makeover_tui/

widget.rs

//! The pieces every terminal app draws, drawn once.
//!
//! Arrived in 0.16.0 out of `quasi-tui`, which had written all of them and was
//! the second consumer to do so. A meter, a badge, a control, a figure and a
//! form field are what a screen is made of below the level [`table`](crate::table)
//! works at, and every one of them had been hand-rolled at least twice in this
//! tree before it was lifted.
//!
//! # What these take, and what they leave alone
//!
//! Each takes a `makeover-layout` description, a [`WidgetStyle`], and whatever
//! the *host* knows that a description never carries. That last part is the
//! shape worth copying: [`field`] takes what is currently typed in the box as a
//! separate argument, because [`Field`] deliberately does not carry a value and
//! is not going to. `makeover-immediate` reached the same seam from the other
//! side with its `Filling`, and [`Held`] is that seam here.
//!
//! Focus is the other one. Nothing in a description says which control the user
//! is on, so every drawing here takes `focused` as an argument and the caller
//! is what counts. What focus *looks like* is this crate's answer and not the
//! caller's, which is the point of it being here: see
//! [`WidgetStyle::focused`].
//!
//! # What they do not do
//!
//! No layout. Each answers rows for a width, or draws into the rect it is
//! given, top-aligned, and never below it. Nothing here measures twice and
//! nothing here places anything relative to anything else, because the moment
//! it did it would be a layout engine with one consumer's flow baked into it.

use makeover_layout::{Act, Field, FieldKind, Figure, Heading, Meter, Token, Tone};
use ratatui::buffer::Buffer;
use ratatui::layout::Rect;
use ratatui::style::{Modifier, Style};
use ratatui::text::{Line, Span};

use crate::text;

/// The colours and marks the drawings below use.
///
/// [`TableStyle`](crate::table::TableStyle)'s shape, for its reasons: an
/// ungated struct of styles with a [`Default`], plus a
/// [`from_theme`](Self::from_theme) that is what a consumer holding a loaded
/// theme should reach for first. A consumer painting bevels and nothing else
/// should not have to supply text tones it never uses, and gating the whole
/// module on `theme` would make these unreachable to anyone hand-picking
/// colours.
///
/// The default is the one that survives a terminal with no colour at all:
/// modifiers only, no foreground anywhere. That is not a placeholder. A
/// two-colour terminal is the case where a `Style` carrying a foreground is a
/// foreground that will not land, and bold-and-reversed is what is left.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct WidgetStyle {
    /// Ordinary content, and what [`Tone::Neutral`] reads as.
    pub content: Style,
    /// Content one step back: a field's label, a quoted run.
    pub secondary: Style,
    /// Content two steps back: a caption, a hint, a meter's reading.
    pub muted: Style,
    /// Something worth knowing and nothing to do about it.
    pub info: Style,
    /// Something finished and it worked.
    pub success: Style,
    /// Something the user should look at.
    pub warning: Style,
    /// Something broken, or about to be destroyed.
    pub danger: Style,
    /// A page title.
    pub page: Style,
    /// A section title.
    pub section: Style,
    /// A subsection title.
    pub subsection: Style,
    /// Text that goes somewhere, and a control's label.
    pub action: Style,
    /// A control filled with the action colour, for the one on a screen that is
    /// the thing to press. A form's submit is the case that has it.
    pub filled: Style,
    /// A surface set back from the one it sits on, by colour and nothing else.
    /// What a code run takes, since every cell is monospace and the thing a
    /// webview says with a typeface cannot be said that way here.
    pub sunken: Style,
    /// What "you are on this one" adds to whatever it lands on.
    ///
    /// Reversed video by default, which is the affordance a cell has left once
    /// colour is spent on tone and bold on weight. A webview says it with an
    /// outline; a terminal has no outline that is not four more cells.
    pub focus: Modifier,
    /// How many cells [`meter`] spends on its bar.
    pub meter_cells: u16,
    /// The filled part of a bar.
    pub meter_full: char,
    /// The empty part of a bar.
    pub meter_empty: char,
    /// What marks a compulsory field, appended to its label.
    ///
    /// A knob for `makeover-immediate`'s reason: it is the one piece of *copy*
    /// here, and copy is not a renderer's call.
    pub required_marker: &'static str,
}

impl Default for WidgetStyle {
    /// Modifiers only, no foreground: what survives a terminal with two
    /// colours.
    fn default() -> Self {
        Self {
            content: Style::new(),
            secondary: Style::new(),
            muted: Style::new().add_modifier(Modifier::DIM),
            info: Style::new(),
            success: Style::new(),
            warning: Style::new(),
            danger: Style::new().add_modifier(Modifier::BOLD),
            page: Style::new().add_modifier(Modifier::BOLD),
            section: Style::new().add_modifier(Modifier::BOLD),
            subsection: Style::new(),
            action: Style::new().add_modifier(Modifier::UNDERLINED),
            filled: Style::new().add_modifier(Modifier::REVERSED),
            sunken: Style::new().add_modifier(Modifier::DIM),
            focus: Modifier::REVERSED,
            meter_cells: 10,
            meter_full: '#',
            meter_empty: '-',
            required_marker: "*",
        }
    }
}

impl WidgetStyle {
    /// The house widgets, from a loaded theme.
    ///
    /// The lift this module exists for. `quasi-tui` carried every line of this
    /// as private methods on its own renderer; a second terminal app wanting a
    /// toned control had no way to reach them and would have picked its own
    /// colours for the same five tones.
    #[cfg(feature = "theme")]
    #[must_use]
    pub fn from_theme(theme: &crate::Theme) -> Self {
        Self {
            content: Style::new().fg(theme.content_primary),
            secondary: Style::new().fg(theme.content_secondary),
            muted: Style::new().fg(theme.content_muted),
            info: Style::new().fg(theme.status_info),
            success: Style::new().fg(theme.status_success),
            warning: Style::new().fg(theme.status_warning),
            danger: Style::new().fg(theme.status_danger),
            // Three depths and two of them are bold, which is the whole of what
            // a terminal has: there is no type scale in a grid of one cell
            // size. A page title takes bold and the accent, a section bold, a
            // subsection the secondary colour. That is the emphasis order a
            // webview's type scale says with size, said with the two axes a
            // cell has.
            page: Style::new()
                .fg(theme.action_primary)
                .add_modifier(Modifier::BOLD),
            section: Style::new()
                .fg(theme.content_primary)
                .add_modifier(Modifier::BOLD),
            subsection: Style::new().fg(theme.content_secondary),
            action: Style::new().fg(theme.action_primary),
            filled: Style::new().fg(theme.selection_on).bg(theme.action_primary),
            sunken: Style::new().bg(theme.surface_sunken),
            focus: Modifier::REVERSED,
            meter_cells: 10,
            meter_full: '#',
            meter_empty: '-',
            required_marker: "*",
        }
    }

    /// The style a tone reads as.
    ///
    /// [`Tone`] is closed and stays closed, so this is total and needs no
    /// fallback arm.
    #[must_use]
    pub const fn tone(&self, tone: Tone) -> Style {
        match tone {
            Tone::Neutral => self.content,
            Tone::Info => self.info,
            Tone::Success => self.success,
            Tone::Warning => self.warning,
            Tone::Danger => self.danger,
        }
    }

    /// The style a heading reads as.
    #[must_use]
    pub const fn heading(&self, level: Heading) -> Style {
        match level {
            Heading::Page => self.page,
            Heading::Section => self.section,
            Heading::Subsection => self.subsection,
        }
    }

    /// `style`, plus the mark that says the user is on this one.
    ///
    /// Takes the flag rather than being called behind an `if`, because every
    /// caller has a bool in hand and the branch is the part that gets forgotten.
    #[must_use]
    pub fn focused(&self, focused: bool, style: Style) -> Style {
        if focused {
            style.add_modifier(self.focus)
        } else {
            style
        }
    }
}

/// What a field currently holds, which a description never carries.
///
/// The terminal counterpart of `makeover_immediate::Filling`, and the same seam:
/// there the widget writes through a `&mut` as the value is edited, and here the
/// caller keeps an edit buffer and lends it out for the draw. Neither is
/// something [`Field`] could carry without becoming a form model.
///
/// An enum rather than a bag of options, for `Filling`'s reason: a checkbox
/// holding a string is unsayable here, where a struct would let it be said and
/// then have to cope.
#[derive(Debug, Default, Clone, Copy, PartialEq, Eq)]
pub enum Held<'a> {
    /// Nothing typed and nothing chosen. The control draws empty.
    #[default]
    Absent,
    /// What is in the box, or the `value` of the chosen [`Choice`].
    ///
    /// [`Choice`]: makeover_layout::Choice
    Text(&'a str),
    /// A checkbox, on or off.
    On(bool),
}

impl<'a> Held<'a> {
    /// What is typed, as a string. A checkbox has no text and answers empty.
    #[must_use]
    pub const fn text(self) -> &'a str {
        match self {
            Self::Text(text) => text,
            Self::Absent | Self::On(_) => "",
        }
    }

    /// Whether a checkbox is ticked.
    #[must_use]
    pub const fn on(self) -> bool {
        matches!(self, Self::On(true))
    }
}

/// A proportion as one line: the bar, then the reading beside it.
///
/// The reading is built here from the two numbers and the noun rather than
/// taken assembled, which is what [`Meter::label`] carrying the noun alone is
/// for: a terminal at one line and a tooltip want different sentence orders.
#[must_use]
pub fn meter(style: &WidgetStyle, meter: &Meter<'_>) -> Line<'static> {
    let cells = u32::from(style.meter_cells);
    let filled = meter
        .done
        .checked_mul(cells)
        .and_then(|reached| reached.checked_div(meter.total))
        .unwrap_or(0)
        .min(cells);
    let bar = format!(
        "{}{}",
        style.meter_full.to_string().repeat(filled as usize),
        style
            .meter_empty
            .to_string()
            .repeat((cells - filled) as usize)
    );
    let reading = match meter.label {
        Some(label) => format!(" {}/{} {label}", meter.done, meter.total),
        None => format!(" {}/{}", meter.done, meter.total),
    };
    Line::from(vec![
        Span::styled(bar, style.tone(meter.tone)),
        Span::styled(reading, style.muted),
    ])
}

/// A badge or a chip as one span.
///
/// Round for a badge, square for a chip. A chip answers a press and a badge does
/// not, and the bracket is the only affordance a cell has left once colour is
/// spent on the tone.
///
/// `latched` is a chip that is switched on, and it reads as reversed. So does
/// focus, which is a collision a terminal cannot avoid: latched is "this filter
/// is on" and focused is "you are here", and there is one spare axis for two
/// facts. Said here rather than resolved by inventing a third look nobody would
/// read.
///
/// A chip's removable half is not drawn. The `x` a webview hangs on a chip is a
/// second control inside one span, and a terminal reaches a control by focusing
/// it; two targets in one cell run is a question for whoever owns the
/// interaction, not for a drawing.
#[must_use]
pub fn token(
    style: &WidgetStyle,
    label: &str,
    kind: Token,
    tone: Tone,
    latched: bool,
    focused: bool,
) -> Span<'static> {
    let painted = style.tone(tone);
    let painted = if latched {
        painted.add_modifier(style.focus)
    } else {
        style.focused(focused, painted)
    };
    match kind {
        Token::Badge => Span::styled(format!("({label})"), painted),
        Token::Chip { .. } => Span::styled(format!("[{label}]"), painted),
    }
}

/// A control as one line.
///
/// `< Label > (key)`, and the key only where the description named one. That
/// member is the one place `makeover-layout` anticipated a terminal before there
/// was one, and this is the renderer that reads it.
///
/// A disabled control is drawn muted and is not marked focused, whatever the
/// caller passed: it is present, visible and not answering, so a focus mark on
/// it would be an affordance that lies. Whether it is reachable at all is the
/// caller's count to keep — ask [`Act::disabled`].
#[must_use]
pub fn act(style: &WidgetStyle, act: &Act<'_>, focused: bool) -> Line<'static> {
    let painted = if act.disabled() {
        style.muted
    } else {
        style.focused(focused, style.tone(act.tone))
    };
    let label = match act.key {
        Some(key) => format!("< {} > ({key})", act.label),
        None => format!("< {} >", act.label),
    };
    Line::from(Span::styled(label, painted))
}

/// A control filled with the action colour, for the one press a screen is about.
///
/// `[ Label ]` rather than `< Label >`, which is the weight difference a webview
/// carries as a primary-versus-secondary button. A form's submit is the case
/// this exists for.
#[must_use]
pub fn filled_act(style: &WidgetStyle, label: &str, focused: bool) -> Line<'static> {
    Line::from(Span::styled(
        format!("[ {label} ]"),
        style.focused(focused, style.filled),
    ))
}

/// The rows [`figure`] wants at `width`.
#[must_use]
pub fn figure_height(figure: &Figure<'_>, width: u16) -> u16 {
    text::height(figure.value, width) + text::height(figure.caption, width)
}

/// A figure: the number, then what it counts under it.
///
/// The tone lands on the value and its change rather than on the caption, which
/// is what [`Figure::tone`] means: the figure is an ordinary fact and it is the
/// movement that reads as good or bad.
pub fn figure(style: &WidgetStyle, figure: &Figure<'_>, area: Rect, buf: &mut Buffer) -> u16 {
    let value = match figure.change {
        Some(change) => format!("{} {change}", figure.value),
        None => figure.value.to_owned(),
    };
    let used = text::draw(
        &value,
        style.tone(figure.tone).add_modifier(Modifier::BOLD),
        area,
        buf,
    );
    used + text::draw(figure.caption, style.muted, below(area, used), buf)
}

/// The rows [`field`] wants at `width`.
///
/// A label row, the control's rows, and a row for whatever went wrong. A hidden
/// field is nothing at all, which is the one field kind a terminal and a webview
/// agree on completely.
#[must_use]
pub fn field_height(style: &WidgetStyle, field: &Field<'_>, width: u16) -> u16 {
    if !field.kind.visible() {
        return 0;
    }
    let label = text::height(&label_of(style, field), width);
    let body = match field.kind {
        FieldKind::Textarea => 3,
        kind if kind.offers_options() => u16::try_from(field.options.len()).unwrap_or(u16::MAX),
        _ => 1,
    };
    let note = note_of(field).map_or(0, |note| text::height(note, width));
    label + body + note
}

/// A question: its label, the box, and its standing help or what is wrong now.
///
/// `held` is what the user has done to it since the screen arrived, which is the
/// argument a description cannot supply. See [`Held`].
///
/// `focused` marks the box rather than the label, because the box is where the
/// typing lands.
pub fn field(
    style: &WidgetStyle,
    field: &Field<'_>,
    held: Held<'_>,
    focused: bool,
    area: Rect,
    buf: &mut Buffer,
) -> u16 {
    // A hidden field is data travelling with the form. There is nothing to
    // draw, and whoever submits carries it.
    if !field.kind.visible() || area.width == 0 || area.height == 0 {
        return 0;
    }

    let mut used = text::draw(&label_of(style, field), style.secondary, area, buf);

    let well = style.focused(focused, style.content);
    let placeholder = field.placeholder.unwrap_or_default();

    used += match field.kind {
        FieldKind::Checkbox => text::draw(
            if held.on() { "[x]" } else { "[ ]" },
            well,
            below(area, used),
            buf,
        ),
        kind if kind.offers_options() => {
            let mut rows = 0;
            for choice in field.options {
                let chosen = held.text() == choice.value;
                let mark = if chosen { "(*)" } else { "( )" };
                rows += text::draw(
                    &format!("{mark} {}", choice.label),
                    if chosen { well } else { style.muted },
                    below(area, used + rows),
                    buf,
                );
            }
            rows
        }
        // A secret's dots come from the caller's buffer and can come from
        // nowhere else: a password that comes back down the wire is a password
        // in a page and in a proxy log, so a description carries nothing to dot
        // out. This is the one control that would be undrawable without `held`.
        FieldKind::Secret if !held.text().is_empty() => {
            let dots = "*".repeat(held.text().chars().count());
            text::draw(&dots, well, below(area, used), buf).max(1)
        }
        // A file field has no way back on a terminal any more than it has on an
        // HTTP host. The name is drawn and picking one belongs to whoever owns
        // the interaction.
        _ if held.text().is_empty() => {
            empty_well(style, placeholder, well, focused, below(area, used), buf)
        }
        _ => text::draw(held.text(), well, below(area, used), buf),
    };

    // The error wins over the hint, the same order a webview uses: a hint is
    // what to type and an error is what went wrong, and once something has gone
    // wrong that is the sentence worth the row.
    match note_of(field) {
        Some(note) => {
            let painted = if field.error.is_some() {
                style.danger
            } else {
                style.muted
            };
            used + text::draw(note, painted, below(area, used), buf)
        }
        None => used,
    }
}

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

/// What goes under the box: what is wrong now, or the standing help.
fn note_of<'a>(field: &Field<'a>) -> Option<&'a str> {
    field.error.or(field.hint)
}

/// A box with nothing in it: the ghost text, and the caret when it has focus.
///
/// The caret is not decoration. An empty field under a style is an empty field,
/// so a focused one with no placeholder drew literally nothing and there was no
/// way to tell the box was where the typing would go. A browser has a blinking
/// bar for this and gets it without asking; a terminal has one cell of reversed
/// video, put on the first column, which is where the first character lands.
fn empty_well(
    style: &WidgetStyle,
    placeholder: &str,
    well: Style,
    focused: bool,
    area: Rect,
    buf: &mut Buffer,
) -> u16 {
    let used = text::draw(placeholder, style.muted, area, buf).max(1);
    if focused
        && area.height > 0
        && area.width > 0
        && let Some(cell) = buf.cell_mut((area.x, area.y))
    {
        cell.set_style(well);
    }
    used
}

/// What is left of `area` after `used` rows from the top.
fn below(area: Rect, used: u16) -> Rect {
    let used = used.min(area.height);
    Rect {
        x: area.x,
        y: area.y + used,
        width: area.width,
        height: area.height - used,
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use makeover_layout::{Choice, State};

    /// The style the drawings are read against: one distinguishable modifier
    /// per role, so a test can say which style landed without a colour.
    fn style() -> WidgetStyle {
        WidgetStyle {
            content: Style::new().add_modifier(Modifier::BOLD),
            muted: Style::new().add_modifier(Modifier::DIM),
            danger: Style::new().add_modifier(Modifier::CROSSED_OUT),
            ..WidgetStyle::default()
        }
    }

    fn buffer(width: u16, height: u16) -> Buffer {
        Buffer::empty(Rect::new(0, 0, width, height))
    }

    /// Everything in the buffer, one string per row.
    fn rows(buf: &Buffer) -> Vec<String> {
        (0..buf.area.height)
            .map(|y| {
                (0..buf.area.width)
                    .map(|x| {
                        buf.cell((x, y))
                            .map_or(' ', |c| c.symbol().chars().next().unwrap_or(' '))
                    })
                    .collect::<String>()
                    .trim_end()
                    .to_owned()
            })
            .collect()
    }

    #[test]
    fn a_bar_fills_in_proportion_and_reads_out_the_two_numbers() {
        let style = style();
        let line = meter(&style, &Meter::new(3, 10).label("subtasks"));
        let drawn: String = line.spans.iter().map(|s| s.content.as_ref()).collect();
        assert_eq!(drawn, "###------- 3/10 subtasks");
        // The noun is optional and the ratio is not, because a bar with no
        // reading is a bar you cannot check.
        let bare = meter(&style, &Meter::new(3, 10));
        let drawn: String = bare.spans.iter().map(|s| s.content.as_ref()).collect();
        assert_eq!(drawn, "###------- 3/10");
    }

    #[test]
    fn an_empty_set_is_an_empty_bar_rather_than_a_divide_by_zero() {
        // `Meter::total` of zero means there is no set, and the checked
        // division is what keeps that from being a panic in a draw.
        let line = meter(&style(), &Meter::new(0, 0));
        let drawn: String = line.spans.iter().map(|s| s.content.as_ref()).collect();
        assert_eq!(drawn, "---------- 0/0");
    }

    #[test]
    fn an_over_run_fills_the_bar_and_still_reports_the_overflow() {
        // The clamp is for drawing only. The reading is what keeps the fact
        // `Meter::percent` destroys.
        let line = meter(&style(), &Meter::new(14, 10));
        let drawn: String = line.spans.iter().map(|s| s.content.as_ref()).collect();
        assert_eq!(drawn, "########## 14/10");
    }

    #[test]
    fn a_badge_is_round_and_a_chip_is_square() {
        // The one affordance a cell has left once colour is spent on the tone,
        // and the whole of how a terminal says "this one answers a press".
        let style = style();
        let badge = token(&style, "draft", Token::Badge, Tone::Neutral, false, false);
        assert_eq!(badge.content.as_ref(), "(draft)");
        let chip = token(
            &style,
            "rust",
            Token::Chip { removable: false },
            Tone::Neutral,
            false,
            false,
        );
        assert_eq!(chip.content.as_ref(), "[rust]");
    }

    #[test]
    fn a_latched_chip_reads_the_same_as_a_focused_one() {
        // The collision a terminal cannot avoid, asserted rather than left to
        // be rediscovered: latched is "this filter is on" and focused is "you
        // are here", and there is one spare axis for two facts.
        let style = style();
        let kind = Token::Chip { removable: false };
        let latched = token(&style, "rust", kind, Tone::Neutral, true, false);
        let focused = token(&style, "rust", kind, Tone::Neutral, false, true);
        assert_eq!(latched.style, focused.style);
        assert!(latched.style.add_modifier.contains(Modifier::REVERSED));
    }

    #[test]
    fn a_control_draws_its_key_only_where_one_was_named() {
        let style = style();
        let line = act(&style, &Act::new("Delete"), false);
        assert_eq!(line.spans[0].content.as_ref(), "< Delete >");
        let line = act(&style, &Act::new("Quit").key("q"), false);
        assert_eq!(line.spans[0].content.as_ref(), "< Quit > (q)");
    }

    #[test]
    fn a_disabled_control_is_never_marked_focused() {
        // Present, visible, and not answering. A focus mark on it would be an
        // affordance that lies, so the flag is overridden rather than trusted.
        let style = style();
        let disabled = Act::new("Save").state(State::Disabled);
        let line = act(&style, &disabled, true);
        assert!(
            !line.spans[0]
                .style
                .add_modifier
                .contains(Modifier::REVERSED)
        );
        assert_eq!(line.spans[0].style, style.muted);
        // The same call on a control the description says nothing about: the
        // mark is this renderer's own focus flag and always was, which is why
        // only `Disabled` can override it.
        let unstated = Act::new("Save");
        let line = act(&style, &unstated, true);
        assert!(
            line.spans[0]
                .style
                .add_modifier
                .contains(Modifier::REVERSED)
        );
    }

    #[test]
    fn a_danger_control_keeps_its_tone_under_focus() {
        // Focus adds a modifier rather than repainting, so the fact that this
        // is the button that destroys something survives being landed on.
        let style = style();
        let line = act(&style, &Act::new("Delete").tone(Tone::Danger), true);
        assert_eq!(
            line.spans[0].style.add_modifier,
            style.danger.add_modifier | Modifier::REVERSED
        );
    }

    #[test]
    fn a_figure_puts_the_number_over_what_it_counts() {
        let style = style();
        let figure_ = Figure::new("42", "open tasks");
        let mut buf = buffer(20, 4);
        let used = figure(&style, &figure_, buf.area, &mut buf);
        assert_eq!(used, 2);
        assert_eq!(rows(&buf)[..2], ["42".to_owned(), "open tasks".to_owned()]);
        assert_eq!(figure_height(&figure_, 20), 2);
    }

    #[test]
    fn a_figures_change_rides_on_the_value_row() {
        // The delta is the toned part and the value is an ordinary fact, so the
        // two share a row rather than the caption growing a second sentence.
        let style = style();
        let figure_ = Figure::new("42", "open tasks")
            .change("+3")
            .tone(Tone::Success);
        let mut buf = buffer(20, 4);
        figure(&style, &figure_, buf.area, &mut buf);
        assert_eq!(rows(&buf)[0], "42 +3");
    }

    #[test]
    fn a_compulsory_field_says_so_in_its_label() {
        let style = style();
        let mut field_ = Field::new(FieldKind::Text, "email", "Email");
        field_.required = true;
        let mut buf = buffer(20, 4);
        field(&style, &field_, Held::Absent, false, buf.area, &mut buf);
        assert_eq!(rows(&buf)[0], "Email *");
    }

    #[test]
    fn a_hidden_field_costs_no_rows_at_all() {
        // The one field kind a terminal and a webview agree on completely.
        let style = style();
        let field_ = Field::new(FieldKind::Hidden, "csrf", "Token");
        let mut buf = buffer(20, 4);
        assert_eq!(
            field(
                &style,
                &field_,
                Held::Text("abc"),
                false,
                buf.area,
                &mut buf
            ),
            0
        );
        assert_eq!(field_height(&style, &field_, 20), 0);
        assert_eq!(rows(&buf)[0], "");
    }

    #[test]
    fn a_secret_is_dotted_from_the_callers_buffer_and_never_from_the_description() {
        // The one control that would be undrawable without `held`: a password
        // that came back down the wire is a password in a page and in a log.
        let style = style();
        let field_ = Field::new(FieldKind::Secret, "password", "Password");
        let mut buf = buffer(20, 4);
        field(
            &style,
            &field_,
            Held::Text("hunter2"),
            false,
            buf.area,
            &mut buf,
        );
        assert_eq!(rows(&buf)[1], "*******");
    }

    #[test]
    fn an_error_takes_the_row_the_hint_would_have_had() {
        // Once something has gone wrong that is the sentence worth the row,
        // which is the order a webview uses too.
        let style = style();
        let mut field_ = Field::new(FieldKind::Text, "email", "Email");
        field_.hint = Some("work address");
        field_.error = Some("not an address");
        let mut buf = buffer(20, 5);
        field(
            &style,
            &field_,
            Held::Text("nope"),
            false,
            buf.area,
            &mut buf,
        );
        assert_eq!(rows(&buf)[2], "not an address");
        assert_eq!(field_height(&style, &field_, 20), 3);
    }

    #[test]
    fn a_focused_empty_box_shows_where_the_typing_will_land() {
        // An empty field under a style is an empty field. Without the caret a
        // focused box with no placeholder drew literally nothing.
        let style = style();
        let field_ = Field::new(FieldKind::Text, "email", "Email");
        let mut buf = buffer(20, 4);
        field(&style, &field_, Held::Absent, true, buf.area, &mut buf);
        let caret = buf.cell((0, 1)).expect("the well's first cell").style();
        assert!(caret.add_modifier.contains(Modifier::REVERSED));
    }

    #[test]
    fn a_choice_field_marks_the_chosen_option_and_costs_a_row_each() {
        let style = style();
        let mut field_ = Field::new(FieldKind::Radio, "size", "Size");
        let options = [Choice::plain("small"), Choice::plain("large")];
        field_.options = &options;
        let mut buf = buffer(20, 5);
        field(
            &style,
            &field_,
            Held::Text("large"),
            false,
            buf.area,
            &mut buf,
        );
        assert_eq!(rows(&buf)[1], "( ) small");
        assert_eq!(rows(&buf)[2], "(*) large");
        assert_eq!(field_height(&style, &field_, 20), 3);
    }

    #[test]
    fn a_checkbox_reads_a_bool_rather_than_a_submitted_string() {
        // `Held::On` exists so a host's own submission convention -- quasi
        // sends "value" -- stays the host's and never reaches a drawing.
        let style = style();
        let field_ = Field::new(FieldKind::Checkbox, "agree", "Agree");
        let mut buf = buffer(20, 4);
        field(&style, &field_, Held::On(true), false, buf.area, &mut buf);
        assert_eq!(rows(&buf)[1], "[x]");
        let mut buf = buffer(20, 4);
        field(&style, &field_, Held::On(false), false, buf.area, &mut buf);
        assert_eq!(rows(&buf)[1], "[ ]");
    }

    #[test]
    fn a_tone_and_a_heading_map_without_a_fallback_arm() {
        // Both source enums are closed, which is what lets these be total. A
        // renderer that had to guess would be picking its own colours again.
        let style = style();
        assert_eq!(style.tone(Tone::Neutral), style.content);
        assert_eq!(style.tone(Tone::Danger), style.danger);
        assert_eq!(style.heading(Heading::Page), style.page);
        assert_eq!(style.heading(Heading::Subsection), style.subsection);
    }

    #[test]
    fn the_default_style_carries_no_colour_at_all() {
        // A two-colour terminal is the case where a foreground will not land,
        // so the default is modifiers only rather than a placeholder palette.
        let style = WidgetStyle::default();
        for painted in [style.content, style.danger, style.page, style.action] {
            assert_eq!(painted.fg, None);
            assert_eq!(painted.bg, None);
        }
    }
}