codex/codex-rs/tui/src/custom_terminal.rs

// This is derived from `ratatui::Terminal`, which is licensed under the following terms:
//
// The MIT License (MIT)
// Copyright (c) 2016-2022 Florian Dehau
// Copyright (c) 2023-2025 The Ratatui Developers
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.
use std::io;
use std::io::Write;

use crossterm::cursor::MoveTo;
use crossterm::queue;
use crossterm::style::Colors;
use crossterm::style::Print;
use crossterm::style::SetAttribute;
use crossterm::style::SetBackgroundColor;
use crossterm::style::SetColors;
use crossterm::style::SetForegroundColor;
use crossterm::terminal::Clear;
use derive_more::IsVariant;
use ratatui::backend::Backend;
use ratatui::backend::ClearType;
use ratatui::buffer::Buffer;
use ratatui::layout::Position;
use ratatui::layout::Rect;
use ratatui::layout::Size;
use ratatui::style::Color;
use ratatui::style::Modifier;
use ratatui::widgets::WidgetRef;
use unicode_width::UnicodeWidthStr;

/// Returns the display width of a cell symbol, ignoring OSC escape sequences.
///
/// OSC sequences (e.g. OSC 8 hyperlinks: `\x1B]8;;URL\x07`) are terminal
/// control sequences that don't consume display columns.  The standard
/// `UnicodeWidthStr::width()` method incorrectly counts the printable
/// characters inside OSC payloads (like `]`, `8`, `;`, and URL characters).
/// This function strips them first so that only visible characters contribute
/// to the width.
fn display_width(s: &str) -> usize {
    // Fast path: no escape sequences present.
    if !s.contains('\x1B') {
        return s.width();
    }

    // Strip OSC sequences: ESC ] ... BEL
    let mut visible = String::with_capacity(s.len());
    let mut chars = s.chars();
    while let Some(ch) = chars.next() {
        if ch == '\x1B' && chars.clone().next() == Some(']') {
            // Consume the ']' and everything up to and including BEL.
            chars.next(); // skip ']'
            for c in chars.by_ref() {
                if c == '\x07' {
                    break;
                }
            }
            continue;
        }
        visible.push(ch);
    }
    visible.width()
}

#[derive(Debug, Hash)]
pub struct Frame<'a> {
    /// Where should the cursor be after drawing this frame?
    ///
    /// If `None`, the cursor is hidden and its position is controlled by the backend. If `Some((x,
    /// y))`, the cursor is shown and placed at `(x, y)` after the call to `Terminal::draw()`.
    pub(crate) cursor_position: Option<Position>,

    /// The area of the viewport
    pub(crate) viewport_area: Rect,

    /// The buffer that is used to draw the current frame
    pub(crate) buffer: &'a mut Buffer,
}

impl Frame<'_> {
    /// The area of the current frame
    ///
    /// This is guaranteed not to change during rendering, so may be called multiple times.
    ///
    /// If your app listens for a resize event from the backend, it should ignore the values from
    /// the event for any calculations that are used to render the current frame and use this value
    /// instead as this is the area of the buffer that is used to render the current frame.
    pub const fn area(&self) -> Rect {
        self.viewport_area
    }

    /// Render a [`WidgetRef`] to the current buffer using [`WidgetRef::render_ref`].
    ///
    /// Usually the area argument is the size of the current frame or a sub-area of the current
    /// frame (which can be obtained using [`Layout`] to split the total area).
    #[allow(clippy::needless_pass_by_value)]
    pub fn render_widget_ref<W: WidgetRef>(&mut self, widget: W, area: Rect) {
        widget.render_ref(area, self.buffer);
    }

    /// After drawing this frame, make the cursor visible and put it at the specified (x, y)
    /// coordinates. If this method is not called, the cursor will be hidden.
    ///
    /// Note that this will interfere with calls to [`Terminal::hide_cursor`],
    /// [`Terminal::show_cursor`], and [`Terminal::set_cursor_position`]. Pick one of the APIs and
    /// stick with it.
    ///
    /// [`Terminal::hide_cursor`]: crate::Terminal::hide_cursor
    /// [`Terminal::show_cursor`]: crate::Terminal::show_cursor
    /// [`Terminal::set_cursor_position`]: crate::Terminal::set_cursor_position
    pub fn set_cursor_position<P: Into<Position>>(&mut self, position: P) {
        self.cursor_position = Some(position.into());
    }

    /// Gets the buffer that this `Frame` draws into as a mutable reference.
    pub fn buffer_mut(&mut self) -> &mut Buffer {
        self.buffer
    }
}

#[derive(Debug, Default, Clone, Eq, PartialEq, Hash)]
pub struct Terminal<B>
where
    B: Backend + Write,
{
    /// The backend used to interface with the terminal
    backend: B,
    /// Holds the results of the current and previous draw calls. The two are compared at the end
    /// of each draw pass to output the necessary updates to the terminal
    buffers: [Buffer; 2],
    /// Index of the current buffer in the previous array
    current: usize,
    /// Whether the cursor is currently hidden
    pub hidden_cursor: bool,
    /// Area of the viewport
    pub viewport_area: Rect,
    /// Last known size of the terminal. Used to detect if the internal buffers have to be resized.
    pub last_known_screen_size: Size,
    /// Last known position of the cursor. Used to find the new area when the viewport is inlined
    /// and the terminal resized.
    pub last_known_cursor_pos: Position,
}

impl<B> Drop for Terminal<B>
where
    B: Backend,
    B: Write,
{
    #[allow(clippy::print_stderr)]
    fn drop(&mut self) {
        // Attempt to restore the cursor state
        if self.hidden_cursor
            && let Err(err) = self.show_cursor()
        {
            eprintln!("Failed to show the cursor: {err}");
        }
    }
}

impl<B> Terminal<B>
where
    B: Backend,
    B: Write,
{
    /// Creates a new [`Terminal`] with the given [`Backend`] and [`TerminalOptions`].
    pub fn with_options(mut backend: B) -> io::Result<Self> {
        let screen_size = backend.size()?;
        let cursor_pos = backend.get_cursor_position().unwrap_or_else(|err| {
            // Some PTYs do not answer CPR (`ESC[6n`); continue with a safe default instead
            // of failing TUI startup.
            tracing::warn!("failed to read initial cursor position; defaulting to origin: {err}");
            Position { x: 0, y: 0 }
        });
        Ok(Self {
            backend,
            buffers: [Buffer::empty(Rect::ZERO), Buffer::empty(Rect::ZERO)],
            current: 0,
            hidden_cursor: false,
            viewport_area: Rect::new(0, cursor_pos.y, 0, 0),
            last_known_screen_size: screen_size,
            last_known_cursor_pos: cursor_pos,
        })
    }

    /// Get a Frame object which provides a consistent view into the terminal state for rendering.
    pub fn get_frame(&mut self) -> Frame<'_> {
        Frame {
            cursor_position: None,
            viewport_area: self.viewport_area,
            buffer: self.current_buffer_mut(),
        }
    }

    /// Gets the current buffer as a reference.
    fn current_buffer(&self) -> &Buffer {
        &self.buffers[self.current]
    }

    /// Gets the current buffer as a mutable reference.
    fn current_buffer_mut(&mut self) -> &mut Buffer {
        &mut self.buffers[self.current]
    }

    /// Gets the previous buffer as a reference.
    fn previous_buffer(&self) -> &Buffer {
        &self.buffers[1 - self.current]
    }

    /// Gets the previous buffer as a mutable reference.
    fn previous_buffer_mut(&mut self) -> &mut Buffer {
        &mut self.buffers[1 - self.current]
    }

    /// Gets the backend
    pub const fn backend(&self) -> &B {
        &self.backend
    }

    /// Gets the backend as a mutable reference
    pub fn backend_mut(&mut self) -> &mut B {
        &mut self.backend
    }

    /// Obtains a difference between the previous and the current buffer and passes it to the
    /// current backend for drawing.
    pub fn flush(&mut self) -> io::Result<()> {
        let updates = diff_buffers(self.previous_buffer(), self.current_buffer());
        let last_put_command = updates.iter().rfind(|command| command.is_put());
        if let Some(&DrawCommand::Put { x, y, .. }) = last_put_command {
            self.last_known_cursor_pos = Position { x, y };
        }
        draw(&mut self.backend, updates.into_iter())
    }

    /// Updates the Terminal so that internal buffers match the requested area.
    ///
    /// Requested area will be saved to remain consistent when rendering. This leads to a full clear
    /// of the screen.
    pub fn resize(&mut self, screen_size: Size) -> io::Result<()> {
        self.last_known_screen_size = screen_size;
        Ok(())
    }

    /// Sets the viewport area.
    pub fn set_viewport_area(&mut self, area: Rect) {
        self.current_buffer_mut().resize(area);
        self.previous_buffer_mut().resize(area);
        self.viewport_area = area;
    }

    /// Queries the backend for size and resizes if it doesn't match the previous size.
    pub fn autoresize(&mut self) -> io::Result<()> {
        let screen_size = self.size()?;
        if screen_size != self.last_known_screen_size {
            self.resize(screen_size)?;
        }
        Ok(())
    }

    /// Draws a single frame to the terminal.
    ///
    /// Returns a [`CompletedFrame`] if successful, otherwise a [`std::io::Error`].
    ///
    /// If the render callback passed to this method can fail, use [`try_draw`] instead.
    ///
    /// Applications should call `draw` or [`try_draw`] in a loop to continuously render the
    /// terminal. These methods are the main entry points for drawing to the terminal.
    ///
    /// [`try_draw`]: Terminal::try_draw
    ///
    /// This method will:
    ///
    /// - autoresize the terminal if necessary
    /// - call the render callback, passing it a [`Frame`] reference to render to
    /// - flush the current internal state by copying the current buffer to the backend
    /// - move the cursor to the last known position if it was set during the rendering closure
    ///
    /// The render callback should fully render the entire frame when called, including areas that
    /// are unchanged from the previous frame. This is because each frame is compared to the
    /// previous frame to determine what has changed, and only the changes are written to the
    /// terminal. If the render callback does not fully render the frame, the terminal will not be
    /// in a consistent state.
    pub fn draw<F>(&mut self, render_callback: F) -> io::Result<()>
    where
        F: FnOnce(&mut Frame),
    {
        self.try_draw(|frame| {
            render_callback(frame);
            io::Result::Ok(())
        })
    }

    /// Tries to draw a single frame to the terminal.
    ///
    /// Returns [`Result::Ok`] containing a [`CompletedFrame`] if successful, otherwise
    /// [`Result::Err`] containing the [`std::io::Error`] that caused the failure.
    ///
    /// This is the equivalent of [`Terminal::draw`] but the render callback is a function or
    /// closure that returns a `Result` instead of nothing.
    ///
    /// Applications should call `try_draw` or [`draw`] in a loop to continuously render the
    /// terminal. These methods are the main entry points for drawing to the terminal.
    ///
    /// [`draw`]: Terminal::draw
    ///
    /// This method will:
    ///
    /// - autoresize the terminal if necessary
    /// - call the render callback, passing it a [`Frame`] reference to render to
    /// - flush the current internal state by copying the current buffer to the backend
    /// - move the cursor to the last known position if it was set during the rendering closure
    /// - return a [`CompletedFrame`] with the current buffer and the area of the terminal
    ///
    /// The render callback passed to `try_draw` can return any [`Result`] with an error type that
    /// can be converted into an [`std::io::Error`] using the [`Into`] trait. This makes it possible
    /// to use the `?` operator to propagate errors that occur during rendering. If the render
    /// callback returns an error, the error will be returned from `try_draw` as an
    /// [`std::io::Error`] and the terminal will not be updated.
    ///
    /// The [`CompletedFrame`] returned by this method can be useful for debugging or testing
    /// purposes, but it is often not used in regular applicationss.
    ///
    /// The render callback should fully render the entire frame when called, including areas that
    /// are unchanged from the previous frame. This is because each frame is compared to the
    /// previous frame to determine what has changed, and only the changes are written to the
    /// terminal. If the render function does not fully render the frame, the terminal will not be
    /// in a consistent state.
    pub fn try_draw<F, E>(&mut self, render_callback: F) -> io::Result<()>
    where
        F: FnOnce(&mut Frame) -> Result<(), E>,
        E: Into<io::Error>,
    {
        // Autoresize - otherwise we get glitches if shrinking or potential desync between widgets
        // and the terminal (if growing), which may OOB.
        self.autoresize()?;

        let mut frame = self.get_frame();

        render_callback(&mut frame).map_err(Into::into)?;

        // We can't change the cursor position right away because we have to flush the frame to
        // stdout first. But we also can't keep the frame around, since it holds a &mut to
        // Buffer. Thus, we're taking the important data out of the Frame and dropping it.
        let cursor_position = frame.cursor_position;

        // Draw to stdout
        self.flush()?;

        match cursor_position {
            None => self.hide_cursor()?,
            Some(position) => {
                self.show_cursor()?;
                self.set_cursor_position(position)?;
            }
        }

        self.swap_buffers();

        Backend::flush(&mut self.backend)?;

        Ok(())
    }

    /// Hides the cursor.
    pub fn hide_cursor(&mut self) -> io::Result<()> {
        self.backend.hide_cursor()?;
        self.hidden_cursor = true;
        Ok(())
    }

    /// Shows the cursor.
    pub fn show_cursor(&mut self) -> io::Result<()> {
        self.backend.show_cursor()?;
        self.hidden_cursor = false;
        Ok(())
    }

    /// Gets the current cursor position.
    ///
    /// This is the position of the cursor after the last draw call.
    #[allow(dead_code)]
    pub fn get_cursor_position(&mut self) -> io::Result<Position> {
        self.backend.get_cursor_position()
    }

    /// Sets the cursor position.
    pub fn set_cursor_position<P: Into<Position>>(&mut self, position: P) -> io::Result<()> {
        let position = position.into();
        self.backend.set_cursor_position(position)?;
        self.last_known_cursor_pos = position;
        Ok(())
    }

    /// Clear the terminal and force a full redraw on the next draw call.
    pub fn clear(&mut self) -> io::Result<()> {
        if self.viewport_area.is_empty() {
            return Ok(());
        }
        self.backend
            .set_cursor_position(self.viewport_area.as_position())?;
        self.backend.clear_region(ClearType::AfterCursor)?;
        // Reset the back buffer to make sure the next update will redraw everything.
        self.previous_buffer_mut().reset();
        Ok(())
    }

    /// Clear terminal scrollback (if supported) and force a full redraw.
    pub fn clear_scrollback(&mut self) -> io::Result<()> {
        if self.viewport_area.is_empty() {
            return Ok(());
        }
        self.backend
            .set_cursor_position(self.viewport_area.as_position())?;
        queue!(self.backend, Clear(crossterm::terminal::ClearType::Purge))?;
        std::io::Write::flush(&mut self.backend)?;
        self.previous_buffer_mut().reset();
        Ok(())
    }

    /// Clears the inactive buffer and swaps it with the current buffer
    pub fn swap_buffers(&mut self) {
        self.previous_buffer_mut().reset();
        self.current = 1 - self.current;
    }

    /// Queries the real size of the backend.
    pub fn size(&self) -> io::Result<Size> {
        self.backend.size()
    }
}

use ratatui::buffer::Cell;

#[derive(Debug, IsVariant)]
enum DrawCommand {
    Put { x: u16, y: u16, cell: Cell },
    ClearToEnd { x: u16, y: u16, bg: Color },
}

fn diff_buffers(a: &Buffer, b: &Buffer) -> Vec<DrawCommand> {
    let previous_buffer = &a.content;
    let next_buffer = &b.content;

    let mut updates = vec![];
    let mut last_nonblank_columns = vec![0; a.area.height as usize];
    for y in 0..a.area.height {
        let row_start = y as usize * a.area.width as usize;
        let row_end = row_start + a.area.width as usize;
        let row = &next_buffer[row_start..row_end];
        let bg = row.last().map(|cell| cell.bg).unwrap_or(Color::Reset);

        // Scan the row to find the rightmost column that still matters: any non-space glyph,
        // any cell whose bg differs from the row’s trailing bg, or any cell with modifiers.
        // Multi-width glyphs extend that region through their full displayed width.
        // After that point the rest of the row can be cleared with a single ClearToEnd, a perf win
        // versus emitting multiple space Put commands.
        let mut last_nonblank_column = 0usize;
        let mut column = 0usize;
        while column < row.len() {
            let cell = &row[column];
            let width = display_width(cell.symbol());
            if cell.symbol() != " " || cell.bg != bg || cell.modifier != Modifier::empty() {
                last_nonblank_column = column + (width.saturating_sub(1));
            }
            column += width.max(1); // treat zero-width symbols as width 1
        }

        if last_nonblank_column + 1 < row.len() {
            let (x, y) = a.pos_of(row_start + last_nonblank_column + 1);
            updates.push(DrawCommand::ClearToEnd { x, y, bg });
        }

        last_nonblank_columns[y as usize] = last_nonblank_column as u16;
    }

    // Cells invalidated by drawing/replacing preceding multi-width characters:
    let mut invalidated: usize = 0;
    // Cells from the current buffer to skip due to preceding multi-width characters taking
    // their place (the skipped cells should be blank anyway), or due to per-cell-skipping:
    let mut to_skip: usize = 0;
    for (i, (current, previous)) in next_buffer.iter().zip(previous_buffer.iter()).enumerate() {
        if !current.skip && (current != previous || invalidated > 0) && to_skip == 0 {
            let (x, y) = a.pos_of(i);
            let row = i / a.area.width as usize;
            if x <= last_nonblank_columns[row] {
                updates.push(DrawCommand::Put {
                    x,
                    y,
                    cell: next_buffer[i].clone(),
                });
            }
        }

        to_skip = display_width(current.symbol()).saturating_sub(1);

        let affected_width = std::cmp::max(
            display_width(current.symbol()),
            display_width(previous.symbol()),
        );
        invalidated = std::cmp::max(affected_width, invalidated).saturating_sub(1);
    }
    updates
}

fn draw<I>(writer: &mut impl Write, commands: I) -> io::Result<()>
where
    I: Iterator<Item = DrawCommand>,
{
    let mut fg = Color::Reset;
    let mut bg = Color::Reset;
    let mut modifier = Modifier::empty();
    let mut last_pos: Option<Position> = None;
    for command in commands {
        let (x, y) = match command {
            DrawCommand::Put { x, y, .. } => (x, y),
            DrawCommand::ClearToEnd { x, y, .. } => (x, y),
        };
        // Move the cursor if the previous location was not (x - 1, y)
        if !matches!(last_pos, Some(p) if x == p.x + 1 && y == p.y) {
            queue!(writer, MoveTo(x, y))?;
        }
        last_pos = Some(Position { x, y });
        match command {
            DrawCommand::Put { cell, .. } => {
                if cell.modifier != modifier {
                    let diff = ModifierDiff {
                        from: modifier,
                        to: cell.modifier,
                    };
                    diff.queue(writer)?;
                    modifier = cell.modifier;
                }
                if cell.fg != fg || cell.bg != bg {
                    queue!(
                        writer,
                        SetColors(Colors::new(cell.fg.into(), cell.bg.into()))
                    )?;
                    fg = cell.fg;
                    bg = cell.bg;
                }

                queue!(writer, Print(cell.symbol()))?;
            }
            DrawCommand::ClearToEnd { bg: clear_bg, .. } => {
                queue!(writer, SetAttribute(crossterm::style::Attribute::Reset))?;
                modifier = Modifier::empty();
                queue!(writer, SetBackgroundColor(clear_bg.into()))?;
                bg = clear_bg;
                queue!(writer, Clear(crossterm::terminal::ClearType::UntilNewLine))?;
            }
        }
    }

    queue!(
        writer,
        SetForegroundColor(crossterm::style::Color::Reset),
        SetBackgroundColor(crossterm::style::Color::Reset),
        SetAttribute(crossterm::style::Attribute::Reset),
    )?;

    Ok(())
}

/// The `ModifierDiff` struct is used to calculate the difference between two `Modifier`
/// values. This is useful when updating the terminal display, as it allows for more
/// efficient updates by only sending the necessary changes.
struct ModifierDiff {
    pub from: Modifier,
    pub to: Modifier,
}

impl ModifierDiff {
    fn queue<W: io::Write>(self, w: &mut W) -> io::Result<()> {
        use crossterm::style::Attribute as CAttribute;
        let removed = self.from - self.to;
        if removed.contains(Modifier::REVERSED) {
            queue!(w, SetAttribute(CAttribute::NoReverse))?;
        }
        if removed.contains(Modifier::BOLD) {
            queue!(w, SetAttribute(CAttribute::NormalIntensity))?;
            if self.to.contains(Modifier::DIM) {
                queue!(w, SetAttribute(CAttribute::Dim))?;
            }
        }
        if removed.contains(Modifier::ITALIC) {
            queue!(w, SetAttribute(CAttribute::NoItalic))?;
        }
        if removed.contains(Modifier::UNDERLINED) {
            queue!(w, SetAttribute(CAttribute::NoUnderline))?;
        }
        if removed.contains(Modifier::DIM) {
            queue!(w, SetAttribute(CAttribute::NormalIntensity))?;
        }
        if removed.contains(Modifier::CROSSED_OUT) {
            queue!(w, SetAttribute(CAttribute::NotCrossedOut))?;
        }
        if removed.contains(Modifier::SLOW_BLINK) || removed.contains(Modifier::RAPID_BLINK) {
            queue!(w, SetAttribute(CAttribute::NoBlink))?;
        }

        let added = self.to - self.from;
        if added.contains(Modifier::REVERSED) {
            queue!(w, SetAttribute(CAttribute::Reverse))?;
        }
        if added.contains(Modifier::BOLD) {
            queue!(w, SetAttribute(CAttribute::Bold))?;
        }
        if added.contains(Modifier::ITALIC) {
            queue!(w, SetAttribute(CAttribute::Italic))?;
        }
        if added.contains(Modifier::UNDERLINED) {
            queue!(w, SetAttribute(CAttribute::Underlined))?;
        }
        if added.contains(Modifier::DIM) {
            queue!(w, SetAttribute(CAttribute::Dim))?;
        }
        if added.contains(Modifier::CROSSED_OUT) {
            queue!(w, SetAttribute(CAttribute::CrossedOut))?;
        }
        if added.contains(Modifier::SLOW_BLINK) {
            queue!(w, SetAttribute(CAttribute::SlowBlink))?;
        }
        if added.contains(Modifier::RAPID_BLINK) {
            queue!(w, SetAttribute(CAttribute::RapidBlink))?;
        }

        Ok(())
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use pretty_assertions::assert_eq;
    use ratatui::layout::Rect;
    use ratatui::style::Style;

    #[test]
    fn diff_buffers_does_not_emit_clear_to_end_for_full_width_row() {
        let area = Rect::new(0, 0, 3, 2);
        let previous = Buffer::empty(area);
        let mut next = Buffer::empty(area);

        next.cell_mut((2, 0))
            .expect("cell should exist")
            .set_symbol("X");

        let commands = diff_buffers(&previous, &next);

        let clear_count = commands
            .iter()
            .filter(|command| matches!(command, DrawCommand::ClearToEnd { y, .. } if *y == 0))
            .count();
        assert_eq!(
            0, clear_count,
            "expected diff_buffers not to emit ClearToEnd; commands: {commands:?}",
        );
        assert!(
            commands
                .iter()
                .any(|command| matches!(command, DrawCommand::Put { x: 2, y: 0, .. })),
            "expected diff_buffers to update the final cell; commands: {commands:?}",
        );
    }

    #[test]
    fn diff_buffers_clear_to_end_starts_after_wide_char() {
        let area = Rect::new(0, 0, 10, 1);
        let mut previous = Buffer::empty(area);
        let mut next = Buffer::empty(area);

        previous.set_string(0, 0, "中文", Style::default());
        next.set_string(0, 0, "中", Style::default());

        let commands = diff_buffers(&previous, &next);
        assert!(
            commands
                .iter()
                .any(|command| matches!(command, DrawCommand::ClearToEnd { x: 2, y: 0, .. })),
            "expected clear-to-end to start after the remaining wide char; commands: {commands:?}"
        );
    }
}