inkferro_core/render/

grid.rs

//! Char grid — plain-frame output assembly.
//!
//! Port of ink's `output.ts` (Output class) — **plain-frame slice only**:
//! no SGR transformers, no style application. The grid holds raw grapheme
//! clusters and produces a trimmed string on `get()`.
//!
//! # Design decisions
//!
//! ## Cell representation
//! ink's `StyledChar` (output.ts:141-151) stores `{type,value,fullWidth,styles}`.
//! The plain slice drops `styles` (no SGR) and folds `fullWidth` into a
//! `wide_placeholder` bool: a cell with `value == ""` is a trailing placeholder
//! for the preceding wide character.
//!
//! ## Wide-char cleanup (output.ts:263-299 citation)
//! When a write lands at a column that is currently a wide-char placeholder
//! (cell.value == "" and cell[x-1] has visual width > 1), ink blanks the
//! **leader** cell before writing (output.ts:263-270). After writing, if the
//! cell immediately after the last written cell is a placeholder, ink blanks it
//! too (output.ts:296-299). We mirror both cleanups exactly.
//!
//! ## Clip-rectangle stack
//! ink uses an operation queue with `clip` / `unclip` ops (output.ts:24-48,
//! 158-226). The plain slice keeps a `Vec<Clip>` stack that is pushed/popped
//! synchronously at write time — same semantics, simpler code.
//!
//! ## Trailing-whitespace trimming (output.ts:305-312 citation)
//! ```ts
//! return styledCharsToString(lineWithoutEmptyItems).trimEnd();
//! ```
//! Each row is right-trimmed before joining with `\n`. We do the same with
//! `trim_end_matches(is_js_trim_end_whitespace)` — JS `trimEnd`'s exact
//! whitespace set, NOT Rust's `char::is_whitespace` (task #123; see the
//! helper's doc for the two-char delta).

use std::rc::Rc;

use compact_str::CompactString;

use crate::text::ansi_tokenize::{
    AnsiToken, StyledChar, empty_styles, styled_chars_from_plain, styled_chars_from_tokens,
    styled_chars_to_string_into, tokenize,
};
use crate::text::slice_ansi::slice_ansi;
use crate::text::string_width::string_width;

/// A post-clip text transformer: `(line, line_index) -> line`.
///
/// Mirrors ink's `OutputTransformer = (s, index) => string`
/// (render-node-to-output.ts:30). The render/napi layer builds the chain
/// `[own, ...inherited]` (render-node-to-output.ts:134-138) — in inkferro the
/// per-node `own` transform (Text.tsx's `colorize`/chalk closure) stays JS-side
/// and is mapped onto this type by the napi boundary. The walk merely THREADS an
/// inherited slice parent→child→write; the write path applies the chain
/// innermost-first AFTER the clip-slice (output.ts:238-240). Borrowed `dyn Fn`
/// refs dodge the `Clone` problem (closures are not `Clone`); the chain is a
/// slice of references rebuilt at each level.
pub type Transformer<'a> = &'a dyn Fn(&str, usize) -> String;

// ─── Clip ────────────────────────────────────────────────────────────────────

/// Axis-independent clip boundary.
///
/// Mirrors ink's `Clip` type (output.ts:39-44).
/// `None` means "no clip on this axis" (output.ts:177-178:
/// `typeof clip?.x1 === 'number' && typeof clip?.x2 === 'number'`).
#[derive(Debug, Clone, Copy, Default)]
pub struct Clip {
    pub x1: Option<i32>,
    pub x2: Option<i32>,
    pub y1: Option<i32>,
    pub y2: Option<i32>,
}

// ─── Cell ────────────────────────────────────────────────────────────────────

/// One terminal cell in the grid.
///
/// `ch` holds the cell's [`StyledChar`] (grapheme `value` + accumulated
/// `styles`). A `value == ""` signals a wide-char trailing placeholder
/// (output.ts:282-290: value='', fullWidth=false, styles=lead.styles).
///
/// `hole` models a JS sparse-array gap. ink's `Output.get()` pre-fills a row
/// with exactly `width` space cells, but writes assign `currentLine[offsetX]`
/// directly — for `offsetX >= width` this *grows* the backing JS array,
/// leaving any skipped index as `undefined` (a hole). At assembly, ink does
/// `line.filter(item => item !== undefined)` (output.ts:308) before
/// `trimEnd`, which DROPS those holes mid-row. A space cell, by contrast,
/// survives the filter and is only removed by `trimEnd` if trailing.
///
/// This distinction is load-bearing for the out-of-bounds artifact: a
/// width-12 box in a width-10 grid materializes a 12-wide top/bottom row
/// (every column written) but 11-wide interior rows (col 10 never written →
/// hole → filtered, while the right border at col 11 survives).
#[derive(Debug, Clone)]
struct Cell {
    /// The styled grapheme in this cell. `value == ""` = wide-char placeholder.
    ch: StyledChar,
    /// True if this cell is a JS sparse-array hole (`undefined`): never
    /// written, beyond the pre-initialized width. Dropped by `get()`'s filter.
    hole: bool,
}

impl Cell {
    /// A space cell, ink's `spaceCell` (output.ts:251-256:
    /// `{value:' ', fullWidth:false, styles:[]}`).
    fn space() -> Self {
        Self {
            ch: StyledChar {
                // `const_new` inlines the 1-byte fill at compile time: a
                // colorless frame's ~900 space cells allocate nothing.
                value: CompactString::const_new(" "),
                full_width: false,
                // Shared empty-style sentinel (Rc::clone, zero heap) so the
                // ~900 space cells stay allocation-free under the Rc styles.
                styles: empty_styles(),
            },
            hole: false,
        }
    }

    /// A wide-char trailing placeholder inheriting the lead char's `styles`
    /// (output.ts:284-289: `{value:'', fullWidth:false, styles:character.styles}`).
    fn placeholder(styles: Rc<[AnsiToken]>) -> Self {
        Self {
            ch: StyledChar {
                value: CompactString::const_new(""),
                full_width: false,
                styles,
            },
            hole: false,
        }
    }

    /// A JS sparse-array gap (`undefined`): exists only to pad a row out to a
    /// written index past the pre-initialized width. Filtered out by `get()`.
    fn hole() -> Self {
        Self {
            ch: StyledChar {
                value: CompactString::const_new(""),
                full_width: false,
                styles: empty_styles(),
            },
            hole: true,
        }
    }

    fn is_placeholder(&self) -> bool {
        // A hole is not a wide-char placeholder; only a real ''-valued cell is.
        !self.hole && self.ch.value.is_empty()
    }
}

// ─── Grid ────────────────────────────────────────────────────────────────────

/// Plain-frame character grid.
///
/// Initialized to spaces (output.ts:141-156: each cell = `{value:' ', fullWidth:false}`).
/// Rows are indexed from top (row 0 = topmost visible line).
///
/// `cols` is the *initial* row width (ink's `Output.width`). Individual rows
/// may grow past `cols` when a write lands at a column beyond the initial
/// width — mirroring JS sparse-array growth in `Output.get()`. Skipped indices
/// become holes ([`Cell::hole`]) and are dropped at assembly.
pub struct Grid {
    rows: usize,
    cells: Vec<Vec<Cell>>,
    clip_stack: Vec<Clip>,
}

impl Grid {
    /// Create a `rows × cols` grid filled with spaces.
    ///
    /// Mirrors `Output` constructor (output.ts:98-103) + the `get()` pre-fill
    /// loop (output.ts:141-156).
    pub fn new(rows: usize, cols: usize) -> Self {
        let cells = (0..rows)
            .map(|_| (0..cols).map(|_| Cell::space()).collect())
            .collect();
        Self {
            rows,
            cells,
            clip_stack: Vec::new(),
        }
    }

    // ── Clip stack (output.ts:126-137) ───────────────────────────────────────

    /// Push a clip rectangle (output.ts:126-131).
    pub fn push_clip(&mut self, clip: Clip) {
        self.clip_stack.push(clip);
    }

    /// Pop the most recent clip rectangle (output.ts:133-137).
    pub fn pop_clip(&mut self) {
        self.clip_stack.pop();
    }

    // ── Write (output.ts:105-124, 169-302) ───────────────────────────────────

    /// Write `text` at grid position `(x, y)`, applying the current clip.
    ///
    /// Plain (no-transformer) entry point — equivalent to
    /// `write_styled(x, y, text, &[])`. Used by `render_border` and any caller
    /// that has no SGR transform to thread.
    pub fn write(&mut self, x: i32, y: i32, text: &str) {
        self.write_styled(x, y, text, &[]);
    }

    /// Write `text` at grid position `(x, y)`, applying the current clip then
    /// the `transformers` chain (innermost-first), then blitting styled chars.
    ///
    /// `text` may contain `\n` to write multiple lines; each line is placed at
    /// `(x, y + line_index)`.
    ///
    /// Mirrors `Output.write` (output.ts:105-124) + the per-operation handler
    /// inside `get()` (output.ts:169-302). In the plain slice, write is
    /// synchronous (no operation queue).
    pub fn write_styled(&mut self, x: i32, y: i32, text: &str, transformers: &[Transformer<'_>]) {
        // output.ts:113-115: skip empty text.
        if text.is_empty() {
            return;
        }

        let clip = self.clip_stack.last().copied();
        // Owned lines: the horizontal-clip `slice_ansi` produces `String`s, and
        // the per-line transformer chain rebinds each line to a fresh `String`.
        let mut lines: Vec<String> = text.split('\n').map(str::to_owned).collect();

        // ── Clip pre-checks (output.ts:175-225) ─────────────────────────────
        if let Some(clip) = clip {
            let clip_h = clip.x1.is_some() && clip.x2.is_some();
            let clip_v = clip.y1.is_some() && clip.y2.is_some();

            // output.ts:185-199: skip if entirely outside clip region.
            if clip_h {
                // widest-line width (output.ts:188)
                let w = lines
                    .iter()
                    .map(|l| string_width(l) as i32)
                    .max()
                    .unwrap_or(0);
                let x1 = clip.x1.unwrap();
                let x2 = clip.x2.unwrap();
                if x + w < x1 || x > x2 {
                    return;
                }
            }
            if clip_v {
                let height = lines.len() as i32;
                let y1 = clip.y1.unwrap();
                let y2 = clip.y2.unwrap();
                if y + height < y1 || y > y2 {
                    return;
                }
            }
        }

        // Mutable copies of x/y for clip-adjusted position (output.ts:171).
        let mut eff_x = x;
        let mut eff_y = y;

        if let Some(clip) = clip {
            let clip_h = clip.x1.is_some() && clip.x2.is_some();
            let clip_v = clip.y1.is_some() && clip.y2.is_some();

            // output.ts:201-213: horizontal clip — slice each line.
            if clip_h {
                let x1 = clip.x1.unwrap();
                let x2 = clip.x2.unwrap();
                lines = lines
                    .iter()
                    .map(|line| {
                        // output.ts:202-208: sliceAnsi(line, from, to) by visible width.
                        let from = if x < x1 { (x1 - x) as usize } else { 0 };
                        let line_w = string_width(line) as i32;
                        let to = if x + line_w > x2 {
                            (x2 - x) as usize
                        } else {
                            line_w as usize
                        };
                        slice_ansi(line, from, Some(to))
                    })
                    .collect();
                if x < x1 {
                    eff_x = x1;
                }
            }

            // output.ts:215-225: vertical clip — trim lines.
            if clip_v {
                let y1 = clip.y1.unwrap();
                let y2 = clip.y2.unwrap();
                let from = if eff_y < y1 { (y1 - eff_y) as usize } else { 0 };
                let height = lines.len() as i32;
                let to = if eff_y + height > y2 {
                    (y2 - eff_y) as usize
                } else {
                    lines.len()
                };
                // output.ts:220 `lines.slice(from, to)`: JS slice CLAMPS — it
                // returns [] when from > to (degenerate/inverted clip, e.g.
                // y1 > y2 with a partially spanning write, where the
                // pre-checks above still pass). Clamp the Rust range the same
                // way instead of panicking; for every valid clip from ≤ to,
                // so this is behavior-neutral there.
                let to = to.min(lines.len());
                lines = lines[from.min(to)..to].to_vec();
                if eff_y < y1 {
                    eff_y = y1;
                }
            }
        }

        // ── Place each line into the grid (output.ts:228-302) ────────────────
        for (line_idx, line) in lines.iter().enumerate() {
            let row_y = eff_y + line_idx as i32;
            if row_y < 0 || row_y as usize >= self.rows {
                continue; // output.ts:231-233: skip if row missing.
            }

            // output.ts:238-240: apply the transformer chain innermost-first.
            // Each transformer takes `(line, index)`; `index` is the per-write
            // line position (`line_idx`), matching ink's `lines.entries()`.
            let mut transformed = line.clone();
            for transformer in transformers {
                transformed = transformer(&transformed, line_idx);
            }

            // output.ts:242: tokenize the (post-transform) line into StyledChars.
            //
            // Fast path: a line with no SGR/OSC opener (no ESC U+001B / C1 CSI
            // U+009B) tokenizes to pure `Token::Char`s with empty styles, so the
            // fused `styled_chars_from_plain` builds the same `Vec<StyledChar>`
            // in one grapheme walk — skipping the intermediate `Vec<Token>` and
            // the per-grapheme `CharToken.value` String. Most grid writes (plain
            // text content, unstyled fills) take this path; styled lines (border
            // SGR, JS-side colorize transforms) fall back to the full tokenizer.
            let chars = if transformed.contains(['\u{1B}', '\u{9B}']) {
                styled_chars_from_tokens(&tokenize(&transformed, None))
            } else {
                styled_chars_from_plain(&transformed)
            };

            // output.ts:246-249: nothing to write (e.g. clipped/transformed away).
            if chars.is_empty() {
                continue;
            }

            let row = &mut self.cells[row_y as usize];
            let mut offset_x = eff_x;

            // output.ts:263-270: wide-char leader cleanup before first write.
            // If we are about to write into a placeholder cell, blank the
            // preceding leader so the terminal never renders half a wide char.
            if offset_x > 0 {
                let col = offset_x as usize;
                if col < row.len()
                    && row[col].is_placeholder()
                    && string_width(&row[col - 1].ch.value) > 1
                {
                    row[col - 1] = Cell::space();
                }
            }

            // Write each styled char.
            //
            // output.ts assigns `currentLine[offsetX] = character` with NO
            // upper bound: in JS this grows the row array, leaving any skipped
            // index as a hole. We mirror that with `grow_to`, which extends the
            // row with `Cell::hole` so a write past the initial width never
            // clips — it materializes the column (and any gap before it).
            for ch in chars {
                // output.ts:276-279: printed width via string-width on the
                // VALUE (not the `full_width` flag) to align with measurement.
                let char_w = string_width(&ch.value).max(1);

                if offset_x < 0 {
                    // Advance without writing (clipped horizontally).
                    offset_x += char_w as i32;
                    continue;
                }
                let col = offset_x as usize;

                // output.ts:272-273: place the styled char.
                grow_to(row, col);

                // output.ts:282-291: fill trailing placeholder cells for wide
                // chars (e.g. CJK), inheriting the lead char's styles.
                //
                // The lead styles only feed the `1..char_w` placeholder loop,
                // which is empty for width-1 graphemes (every ASCII/box-drawing
                // glyph). Snapshot them ONLY when there are placeholders to fill
                // so the common width-1 path never clones the style Vec.
                if char_w > 1 {
                    // Share the lead char's style run into each placeholder by
                    // `Rc::clone` (refcount bump, zero heap) instead of a deep
                    // `Vec<AnsiToken>` copy.
                    let lead_styles = Rc::clone(&ch.styles);
                    row[col] = Cell { ch, hole: false };
                    for extra in 1..char_w {
                        let next_col = col + extra;
                        grow_to(row, next_col);
                        row[next_col] = Cell::placeholder(Rc::clone(&lead_styles));
                    }
                } else {
                    row[col] = Cell { ch, hole: false };
                }

                offset_x += char_w as i32;
            }

            // output.ts:296-299: wide-char trailer cleanup after last write.
            // If the cell immediately after what we wrote is a placeholder,
            // blank it (the wide char it belonged to was overwritten).
            let after = offset_x as usize;
            if after < row.len() && row[after].is_placeholder() {
                row[after] = Cell::space();
            }
        }
    }

    // ── Get (output.ts:139-318) ───────────────────────────────────────────────

    /// Serialize the grid to a string.
    ///
    /// Each row is right-trimmed (output.ts:309-310:
    /// `styledCharsToString(lineWithoutEmptyItems).trimEnd()`),
    /// then rows are joined with `\n`.
    ///
    /// Returns `(output_string, height)` mirroring ink's `{output, height}`.
    /// Height is always `self.rows` (the pre-initialized row count —
    /// output.ts:315-316).
    pub fn get(&self) -> (String, usize) {
        // Single reused buffer across every row. Each row is serialized in place
        // and its trailing spaces truncated before the next `\n` separator, so
        // the whole frame is built in ONE growing allocation instead of a fresh
        // trimmed `String` per row (~one alloc per grid row, every frame).
        let mut output = String::new();
        for (row_idx, row) in self.cells.iter().enumerate() {
            // Rows are joined with `\n` (output.ts joins lines): emit the
            // separator BEFORE every row but the first.
            if row_idx != 0 {
                output.push('\n');
            }
            // Mark where this row's serialized segment begins so we trim only
            // its own trailing spaces (never the prior row or the separator).
            let row_start = output.len();

            // output.ts:308: `line.filter(item => item !== undefined)` —
            // drop JS sparse-array holes (cells past the initial width that
            // were never written). Wide-char placeholders (value "") are
            // NOT holes and survive the filter, exactly as in ink.
            let survivors = row.iter().filter(|c| !c.hole).map(|c| &c.ch);
            // output.ts:310: styledCharsToString(...).trimEnd().
            //
            // No-byte-movement invariant: for a colorless row every cell's
            // `styles` is empty, so the serializer degenerates to a plain
            // concatenation of `.value` (no SGR opened or closed) —
            // byte-identical to the old plain path. The trailing trim below
            // uses JS `trimEnd`'s exact whitespace set (see
            // [`is_js_trim_end_whitespace`]), collapsing the styled trailing
            // spaces a colored frame would otherwise carry (a styled tail
            // ends in an SGR close byte, which is not whitespace, so the
            // serialize-then-trim order keeps the #119 contract intact).
            //
            // Borrowed serialization: `survivors` yields `&StyledChar` straight
            // from the grid cells, so the per-cell `ch.clone()` is gone — the
            // serializer appends the borrows directly into the shared buffer.
            styled_chars_to_string_into(survivors, &mut output);

            // In-place trim of this row's segment with JS `trimEnd` semantics
            // (probe-verified: ink passes `\t`/NBSP/U+3000/thin-space through
            // layout untouched — "A\tB" survives interior — and `trimEnd` at
            // output.ts:310 then strips them from the tail, so a 0x20-only
            // trim diverged byte-wise; task #123). `trim_end_matches` removes
            // whole `char`s, so the boundary always lands on a UTF-8 char
            // boundary and `truncate` is sound. Never trims past `row_start`,
            // so an empty / all-space row collapses to "" exactly as the
            // oracle's, leaving earlier rows untouched.
            let trimmed_len = output[row_start..]
                .trim_end_matches(is_js_trim_end_whitespace)
                .len();
            output.truncate(row_start + trimmed_len);
        }
        (output, self.rows)
    }
}

// ─── Test-only accessors ─────────────────────────────────────────────────────

#[cfg(test)]
impl Grid {
    /// Return the raw content (grapheme value) of the cell at `(row, col)`.
    ///
    /// Exposed only for tests that need to inspect cell state before `get()`
    /// applies `trimEnd` (e.g. to verify wide-char trailer cleanup).
    pub fn cell_content(&self, row: usize, col: usize) -> &str {
        &self.cells[row][col].ch.value
    }

    /// Return the accumulated `styles` of the cell at `(row, col)`.
    ///
    /// Exposed only for tests asserting style inheritance (e.g. wide-char
    /// trailing placeholders inherit the lead char's styles).
    pub fn cell_styles(&self, row: usize, col: usize) -> &[crate::text::ansi_tokenize::AnsiToken] {
        &self.cells[row][col].ch.styles
    }
}

// ─── Helpers ─────────────────────────────────────────────────────────────────

/// JS `String.prototype.trimEnd`'s exact whitespace set (ECMA-262
/// `TrimString`: WhiteSpace ∪ LineTerminator) — the set ink's row trim at
/// output.ts:310 uses. Probe-enumerated against Node (task #123):
/// `\t \n \v \f \r 0x20 U+00A0 U+1680 U+2000–U+200A U+2028 U+2029 U+202F
/// U+205F U+3000 U+FEFF` are trimmed; `U+0085` (NEL), `U+180E`, `U+200B`
/// are kept.
///
/// Rust's `char::is_whitespace` (Unicode `White_Space`) differs from that
/// oracle set in exactly two chars: it INCLUDES U+0085 (NEL, not JS
/// whitespace) and EXCLUDES U+FEFF (BOM/ZWNBSP, which JS trims). We match
/// the ORACLE bytes, not either spec ideal.
fn is_js_trim_end_whitespace(c: char) -> bool {
    c == '\u{FEFF}' || (c != '\u{0085}' && c.is_whitespace())
}

/// Extend `row` with holes so index `col` is addressable.
///
/// Mirrors JS sparse-array growth in `Output.get()`: assigning
/// `currentLine[offsetX]` for `offsetX >= row.len()` grows the array, leaving
/// any skipped index as `undefined`. We materialize those skipped indices as
/// [`Cell::hole`] so they can be filtered out at assembly. No-op when `col`
/// is already in bounds.
fn grow_to(row: &mut Vec<Cell>, col: usize) {
    while row.len() <= col {
        row.push(Cell::hole());
    }
}

// ─── Tests ───────────────────────────────────────────────────────────────────

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

    // ── grid primitives ──────────────────────────────────────────────────────

    // A fresh 3×5 grid: all spaces, trimEnd gives empty per row.
    // output.ts:141-156: initialized to spaces; output.ts:309-310: trimEnd.
    #[test]
    fn empty_grid_trims_to_empty_lines() {
        let g = Grid::new(3, 5);
        let (out, h) = g.get();
        assert_eq!(h, 3);
        // Each of 3 rows is all spaces → trimEnd → empty → "\n\n"
        assert_eq!(out, "\n\n");
    }

    // ── styled-char grid (M2-D) ──────────────────────────────────────────────

    // (a) Two-deep transformer chain: own-before-inherited with the correct
    // per-line index. Oracle-pinned via ink's Output class:
    //   out.write(0,0,'ab\ncd',{transformers:[inner,outer]}) →
    //   "O0{I0{ab}}\nO1{I1{cd}}" (scratch probe in /home/alpha/rewrite/ink,
    //   deleted after). The chain is `[inner, outer]` and applied
    //   `for t in transformers { line = t(line, index) }`, so the innermost
    //   (own) transform runs FIRST and `index` is the per-write line position.
    #[test]
    fn transformer_chain_own_before_inherited_with_index() {
        let inner = |s: &str, i: usize| format!("I{i}{{{s}}}");
        let outer = |s: &str, i: usize| format!("O{i}{{{s}}}");
        let chain: [Transformer<'_>; 2] = [&inner, &outer];

        let mut g = Grid::new(2, 40);
        g.write_styled(0, 0, "ab\ncd", &chain);
        let (out, _) = g.get();
        assert_eq!(out, "O0{I0{ab}}\nO1{I1{cd}}");
    }

    // (b) Wide-char trailing placeholder inherits the lead char's styles
    // (output.ts:284-289: `{value:'', styles: character.styles}`). Write a red
    // "中" (width 2): col 0 is the red lead, col 1 the trailing placeholder —
    // its `styles` must equal the lead's (the red SGR token), NOT empty.
    #[test]
    fn wide_char_placeholder_inherits_lead_styles() {
        let mut g = Grid::new(1, 6);
        g.write(0, 0, "\x1b[31m中\x1b[39m");
        // Lead at col 0: value "中", one red style.
        assert_eq!(g.cell_content(0, 0), "中");
        let lead_styles = g.cell_styles(0, 0).to_vec();
        assert_eq!(lead_styles.len(), 1, "lead carries the red SGR");
        assert_eq!(lead_styles[0].code, "\x1b[31m");
        // Placeholder at col 1: value "", styles inherited from the lead.
        assert_eq!(g.cell_content(0, 1), "", "col 1 is a wide-char placeholder");
        assert_eq!(
            g.cell_styles(0, 1),
            lead_styles.as_slice(),
            "placeholder inherits the lead char's styles (output.ts:288)"
        );
    }

    // (c) A styled trailing-space line trims byte-identically to the plain path.
    // The colorized CONTENT ("Hi" in red) is followed by the grid's own
    // UNSTYLED pad spaces (Cell::space, styles=[]); `get()`'s trimEnd-set
    // trim removes those trailing spaces, leaving exactly the
    // closed red span — byte-identical to ink (no dangling trailing spaces, no
    // SGR re-open over the pad). This is the real grid state: trailing spaces
    // are unstyled, so they collapse just like the plain frame.
    #[test]
    fn styled_trailing_spaces_trim_byte_identical() {
        let red = |s: &str, _i: usize| format!("\x1b[31m{s}\x1b[39m");
        let chain: [Transformer<'_>; 1] = [&red];

        let mut g = Grid::new(1, 10);
        g.write_styled(0, 0, "Hi", &chain); // cols 2..9 stay unstyled spaces
        let (out, _) = g.get();
        // Closed red span, no trailing spaces, no SGR over the pad.
        assert_eq!(out, "\x1b[31mHi\x1b[39m");
    }

    // (d) Task #119: a trailing space that CARRIES styling is CONTENT — it must
    // survive `get()`'s trim. Oracle (output.ts:310): `styledCharsToString(...)
    // .trimEnd()` serializes FIRST, so a style-bearing space ends in its SGR
    // close code (`\x1b[27m`), never in a literal space — `trimEnd` cannot
    // reach it. This is ink-text-input's inverse-video cursor cell
    // (`chalk.inverse(' ')` after the value). Oracle-captured (live ink 7.0.5,
    // fake non-TTY stdout): `AB` + inverse space → "AB\x1b[7m \x1b[27m"
    // byte-exact, even with unstyled pad cells after it.
    //
    // MUTATION (verified): trimming trailing space CELLS before serialization
    // regardless of `styles` collapses this to "AB" and flips both asserts,
    // while (c)'s unstyled-pad control above stays green.
    #[test]
    fn styled_trailing_space_survives_trim() {
        // Grid pad cells (cols 5..9) are unstyled spaces: trimmed as usual.
        let mut g = Grid::new(1, 10);
        g.write(0, 0, "AB\x1b[7m \x1b[27m");
        let (out, _) = g.get();
        assert_eq!(
            out, "AB\x1b[7m \x1b[27m",
            "style-bearing trailing space survives; unstyled pad is trimmed"
        );

        // Mixed tail: explicit UNSTYLED spaces written after the styled one
        // are still trimmed — the trim rule keys on styling, not position.
        let mut g2 = Grid::new(1, 10);
        g2.write(0, 0, "AB\x1b[7m \x1b[27m  ");
        let (out2, _) = g2.get();
        assert_eq!(
            out2, "AB\x1b[7m \x1b[27m",
            "unstyled spaces after the styled space are trimmed exactly as before"
        );
    }

    // (e) Task #123: the tail trim uses JS `trimEnd`'s WHOLE whitespace set,
    // not just 0x20. Oracle-captured (live ink 7.0.5 build, fake non-TTY
    // stdout, /tmp/t123 probes): `<Text>` tails of `\t`, NBSP (U+00A0),
    // ideographic space (U+3000), thin space (U+2009), the mixed `"  \t"`
    // tail, and ZWNBSP/BOM (U+FEFF) ALL wrote exactly "AB\n" — and the
    // interior control `"A\tB"` survived ink's layout verbatim ("A\tB\n"),
    // proving the chars reach the composed row and are removed by
    // output.ts:310's `.trimEnd()`, not normalized earlier.
    //
    // MUTATION (verified): reverting the trim to `trim_end_matches(' ')`
    // keeps every one of these tails (the mixed case even keeps the 0x20s
    // BEFORE the `\t`) and flips all six asserts.
    #[test]
    fn row_tail_trims_full_js_trim_end_set() {
        let cases: [(&str, &str); 6] = [
            ("AB\t", "tab"),
            ("AB\u{a0}", "nbsp"),
            ("AB\u{3000}", "ideographic space"),
            ("AB\u{2009}", "thin space"),
            ("AB  \t", "mixed space+tab tail"),
            (
                "AB\u{feff}",
                "ZWNBSP/BOM (JS trims; Rust is_whitespace does NOT)",
            ),
        ];
        for (text, name) in cases {
            let mut g = Grid::new(1, 20);
            g.write(0, 0, text);
            let (out, _) = g.get();
            assert_eq!(out, "AB", "oracle trims the {name} tail to \"AB\"");
        }
    }

    // (f) Task #123 inverse controls: chars JS `trimEnd` KEEPS must survive.
    // Node-enumerated oracle set (probe /tmp/t123): U+200B ZWSP and U+0085 NEL
    // are NOT JS whitespace — `"AB\u{200b}".trimEnd()` keeps both. The
    // end-to-end oracle keeps ZWSP ("AB\u{200b}\n"); NEL never reaches the
    // row in ink AT ALL (sanitize-ansi.ts strips standalone C1 controls at
    // the squash boundary — an upstream seam, not the trim; at THIS seam the
    // trim must mirror `trimEnd` and keep it).
    //
    // MUTATION (verified): trimming with plain `char::is_whitespace` (Unicode
    // White_Space) removes NEL — flipping the NEL assert — while (e)'s U+FEFF
    // case flips under the same mutation in the other direction (kept when it
    // must be trimmed). Together (e)+(f) pin the exact two-char delta between
    // the JS set and Rust's.
    #[test]
    fn row_tail_keeps_non_js_whitespace() {
        let cases: [(&str, &str, &str); 2] = [
            ("AB\u{200b}", "AB\u{200b}", "ZWSP"),
            ("AB\u{85}", "AB\u{85}", "NEL (C1; JS trimEnd keeps it)"),
        ];
        for (text, expected, name) in cases {
            let mut g = Grid::new(1, 20);
            g.write(0, 0, text);
            let (out, _) = g.get();
            assert_eq!(out, expected, "{name} tail is NOT JS whitespace — kept");
        }
    }

    // (g) Task #123 × #119: a STYLED non-0x20 whitespace tail is content.
    // Oracle-captured (live ink 7.0.5, FORCE_COLOR=3, fake non-TTY stdout):
    // `<Text>AB<Text inverse>{NBSP}</Text></Text>` wrote exactly
    // "AB\x1b[7m\u{a0}\x1b[27m\n" — serialize-then-trim means the styled NBSP
    // ends in the SGR close byte, out of `trimEnd`'s reach, identical to the
    // #119 styled-space contract.
    #[test]
    fn styled_trailing_nbsp_survives_trim() {
        let mut g = Grid::new(1, 10);
        g.write(0, 0, "AB\x1b[7m\u{a0}\x1b[27m");
        let (out, _) = g.get();
        assert_eq!(
            out, "AB\x1b[7m\u{a0}\x1b[27m",
            "style-bearing trailing NBSP survives; unstyled pad is trimmed"
        );
    }

    // Write "Hi" at (0,0) and check it appears.
    #[test]
    fn write_simple_text() {
        let mut g = Grid::new(2, 10);
        g.write(0, 0, "Hi");
        let (out, _) = g.get();
        let lines: Vec<&str> = out.split('\n').collect();
        assert_eq!(lines[0], "Hi");
    }

    // Write at x=2 to check offset.
    #[test]
    fn write_at_offset_x() {
        let mut g = Grid::new(1, 10);
        g.write(2, 0, "AB");
        let (out, _) = g.get();
        assert_eq!(out, "  AB");
    }

    // Write two lines via \n.
    #[test]
    fn write_multiline() {
        let mut g = Grid::new(3, 10);
        g.write(0, 0, "line1\nline2");
        let (out, _) = g.get();
        let lines: Vec<&str> = out.split('\n').collect();
        assert_eq!(lines[0], "line1");
        assert_eq!(lines[1], "line2");
    }

    // ── wide-char cleanup (output.ts:263-299 citations) ─────────────────────

    // Place a CJK char "中" (width 2) at col 0; then write "X" at col 1.
    // output.ts:263-270: writing at col 1 finds placeholder at col 1 and
    // blanks the leader at col 0.
    // ink:output.ts: "if (currentLine[offsetX]?.value === '' && offsetX > 0 &&
    //   this.caches.getStringWidth(currentLine[offsetX - 1]?.value ?? '') > 1)"
    //   → currentLine[offsetX - 1] = spaceCell;
    #[test]
    fn wide_char_leader_blanked_on_overwrite() {
        let mut g = Grid::new(1, 6);
        g.write(0, 0, "中"); // "中" occupies cols 0 and 1 (width 2)
        g.write(1, 0, "X"); // writes at placeholder col 1 → leader col 0 must become ' '
        let (out, _) = g.get();
        // col 0 → ' ' (blanked leader), col 1 → 'X'
        assert_eq!(&out[..2], " X");
    }

    // Place "中" at col 2 then write "Y" at col 2 (overwriting the leader).
    // Before writing: col 2 holds "中" (not a placeholder), so leader-blank
    // does NOT fire. After writing "Y" (width 1), the next cell at col 3 is
    // still the old placeholder → trailer-blank fires: output.ts:296-299.
    // Assert against the raw cells (not get() output) to avoid trimEnd
    // consuming the blanked trailing space at col 3 before we can check it.
    // output.ts:296-299: "if (currentLine[offsetX]?.value === '') currentLine[offsetX] = spaceCell"
    #[test]
    fn wide_char_trailer_blanked_after_write() {
        let mut g = Grid::new(1, 8);
        g.write(2, 0, "中"); // "中" = cols 2 (leader), 3 (placeholder)
        // Write "Y" at col 2 — overwrites leader; col 3 placeholder remains.
        // The trailer-blank (output.ts:296-299) must convert col 3 from
        // placeholder ("") to space (" ").
        g.write(2, 0, "Y");
        // Inspect raw cells via the test accessor to bypass trimEnd.
        assert_eq!(
            g.cell_content(0, 2),
            "Y",
            "col 2 must hold the written char"
        );
        assert_eq!(
            g.cell_content(0, 3),
            " ",
            "col 3 placeholder must be blanked to space"
        );
    }

    // CJK char at clip boundary: "中" spans cols 4-5 in a grid clipped at x2=5.
    // The right edge of the clip cuts through the wide char, leaving only the
    // leader visible. The trailing placeholder at col 5 gets blanked by the
    // clip logic (the write is clipped at col 5 so col 5 placeholder is not
    // filled with the wide char's placeholder, it stays space).
    // Hand-derived: grid width=8, clip x1=0 x2=5, write "中" at x=4.
    // sliceAnsi("中", from=0, to=1) → "" (first grapheme has width 2 > 1 col).
    // The char is wider than the remaining clip space → nothing placed past x2.
    #[test]
    fn wide_char_clipped_at_boundary() {
        let mut g = Grid::new(1, 8);
        g.push_clip(Clip {
            x1: Some(0),
            x2: Some(5),
            y1: None,
            y2: None,
        });
        g.write(4, 0, "中"); // "中" needs 2 cols, only 1 remains within clip
        g.pop_clip();
        let (out, _) = g.get();
        // col 4 should remain ' ' (can't fit width-2 char in 1-col space)
        // or "中" is placed but clipped to 1 col — either way col 4-5 are not
        // a half-rendered wide char.
        // slice_ansi("中", 0, Some(1)) returns "" because the grapheme's
        // width 2 overshoots to=1, so we get empty string: col 4 stays space.
        assert!(!out.contains('\0'), "no null bytes");
        // At minimum, the grid must not contain a dangling placeholder.
        let cells: Vec<&str> = g.cells[0].iter().map(|c| c.ch.value.as_str()).collect();
        assert!(
            !cells[5].is_empty() || cells[4] != "中",
            "no dangling wide-char placeholder at boundary"
        );
    }

    // ── clip stack ───────────────────────────────────────────────────────────

    // Write outside the horizontal clip region: nothing should appear.
    // output.ts:185-199 — skip if entirely outside clip.
    #[test]
    fn clip_horizontal_skips_entirely_outside() {
        let mut g = Grid::new(1, 20);
        g.push_clip(Clip {
            x1: Some(5),
            x2: Some(10),
            y1: None,
            y2: None,
        });
        g.write(15, 0, "hello"); // entirely to the right of x2=10
        g.pop_clip();
        let (out, _) = g.get();
        assert_eq!(out, ""); // all spaces → trimEnd → ""
    }

    // Write that straddles the left edge of the clip: trimmed on the left.
    // output.ts:201-213: clip from x1.
    #[test]
    fn clip_horizontal_trims_left() {
        let mut g = Grid::new(1, 20);
        g.push_clip(Clip {
            x1: Some(3),
            x2: Some(10),
            y1: None,
            y2: None,
        });
        g.write(1, 0, "ABCDE"); // starts at x=1; clip starts at x1=3
        // Visible: from = 3-1 = 2 → "CDE"
        g.pop_clip();
        let (out, _) = g.get();
        let expected: String = "   CDE".to_owned(); // 3 spaces then "CDE"
        assert_eq!(out, expected);
    }

    // Vertical clip skips rows outside range.
    // output.ts:215-225.
    #[test]
    fn clip_vertical_skips_rows_outside() {
        let mut g = Grid::new(4, 10);
        g.push_clip(Clip {
            x1: None,
            x2: None,
            y1: Some(1),
            y2: Some(2),
        });
        g.write(0, 0, "row0\nrow1\nrow2\nrow3"); // all 4 rows
        g.pop_clip();
        let (out, _) = g.get();
        let lines: Vec<&str> = out.split('\n').collect();
        assert_eq!(lines[0], ""); // row 0: outside clip, stays empty
        assert_eq!(lines[1], "row1"); // inside clip
        assert_eq!(lines[2], ""); // row 2: y2=2 means exclusive, so row2 is outside
        assert_eq!(lines[3], ""); // row 3: outside
    }

    // Pop clip restores previous state.
    #[test]
    fn clip_push_pop_restores() {
        let mut g = Grid::new(1, 20);
        g.push_clip(Clip {
            x1: Some(5),
            x2: Some(10),
            y1: None,
            y2: None,
        });
        g.pop_clip();
        // After pop, no clip — write should succeed anywhere.
        g.write(0, 0, "hello");
        let (out, _) = g.get();
        assert!(out.starts_with("hello"));
    }

    // ── trimEnd semantics (output.ts:309-310) ────────────────────────────────

    // A row with trailing spaces must be trimmed.
    #[test]
    fn get_trims_trailing_spaces() {
        let mut g = Grid::new(1, 10);
        g.write(0, 0, "Hi"); // cols 2-9 remain spaces
        let (out, _) = g.get();
        assert_eq!(out, "Hi");
    }

    // A row with only spaces trims to empty string.
    #[test]
    fn get_all_spaces_trims_to_empty() {
        let g = Grid::new(1, 5);
        let (out, _) = g.get();
        assert_eq!(out, "");
    }

    // ── off-grid clip behavior ────────────────────────────────────────────────

    // write(-1, 0, "AB"): x starts at -1 so "A" (width 1) is consumed off-left
    // without being placed; offset_x advances to 0 and "B" lands at col 0.
    // Pins: off-left graphemes are clipped (skipped), not panicked or dropped.
    #[test]
    fn write_negative_x_clips_left_edge() {
        let mut g = Grid::new(1, 5);
        g.write(-1, 0, "AB");
        // "A" consumed off-left (x=-1 → advance to 0), "B" placed at col 0.
        assert_eq!(g.cell_content(0, 0), "B", "B must land at col 0");
        // col 1 onwards untouched — remains space.
        assert_eq!(g.cell_content(0, 1), " ", "col 1 must stay space");
    }

    // write longer than cols: ink's Output.get() has NO right-edge clip — a
    // write past the initial width grows the JS row array. 1×3 grid, write
    // "ABCDE" at x=0 → row grows to 5 cells, all written, get() gives "ABCDE".
    // This is the content-extent (off-grid) semantics: the grid materializes
    // every written column, never clips at the initial width.
    // ink:output.ts:272-273 `currentLine[offsetX] = character` (unbounded).
    #[test]
    fn write_over_width_grows_row_no_right_clip() {
        let mut g = Grid::new(1, 3);
        g.write(0, 0, "ABCDE"); // 5 chars into 3-wide grid — grows to 5 cols
        assert_eq!(g.cell_content(0, 0), "A");
        assert_eq!(g.cell_content(0, 4), "E");
        let (out, _) = g.get();
        assert_eq!(out, "ABCDE");
    }

    // ── off-grid overlap: last-writer-wins (output.ts:272-273) ───────────────

    // Two overlapping writes: the later write's cells overwrite the earlier
    // one cell-for-cell (output.ts assigns `currentLine[offsetX] = character`
    // unconditionally, so the last write at a column wins).
    #[test]
    fn off_grid_overlap_last_writer_wins() {
        let mut g = Grid::new(1, 6);
        g.write(0, 0, "ABCDEF");
        g.write(2, 0, "xy"); // overwrites cols 2,3
        let (out, _) = g.get();
        assert_eq!(out, "ABxyEF");
    }

    // ── jagged out-of-bounds row shape (byte match to ink artifact) ──────────

    // Reproduce ink's overflow.tsx "out of bounds writes do not crash" row
    // shape WITHOUT a width formula — purely via the real blit + hole filter.
    // Grid is width=10 (the viewport), height=3. Border writes (from
    // render-border.ts) for a width-12 box:
    //   top    "╭──────────╮" (12 chars) at (0,0)  → cols 0..=11 all written
    //   left   "│"            at (0,1)             → col 0
    //   right  "│"            at (11,1)            → col 11; col 10 SKIPPED → hole
    //   bottom "╰──────────╯" (12 chars) at (0,2)  → cols 0..=11 all written
    // get() drops the col-10 hole on the interior row → 11 chars, but keeps
    // every (written) column on top/bottom → 12 chars. ASYMMETRIC by artifact,
    // not by formula.
    #[test]
    fn jagged_oob_row_shape_byte_match() {
        let mut g = Grid::new(3, 10);
        g.write(0, 0, "╭──────────╮"); // top: 12 wide
        g.write(0, 1, "│"); // left border, interior row
        g.write(11, 1, "│"); // right border at col 11 → col 10 is a hole
        g.write(0, 2, "╰──────────╯"); // bottom: 12 wide
        let (out, _) = g.get();
        let lines: Vec<&str> = out.split('\n').collect();
        assert_eq!(lines[0], "╭──────────╮", "top row materializes 12 cols");
        assert_eq!(
            lines[1], "│         │",
            "interior row drops the col-10 hole → 11 cols (│ + 9 spaces + │)"
        );
        assert_eq!(lines[2], "╰──────────╯", "bottom row materializes 12 cols");
        // The interior right border survives at col 11 (the col-10 hole is
        // filtered, so it renders as the 11th visible char).
        assert_eq!(lines[1].chars().count(), 11);
        assert_eq!(lines[0].chars().count(), 12);
    }

    // ── innermost-clip-only nesting (output.ts:174 `clips.at(-1)`) ───────────

    // ink applies ONLY the innermost clip; the active clip is always the top
    // of stack. Push outer {0,8} then inner {0,2}; a write under the inner
    // clip yields "AB". After popping the inner clip, the SAME write now sees
    // only the outer {0,8} → "CDEFGH" survives where the inner clip had
    // dropped it — proving pop restores the ancestor as the active (sole) clip.
    #[test]
    fn innermost_clip_is_top_of_stack_after_pop() {
        let mut g = Grid::new(2, 10);
        g.push_clip(Clip {
            x1: Some(0),
            x2: Some(8),
            y1: None,
            y2: None,
        });
        g.push_clip(Clip {
            x1: Some(0),
            x2: Some(2),
            y1: None,
            y2: None,
        });
        g.write(0, 0, "ABCDEFGH"); // innermost {0,2} → row 0 = "AB"
        g.pop_clip();
        g.write(0, 1, "ABCDEFGH"); // now outer {0,8} → row 1 = "ABCDEFGH"
        g.pop_clip();
        let (out, _) = g.get();
        let lines: Vec<&str> = out.split('\n').collect();
        assert_eq!(lines[0], "AB", "under inner clip {{0,2}}");
        assert_eq!(
            lines[1], "ABCDEFGH",
            "after pop, outer {{0,8}} alone is the active clip"
        );
    }

    // Companion: prove the innermost clip can be WIDER than an ancestor — an
    // intersection model would wrongly narrow it. Outer {0,2}, inner {0,8};
    // a write spanning 0..8 must yield the full 8 chars (innermost wins),
    // NOT 2 (which an ancestor-intersection would force).
    #[test]
    fn innermost_clip_wider_than_ancestor_wins() {
        let mut g = Grid::new(1, 10);
        g.push_clip(Clip {
            x1: Some(0),
            x2: Some(2),
            y1: None,
            y2: None,
        });
        g.push_clip(Clip {
            x1: Some(0),
            x2: Some(8),
            y1: None,
            y2: None,
        });
        g.write(0, 0, "ABCDEFGH");
        g.pop_clip();
        g.pop_clip();
        let (out, _) = g.get();
        assert_eq!(
            out, "ABCDEFGH",
            "innermost {{0,8}} wins over wider ancestor {{0,2}}"
        );
    }

    // ── degenerate / inverted clip rects (direct Grid API hardening) ─────────
    //
    // Oracle: live ink Output probe (/tmp/ink-degenerate-clip-probe.mjs against
    // /home/alpha/rewrite/ink/build/output.js, 2026-06-10). Every degenerate
    // shape below produced `output="\n\n\n\n"` (a 5-row grid with NOTHING
    // written) and no throw, while positive controls (valid clip / no clip)
    // wrote normally. Source derivation: output.ts:220 `lines.slice(from, to)`
    // — JS Array.prototype.slice returns [] when from > to; output.ts:207
    // `sliceAnsi(line, from, to)` returns '' when from > to. Degenerate clip
    // ⇒ empty visible region ⇒ the write contributes nothing.

    // y-inverted clip (y1=2 > y2=1) partially spanned by the write: the exact
    // pre-fix panic shape. With y=1, height=3 the pre-checks pass
    // (y+height=4 ≥ y1, y=1 ≤ y2), then from = y1-y = 1, to = y2-y = 0 and
    // `lines[1..0]` panicked: "slice index starts at 1 but ends at 0".
    // Discriminates: the vertical-clip range must clamp like JS slice
    // (output.ts:220 → []) instead of panicking. PANICS pre-fix.
    #[test]
    fn clip_y_inverted_partial_span_writes_nothing() {
        let mut g = Grid::new(5, 10);
        g.push_clip(Clip {
            x1: None,
            x2: None,
            y1: Some(2),
            y2: Some(1),
        });
        g.write(0, 1, "aa\nbb\ncc");
        g.pop_clip();
        let (out, h) = g.get();
        assert_eq!(h, 5);
        // ink probe: output="\n\n\n\n" — nothing written.
        assert_eq!(out, "\n\n\n\n");
    }

    // x-inverted clip (x1=5 > x2=2) partially spanned by the write (x=2,
    // width=4 passes both pre-checks). from = x1-x = 3 > to = x2-x = 0;
    // sliceAnsi(line, 3, 0) === '' (output.ts:207; slice-ansi returns '' for
    // begin > end) → every line empties → nothing placed.
    // Discriminates: a horizontal-clip regression that underflowed/swapped the
    // slice bounds and emitted text (or panicked) instead of an empty line.
    #[test]
    fn clip_x_inverted_partial_span_writes_nothing() {
        let mut g = Grid::new(5, 10);
        g.push_clip(Clip {
            x1: Some(5),
            x2: Some(2),
            y1: None,
            y2: None,
        });
        g.write(2, 0, "abcd");
        g.pop_clip();
        let (out, _) = g.get();
        // ink probe: output="\n\n\n\n" — nothing written.
        assert_eq!(out, "\n\n\n\n");
    }

    // Valid (non-inverted) clip lying entirely OUTSIDE the grid bounds
    // (y1=100..y2=200 on a 5-row grid), write straddling its top edge.
    // The clip slice keeps lines but bumps y to y1=100 (output.ts:222-223);
    // every target row is missing → per-row skip (output.ts:231-233 /
    // grid.rs row bounds check) → nothing written, no panic.
    // Discriminates: out-of-grid clip-adjusted rows must be skipped, not
    // indexed (a direct `self.cells[row_y]` without the bounds check panics).
    #[test]
    fn clip_fully_outside_grid_writes_nothing() {
        let mut g = Grid::new(5, 10);
        g.push_clip(Clip {
            x1: None,
            x2: None,
            y1: Some(100),
            y2: Some(200),
        });
        g.write(0, 99, "aa\nbb\ncc");
        g.pop_clip();
        let (out, _) = g.get();
        // ink probe: output="\n\n\n\n" — nothing written.
        assert_eq!(out, "\n\n\n\n");
    }

    // Zero-area clips: x1==x2, y1==y2, and both. ink's clip bounds behave
    // half-open here: from == to on each axis → sliceAnsi(line, n, n) === ''
    // and lines.slice(n, n) === [] → empty visible region.
    // Discriminates: an off-by-one in the clamp (e.g. treating x1==x2 / y1==y2
    // as a 1-cell/1-row window) would write a column or row where ink writes
    // nothing.
    #[test]
    fn clip_zero_area_writes_nothing() {
        // x1==x2==2 (ink probe: nothing written).
        let mut g = Grid::new(5, 10);
        g.push_clip(Clip {
            x1: Some(2),
            x2: Some(2),
            y1: None,
            y2: None,
        });
        g.write(0, 0, "abcd");
        g.pop_clip();
        assert_eq!(g.get().0, "\n\n\n\n", "zero-area x clip");

        // y1==y2==1 (ink probe: nothing written).
        let mut g = Grid::new(5, 10);
        g.push_clip(Clip {
            x1: None,
            x2: None,
            y1: Some(1),
            y2: Some(1),
        });
        g.write(0, 0, "aa\nbb\ncc");
        g.pop_clip();
        assert_eq!(g.get().0, "\n\n\n\n", "zero-area y clip");

        // Both axes zero-area (ink probe: nothing written).
        let mut g = Grid::new(5, 10);
        g.push_clip(Clip {
            x1: Some(2),
            x2: Some(2),
            y1: Some(1),
            y2: Some(1),
        });
        g.write(0, 0, "abcd\nefgh\nijkl");
        g.pop_clip();
        assert_eq!(g.get().0, "\n\n\n\n", "zero-area x+y clip");
    }
}