makeover_layout/

region.rs

use crate::Depth;

// Names this module's prose links to, resolved for rustdoc.
#[allow(unused_imports)]
use crate::{Fill, Heading, Readiness, RowPart, Selector, Sort, Track};

/// 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::Handover`] and [`Region::Ceded`] 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.
///
/// `#[non_exhaustive]`, for [`RowPart`]'s and [`Readiness`]' reason: the member
/// after this one should not be a lockstep event across three renderers.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
#[non_exhaustive]
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,
    /// Things that belong together, and nothing else.
    ///
    /// The block [`Heading::Section`] names, which the vocabulary otherwise
    /// cannot contain. A section heading is a leaf sitting
    /// *beside* the things it names, so nothing said where a section started or
    /// ended and a renderer learned one had ended only because the next heading
    /// arrived.
    ///
    /// # The measurement
    ///
    /// 41 [`Heading::Section`] sites across the ten screens described through
    /// the router, not one of them contained. audiofiles' settings screen is the
    /// clearest: one pane holding a heading, a field, a heading, two toggles, a
    /// heading, a toggle and a heading, which is four sections and no
    /// containers. Under the hand-written CSS the ports are replacing the same
    /// block is spelled `.settings-section` in goingson, `.form-section` and
    /// `.content-section` in the MNW server, `.help-section` in Balanced
    /// Breakfast: three apps, four names, one shape.
    ///
    /// # Why the existing members were the wrong answer
    ///
    /// [`Pane`](Self::Pane) is what apps reach for, and it claims a scroll of its own and
    /// [`Depth::Well`], so four settings groups inside a pane are four wells
    /// inside a well and four scroll contexts. Neither claim is true of a group.
    ///
    /// [`Widget`](Self::Widget) is wrong from the other side. Its own docs say a
    /// widget is never how a primitive gets added by the back door, and a run of
    /// related controls under a heading is furniture any app would have, which
    /// is the generic-against-bespoke bar a primitive has to clear.
    ///
    /// # What it does not carry
    ///
    /// **A heading.** A group usually has one and it is an ordinary node in the
    /// body, the way it already was. A group of related toggles with no heading
    /// is a real thing and a mandatory slot would forbid it.
    ///
    /// **A depth.** [`Depth::Flat`], on [`Handover`](Self::Handover)'s reasoning:
    /// it inherits, and an app that wants its group in a well puts it in a
    /// [`Pane`](Self::Pane), which composes rather than adding a knob here.
    ///
    /// **A colour.** Distinguishing sibling groups by colour is the thing this
    /// member was asked for and it is deliberately not stated here. The
    /// description says these things belong together; which of the theme's
    /// categorical colours a renderer reaches for, and whether it reaches for
    /// one at all, is derived from sibling order at the renderer. A terminal
    /// that tints nothing and separates with a rule is honouring this.
    Group,
    /// Two panes side by side, where the left chooses what the right shows.
    Split,
    /// Peer regions across, all of them equals.
    ///
    /// A kanban board's columns, and the shape [`Split`](Self::Split) is not:
    /// a split's two panes stand in a master-detail relationship, where the
    /// left chooses what the right shows. These choose nothing about each
    /// other. Each is a whole region and the set is the arrangement.
    ///
    /// # What it does not carry
    ///
    /// **How many.** The children say, and a count here would be a second
    /// source for something the description already states by containing them.
    ///
    /// **How wide.** Peers are equal by definition, so there is no [`Share`] to
    /// state. A board whose columns wanted different widths would be a
    /// different member, and no app has one.
    ///
    /// **What happens when there is no room.** Scroll across, wrap, or collapse
    /// to one column at a time: all three are right on some host, none is
    /// derivable from the description, and every one of them is presentation.
    /// A terminal that stacks them vertically is honouring this, not degrading
    /// it.
    ///
    /// # Why it is not an `Arrangement`
    ///
    /// [`Arrangement`] is the page's shape, and a board is usually a region
    /// *inside* a page that also has a band over it. Naming it here composes;
    /// naming it there would make a screen either a board or a list-detail and
    /// never a band above a board. It also keeps [`Arrangement::share`]
    /// meaningful, which a peer arrangement has no answer for.
    Columns,
    /// A set of panes, one visible at a time, and a [`Selector::Tabs`] that
    /// chooses between them.
    ///
    /// Which way the strip runs is a [`Strip`], stated by whoever holds the
    /// region and carried beside it, the way [`Showing`] carries the index.
    /// Where the strip goes when there is no room is still the renderer's to
    /// measure, the same as its overflow.
    TabGroup,
    /// Content over a scrim, taking input until dismissed.
    Modal,
    /// A region this crate names the *place* of, whose contents the app still
    /// owes every host.
    ///
    /// 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.
    ///
    /// # What separates it from [`Ceded`](Self::Ceded)
    ///
    /// **A fill is owed here in every host's currency.** A renderer handed one
    /// of these and given nothing to put in it is looking at a hole the app
    /// meant to fill, and saying so is the honest drawing. [`owed`](Self::owed)
    /// is how it asks.
    ///
    /// That is the whole of the split. Before it there was one opaque member,
    /// so a region the description had given up on and a region nobody had
    /// converted yet were the same value, and both drew as a silently empty
    /// box on the two renderers that answer no fill.
    Handover {
        /// What the app calls it. Never interpreted here.
        name: &'a str,
    },
    /// A region this crate names the place of, whose contents no host is owed.
    ///
    /// The other half of the old single opaque member. The app has decided this
    /// space is not the description's to fill and is not going to become so:
    /// a chart, a waveform, a rendered picture of domain data with marks
    /// painted over it at positions no description knows.
    ///
    /// **Silence is the correct drawing.** A renderer with no fill for this
    /// draws nothing and is right to; unlike [`Handover`](Self::Handover) there
    /// is nothing missing. That is what makes the pair worth two members rather
    /// than a flag: the two want opposite behaviour from a renderer that cannot
    /// fill them, and one name cannot carry both.
    ///
    /// The measured sites are MNW's analytics charts, which already carry the
    /// ruling that a bar chart is not describable and should not be, and
    /// audiofiles' waveform, whose exclusion had no vocabulary to live in and
    /// was recorded in a doc comment instead.
    Ceded {
        /// What the app calls it. Never interpreted here.
        name: &'a str,
    },
    /// A named assembly of things the vocabulary already says.
    ///
    /// The third tier, between a primitive and the two opaque members.
    ///
    /// # What separates it from the two members either side
    ///
    /// A primitive is a thing every renderer draws from scratch, and the test
    /// it has to pass is that every host has an honest answer. A carousel fails
    /// that test (a terminal has no carousel) which is the same refusal
    /// `Node::Html` got and is why the carousel sat unsayable for months.
    ///
    /// [`Handover`](Self::Handover) fails it from the other side. It is for
    /// what one app owns and nobody will build twice, and it carries *no*
    /// contents: the description names the place and stops. A carousel is
    /// furniture any app would have, and every part of it (an ordered set of
    /// frames, a position, prev and next, a strip of position indicators) is
    /// already sayable. Only the assembly had no name.
    ///
    /// So this member is the pair the other two are not: a name **and**
    /// contents. The contents are the assembly, in the region's own body, said
    /// in members that already exist.
    ///
    /// # Why the name does not have to be understood
    ///
    /// A renderer that recognises the name draws it the way its host does it: a
    /// carousel in a webview, a pager with a count in a terminal, a selector in
    /// egui. A renderer that does not recognise it walks the body, which is
    /// primitives all the way down and which it can already draw.
    ///
    /// That is what lets the widget set be **open** without every renderer
    /// knowing every widget. An unrecognised widget degrades to its assembly
    /// instead of failing, so a second or third party can name one without
    /// three renderers releasing in lockstep to accept it. Contrast
    /// [`Handover`](Self::Handover), which no renderer can degrade: there is
    /// nothing under it to fall back to.
    ///
    /// # What it does not do
    ///
    /// A widget is an assembly of things the vocabulary *already* says, so it
    /// buys no expressive power. Anything needing a member the vocabulary does
    /// not have is a finding about the vocabulary, and the answer to a finding
    /// is to add the member. A widget is never the way a primitive gets added
    /// by the back door. A timeline is describable because [`Track`] was added
    /// to say it, not because a screen was dressed up as an assembly.
    Widget {
        /// What the assembly is called. This crate never interprets it, and a
        /// renderer is free not to know it.
        name: &'a str,
    },
}

impl<'a> Region<'a> {
    /// 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,
            // Flat, and it inherits. A group says its contents belong together
            // and says nothing about the surface they sit on, so a group in a
            // pane is in a well and a group on the page is on the page. An app
            // wanting one lifted puts it in a `Pane`.
            Self::Group => Depth::Flat,
            // Flat, and it is the container rather than the columns. Each
            // column is its own region and brings its own depth; a well here
            // would put a second edge around a row of wells.
            Self::Columns => 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.
            //
            // A widget inherits for the same reason and it matters more here,
            // because a widget is drawn by whichever renderer recognises it. A
            // depth set here would be this crate deciding that a carousel is
            // raised on every host, which is the kind of value the deferral
            // rule exists to refuse.
            Self::Handover { .. } | Self::Ceded { .. } | Self::Widget { .. } => 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.
    ///
    /// [`Widget`](Self::Widget) is described, and that is the whole of what
    /// separates it from the two opaque members here. All three carry a name
    /// this crate never interprets; only the widget carries contents under it.
    /// A renderer that does not recognise a widget's name still walks its body,
    /// so there is nothing for it to hand over and nothing it cannot draw.
    #[must_use]
    pub const fn described(self) -> bool {
        !matches!(self, Self::Handover { .. } | Self::Ceded { .. })
    }

    /// Whether a fill is owed here, for a renderer that has none.
    ///
    /// The question the single opaque member could not answer. True for
    /// [`Handover`](Self::Handover): the app meant to fill this and a renderer
    /// with nothing to put in it should say so. False for everything else,
    /// [`Ceded`](Self::Ceded) included, where silence is the correct drawing
    /// because nothing is missing.
    ///
    /// A method rather than a `matches!` at each renderer, for
    /// [`described`](Self::described)'s reason: three renderers writing the
    /// same match is three chances to disagree about what an empty region
    /// means.
    #[must_use]
    pub const fn owed(self) -> bool {
        matches!(self, Self::Handover { .. })
    }

    /// The name an app gave this region, if it gave one.
    ///
    /// [`Handover`](Self::Handover), [`Ceded`](Self::Ceded) and
    /// [`Widget`](Self::Widget) are the members that carry a name, for two
    /// different purposes: the first two say what the app puts in the space,
    /// the third says what the assembly under it is called. A renderer dispatching on either wants the string without
    /// caring which member it came from, and writing that `matches!` at each
    /// renderer is how the two drift apart.
    #[must_use]
    pub const fn name(self) -> Option<&'a str> {
        match self {
            Self::Handover { name } | Self::Ceded { name } | Self::Widget { name } => Some(name),
            // Spelled out rather than a wildcard, so a member added later has
            // to answer whether it carries a name instead of inheriting `None`
            // by sitting under a `_`.
            Self::Band
            | Self::Sidebar
            | Self::Pane
            | Self::Group
            | Self::Split
            | Self::Columns
            | Self::TabGroup
            | Self::Modal => None,
        }
    }
}

/// How many of a region's children are visible at once.
///
/// One sentence covering three shapes: *this region holds several children and
/// shows some of them, and the reader can change which.* A tab group, a
/// carousel and a disclosure all need it, and without it a renderer has two
/// moves: hardcode a widget name, or draw every child. That is what puts
/// per-widget code in renderers.
///
/// # What is here and what is not
///
/// The *kind*, and only the kind. Which child is currently up is the current
/// answer, and a layer that defers every address does not hold the current
/// answer either, the split [`Selector`] already makes, where this crate says
/// what kind of chooser a thing is and the router says which option is picked.
/// So a holder of regions carries the index and the per-child label beside this.
///
/// # What a renderer does with it
///
/// Derives its chrome, once, for every widget rather than per name:
///
/// - Children carrying labels get a strip of the labels, the current one marked.
/// - Children carrying none get previous, position, next.
/// - [`AtMostOne`](Self::AtMostOne) over one child gets a summary line that
///   opens.
///
/// The name on [`Region::Widget`] survives as app vocabulary, for a renderer
/// that wants to do something *special* with one, which is what it should have
/// been from the start.
///
/// Degradation runs the way it already did: a renderer ignoring this draws every
/// child, which is more content rather than less.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Default)]
#[non_exhaustive]
pub enum Showing {
    /// Every child, in order. What every region did before this existed.
    #[default]
    All,
    /// Exactly one. A carousel, a tab group.
    One,
    /// One, or none. A disclosure, which is closed until it is opened.
    AtMostOne,
}

impl Showing {
    /// Whether the reader can change which child is up.
    ///
    /// The question every renderer's region arm asks before deriving any
    /// chrome, and a method rather than a `matches!` at each renderer for
    /// [`Region::name`]'s reason: three renderers writing the same comparison is
    /// how they come to disagree about a member added later.
    #[must_use]
    pub const fn selective(self) -> bool {
        !matches!(self, Self::All)
    }

    /// Whether showing nothing is a legal state.
    ///
    /// True only for [`AtMostOne`](Self::AtMostOne). A renderer needs this to
    /// know whether its control closes as well as moves: a carousel's row moves
    /// between frames and never reaches empty, and a disclosure's summary line
    /// is the same control wearing its closed state.
    #[must_use]
    pub const fn dismissible(self) -> bool {
        matches!(self, Self::AtMostOne)
    }
}

/// Which way a [`Region::TabGroup`]'s strip runs.
///
/// Across is a row of tabs over the panes it opens; down is a column of them
/// beside the panes. Both are the folder semantic, one pane showing at a time,
/// and both put the strip first, so a tab is always read before the pane it
/// opens.
///
/// # Why the description says it
///
/// A renderer left to choose picks across every time, so a tab group that
/// wants a column has no way to get one. Both shapes are in use: strips across
/// a screen, and long settings panels with their sections listed down beside
/// them. The language says both.
///
/// # What is here and what is not
///
/// The direction, carried beside the region the way [`Showing`] is: this crate
/// names the fact and the holder of the region states it, because the region
/// member is a place and not a record of how it is dressed.
///
/// **No default.** A tab group that says nothing about its strip is the silence
/// this type exists to end, so a holder has to pick one.
///
/// **No breakpoint.** Room is still the renderer's to measure. A strip that
/// runs down may fold over its panes when there is no room beside them, the
/// way a row folds into a menu; what it may not do is run across when there
/// is room, because then the description stopped being what was drawn.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
#[non_exhaustive]
pub enum Strip {
    /// A row over the panes.
    Across,
    /// A column beside the panes.
    Down,
}

impl Strip {
    /// Whether the tabs stack vertically.
    ///
    /// What a renderer asks for the two things that turn on direction rather
    /// than on placement: the accessibility orientation a strip announces, and
    /// which pair of arrow keys moves between its tabs. A method for
    /// [`Region::name`]'s reason, so three renderers do not each write the
    /// comparison.
    #[must_use]
    pub const fn runs_down(self) -> bool {
        matches!(self, Self::Down)
    }
}

/// A window onto a sequence: where it starts, how much it covers, and how long
/// the sequence is when that is known.
///
/// The mechanism under two things the vocabulary deliberately keeps apart. A
/// carousel is a window of one frame over children that are all present; a
/// paged list is a window of a page over rows most of which were never fetched.
/// Those are different facts and they stay different types ([`Showing`] says
/// which child is up, [`Paging`] says where a reader is in a query) but the
/// arithmetic underneath is one piece of code, so a terminal and a browser
/// cannot come to disagree about which frame is last.
///
/// # Why `of` is optional and `count` is not
///
/// `count` is what is on screen and is therefore always known. `of` is the
/// length of the thing being windowed, and a host that cannot count says so by
/// leaving it empty **for the life of the screen**. It is never "not counted
/// yet": see "First paint is final paint" in the crate header. A total that
/// turns up on a later pass widens the text that prints it.
///
/// # Clamping
///
/// Every derivation clamps rather than refusing, and a zero `count` answers
/// `None` rather than dividing. A window past the end is a bug in the host, and
/// a renderer that answered it by drawing nothing would report a region that
/// vanished, which is the hardest kind of bug to find from what is on screen.
/// [`Share::percent`] clamps for the same reason.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
#[non_exhaustive]
pub struct Window {
    from: usize,
    count: usize,
    of: Option<usize>,
}

impl Window {
    /// A window of `count`, starting at `from`, over a sequence of unknown
    /// length.
    #[must_use]
    pub const fn new(from: usize, count: usize) -> Self {
        Self {
            from,
            count,
            of: None,
        }
    }

    /// How long the sequence is.
    #[must_use]
    pub const fn of(mut self, of: usize) -> Self {
        self.of = Some(of);
        self
    }

    /// The index into the sequence where the window starts.
    #[must_use]
    pub const fn from(self) -> usize {
        self.from
    }

    /// How many the window covers. One, for a carousel.
    #[must_use]
    pub const fn count(self) -> usize {
        self.count
    }

    /// How long the sequence is, when the host can say.
    #[must_use]
    pub const fn length(self) -> Option<usize> {
        self.of
    }

    /// One item of a sequence whose length is known. A carousel frame.
    #[must_use]
    pub const fn frame(at: usize, of: usize) -> Self {
        Self {
            from: at,
            count: 1,
            of: Some(of),
        }
    }

    /// Which window this is, counting from zero.
    ///
    /// `None` when `count` is zero, which is the only input with no answer
    /// rather than a clamped one.
    ///
    /// A window that does not start on a multiple of `count` has a short
    /// window before it, and that one counts: rows 5 to 14 of 20 in tens is the
    /// second of three, not the first of two, which is what agrees with
    /// [`has_before`](Self::has_before). An aligned window reads as it always
    /// did.
    #[must_use]
    pub const fn index(self) -> Option<usize> {
        if self.count == 0 {
            return None;
        }
        Some(self.from.div_ceil(self.count))
    }

    /// How many windows the sequence holds.
    ///
    /// `None` unless both the length and a non-zero `count` are known. A
    /// partial answer here would be a renderer drawing "of 0".
    #[must_use]
    pub const fn windows(self) -> Option<usize> {
        match self.of {
            Some(of) if self.count > 0 => Some(
                self.from.div_ceil(self.count) + of.saturating_sub(self.from).div_ceil(self.count),
            ),
            _ => None,
        }
    }

    /// Whether anything sits before this window.
    #[must_use]
    pub const fn has_before(self) -> bool {
        self.from > 0
    }

    /// How many sit after this window, when the length is known.
    ///
    /// Here rather than in each renderer for [`Showing::selective`]'s reason:
    /// three of them writing the same subtraction is how they come to disagree,
    /// and this one has an underflow in it for whoever writes it fourth.
    #[must_use]
    pub const fn after(self) -> Option<usize> {
        match self.of {
            Some(of) => Some(of.saturating_sub(self.from.saturating_add(self.count))),
            None => None,
        }
    }

    /// Whether anything sits after it.
    ///
    /// `true` when the length is unknown: a host that cannot count cannot rule
    /// out more, and offering a way forward that turns out to be empty is the
    /// cheaper of the two mistakes.
    #[must_use]
    pub const fn has_after(self) -> bool {
        match self.of {
            Some(of) => self.from.saturating_add(self.count) < of,
            None => true,
        }
    }

    /// The window with `from` brought inside the sequence.
    ///
    /// A no-op when the length is unknown, since there is nothing to clamp
    /// against.
    #[must_use]
    pub const fn clamped(mut self) -> Self {
        if let Some(of) = self.of
            && self.from >= of
        {
            // `max(1)` by hand: `Ord::max` is not const yet, and a zero-count
            // window would otherwise clamp onto the end rather than inside it.
            let step = if self.count == 0 { 1 } else { self.count };
            self.from = of.saturating_sub(step);
        }
        self
    }
}

/// Where a reader is in a set that arrived in parts.
///
/// A [`Window`] wearing the paged reading of itself. Distinct from a carousel's
/// window at the top level on purpose, because the intent differs and a call
/// site should say which one it means, while the arithmetic below is shared so
/// the two cannot drift apart.
///
/// # The two idioms, and which one a renderer may draw
///
/// Load-more and numbered pages are both this type. Which is honest is
/// [`kind`](Self::kind): a set whose page size is known can be drawn as
/// "Page 3 of 8", and one without can only be drawn as "150 of 400" and a way
/// forward. Saying it here rather than letting each renderer guess is the point
///, three renderers inferring it from the numbers is how they come to disagree.
///
/// # What it does not carry
///
/// No addresses. `makeover-layout` cannot name an action, and the way to ask for
/// the next part is the host's: `quasi_router` pairs this with the addresses the
/// same way `Row` pairs its parts with `Row::activate`. That split is the reason
/// this type is reusable by a carousel, which has nothing to ask.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
#[non_exhaustive]
pub struct Paging {
    /// The window onto the set.
    pub window: Window,
    /// Whether the parts are a fixed size, and so whether pages are countable.
    pub kind: PageKind,
}

/// Whether a paged set arrives in pages of a fixed size or as one window that
/// grows.
///
/// The fact [`Paging`] carries so no renderer infers it from the numbers.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Default)]
#[non_exhaustive]
pub enum PageKind {
    /// Parts of a fixed size, so "Page 3 of 8" names something.
    #[default]
    Numbered,
    /// Load-more: the window starts at the beginning and grows, and "page 2"
    /// would name nothing.
    Growing,
}

impl Paging {
    /// A page of `per`, starting at `from`.
    #[must_use]
    pub const fn pages(from: usize, per: usize) -> Self {
        Self {
            window: Window::new(from, per),
            kind: PageKind::Numbered,
        }
    }

    /// The first `shown`, with more behind them.
    ///
    /// The load-more shape: the window starts at the beginning and grows, so
    /// there is no page to number.
    #[must_use]
    pub const fn more(shown: usize) -> Self {
        Self {
            window: Window::new(0, shown),
            kind: PageKind::Growing,
        }
    }

    /// How many there are altogether.
    ///
    /// Left unsaid by a host that cannot count, and left unsaid **for good**:
    /// a total arriving later widens whatever prints it. See "First paint is
    /// final paint" in the crate header.
    #[must_use]
    pub const fn of(mut self, of: usize) -> Self {
        self.window = self.window.of(of);
        self
    }

    /// Which page this is, counting from one, when pages are countable.
    ///
    /// One-based because it is read aloud. [`Window::index`] is the zero-based
    /// form for anyone indexing with it.
    #[must_use]
    pub const fn page(self) -> Option<usize> {
        if matches!(self.kind, PageKind::Growing) {
            return None;
        }
        match self.window.index() {
            Some(index) => Some(index + 1),
            None => None,
        }
    }

    /// How many pages there are, when that is countable.
    #[must_use]
    pub const fn pages_total(self) -> Option<usize> {
        if matches!(self.kind, PageKind::Growing) {
            return None;
        }
        self.window.windows()
    }

    /// How many are on screen.
    #[must_use]
    pub const fn shown(self) -> usize {
        self.window.count
    }

    /// How many there are, when the host counted.
    #[must_use]
    pub const fn total(self) -> Option<usize> {
        self.window.of
    }

    /// How many are not shown yet, when the host counted.
    ///
    /// The figure a load-more control puts in its label. `None` is the honest
    /// and common case: a set that cannot say how many more there are still has
    /// a way to ask for them.
    #[must_use]
    pub const fn remaining(self) -> Option<usize> {
        self.window.after()
    }

    /// Whether there is anything further on.
    #[must_use]
    pub const fn has_more(self) -> bool {
        self.window.has_after()
    }

    /// Whether there is anything back the other way.
    #[must_use]
    pub const fn has_previous(self) -> bool {
        self.window.has_before()
    }
}

/// How much of the width an arrangement's first region takes.
///
/// Nothing said how much room a region got, so every renderer invented its own
/// number and two hosts showing one screen disagreed about its proportions. A
/// webview never noticed, because the stylesheet answered once for every
/// consumer; a terminal has no stylesheet to inherit from, so `quasi-tui`
/// picked 24 columns for a sidebar and 40% for a list pane and neither had
/// anything behind it.
///
/// # A proportion, never a unit
///
/// Held as a percentage, and that is the only form it comes in. A description
/// carrying columns would be describing a terminal and one carrying pixels a
/// webview, and the whole point is that both honour the same fact: a terminal
/// resolves it against a column count, a webview writes it into a grid, and
/// neither has to know what the other did.
///
/// It is not [`makeover_geometry::Ratio`]'s job either, which was the first
/// guess. Geometry is scales that answer the same for every screen and takes
/// no input that would let a sidebar screen differ from a list-detail one.
///
/// [`makeover_geometry::Ratio`]: https://docs.rs/makeover-geometry
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, PartialOrd, Ord)]
#[non_exhaustive]
pub struct Share(u8);

impl Share {
    /// What a sidebar takes, when nobody says otherwise.
    ///
    /// A quarter. `quasi-tui` drew 24 columns, which is a quarter of a
    /// 96-column terminal and about a fifth of a wide one; a quarter is that
    /// number said in the form a webview can honour too.
    pub const SIDEBAR: Self = Self(25);

    /// What the list side of a list-detail takes, when nobody says otherwise.
    ///
    /// Sixty percent. A webview's list side is usually a table of four or five
    /// columns, and less than this is not enough room for one; a terminal's
    /// list of titles loses nothing it needs at this share. A screen that wants
    /// something else says so with the `share` word in `quasi-declare`'s
    /// screen block.
    pub const LIST: Self = Self(60);

    /// A share of the width, as a percentage.
    ///
    /// Clamped to 5..=95 rather than refused. A description that asked for a
    /// region of nothing is a bug in the app, and a renderer drawing a region
    /// zero cells wide reports it as a region that vanished, which is the
    /// hardest kind of bug to find from what is on the screen.
    #[must_use]
    pub const fn percent(percent: u8) -> Self {
        Self(if percent < 5 {
            5
        } else if percent > 95 {
            95
        } else {
            percent
        })
    }

    /// The share as a percentage.
    #[must_use]
    pub const fn as_percent(self) -> u8 {
        self.0
    }

    /// This share of a width, rounded to the nearest whole unit.
    ///
    /// What a terminal calls to turn the proportion into columns. At least one
    /// when there is any width at all, because a region the description named
    /// should be visible: a screen 3 columns wide is unusable either way, and a
    /// sidebar that is there is a truer picture of the description than a
    /// sidebar that is not. Nothing of nothing is nothing.
    #[must_use]
    pub const fn of(self, whole: u16) -> u16 {
        if whole == 0 {
            return 0;
        }
        // Half up: 50 hundredths is the half.
        let taken = (whole as u32 * self.0 as u32 + 50) / 100;
        if taken == 0 { 1 } else { taken as u16 }
    }
}

/// How a screen is laid out.
///
/// Three, and no one of them is a variant of another. goingson is list-detail,
/// Balanced Breakfast is sidebar plus content, and MNW's embeds are one region
/// filling the document. 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.
///
/// # Why the share rides here
///
/// A share is per-arrangement: how much a sidebar takes and how much a list
/// side takes are different questions, and this enum is the only thing that
/// knows which one is being asked. Geometry would have had to invent a channel
/// to be told.
///
/// [`list_detail`](Self::list_detail) and
/// [`sidebar_content`](Self::sidebar_content) build these with the default
/// shares, so a screen that has no opinion does not have to have one.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
#[non_exhaustive]
pub enum Arrangement {
    /// A list that chooses what the detail beside it shows.
    ListDetail {
        /// Whether the detail side is a [`Region::TabGroup`].
        tabbed: bool,
        /// How much of the width the list side takes.
        share: Share,
    },
    /// Navigation down the side, content filling the rest.
    SidebarContent {
        /// How much of the width the sidebar takes.
        share: Share,
    },
    /// One region, filling the document.
    ///
    /// The other two are both about dividing a width between two regions, so a
    /// screen that is one region had to borrow one of them and then undo it:
    /// MNW's five embeds said `list_detail(title, false)` and the host spent a
    /// `display: block` cancelling the grid that produced. A host writing CSS
    /// to contradict the description rather than to add to it is the thing
    /// this member ends.
    ///
    /// Carries no [`Share`], because there is no division to describe. That is
    /// why [`share`](Self::share) answers `None` here.
    Single,
}

impl Arrangement {
    /// A list and a detail beside it, at the default share.
    #[must_use]
    pub const fn list_detail(tabbed: bool) -> Self {
        Self::ListDetail {
            tabbed,
            share: Share::LIST,
        }
    }

    /// A sidebar and content beside it, at the default share.
    #[must_use]
    pub const fn sidebar_content() -> Self {
        Self::SidebarContent {
            share: Share::SIDEBAR,
        }
    }

    /// How much of the width the first region takes, when two regions divide it.
    ///
    /// `None` for [`Single`](Self::Single): one region takes the width, and a
    /// renderer that asked how to divide it was asking the wrong question. It
    /// answers `Option` rather than a full-width `Share` so that a host cannot
    /// quietly draw a one-region screen as a grid with an empty second column.
    #[must_use]
    pub const fn share(self) -> Option<Share> {
        match self {
            Self::ListDetail { share, .. } | Self::SidebarContent { share } => Some(share),
            Self::Single => None,
        }
    }

    /// The same arrangement, at this share.
    ///
    /// [`Single`](Self::Single) is returned unchanged: it has no division to
    /// set, so a share named for it is a statement about nothing rather than an
    /// error worth refusing a screen over.
    #[must_use]
    pub const fn with_share(self, share: Share) -> Self {
        match self {
            Self::ListDetail { tabbed, .. } => Self::ListDetail { tabbed, share },
            Self::SidebarContent { .. } => Self::SidebarContent { share },
            Self::Single => Self::Single,
        }
    }
}

/// How wide the content of a whole screen runs.
///
/// Both are the description's, which is what answering the two together
/// settled.
///
/// Measured in the MNW server, where 69 of 72 templates carry exactly one of
/// three mutually exclusive classes and the choice is per screen. GoingsOn
/// reaches for `max-width` 56 times and Balanced Breakfast 12, neither with a
/// token for it, so three apps were solving one thing by hand.
///
/// # Named for the measure, not for MNW's classes
///
/// A renderer that is not a browser has to answer this too, and `padded-page`
/// tells a terminal nothing. The three say how wide the text runs, which is a
/// question every renderer can answer: a webview with a `max-width`, a terminal
/// with gutters, an immediate-mode frame with its own width.
///
/// `#[non_exhaustive]` for [`Fill`]'s reason. The set is closed today because
/// the measurement found three, and a fourth arriving should not be a lockstep
/// release across nine repos.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Default)]
#[non_exhaustive]
pub enum Measure {
    /// The whole width, with gutters. The default, and 53 of the 69.
    ///
    /// What a dashboard, a table and a settings screen want: the content is
    /// wide because the content *is* wide, and constraining it would waste the
    /// window.
    #[default]
    Wide,
    /// Capped at a comfortable page width, centred. 13 of the 69.
    ///
    /// A form, a sign-in, a purchase. Content that does not get better by
    /// getting wider, but is not prose either.
    Contained,
    /// Capped at a line length that reads well. 3 of the 69.
    ///
    /// Prose. The narrowest of the three, and the one with a reason outside
    /// taste: a line of text past roughly 75 characters costs the reader the
    /// return sweep.
    Reading,
}

impl Measure {
    /// A stable name, for a renderer that needs to spell it.
    ///
    /// Here rather than in each renderer for [`Sort::as_str`]'s reason: three
    /// renderers spelling one enum is three chances to spell it differently.
    #[must_use]
    pub const fn as_str(self) -> &'static str {
        match self {
            Self::Wide => "wide",
            Self::Contained => "contained",
            Self::Reading => "reading",
        }
    }
}