group and Balanced Breakfast does not, so goingson's shape is the
    /// one taken here.
    #[must_use]
    pub const fn invalid(&self) -> bool {
        self.error.is_some()
    }
}

/// How much room a column asks for.
///
/// An intent, so the actual floor stays with `makeover-geometry`. goingson's
/// task table spells these as `minmax(200px, 1fr)`, `140px` and content-sized;
/// only the first three words of that survive deferral.
/// `#[non_exhaustive]`, for the reason [`Fill`] and [`FieldKind`] are: a
/// renderer matches on this and a vocabulary that grows must not break every
/// renderer when it does.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
#[non_exhaustive]
pub enum Width {
    /// Takes what it needs and no more.
    Content,
    /// A fixed share, the same at every width.
    Fixed,
    /// Absorbs whatever is left over.
    Fill,
}

/// What a column is worth when there is not room for all of them.
///
/// Ordered: [`Priority::Optional`] drops first, [`Priority::Essential`] never
/// drops. This replaces addressing columns by position, which is what both
/// webview apps do today and is a live bug rather than only verbosity. goingson
/// hides mobile columns with `nth-child(n+5)` against a seven-column table, so
/// inserting a column silently hides the wrong one.
/// `#[non_exhaustive]`, same reasoning as [`Width`]. Note the ordering is the
/// whole point of the type, so a new tier has to be declared in its place in
/// the sequence rather than appended.
#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
#[non_exhaustive]
pub enum Priority {
    /// Dropped first.
    Optional,
    /// Dropped once the optional columns are gone.
    Secondary,
    /// Never dropped. Without it the row does not identify itself.
    Essential,
}

/// One column of a table.
///
/// Described once. The grid track, the cell order and the drop behaviour are
/// all derived from this, rather than being three hand-written encodings that
/// must agree and are never checked against each other.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub struct Column<'a> {
    /// The heading, and the name the cell is addressed by.
    pub name: &'a str,
    /// How much room it asks for.
    pub width: Width,
    /// What it is worth when room runs out.
    pub priority: Priority,
}

impl<'a> Column<'a> {
    /// A column that absorbs slack and drops after the optional ones.
    #[must_use]
    pub const fn new(name: &'a str) -> Self {
        Self {
            name,
            width: Width::Fill,
            priority: Priority::Secondary,
        }
    }

    /// Whether this column survives at the given cutoff.
    ///
    /// A renderer narrows by raising the cutoff, and never by counting
    /// positions.
    #[must_use]
    pub const fn kept_at(&self, cutoff: Priority) -> bool {
        (self.priority as u8) >= (cutoff as u8)
    }
}

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

    #[test]
    fn inset_is_raised_with_the_light_moved() {
        let (rl, rd) = Bevel::Raised.edges();
        let (il, id) = Bevel::Inset.edges();
        assert_eq!((rl, rd), (Edge::Light, Edge::Dark));
        assert_eq!((il, id), (rd, rl));
    }

    #[test]
    fn pressing_twice_is_a_no_op() {
        for b in [Bevel::Raised, Bevel::Inset] {
            assert_eq!(b.pressed().pressed(), b);
        }
    }

    #[test]
    fn a_raised_region_is_never_filled_with_a_recessed_surface() {
        // The bug this vocabulary exists to make unrepresentable.
        assert_eq!(Depth::Raised.fill(), Some(Fill::Raised));
        assert_eq!(Depth::Raised.bevel(), Some(Bevel::Raised));
        assert_eq!(Depth::Well.bevel(), Some(Bevel::Inset));
        assert_ne!(Depth::Well.fill(), Depth::Raised.fill());
    }

    #[test]
    fn state_is_orthogonal_to_depth() {
        // The reason State is its own axis and not a Depth member: a disabled
        // button and a disabled field are both disabled and are not the same
        // shape, which one shared variant could not have said.
        assert_eq!(Depth::Raised.fill(), Some(Fill::Raised));
        assert_eq!(Depth::Well.fill(), Some(Fill::Well));
        assert!(State::Disabled.suppresses_interaction());
    }

    #[test]
    fn only_disabled_stops_answering() {
        // Focus is a thing you can still click. Getting this backwards is how
        // a focus ring ends up on something inert.
        assert!(!State::Focus.suppresses_interaction());
        assert!(State::Disabled.suppresses_interaction());
    }

    #[test]
    fn both_states_resolve_against_intents_makeover_already_derives() {
        // Neither needs a new token, so this costs no `makeover` release.
        assert_eq!(State::Focus.token(), "focus-ring");
        assert_eq!(State::Disabled.token(), "content-muted");
    }

    #[test]
    fn flat_has_neither_edge_nor_fill() {
        assert_eq!(Depth::Flat.bevel(), None);
        assert_eq!(Depth::Flat.fill(), None);
    }

    #[test]
    fn sunken_is_recessed_by_colour_with_no_edge() {
        // The one member carrying a fill without a bevel. A renderer that
        // assumes the two arrive together drops the fill silently, which is
        // exactly what makeover-webview did before 0.3.0.
        assert_eq!(Depth::Sunken.fill(), Some(Fill::Sunken));
        assert_eq!(Depth::Sunken.bevel(), None);
    }

    #[test]
    fn sunken_and_flat_are_different_claims() {
        // Both edgeless, and only one of them needs a colour. Collapsing them
        // is what left an unchosen tab unsayable.
        assert_eq!(Depth::Flat.bevel(), Depth::Sunken.bevel());
        assert_ne!(Depth::Flat.fill(), Depth::Sunken.fill());
    }

    #[test]
    fn a_sunken_surface_is_not_a_well() {
        // Authored in opposite directions: makeover derives surface-well by
        // inverting against the theme's content colour, while surface-sunken is
        // authored and may sit darker than raised.
        assert_ne!(Fill::Sunken, Fill::Well);
        assert_eq!(Fill::Sunken.token(), "surface-sunken");
        assert_eq!(Fill::Well.token(), "surface-well");
    }

    #[test]
    fn every_selector_describes_both_of_its_states() {
        // The gap 0.3.0 closed. Before it, only `chosen` existed and the
        // unchosen option fell through to Flat at every renderer.
        for s in [Selector::Tabs, Selector::Segmented, Selector::Toggle] {
            assert_ne!(
                s.chosen(),
                s.unchosen(),
                "{s:?} cannot tell picked from unpicked"
            );
        }
    }

    #[test]
    fn only_a_tab_inverts_the_other_way() {
        // Tabs recede so the chosen one comes forward; a segment and a toggle
        // stand up so the chosen one is held in. That inversion is the whole
        // content of "picked" once colour is deferred, and it is why the three
        // are not one member with a flag.
        assert_eq!(Selector::Tabs.unchosen(), Depth::Sunken);
        assert_eq!(Selector::Tabs.chosen(), Depth::Raised);

        for s in [Selector::Segmented, Selector::Toggle] {
            assert_eq!(s.unchosen(), Depth::Raised);
            assert_eq!(s.chosen(), Depth::Well);
            // Held in is what pressing produces: one appearance, two reasons.
            assert_eq!(s.unchosen().pressed(), s.chosen());
        }
    }

    #[test]
    fn pressing_a_card_makes_a_well() {
        assert_eq!(Depth::Raised.pressed(), Depth::Well);
        assert_eq!(
            Depth::Raised.pressed().bevel(),
            Depth::Raised.bevel().map(Bevel::pressed)
        );
        // Only raised regions respond to being pressed.
        assert_eq!(Depth::Flat.pressed(), Depth::Flat);
        assert_eq!(Depth::Well.pressed(), Depth::Well);
    }

    #[test]
    fn intents_name_makeover_tokens_and_nothing_else() {
        assert_eq!(Edge::Light.token(), "bevel-light");
        assert_eq!(Edge::Dark.token(), "bevel-dark");
        assert_eq!(Fill::Raised.token(), "surface-raised");
        assert_eq!(Fill::Well.token(), "surface-well");
        // No value ever leaves this crate.
        for t in [
            Edge::Light.token(),
            Edge::Dark.token(),
            Tone::Danger.token(),
            Tone::Neutral.token(),
            State::Focus.token(),
            State::Disabled.token(),
        ] {
            assert!(!t.starts_with('#'), "{t} looks like a value");
            assert!(
                !t.chars().next().unwrap().is_ascii_digit(),
                "{t} is a value"
            );
        }
    }

    #[test]
    fn a_badge_cannot_be_pressed_and_a_chip_latches() {
        // The one line that runs through all three apps' taxonomies.
        assert!(!Token::Badge.interactive());
        assert!(Token::Chip { removable: false }.interactive());
        assert!(Token::Chip { removable: true }.interactive());

        // A badge is a label, so giving it an edge would lie about it.
        assert_eq!(Token::Badge.depth(false), Depth::Flat);
        assert_eq!(Token::Badge.depth(true), Depth::Flat);

        // A latched chip wears the same shape a pressed one does.
        let chip = Token::Chip { removable: false };
        assert_eq!(chip.depth(false), Depth::Raised);
        assert_eq!(chip.depth(true), Depth::Raised.pressed());
    }

    #[test]
    fn a_toast_and_a_banner_differ_in_more_than_placement() {
        assert!(Notice::Toast.transient());
        assert!(!Notice::Banner.transient());
        // A toast floats above the page; a banner rests in the flow.
        assert_eq!(Notice::Toast.fill(), Fill::Overlay);
        assert_eq!(Notice::Banner.fill(), Fill::Raised);
    }

    #[test]
    fn only_the_actions_part_hides_until_hovered() {
        for p in [
            RowPart::Primary,
            RowPart::Secondary,
            RowPart::Meta,
            RowPart::Tokens,
        ] {
            assert!(!p.revealed_on_hover(), "{p:?} should always be visible");
        }
        assert!(RowPart::Actions.revealed_on_hover());
        // Emphasis falls off down the row, and never rises again.
        assert_eq!(RowPart::Primary.intent(), "content");
        assert_eq!(RowPart::Secondary.intent(), "content-secondary");
        assert_eq!(RowPart::Meta.intent(), "content-muted");
    }

    #[test]
    fn a_token_part_carries_no_intent_of_its_own() {
        // Each token carries its own tone, so a part-level intent underneath
        // would fight the thing sitting on it. Same reasoning as actions, which
        // is why they answer alike.
        assert_eq!(RowPart::Tokens.intent(), RowPart::Actions.intent());
        assert_eq!(RowPart::Tokens.intent(), "content");
    }

    #[test]
    fn a_separator_is_what_tells_a_section_from_a_subsection() {
        assert!(Heading::Section.separated());
        assert!(!Heading::Subsection.separated());
        assert!(!Heading::Page.separated());
    }

    #[test]
    fn a_chosen_segment_is_held_in_and_a_chosen_tab_comes_forward() {
        assert_eq!(Selector::Segmented.chosen(), Depth::Well);
        assert_eq!(Selector::Toggle.chosen(), Depth::Well);
        // The exception, and the whole folder semantic: the open tab joins its
        // pane rather than sinking away from it.
        assert_eq!(Selector::Tabs.chosen(), Depth::Raised);

        // A held-in segment is indistinguishable from a pressed raised one,
        // which is the economy the light model buys over a colour swap.
        assert_eq!(Selector::Segmented.chosen(), Depth::Raised.pressed());

        // A toggle stands alone; the other two are built out of parts that
        // touch.
        assert!(Selector::Segmented.abutting());
        assert!(Selector::Tabs.abutting());
        assert!(!Selector::Toggle.abutting());
    }

    #[test]
    fn a_pane_is_looked_into_and_a_band_is_not() {
        assert_eq!(Region::Pane.depth(), Depth::Well);
        assert_eq!(Region::Modal.depth(), Depth::Raised);
        for r in [
            Region::Band,
            Region::Sidebar,
            Region::Split,
            Region::TabGroup,
        ] {
            assert_eq!(r.depth(), Depth::Flat, "{r:?} should carry no edge");
        }
    }

    #[test]
    fn exactly_one_region_is_opaque() {
        // The escape hatch is one member and stays one member. If a second
        // undescribed region ever appears, the description has started
        // conceding rather than deferring.
        for r in [
            Region::Band,
            Region::Sidebar,
            Region::Pane,
            Region::Split,
            Region::TabGroup,
            Region::Modal,
        ] {
            assert!(r.described(), "{r:?} should be describable");
        }
        assert!(!Region::Bespoke { name: "day-plan" }.described());
    }

    #[test]
    fn a_bespoke_region_inherits_its_depth_rather_than_choosing_one() {
        // The app owns the contents, not the placement. An app that wants its
        // timeline in a well frames it in a Pane.
        assert_eq!(Region::Bespoke { name: "day-plan" }.depth(), Depth::Flat);
        assert_eq!(Region::Bespoke { name: "kanban" }.depth(), Depth::Flat);
    }

    #[test]
    fn a_screen_with_a_bespoke_region_is_still_a_whole_screen() {
        // The argument the member exists for: goingson's day-plan has to be
        // routable, or the description covers only the boring screens and the
        // interesting four need a second path beside the router.
        let day_plan = [
            Region::Band,
            Region::Bespoke { name: "day-plan" },
            Region::Sidebar,
        ];
        assert_eq!(day_plan.iter().filter(|r| r.described()).count(), 2);
        assert_eq!(day_plan.iter().filter(|r| !r.described()).count(), 1);
    }

    #[test]
    fn a_secret_field_is_marked_as_one_and_a_hidden_field_is_not_drawn() {
        let secret = Field::new(FieldKind::Secret, "password", "Password");
        assert!(secret.kind.confidential());
        assert!(secret.kind.visible());

        assert!(!FieldKind::Hidden.visible());
        // Nothing else is confidential, or the marker means nothing.
        for k in [
            FieldKind::Text,
            FieldKind::Number,
            FieldKind::Textarea,
            FieldKind::Select,
            FieldKind::Checkbox,
            FieldKind::Hidden,
        ] {
            assert!(!k.confidential(), "{k:?} should not be confidential");
        }

        // Only a checkbox carries its own label.
        assert!(FieldKind::Checkbox.labels_itself());
        assert!(!FieldKind::Text.labels_itself());
    }

    #[test]
    fn a_plain_field_offers_nothing_and_a_select_offers_its_options() {
        let text = Field::new(FieldKind::Text, "title", "Title");
        assert!(text.options.is_empty());
        assert_eq!(text.placeholder, None);

        let sizes = [Choice::plain("small"), Choice::plain("large")];
        let select = Field::select("size", "Size", &sizes);
        assert_eq!(select.kind, FieldKind::Select);
        assert_eq!(select.options.len(), 2);
    }

    #[test]
    fn a_choice_says_what_submits_and_what_is_read_apart() {
        // The whole reason it is two strings. `plain` is the case where they
        // coincide, and it is a shorthand rather than the general shape.
        let plain = Choice::plain("7");
        assert_eq!((plain.value, plain.label), ("7", "7"));

        let spelled = Choice {
            value: "7",
            label: "One week",
        };
        assert_ne!(spelled.value, spelled.label);
    }

    #[test]
    fn a_radio_asks_the_same_question_as_a_select_and_is_not_the_same_kind() {
        // Both offer a fixed set and both read `options`, so the two
        // constructors differ in exactly one thing. That one thing is the
        // point: a renderer decides whether the alternatives are readable
        // without opening anything, and it can only decide that if the
        // description said which question was asked.
        let styles = [
            Choice {
                value: "copy",
                label: "Copy samples in",
            },
            Choice {
                value: "reference",
                label: "Reference in place",
            },
        ];
        let radio = Field::radio("storage", "Storage style", &styles);
        let select = Field::select("storage", "Storage style", &styles);

        assert_eq!(radio.kind, FieldKind::Radio);
        assert_ne!(radio.kind, select.kind);
        assert_eq!(radio.options, select.options);
        assert_eq!(
            Field {
                kind: select.kind,
                ..radio
            },
            select
        );
    }

    #[test]
    fn exactly_the_option_taking_kinds_say_so() {
        // The renderers branch on this rather than on a list of their own, so
        // a kind added without a decision here renders its options nowhere.
        assert!(FieldKind::Select.offers_options());
        assert!(FieldKind::Radio.offers_options());
        for kind in [
            FieldKind::Text,
            FieldKind::Secret,
            FieldKind::Number,
            FieldKind::Email,
            FieldKind::Url,
            FieldKind::Tel,
            FieldKind::Textarea,
            FieldKind::Checkbox,
            FieldKind::Hidden,
        ] {
            assert!(!kind.offers_options(), "{kind:?} does not offer options");
        }
    }

    #[test]
    fn a_radio_group_takes_a_label_even_though_its_options_carry_their_own() {
        // The near-miss: each option is labelled beside its own button, so a
        // renderer could plausibly read the group as self-labelling and drop
        // the question. Checkbox is the only kind that does that.
        assert!(!FieldKind::Radio.labels_itself());
        assert!(FieldKind::Checkbox.labels_itself());
    }

    #[test]
    fn a_select_with_no_options_is_sayable() {
        // An app whose option list has not loaded has exactly this. Making it
        // unrepresentable would push the state somewhere less visible, and a
        // renderer drawing an empty select reports it on screen.
        let loading = Field::select("project", "Project", &[]);
        assert!(loading.options.is_empty());
    }

    #[test]
    fn the_description_carries_the_question_and_never_the_answer() {
        // The line 0.8.0 drew. Placeholder and options are properties of what
        // is being asked; the current value is what came back, and no field
        // here holds one.
        let f = Field {
            placeholder: Some("yyyy-mm-dd"),
            ..Field::new(FieldKind::Text, "due", "Due")
        };
        assert_eq!(f.placeholder, Some("yyyy-mm-dd"));
        // A placeholder is not a label, and having one does not excuse the
        // field from carrying the other.
        assert_eq!(f.label, "Due");
    }

    #[test]
    fn a_field_reports_its_own_error_state() {
        let mut f = Field::new(FieldKind::Text, "title", "Title");
        assert!(!f.invalid());
        f.error = Some("Required");
        assert!(f.invalid());
    }

    #[test]
    fn columns_drop_by_priority_and_never_by_position() {
        let cols = [
            Column {
                name: "Title",
                width: Width::Fill,
                priority: Priority::Essential,
            },
            Column {
                name: "Due",
                width: Width::Fixed,
                priority: Priority::Secondary,
            },
            Column {
                name: "Estimate",
                width: Width::Fixed,
                priority: Priority::Optional,
            },
        ];

        // Widest: everything survives.
        assert_eq!(
            cols.iter()
                .filter(|c| c.kept_at(Priority::Optional))
                .count(),
            3
        );
        // Narrower: the optional column goes first.
        let kept: Vec<_> = cols
            .iter()
            .filter(|c| c.kept_at(Priority::Secondary))
            .map(|c| c.name)
            .collect();
        assert_eq!(kept, ["Title", "Due"]);
        // Narrowest: only what identifies the row.
        let kept: Vec<_> = cols
            .iter()
            .filter(|c| c.kept_at(Priority::Essential))
            .map(|c| c.name)
            .collect();
        assert_eq!(kept, ["Title"]);
    }

    #[test]
    fn inserting_a_column_does_not_move_what_gets_dropped() {
        // The bug the ordinal form has and this form cannot: goingson hides
        // `nth-child(n+5)` against a seven-column table, so a column inserted
        // anywhere to the left silently hides a different one.
        let before = [
            Column::new("Title"),
            Column {
                name: "Estimate",
                width: Width::Fixed,
                priority: Priority::Optional,
            },
        ];
        let after = [
            Column::new("Title"),
            Column::new("Project"), // inserted
            Column {
                name: "Estimate",
                width: Width::Fixed,
                priority: Priority::Optional,
            },
        ];

        fn dropped<'a>(cols: &[Column<'a>]) -> Vec<&'a str> {
            cols.iter()
                .filter(|c| !c.kept_at(Priority::Secondary))
                .map(|c| c.name)
                .collect()
        }
        assert_eq!(dropped(&before), ["Estimate"]);
        assert_eq!(dropped(&after), ["Estimate"]);
    }

    #[test]
    fn an_arrangement_carries_the_tab_group_as_a_modifier() {
        // goingson uses the tab group inside the content region rather than
        // instead of one, so it is not a third arrangement.
        let go = Arrangement::ListDetail { tabbed: true };
        let plain = Arrangement::ListDetail { tabbed: false };
        assert_ne!(go, plain);
        assert_ne!(go, Arrangement::SidebarContent);
    }

    #[test]
    fn readiness_names_the_state_and_not_the_shimmer() {
        // Two members and no third. If a skeleton ever appears in this enum,
        // the deferral rule has been broken.
        assert_ne!(Readiness::Ready, Readiness::Pending);
    }
}