//! The immediate-mode renderer for [`makeover_layout`].
//!
//! <!-- wiki: makeover-immediate -->
//!
//! Named for the mode, not the library, the way `makeover-tui` is named for
//! the target and not for ratatui. Immediate mode is the constraint that
//! actually separates this renderer from the other two, and egui is the
//! backend it is written against.
//!
//! It is the harshest renderer the description has to survive: no
//! `box-shadow`, no `inset`, no cascade, no retained tree to mutate, and
//! `Visuals.widgets.*.bg_stroke` is a single stroke with no per-side control.
//! A two-tone lit edge is not something egui can be configured into producing,
//! so it gets painted by hand here, once, instead of in every consuming app.
//!
//! # What this crate does and does not own
//!
//! It owns the *expression*: two mitred polylines for a bevel and a `Frame`
//! for a filled region. It owns no colours, no sizes and no substitutions:
//! makeover derives `surface-well` and every consumer reads the real token.
//! [`Palette`] is supplied by the caller,
//! already resolved, and every radius, margin and stroke width arrives in
//! [`FrameStyle`].
//!
//! That split is why the crate has no dependency on `makeover` itself: the app
//! already resolves a theme, and coupling a renderer to a colour crate's
//! version would buy nothing.
//!
//! # The cascade is the real difference
//!
//! A stylesheet can say "a pressed button inverts its bevel" once and let the
//! cascade carry it. An immediate-mode renderer has nowhere to put that, so
//! every call site decides. [`makeover_layout::Depth::pressed`] is what keeps
//! the decision from being re-derived per widget.
//!
//! # Overlays
//!
//! [`Palette::cast`] is what overlaying means in immediate mode. [`frame`] hands
//! the cast shadow to the `egui::Frame` for any depth whose fill is
//! [`Fill::Overlay`], keyed off the fill rather than the variant.
//!
//! # The table
//!
//! [`table`] draws `makeover_layout::CellPart`. Two things it forces, both named
//! where they land:
//!
//! - **`egui_extras`**, this crate's one dependency past egui. egui has no
//!   table, and `Grid` gives no per-column sizing, no sticky header and no
//!   scroll sync. A third answer here would reimplement that crate worse.
//! - **[`Palette::action`]**, on the footing [`Palette::content`] sits on: a
//!   link in a cell needs the action intent.
//!
//! Narrowing works differently from the terminal's and the module header says
//! why: a content column cannot be measured before the app's closure has drawn
//! it, so `egui_extras` sizes it and the declared floor budgets it.
//!
//! Three things are host idiom rather than description, which is why they land
//! here and not in `makeover-layout`, and all three are answered on a handle the
//! app never sees: the `egui_extras` row and builder this crate owns. That is
//! [`table::cell`]'s reasoning again: what the app cannot reach, the renderer
//! owes it.
//!
//! - **A selected row.** [`table::Body::selected`], a predicate asked per row,
//!   because `set_selected` is a method on the row. Without it a file list has
//!   no way to show what is selected, which is most of what a file list does.
//! - **Scrolling a row into view.** [`table::Body::scroll_to`], because
//!   `scroll_to_row` is a method on the builder. A keyboard cursor that moves
//!   off-screen and stays there is the bug this prevents.
//! - **Dragging a divider.** [`table::TableStyle::resizable`], which is a knob
//!   because egui_extras offers two settings here and a renderer can honestly
//!   make either choice.
//!
//! Cells are centred on the row's centre line, always, because there is no
//! second honest answer and egui's own default (top-aligned) is the one thing it
//! cannot be. That is not a knob.
//!
//! [`table::Body`] is also what splits a table's per-frame facts from its
//! description and from its style. A row count, a selection and a scroll request
//! are none of them style, and none of them survive the frame.
//!
//! # The nodes that are not fields, tables or frames
//!
//! [`widget`] draws a meter, a token, a control and a figure. The four are
//! ordinary nodes, so without them a screen walk has to draw them itself, one
//! copy per consumer.
//!
//! [`Palette`] carries the three status intents together rather than one per
//! widget, for the reason [`Palette::fill`] is an `Option`: `Tone` is five
//! members wide, and a resolver missing one has to invent a colour, which is a
//! substitution this crate does not make.
//! # Forms
//!
//! The field vocabulary sits on top of the depth vocabulary:
//! [`makeover_layout::Field`] rendered to egui widgets, in [`field`], and a set
//! of them laid down a column in [`group`].
//!
//! `makeover-webview`'s form emitter is the precedent, followed rather than
//! re-derived, including the parts that are bug fixes: a
//! select handed a value none of its options carries keeps that value visible
//! instead of silently reading as the first option, which is a save-the-wrong-
//! thing bug goingson hit for real.
//!
//! What differs is forced by the mode and not chosen:
//!
//! - **The value arrives as a `&mut`.** [`Filling`] borrows the app's own field
//!   and the widget writes through it. There is no DOM to read back out of,
//!   which is also why the description deliberately does not carry the value.
//! - **A text control is drawn as a well and a select is not.** The description
//!   holds that a well is for anything the user looks *into*, and a text field
//!   is its own example; a select and a checkbox are pressed rather than looked
//!   into, so they keep egui's own control painting.
//! - **Focus is not describable, and egui owns all of it here.** **Reach**,
//!   **focus** and the **focus ring** are this renderer's three answers and
//!   egui already has all three: its own id stack decides what is reachable,
//!   its own state decides what holds the keyboard, and it paints exactly one
//!   ring. A description states none of them, and drawing a second ring on top
//!   of egui's would break the one-ring rule. The terms
//!   are defined once in `makeover_layout`'s crate header, "Reach, focus and
//!   the focus ring". [`makeover_layout::State::Disabled`] *is* drawn, because
//!   egui has no opinion about it until told.
//! - **App-level chrome is not drawn here, and it is not this crate's to
//!   draw.** `quasi-router` names the affordances that outlive one screen: a
//!   `Chrome` of key bindings, and an `Outcome::Over` for a screen drawn over
//!   another. Both are answered by `quasi-webview` and `quasi-tui`, and neither
//!   is answerable here, because this crate depends on `makeover-layout` and
//!   not on `quasi-router` — it is the peer of `makeover-webview` and
//!   `makeover-tui`, one layer below the renderers that consume a `Screen`.
//!   What is missing is the egui crate at *that* layer, which does not exist:
//!   nothing renders a quasi `Screen` in egui at all, and chrome is one item on
//!   the list such a crate would owe. Said here because this is where a reader
//!   looks for it, and because the silent version reads as "egui does not need
//!   a palette" rather than "nobody has built the renderer yet".

//! # An interval is a sixth control shape
//!
//! [`FieldKind::Interval`] is drawn as `Control::Spanned`: two drag boxes on one
//! row with the word `to` between them.
//!
//! - **Dragged rather than typed**, because that is what these controls already
//!   were. audiofiles' six filter axes are `DragValue` pairs sharing an extent,
//!   a speed and a suffix, and describing them into two text boxes would be a
//!   port that cost the app a control.
//! - **One row, not two wells stacked.** Two wells are two questions on screen
//!   whatever the description says, and the arrangement is the whole content of
//!   the kind.
//! - **An empty end reads as the bound it stands for.** An unset minimum sits
//!   on the low edge and stores no filter, which is what the shipped control
//!   did; egui's `DragValue` has no empty state, and a text box in its place
//!   would cost the app a control. With no extent to fall back on it reads
//!   zero -- the one number this renderer invents, invented where the
//!   description declined to say anything.
//! - **The word rather than a dash**, which on a signed axis is a minus sign.
//!   audiofiles filters loudness in dBFS.
//!
//! `Axis` holds the four facts both boxes share, because they are one axis:
//! reading `min`, `max`, `step` and `unit` once is what stops the two ends
//! drifting apart.
//!
//! # A number draws its unit
//!
//! [`Field::unit`], and this host is the one with somewhere better than the
//! label to put it: egui's `Slider` draws a suffix beside its readout.
//!
//! So a slider takes it as a suffix, inside the control. A typed number has no
//! readout of its own and takes it as a muted label after the box. Every other
//! kind ignores it, and the description says which those are --
//! `FieldKind::measurable`, rather than a `matches!` kept here.
//!
//! # The slider's track is a curve
//!
//! `makeover-layout` says what a slider is: a fraction and a function
//! taking numbers to numbers, with `min` and `max` being `f(0)` and `f(1)`
//! rather than the control's extent. This host has the easiest job of the
//! three, because egui already has the control -- `Slider::logarithmic` is a
//! constant-ratio track, so the mapping is a builder call rather than an
//! arithmetic of its own.
//!
//! Two things worth knowing. The granularity rides on the curve, so a range
//! reads `Field::curve.step()` and every other kind reads `Field::step`; the
//! step is in the value's own units under either curve, so the display
//! precision derives from it directly. And the fallback for a ratio curve
//! across zero is asked of `Curve::is_ratio` rather than matched on the
//! variant, so this renderer and a terminal cannot disagree about when a
//! logarithmic request is honoured.
//!
//! # The slider, the unanswered chooser, and the option that is not offered
//! yet
//!
//! Three things a description can say here.
//!
//! - **[`FieldKind::Range`] is a fifth control shape**, `Control::Slid`, drawn
//!   with egui's `Slider`. A range missing an end falls back to a well rather
//!   than to invented bounds, which is what
//!   `makeover_layout::Field::bounded` is for.
//! - **`Field::placeholder` reads on a chooser**, so a select with nothing
//!   chosen does not show an empty box.
//! - **`Choice::unavailable` is drawn rather than dropped.** The option stays
//!   in the list, inert, with its precondition beside it instead of behind a
//!   hover — a greyed row with no reason reads as a dead end, which is the
//!   whole finding.
//! - **`Choice::detail` is drawn under the option in a
//!   radio group and inside the row in a combo.** A closed chooser hides its
//!   list, so everything an option carries has to travel with its row; a group
//!   has a line to spare and putting a sentence beside the control instead
//!   would push every option's radio out of line with its neighbours.
//!
//! The value still arrives as a `&mut String` and a slider is a number, so the
//! parse and the write-back are this renderer's, and the write happens only on
//! a real drag: a value the app put there that this host cannot read survives
//! being looked at.

#![forbid(unsafe_code)]

use egui::{
    Color32, ComboBox, CornerRadius, DragValue, Margin, Painter, Rect, Response, RichText, Shape,
    Slider, Stroke, TextEdit, Ui,
};
use makeover_layout::{
    Bevel, Choice, Depth, Edge, Field, FieldKind, Fill, State, ThemeVariant, Tone,
};
use std::ops::RangeInclusive;

/// Columns, narrowing, cell parts and the sort caret, over `egui_extras`.
pub mod table;
pub mod widget;

/// The resolved colours this renderer needs, as flat values.
///
/// Built by the app from whatever it already uses to resolve a theme, then
/// held and reused. Deliberately not a trait and not string-keyed: a bevel is
/// painted per widget per frame, and a map lookup per edge is a cost with
/// nothing to show for it.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct Palette {
    /// `surface-page`.
    pub page: Color32,
    /// `surface-raised`.
    pub raised: Color32,
    /// `surface-overlay`.
    pub overlay: Color32,
    /// `surface-well`.
    ///
    /// Required, not optional. makeover derives it for every theme, so a
    /// resolved palette without a well is not a thing that exists here.
    /// `makeover-tui` keeps its own `Option` for a different reason, since a
    /// terminal can have the colour and still be unable to show it.
    pub well: Color32,
    /// `surface-sunken`.
    ///
    /// 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 this has no edge. An
    /// immediate-mode renderer paints an arbitrary rect, so unlike
    /// `makeover-tui` it has no excuse for declining this one.
    ///
    /// Required rather than optional, on the same footing as `well`: all 31
    /// themes makeover embeds author it.
    pub sunken: Color32,
    /// `bevel-light`.
    pub bevel_light: Color32,
    /// `bevel-dark`.
    pub bevel_dark: Color32,
    /// `elevation`.
    ///
    /// What a surface that floats OVER the page is cast onto it with. The one
    /// intent here that is about a surface's relationship to the page rather
    /// than about the surface, which is why it is a translucent near-black on
    /// every theme rather than something read off the palette's own ramp.
    ///
    /// **Only for a surface that overlays.** A menu, a tooltip, a modal. A
    /// surface *in* the layout takes a bevel, and reaching for this on a panel
    /// or a card is how a pre-Platinum look survives a conversion under a new
    /// name.
    ///
    /// egui has a real answer for this where a terminal does not: see
    /// [`Palette::cast`], which is the shadow to hand an
    /// [`egui::Frame`](egui::Frame).
    pub elevation: Color32,
    /// `content`.
    ///
    /// Ordinary text.
    pub content: Color32,
    /// `content-secondary`.
    ///
    /// Inactive but usable: it still answers a press. The middle tone of the
    /// three (wiki `three-tone-convention`), and the one an unchosen option in
    /// a choice field takes.
    ///
    /// Not [`content_muted`](Self::content_muted), which carries a claim:
    /// `State::Disabled` resolves to it, so a live control wearing it tells the
    /// user it will not answer. `makeover-tui` draws the same widget the same
    /// way from `makeover-tui@230bf63`.
    ///
    /// A step of `content` toward the page, derived at load by `makeover`
    /// rather than authored, so it is read off the resolved theme here like
    /// any other token and never re-derived.
    pub content_secondary: Color32,
    /// `content-muted`.
    ///
    /// A field's hint, and what
    /// [`makeover_layout::State::Disabled`](makeover_layout::State::Disabled)
    /// resolves to. Both readings come from the description rather than from
    /// here: `State::Disabled` names this intent by token.
    pub content_muted: Color32,
    /// `action-primary`.
    ///
    /// What a control is drawn in.
    ///
    /// This is the intent [`CellPart`](makeover_layout::CellPart) exists to
    /// separate. A cell holding a control that takes the cell's text colour is
    /// the drift `CellPart` names.
    pub action: Color32,
    /// `danger`.
    ///
    /// A field's error message, a destructive control, a bar that has run over.
    pub danger: Color32,
    /// `success`.
    ///
    /// The three status intents arrive together and not one at a time:
    /// [`Tone`] is five members wide and a resolver missing one has to invent
    /// a colour for it, which is the substitution [`Palette::fill`] refuses.
    pub success: Color32,
    /// `warning`.
    pub warning: Color32,
    /// `info`.
    pub info: Color32,
}

impl Palette {
    /// Resolve a surface intent, or `None` for one this renderer does not know.
    ///
    /// A plain lookup, and no substitution.
    ///
    /// `Option`, because [`Fill`] is `#[non_exhaustive]` and a total function
    /// over an open enum can only stay total by inventing a colour for a member
    /// it has never heard of. Every member the description has today is
    /// answered with `Some`.
    #[must_use]
    pub const fn fill(&self, fill: Fill) -> Option<Color32> {
        match fill {
            Fill::Page => Some(self.page),
            Fill::Raised => Some(self.raised),
            Fill::Overlay => Some(self.overlay),
            Fill::Well => Some(self.well),
            Fill::Sunken => Some(self.sunken),
            _ => None,
        }
    }

    /// The colour a [`Tone`] reads as.
    ///
    /// Total, unlike [`fill`](Self::fill), and the difference is not an
    /// inconsistency. `Fill` is `#[non_exhaustive]` and `Tone` is not: the
    /// description layer settled tone at five members and grows surfaces, so a
    /// total function here cannot be made to invent a colour by an upstream
    /// release the way a total `fill` could.
    ///
    /// [`Tone::Neutral`] is [`content`](Self::content) rather than a colour of
    /// its own, which is what "an ordinary fact" means: a neutral badge is text
    /// in a box, not a fifth status.
    #[must_use]
    pub const fn tone(&self, tone: Tone) -> Color32 {
        match tone {
            Tone::Neutral => self.content,
            Tone::Info => self.info,
            Tone::Success => self.success,
            Tone::Warning => self.warning,
            Tone::Danger => self.danger,
        }
    }

    /// The cast shadow for a surface that overlays the page.
    ///
    /// What "overlaying" means in immediate mode, answered rather than skipped.
    /// egui already paints shadows for its menus and windows through
    /// [`egui::Frame::shadow`], so the honest port is to hand that machinery the
    /// theme's tone instead of egui's own default, not to invent a painter here
    /// the way [`paint_bevel`] had to.
    ///
    /// The geometry matches what `makeover-webview` composes, in points rather
    /// than pixels: a small downward offset and a wide soft blur. A Platinum-era
    /// menu sits just off the page rather than hovering above it.
    ///
    /// ```no_run
    /// # let palette: makeover_immediate::Palette = unimplemented!();
    /// # let ui: &mut egui::Ui = unimplemented!();
    /// egui::Frame::popup(ui.style())
    ///     .shadow(palette.cast())
    ///     .show(ui, |ui| { ui.label("over the page"); });
    /// ```
    #[must_use]
    pub const fn cast(&self) -> egui::Shadow {
        egui::Shadow {
            offset: [0, 2],
            blur: 24,
            spread: 0,
            color: self.elevation,
        }
    }

    /// Resolve a bevel edge intent.
    #[must_use]
    pub const fn edge(&self, edge: Edge) -> Color32 {
        match edge {
            Edge::Light => self.bevel_light,
            Edge::Dark => self.bevel_dark,
        }
    }
}

/// The geometry a framed region is drawn with.
///
/// Every field is a value, which is why they all arrive from the caller:
/// radius and border width belong to `makeover-geometry`, and margins come
/// from its relational gaps.
#[derive(Debug, Clone, Copy, PartialEq)]
pub struct FrameStyle {
    /// Corner radius. Square under the Platinum default.
    pub radius: CornerRadius,
    /// Inner margin between the frame and its contents.
    pub margin: Margin,
    /// Bevel stroke width, in points.
    pub stroke: f32,
}

impl Default for FrameStyle {
    /// A one-point square frame with no inner margin.
    fn default() -> Self {
        Self {
            radius: CornerRadius::ZERO,
            margin: Margin::ZERO,
            stroke: 1.0,
        }
    }
}

/// Paint a two-tone edge just inside `rect`.
///
/// Fill first, bevel after: this adds two polylines and nothing else, so it
/// composes over whatever is already there. That is what lets it go over an
/// [`egui::TextEdit`] after `ui.add`, where the widget's own fill has landed.
///
/// Two three-point polylines meeting at opposite corners, rather than four
/// segments, so egui mitres the corner joins instead of leaving a notch.
///
/// The dark polyline is drawn second, so the two corners where the runs meet
/// take its tone. That is the right answer here rather than a concession.
/// [`makeover_layout::Bevel`] holds those corners to belong to both edges, and
/// a renderer with room to divide one should; at the default one-point stroke
/// the corner is a one-point square, so the division is sub-pixel and
/// antialiasing resolves it to the same blend the mitre already gives. Splitting
/// it would add a seam and no information. `makeover-tui` does split, because a
/// terminal cell is large enough that not splitting costs a visible cell of edge
/// weight — the same rule, at a resolution where it has something to say.
pub fn paint_bevel(painter: &Painter, rect: Rect, bevel: Bevel, palette: &Palette, stroke: f32) {
    let (top_left, bottom_right) = bevel.edges();

    // Inset by half a stroke so the line lands inside `rect` rather than
    // straddling its edge, which on a fractional-scale display is the
    // difference between one crisp pixel and two dim ones.
    let r = rect.shrink(stroke / 2.0);

    painter.add(Shape::line(
        vec![r.left_bottom(), r.left_top(), r.right_top()],
        Stroke::new(stroke, palette.edge(top_left)),
    ));
    painter.add(Shape::line(
        vec![r.right_top(), r.right_bottom(), r.left_bottom()],
        Stroke::new(stroke, palette.edge(bottom_right)),
    ));
}

/// Draw a region at a given [`Depth`]: its fill and its edge, together.
///
/// [`Depth::Flat`] gets neither, and inherits whatever it sits on. That is the
/// difference between level-with and painted-the-same-colour, and it is the
/// reason `Depth::fill` returns an [`Option`] rather than defaulting to the
/// page.
pub fn frame<R>(
    ui: &mut Ui,
    depth: Depth,
    palette: &Palette,
    style: FrameStyle,
    add_contents: impl FnOnce(&mut Ui) -> R,
) -> R {
    let mut f = egui::Frame::new()
        .corner_radius(style.radius)
        .inner_margin(style.margin);
    // Two ways there is no fill to paint, and they collapse to the same
    // outcome: the depth names none (Depth::Flat), or it names one this
    // renderer cannot resolve. Either way the frame goes unfilled and the
    // bevel below carries the depth on its own, which is the rule this
    // module already documents for Flat.
    if let Some(fill) = depth.fill().and_then(|f| palette.fill(f)) {
        f = f.fill(fill);
    }
    // A surface that overlays the page is cast onto it. [`Palette::cast`] has
    // answered what that means here since 0.10.0 and nothing could reach it: a
    // description had no way to say Overlay until makeover-layout 0.14.0, so
    // the answer sat beside the question. Keyed off the fill rather than the
    // variant, so it stays right for whatever else the description calls an
    // overlay later.
    if depth.fill() == Some(Fill::Overlay) {
        f = f.shadow(palette.cast());
    }
    let framed = f.show(ui, add_contents);
    if let Some(bevel) = depth.bevel() {
        paint_bevel(
            ui.painter(),
            framed.response.rect,
            bevel,
            palette,
            style.stroke,
        );
    }
    framed.inner
}

/// The geometry a field group is drawn with.
///
/// Values again, for the reason [`FrameStyle`] is: every number here belongs to
/// `makeover-geometry` and arrives already resolved.
#[derive(Debug, Clone, Copy, PartialEq)]
pub struct FieldStyle {
    /// The well a text control sits in.
    pub frame: FrameStyle,
    /// Between a field's own parts: its label, its control, its hint and its
    /// error.
    pub gap: f32,
    /// Between one field and the next.
    pub group_gap: f32,
    /// What marks a required field, appended to its label.
    ///
    /// A knob rather than a constant, because it is the one piece of *copy* in
    /// this crate and copy is not a renderer's call. A webview does not need it
    /// at all — it emits the `required` attribute and the browser answers — so
    /// this renderer is the first place where a compulsory field either shows
    /// that it is or silently does not.
    pub required_marker: &'static str,
}

impl Default for FieldStyle {
    /// The default frame, no gaps, and an asterisk.
    fn default() -> Self {
        Self {
            frame: FrameStyle::default(),
            gap: 0.0,
            group_gap: 0.0,
            required_marker: "*",
        }
    }
}

/// What the field currently holds, borrowed from wherever the app keeps it.
///
/// The immediate-mode counterpart of `makeover_webview::form::Value`, and the
/// place the two renderers are forced apart: there the value is read back out
/// of the DOM after the fact, and here the widget writes through this borrow as
/// it is edited. Same reason the description carries neither.
///
/// An enum rather than a bag of options, on the reasoning
/// `makeover_webview::form::Value` records: a checkbox holding a string is
/// unsayable here, where a struct would let it be said and then have to cope.
#[derive(Debug, Default)]
pub enum Filling<'a> {
    /// Nothing to edit. The control is drawn and does not answer.
    #[default]
    Absent,
    /// The buffer behind anything that takes typed text, a select included:
    /// what a select holds is the `value` of one of its [`Choice`]s.
    ///
    /// [`Choice`]: makeover_layout::Choice
    Text(&'a mut String),
    /// A checkbox, on or off.
    On(&'a mut bool),
    /// The two buffers behind a [`FieldKind::Interval`], lower first.
    ///
    /// Two buffers rather than one string with a separator, which is
    /// [`makeover_layout::Field::upper_name`]'s reason one level down: an
    /// interval is submitted under two names, so it is edited as two values,
    /// and a delimiter this crate owned could appear inside either of them.
    ///
    /// Either end may be empty while the other stands. An open end is an
    /// answer -- "over 120 BPM" -- rather than a half-filled box.
    Between {
        /// The lower end's buffer.
        lower: &'a mut String,
        /// The upper end's buffer.
        upper: &'a mut String,
    },
}

/// The label, marked if the field is compulsory.
fn label_text(field: &Field<'_>, style: &FieldStyle) -> String {
    if field.required {
        format!("{} {}", field.label, style.required_marker)
    } else {
        field.label.to_owned()
    }
}

/// The four shapes a control comes in here, which is fewer than there are
/// kinds.
///
/// [`FieldKind`] is `#[non_exhaustive]` and grows; this does not, because the
/// ways egui has of asking for a value do not. Reducing the open set to this
/// closed one in one total function is what keeps a new kind from needing a new
/// arm at every match below.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum Control {
    /// Typed into, so it is drawn as a well: the user looks into it.
    Typed,
    /// Picked from a control that shows one option at a time. Pressed rather
    /// than looked into, so egui's own control painting stands.
    Chosen,
    /// Picked from options that are all on screen at once.
    ///
    /// Apart from [`Chosen`](Self::Chosen) because the description holds them
    /// apart, and holding them apart is the whole content of
    /// [`FieldKind::Radio`]: same question, and an answer the user can read
    /// without opening anything.
    Listed,
    /// Held on or off.
    Toggled,
    /// Dragged across an extent that is on screen the whole time.
    ///
    /// Apart from [`Typed`](Self::Typed) for the reason
    /// [`FieldKind::Range`] is apart from `Number`: the two ends are what the
    /// question means, so a well with a figure in it is not a quieter version
    /// of this control, it is a different one.
    Slid,
    /// Picked from a list of themes that arrives grouped and marked.
    ///
    /// Apart from [`Chosen`](Self::Chosen) rather than folded into it, and the
    /// distinction is the same one [`Listed`](Self::Listed) draws: it is not a
    /// different question, it is a different amount of structure on screen. A
    /// theme picker's rows carry a group heading and a contrast mark, and both
    /// come from members [`Field::options`] does not have, so a shared arm
    /// would be a `matches!` on the kind inside the loop rather than one arm
    /// less.
    Themed,
    /// Two values dragged across one axis, drawn as one question.
    ///
    /// Apart from [`Typed`](Self::Typed) for the reason
    /// [`FieldKind::Interval`] is apart from `Number`: two wells one under the
    /// other are two questions on screen, whatever the description says, and
    /// the arrangement is the whole content of the kind.
    Spanned,
}

/// Which shape a kind takes.
///
/// The wildcard falls to [`Control::Typed`] on purpose: a kind added to the
/// description since this renderer was built degrades to a text box, which
/// accepts any value the others would, rather than to nothing drawn at all.
///
/// `FieldKind::File` lands there rather than growing a shape of its own. egui's
/// honest answer is a button that opens a native picker, which is a fifth
/// control and a file-dialog dependency, and no consumer of this crate asks for
/// a file field. The membership test is that every renderer *could* answer
/// honestly, not that each one does on the day. A path in a text box is not
/// nothing.
///
/// `Field::accept` and `Field::multiple` land on the same position: both are the picker's arguments, and this
/// renderer has no picker to give them to. They are not lost — the description
/// still carries them, and the day the native dialog arrives here it is opened
/// with them rather than with a filter written twice.
///
/// `FieldKind::Date` and `FieldKind::DateTime` land there too, on the same
/// footing and with one thing owed. A
/// calendar is a sixth control and bare `egui` has none, so a typed value is
/// the honest answer here; what the app gets is the format the description
/// names, `makeover_layout::DATE_FORMAT` and `DATETIME_FORMAT`, which is why
/// those are constants rather than a sentence. audiofiles is the only consumer
/// of this crate and asks for neither today. A calendar popup is the upgrade
/// whenever one does.
///
/// `Field::as_instant` is carried and not honoured, on
/// the same footing. It asks for the typed wall-clock value to be submitted as
/// the moment it names, and this renderer has no submission to convert on: it
/// draws the control and the app reads the value back, so the conversion would
/// belong wherever that read happens rather than here. What the app gets is the
/// local value in `DATETIME_FORMAT`, which is what it got before the member
/// existed. No described site on this host asks for it today.
/// One row of a closed chooser, as the single string it has room for.
///
/// A combo hides its list, so everything an option carries has to travel with
/// the row it belongs to: there is no second line to put a detail on and no
/// space beside the row to put a reason in. That is the same constraint a
/// `<select>`'s option has in the webview, and it takes the same answer.
///
/// The order is what the option *is* before why it cannot be picked, which is
/// the order the two read in and the order the radio group draws them in.
fn combo_row(opt: &Choice<'_>) -> String {
    let mut text = opt.label.to_owned();
    for extra in [opt.detail, opt.unavailable].into_iter().flatten() {
        text.push_str("  ");
        text.push_str(extra);
    }
    text
}

/// How far an option's second line is inset, in points.
///
/// The width of egui's radio button plus the gap after it, so the line starts under the label rather than under the control. A
/// magnitude, which is `makeover-geometry`'s subject and not this crate's --
/// but this one is measured off a widget egui draws and sizes, so there is
/// nothing for a spacing scale to say about it.
const OPTION_DETAIL_INDENT: f32 = 22.0;

const fn control_shape(kind: FieldKind) -> Control {
    match kind {
        FieldKind::Select => Control::Chosen,
        FieldKind::Radio => Control::Listed,
        FieldKind::Checkbox => Control::Toggled,
        FieldKind::Range => Control::Slid,
        FieldKind::Interval => Control::Spanned,
        FieldKind::Theme => Control::Themed,
        _ => Control::Typed,
    }
}

/// The shape the field actually gets, which is the kind's unless the field is
/// missing what that shape needs.
///
/// One case, and `makeover-layout` names it: a [`FieldKind::Range`] carries its
/// extent in [`Field::min`] and [`Field::max`], and a range missing an end has
/// nothing to slide across. egui's `Slider` demands a `RangeInclusive`, so
/// inventing one would be this renderer picking bounds the app never stated and
/// the user then dragging against them.
///
/// It falls back to [`Control::Typed`], which is where every kind this renderer
/// cannot draw natively already lands: a number in a well is a true report of
/// the value and takes any answer the slider would.
fn shape_of(field: &Field<'_>) -> Control {
    match control_shape(field.kind) {
        Control::Slid if !field.bounded() => Control::Typed,
        shape => shape,
    }
}

/// The unit to draw beside this field's value, if there is one to draw.
///
/// Two conditions rather than one: the field has to carry a unit and its kind
/// has to be one that means anything by it. `FieldKind::measurable` is the
/// description answering the second, so this renderer does not keep its own
/// list of which kinds are quantities -- which is the drift that predicate
/// exists to stop.
fn unit_of<'a>(field: &Field<'a>) -> Option<&'a str> {
    field.unit.filter(|_| field.kind.measurable())
}

/// The two ends of a range, as egui wants them.
///
/// `None` when either end is missing or is not a number this host can read.
/// The description carries the bounds as text on purpose — the bound of a date
/// is a date — so parsing them is the renderer's job and failing to is a real
/// outcome rather than an assertion.
fn extent(field: &Field<'_>) -> Option<RangeInclusive<f64>> {
    let min = field.min?.parse::<f64>().ok()?;
    let max = field.max?.parse::<f64>().ok()?;
    Some(min..=max)
}

/// How many decimals to write a dragged value back with.
///
/// Read off [`Field::step`], which is the only thing that says what
/// granularity the question has: a step of `0.01` is a two-decimal question and
/// a step of `1` is a whole-number one. Without a step the host's own
/// granularity stands, and egui's is continuous, so the value is written back
/// at whatever precision it round-trips at.
fn decimals(step: Option<&str>) -> Option<usize> {
    let step = step?;
    Some(match step.split_once('.') {
        Some((_, fraction)) => fraction.trim_end_matches('0').len(),
        None => 0,
    })
}

/// What a select shows for the value it currently holds.
///
/// A value no option carries stays on screen as itself rather than reading as
/// whichever option happens to be first. goingson saved a backup retention of
/// 10 against a 1/3/7/14/0 list and the browser silently showed it as 1, so the
/// next save wrote a value nobody chose; `makeover-webview` grew the fix as a
/// stray `<option>` and this is the same fix in the shape egui allows.
///
/// The empty value is the one case that reads as unanswered rather than as an
/// answer, and [`chosen_text`] is what puts the field's ghost text there.
fn shown_label<'a>(options: &'a [Choice<'a>], value: &'a str) -> &'a str {
    options
        .iter()
        .find(|opt| opt.value == value)
        .map_or(value, |opt| opt.label)
}

/// What a select's closed control reads, and in which tone.
///
/// A chooser with nothing chosen showed an empty box: `shown_label` falls back
/// to the value, and the unanswered value is the empty string. So an app with
/// an instruction to give — audiofiles' "Select device..." — had nowhere to put
/// it but a disabled button elsewhere on the screen, which is the affordance
/// this vocabulary keeps moving messages *off*.
///
/// [`Field::placeholder`] is already the description's word for "what the field
/// reads while it is empty" and was honoured by the typed kinds alone, so
/// nothing new is said here; the renderer is what had not caught up. Muted
/// because it is not an answer, the same tone the typed kinds' ghost text takes
/// three lines up.
///
/// A value no option carries but that is *not* empty stays as itself, in
/// `content`: that is the goingson retention bug and it is a wrong answer
/// rather than an absent one.
///
/// Returns the words and the tone rather than a built [`RichText`], because
/// what it decides is both of them and only one of them is readable back off a
/// `RichText`.
///
/// [`Field::placeholder`]: makeover_layout::Field::placeholder
fn chosen_text<'a>(field: &'a Field<'a>, value: &'a str, palette: &Palette) -> (&'a str, Color32) {
    match field.placeholder {
        Some(ghost) if value.is_empty() => (ghost, palette.content_muted),
        _ => (shown_label(field.options, value), palette.content),
    }
}

/// What one option in a choice field is drawn in.
///
/// The chosen one is the emphasised thing and takes `content`; the rest take
/// [`content_secondary`](Palette::content_secondary), because an option that is
/// not chosen is still an option and pressing it chooses it. Muted would be the
/// lie: [`State::Disabled`] resolves to it, so a five-option field read as one
/// live row and four dead ones. `makeover-tui` draws it the same way
/// (`makeover-tui@230bf63`); wiki `three-tone-convention` is the table.
fn option_color(value: &str, option: &str, palette: &Palette) -> Color32 {
    if value == option {
        palette.content
    } else {
        palette.content_secondary
    }
}

/// Which end of an interval a box is.
///
/// Named rather than a bool, because what it selects is not a side but a
/// fallback: an empty end reads as the bound it stands for, and which bound
/// that is depends on the end.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum Bound {
    /// The lower end, falling back to the start of the extent.
    Low,
    /// The upper end, falling back to its end.
    High,
}

/// The facts an interval's two boxes share.
///
/// One struct because they are one axis: [`Field::min`], [`Field::max`],
/// [`Field::step`] and [`Field::unit`] describe the question rather than either
/// end of it, so reading them once is what stops the two boxes drifting apart.
struct Axis<'a> {
    /// The extent both ends are dragged inside, when it is one this host can
    /// read.
    extent: Option<RangeInclusive<f64>>,
    /// The granularity, as the description writes it.
    step: Option<&'a str>,
    /// What the axis is measured in.
    unit: Option<&'a str>,
}

impl Axis<'_> {
    /// What an empty end reads as: the bound it stands for.
    ///
    /// Zero with no extent to fall back on. That is the one number this
    /// renderer invents, and it invents it where the description declined to
    /// say anything: an unbounded interval has no edge for the end to sit on,
    /// and a drag box has to start somewhere.
    fn edge(&self, which: Bound) -> f64 {
        self.extent.as_ref().map_or(0.0, |extent| match which {
            Bound::Low => *extent.start(),
            Bound::High => *extent.end(),
        })
    }

    /// One end of the interval, as a drag box.
    ///
    /// # An empty end reads as its bound
    ///
    /// Which is what the shipped control did before it was described: an unset
    /// minimum sits on the low edge and stores no filter. egui's `DragValue`
    /// holds a number and has no empty state to offer, so the alternative was a
    /// text box, and that would cost the app a control on the way into being
    /// described.
    ///
    /// With no extent to fall back on, an empty end reads zero. That is the one
    /// number this renderer invents, and it invents it where the description
    /// declined to say anything: an unbounded interval has no edge for the end
    /// to sit on, and a drag box has to start somewhere.
    ///
    /// Nothing is written back until the user drags, so a value the app put
    /// there survives being looked at -- the same guarantee the slider makes.
    fn end(&self, ui: &mut Ui, value: &mut String, which: Bound) -> Response {
        let mut number = value.parse::<f64>().unwrap_or(self.edge(which));
        let mut drag = DragValue::new(&mut number);
        if let Some(extent) = self.extent.clone() {
            drag = drag.range(extent);
        }
        if let Some(places) = decimals(self.step) {
            drag = drag.max_decimals(places);
        }
        if let Some(step) = self.step.and_then(|s| s.parse::<f64>().ok()) {
            drag = drag.speed(step);
        }
        // Inside the control, beside the readout, which is where `Field::unit`
        // was decided to belong and where these boxes already put it.
        if let Some(unit) = self.unit {
            drag = drag.suffix(format!(" {unit}"));
        }
        let response = ui.add(drag);
        if response.changed() {
            *value = match decimals(self.step) {
                Some(places) => format!("{number:.places$}"),
                None => number.to_string(),
            };
        }
        response
    }
}

/// The control alone, without its label, hint or error.
fn control(
    ui: &mut Ui,
    field: &Field<'_>,
    filling: Filling<'_>,
    palette: &Palette,
    style: &FieldStyle,
    named_by: Option<egui::Id>,
) -> Response {
    // The mismatch path: described as one thing and filled as another. Nothing
    // here can fix it, so it is drawn as the empty, inert version of what was
    // described — visible on screen, in the way an empty select is at the
    // webview renderer, rather than reported in a log nobody reads.
    let mut discard = String::new();
    // The interval's second scratch buffer. Two ends means the mismatch path
    // needs two places to write nothing to.
    let mut spare = String::new();
    let mut off = false;

    match shape_of(field) {
        Control::Slid => {
            let value = match filling {
                Filling::Text(text) => text,
                _ => &mut discard,
            };
            // `shape_of` has already refused an unbounded range, so the extent
            // is only missing here if a bound is not a number — a date range,
            // say, which this control cannot draw either.
            let Some(extent) = extent(field) else {
                return ui.label(RichText::new(value.as_str()).color(palette.content));
            };

            // A value the host cannot read starts at the low end rather than at
            // zero, which may be outside the extent entirely. Nothing is
            // written back until the user drags, so an unreadable value the app
            // put there survives being looked at.
            let mut number = value.parse::<f64>().unwrap_or(*extent.start());
            // The granularity is the curve's as of makeover-layout 0.32.0. It
            // is still in the value's own units, so the display precision is
            // read off it exactly as before.
            let step = field.curve.step();
            let mut slider = Slider::new(&mut number, extent.clone()).text("");
            if let Some(places) = decimals(step) {
                slider = slider.max_decimals(places);
            }
            if let Some(step) = step.and_then(|s| s.parse::<f64>().ok()) {
                slider = slider.step_by(step);
            }
            // egui's own constant-ratio track, which is this host's answer to
            // `Curve::Logarithmic`. `is_ratio` rather than a match on the
            // variant, because a ratio across zero is not one: makeover-layout
            // decides the fallback so that four renderers cannot disagree about
            // when it applies.
            if field.curve.is_ratio(*extent.start(), *extent.end()) {
                slider = slider.logarithmic(true);
            }
            // The unit goes inside the control, beside the readout egui already
            // draws. That placement is the argument `Field::unit` was decided
            // on: it is where these controls put it before they were described,
            // and it is the one a label could never reach.
            if let Some(unit) = unit_of(field) {
                slider = slider.suffix(format!(" {unit}"));
            }
            let response = ui.add(slider);
            if response.changed() {
                *value = match decimals(step) {
                    Some(places) => format!("{number:.places$}"),
                    None => number.to_string(),
                };
            }
            response
        }
        // One question, so one row. Two wells stacked would be two questions on
        // screen whatever the description said, which is the reading
        // `FieldKind::Interval` exists to prevent.
        //
        // Dragged rather than typed, because that is what these controls
        // already were: audiofiles' six filter axes are `DragValue` pairs with
        // a shared extent, a speed and a suffix, and a port that turned them
        // into text boxes would be a description costing the app a control.
        Control::Spanned => {
            let (lower, upper) = match filling {
                Filling::Between { lower, upper } => (lower, upper),
                _ => (&mut discard, &mut spare),
            };
            let axis = Axis {
                extent: extent(field),
                step: field.step,
                unit: unit_of(field),
            };
            ui.horizontal(|ui| {
                let low = axis.end(ui, lower, Bound::Low);
                // The word rather than a dash. A dash between two numbers is a
                // minus sign on a signed axis, and audiofiles filters loudness
                // in dBFS.
                ui.label(RichText::new("to").color(palette.content_secondary));
                let high = axis.end(ui, upper, Bound::High);
                // Both ends, by name. A union response carries the first id, so
                // labelling the union outside this arm names the lower box and
                // leaves the upper one announced as whatever number is in it --
                // which is how an interval half-kept the fix that gave every
                // other shape its question.
                if let Some(id) = named_by {
                    low.clone().labelled_by(id);
                    high.clone().labelled_by(id);
                }
                low | high
            })
            .inner
        }
        Control::Typed => {
            let text = match filling {
                Filling::Text(text) => text,
                _ => &mut discard,
            };
            // An empty frame and no margin: the well is this crate's, and egui's
            // own control background and padding would sit underneath it saying
            // something different about both.
            // Keyed on the description's own `multiline` and not on the
            // member: a markdown field is several lines by definition, and a
            // single-line edit would be a control the value cannot fit in. egui
            // does nothing else with the markdown, which is the honest answer
            // rather than a gap -- the source is text, and editing it as text
            // loses none of it.
            let mut edit = if field.kind.multiline() {
                TextEdit::multiline(text)
            } else {
                TextEdit::singleline(text)
            }
            .frame(egui::Frame::NONE)
            .margin(Margin::ZERO)
            .text_color(palette.content)
            .password(field.kind.confidential());
            if let Some(ghost) = field.placeholder {
                edit = edit.hint_text(RichText::new(ghost).color(palette.content_muted));
            }
            let response = frame(ui, Depth::Well, palette, style.frame, |ui| ui.add(edit));
            // A typed number has no readout of its own to sit beside, so the
            // unit follows the box. Muted, because it is a fact about the value
            // rather than a second thing to read.
            match unit_of(field) {
                Some(unit) => {
                    ui.label(RichText::new(unit).color(palette.content_muted));
                    response
                }
                None => response,
            }
        }
        Control::Toggled => {
            let on = match filling {
                Filling::On(on) => on,
                _ => &mut off,
            };
            ui.checkbox(on, RichText::new(field.label).color(palette.content))
        }
        Control::Listed => {
            let value = match filling {
                Filling::Text(text) => text,
                _ => &mut discard,
            };
            // No `shown_label` counterpart, and none is needed: a value no
            // option carries leaves every button unfilled, which is already
            // the honest report on screen. The select needs the fix because it
            // has one slot and must put *something* in it.
            let group = ui.vertical(|ui| {
                let mut answered: Option<Response> = None;
                for opt in field.options {
                    // An option that cannot be picked yet is drawn and does not
                    // answer, with the precondition beside it rather than
                    // behind a hover: a greyed row with no reason reads as a
                    // dead end, which is the state `Choice::unavailable` exists
                    // to stop being sayable.
                    let picked = if let Some(reason) = opt.unavailable {
                        ui.horizontal(|ui| {
                            let picked = ui
                                .add_enabled_ui(false, |ui| {
                                    ui.radio_value(
                                        value,
                                        opt.value.to_owned(),
                                        RichText::new(opt.label).color(palette.content_muted),
                                    )
                                })
                                .inner;
                            ui.label(RichText::new(reason).color(palette.content_muted));
                            picked
                        })
                        .inner
                    } else {
                        ui.radio_value(
                            value,
                            opt.value.to_owned(),
                            RichText::new(opt.label).color(option_color(value, opt.value, palette)),
                        )
                    };
                    // What picking it means, under the option rather than
                    // beside it. makeover-layout 0.39.0, and the placement is
                    // the difference from the reason above: a precondition is
                    // short enough to sit on the line, and a sentence saying
                    // what a tier is would push every option's control out of
                    // line with its neighbours.
                    //
                    // Indented past the radio, so it reads as belonging to the
                    // option above rather than as a label of its own. Muted,
                    // the same reading `.form-option-detail` takes in the
                    // webview: the line orients the label, it does not compete
                    // with it.
                    if let Some(detail) = opt.detail {
                        ui.horizontal(|ui| {
                            ui.add_space(OPTION_DETAIL_INDENT);
                            ui.label(RichText::new(detail).color(palette.content_muted));
                        });
                    }
                    answered = Some(match answered {
                        Some(prev) => prev.union(picked),
                        None => picked,
                    });
                }
                answered
            });
            // A group described with no options answers as its own empty area
            // rather than as no response at all, which keeps the caller's
            // `.changed()` chain working on a field whose option list has not
            // loaded yet.
            group.inner.unwrap_or(group.response)
        }
        Control::Chosen => {
            let value = match filling {
                Filling::Text(text) => text,
                _ => &mut discard,
            };
            let (shown, tone) = chosen_text(field, value, palette);
            ComboBox::from_id_salt(field.name)
                .selected_text(RichText::new(shown).color(tone))
                .show_ui(ui, |ui| {
                    for opt in field.options {
                        // Same rule as the radio group: shown, inert, and
                        // saying why. A closed control hides its list, so the
                        // reason has to travel with the row it belongs to.
                        // A closed control hides its list, so everything an
                        // option carries has to travel with the row it belongs
                        // to. That is why both extra strings run into the text
                        // here and neither does in the group above: a combo
                        // row is a row, the way a `<select>`'s option is.
                        let text = combo_row(opt);
                        if opt.unavailable.is_some() {
                            ui.add_enabled_ui(false, |ui| {
                                ui.selectable_value(
                                    value,
                                    opt.value.to_owned(),
                                    RichText::new(text).color(palette.content_muted),
                                );
                            });
                            continue;
                        }
                        ui.selectable_value(
                            value,
                            opt.value.to_owned(),
                            RichText::new(text).color(option_color(value, opt.value, palette)),
                        );
                    }
                })
                .response
        }
        // The grouping comes out of the order rather than out of a group list:
        // `Field::themes` arrives sorted by variant, so the run of one variant
        // is the group and a heading opens whenever the variant changes. Same
        // walk as `makeover-webview`'s `<optgroup>` emission, which is what
        // keeps two renderers from disagreeing about where a group starts.
        Control::Themed => {
            let value = match filling {
                Filling::Text(text) => text,
                _ => &mut discard,
            };
            let shown = themed_text(field, value);
            ComboBox::from_id_salt(field.name)
                .selected_text(RichText::new(shown).color(palette.content))
                .show_ui(ui, |ui| {
                    if let Some(follow) = field.follows {
                        // First and outside every heading. It names no theme
                        // and sits in no variant, so a heading over it would be
                        // inventing a fourth variant for one row.
                        ui.selectable_value(
                            value,
                            follow.value.to_owned(),
                            RichText::new(follow.label).color(option_color(
                                value,
                                follow.value,
                                palette,
                            )),
                        );
                    }
                    let mut open: Option<ThemeVariant> = None;
                    for theme in field.themes {
                        if open != Some(theme.variant) {
                            // A heading rather than a `selectable_value`: it is
                            // not pickable, and egui has no inert row that
                            // still reads as a row. `content_muted` is the tone
                            // for something that is not an answer, which is the
                            // ghost text's tone eight lines up.
                            if open.is_some() {
                                ui.separator();
                            }
                            ui.label(
                                RichText::new(theme.variant.heading()).color(palette.content_muted),
                            );
                            open = Some(theme.variant);
                        }
                        ui.selectable_value(
                            value,
                            theme.id.to_owned(),
                            RichText::new(format!("{}  {}", theme.name, theme.contrast.badge()))
                                .color(option_color(value, theme.id, palette)),
                        );
                    }
                })
                .response
        }
    }
}

/// What a theme picker's closed control reads.
///
/// [`chosen_text`]'s counterpart, and it is separate for the reason
/// [`Control::Themed`] is: the label lives on a [`makeover_layout::ThemeChoice`]
/// rather than on a [`Choice`], and the follow row is a third place to look.
///
/// No placeholder arm. A theme picker is never unanswered in the way a select
/// is — an app that resolved a theme to paint this control with has one — and
/// falling back to the raw value is the honest report on a stored id whose
/// theme has since been deleted.
fn themed_text<'a>(field: &'a Field<'a>, value: &'a str) -> &'a str {
    if let Some(follow) = field.follows
        && follow.value == value
    {
        return follow.label;
    }
    field
        .themes
        .iter()
        .find(|theme| theme.id == value)
        .map_or(value, |theme| theme.name)
}

/// One field, as the column the app drops into its form.
///
/// The anatomy is `makeover-webview`'s, so the two renderers put a form
/// together the same way: label, control, hint, error, top to bottom, with a
/// checkbox labelling itself instead of taking a label above.
///
/// Returns [`None`] for a [`FieldKind::Hidden`] field, which is what
/// [`FieldKind::visible`] means and is the honest answer here: a webview still
/// emits an input for it because the form submits, and an immediate-mode
/// renderer has no form and no submission, so a hidden field is a value the app
/// already holds and there is nothing to draw or to respond to.
///
/// `state` is the description's interaction axis.
/// [`State::Disabled`] greys the field and stops it answering, through
/// [`State::suppresses_interaction`] rather than through a second reading of
/// what disabled means. Focus is not on that axis and never reaches here: egui
/// owns reach, focus and the ring for this renderer, and one ring means not a
/// second one per renderer that happens to have opinions.
pub fn field(
    ui: &mut Ui,
    field: &Field<'_>,
    filling: Filling<'_>,
    state: Option<State>,
    palette: &Palette,
    style: &FieldStyle,
) -> Option<Response> {
    if !field.kind.visible() {
        return None;
    }
    let enabled = !state.is_some_and(State::suppresses_interaction);
    let text = if enabled {
        palette.content
    } else {
        palette.content_muted
    };

    let response = ui
        .vertical(|ui| {
            ui.spacing_mut().item_spacing.y = style.gap;

            // A checkbox labels itself, on the right of the box.
            // `FieldKind::labels_itself` is the description saying so, and both
            // webview apps special-cased it inline before it did.
            let named_by = (!field.kind.labels_itself())
                .then(|| ui.label(RichText::new(label_text(field, style)).color(text)));

            let response = ui
                .add_enabled_ui(enabled, |ui| {
                    control(
                        ui,
                        field,
                        filling,
                        palette,
                        style,
                        named_by.as_ref().map(|l| l.id),
                    )
                })
                .inner;

            // The label, attached rather than merely adjacent.
            //
            // Drawing it above the control and stopping there is what this did
            // until 2026-08-22, and it put an unnamed box in the accessibility
            // tree with some text near it: a screen reader announced a text
            // field with no question, and a prefilled one announced its own
            // contents instead. audiofiles' four name modals were the site that
            // measured it, through a harness reading what the panel drew.
            //
            // Worth stating why it was worth fixing here rather than in each
            // app: `Field::label` is a member the description carries so a
            // renderer does not have to guess, `makeover-webview` has always
            // named it in `aria-describedby`, and the two renderers disagreeing
            // about a fact the description states is the one thing this layer
            // exists to prevent. A checkbox is unaffected -- egui names one from
            // its own text, which is what `labels_itself` already says.
            let response = match named_by {
                Some(label) => response.labelled_by(label.id),
                None => response,
            };

            // Standing help, then what the answer costs, then what is wrong
            // now. All three, in that order, for the reason the webview
            // renderer names all three in `aria-describedby`: an error
            // appearing must not take the hint away with it. This host has the
            // room, so unlike makeover-tui it never has to choose -- the
            // precedence rule on `Field::note` is for the renderer that does.
            if let Some(hint) = field.hint {
                ui.label(RichText::new(hint).color(palette.content_muted));
            }
            // The note carries its own tone, and `Palette::tone` is what
            // resolves it, so a Neutral note is ordinary content rather than
            // a colour this renderer picked.
            if let Some((tone, note)) = field.note {
                ui.label(RichText::new(note).color(palette.tone(tone)));
            }
            if let Some(error) = field.error {
                ui.label(RichText::new(error).color(palette.danger));
            }
            response
        })
        .inner;

    Some(response)
}

/// A set of fields, laid down a column.
///
/// `show_extended` is the disclosure, and it is a parameter rather than state
/// held here because the disclosure belongs to the *form* and not to any field:
/// [`Field::extended`] marks which fields are behind one, and the app owns
/// whether it is open. That is the same division `makeover-webview` draws when
/// it marks the group `data-extended` and emits no control to toggle it.
///
/// `draw` is called once per field that should be visible, in order. Taking a
/// callback rather than a slice of [`Filling`]s is what keeps the app's own
/// values borrowed one at a time: a form's fields usually live in different
/// structs, and a parallel array would have to be built each frame and kept in
/// step with the description by hand.
pub fn group<'a>(
    ui: &mut Ui,
    fields: &'a [Field<'a>],
    show_extended: bool,
    style: &FieldStyle,
    mut draw: impl FnMut(&mut Ui, &'a Field<'a>),
) {
    ui.vertical(|ui| {
        ui.spacing_mut().item_spacing.y = style.group_gap;
        for f in fields {
            if f.extended && !show_extended {
                continue;
            }
            draw(ui, f);
        }
    });
}

#[cfg(test)]
mod tests;