turbo_debug_console/

streamview.rs

// Copyright (c) 2026 Enzo Lombardi
// SPDX-License-Identifier: MIT

//! A scrollback view over styled cells, one `Vec<Cell>` per line.

use unicode_width::{UnicodeWidthChar, UnicodeWidthStr};

use turbo_vision::core::draw::{Cell, DrawBuffer};
use turbo_vision::core::event::{
    Event, EventType, KB_DOWN, KB_END, KB_ESC, KB_HOME, KB_PGDN, KB_PGUP, KB_UP, MB_LEFT_BUTTON,
};
use turbo_vision::core::geometry::{Point, Rect};
use turbo_vision::core::palette::{Attr, TvColor};
use turbo_vision::core::state::{GF_GROW_HI_X, GF_GROW_HI_Y, GrowFlags};
use turbo_vision::terminal::Terminal;
use turbo_vision::views::view::{View, write_line_to_terminal};

/// A caret position in the wrapped scrollback: an absolute display-row index
/// (into `iter_rows()`, so it survives scrolling) and a column, where the
/// column is a cell index in that row (`draw` maps cell index 1:1 to screen
/// column). A caret at `col` sits just before the cell at `col`.
#[derive(Clone, Copy, PartialEq, Eq, Debug)]
struct SelPos {
    row: usize,
    col: usize,
}

/// An active (continuous) selection between two carets.
#[derive(Clone, Copy, PartialEq, Eq, Debug)]
struct Selection {
    anchor: SelPos,
    head: SelPos,
}

/// Swaps foreground and background, preserving text style — the highlight for
/// a selected cell.
fn reverse(attr: Attr) -> Attr {
    Attr::new(attr.bg, attr.fg).with_style(attr.style)
}

/// The two carets in reading order (top-to-bottom, left-to-right).
fn order(a: SelPos, b: SelPos) -> (SelPos, SelPos) {
    if (a.row, a.col) <= (b.row, b.col) {
        (a, b)
    } else {
        (b, a)
    }
}

/// A base character immediately followed by U+FE0F (the emoji presentation
/// selector, VS-16) or U+FE0E (the text presentation selector, VS-15) forms
/// one *presentation sequence* whose combined width can differ from the
/// base character's own width in isolation. This is exactly the shape of
/// plank's tool-call banner glyph (`🛠️` = U+1F6E0 + U+FE0F): the bare
/// wrench is East-Asian-Width `Neutral` (width 1), but the fully-qualified
/// emoji sequence the model actually emits is double-width. `unicode_width`
/// only resolves this at the *string* level (`UnicodeWidthStr`), not per
/// `char`, so a two-character lookahead is required to catch it — this is
/// still the crate doing the Unicode-correctness work; nothing here is a
/// hand-rolled codepoint table.
const PRESENTATION_SELECTORS: [char; 2] = ['\u{FE0F}', '\u{FE0E}'];

/// Normalizes a naive, one-`Cell`-per-`char` line into one `Cell` per
/// terminal *column* — the invariant every other method in this module
/// relies on (row width, wrapping's column-accurate break points, and
/// `draw`'s column count).
///
/// A double-width character (an emoji, a CJK glyph) keeps its real `char`
/// in the first cell and gets a filler cell for each additional column,
/// mirroring `turbo_vision`'s own `DrawBuffer::move_str` convention: the
/// terminal's cell-diffing flush already knows to skip a `'\0'` when
/// encoding output, so an invented filler paints as blank if ever exposed
/// (e.g. wrapping is careful never to cut a wide character in half, but if
/// it ever did, this is what would be exposed) rather than emitting half a
/// glyph. When the second column instead comes from a real
/// trailing presentation selector, that selector's own character is kept
/// as the filler — it is a genuine, zero-advance character, not a padding
/// artifact, so `plain_text` must still hand it back on Save As.
///
/// A zero-width character (a combining mark, a selector whose sequence
/// collapses to width 0) occupies no column and is dropped — again
/// matching `move_str`, and this module's only way to keep the stored
/// column count equal to the true rendered width without a codepoint-range
/// table of our own.
///
/// Idempotent: a spacer cell (`ch == '\0'`) already produced by a previous
/// call passes through unchanged, so re-normalizing already-normalized
/// cells (e.g. lines rebuilt from `styled_lines()`) is harmless.
fn normalize_line(cells: &[Cell]) -> Vec<Cell> {
    let mut out = Vec::with_capacity(cells.len());
    let mut i = 0;
    while i < cells.len() {
        let cell = cells[i];
        if cell.ch == '\0' {
            out.push(cell);
            i += 1;
            continue;
        }

        let next = cells.get(i + 1).copied();
        let selector = next.filter(|n| PRESENTATION_SELECTORS.contains(&n.ch));

        let width = if let Some(sel) = selector {
            let mut seq = String::with_capacity(cell.ch.len_utf8() + sel.ch.len_utf8());
            seq.push(cell.ch);
            seq.push(sel.ch);
            seq.width()
        } else {
            cell.ch.width().unwrap_or(0)
        };

        if width == 0 {
            i += if selector.is_some() { 2 } else { 1 };
            continue;
        }

        out.push(cell);
        if let Some(sel) = selector {
            out.push(sel);
            for _ in 2..width {
                out.push(Cell::new('\0', cell.attr));
            }
            i += 2;
        } else {
            for _ in 1..width {
                out.push(Cell::new('\0', cell.attr));
            }
            i += 1;
        }
    }
    out
}

/// Default scrollback depth.
pub const DEFAULT_MAX_LINES: usize = 10_000;

/// Splits one width-normalized logical line (one `Cell` per terminal column,
/// per `normalize_line`'s invariant) into the display rows it wraps to at
/// `width` columns.
///
/// Breaks at the last whitespace cell at or before the width boundary when
/// one exists in the row being filled; otherwise breaks exactly at `width`.
/// Because `cells` is already column-normalized, a wrap point chosen this
/// way always falls on a column boundary and never between a double-width
/// character's leading cell and its filler, since a filler cell (`ch ==
/// '\0'`) is never itself whitespace and so is never chosen as, or split
/// from, a break point ahead of its owner.
///
/// An empty line still yields one (empty) row, matching a real terminal:
/// a blank logical line occupies one blank display row, not zero.
fn wrap_cells(cells: &[Cell], width: usize) -> Vec<Vec<Cell>> {
    if width == 0 || cells.is_empty() {
        return vec![cells.to_vec()];
    }

    let mut rows = Vec::new();
    let mut rest = cells;
    while rest.len() > width {
        // Search for a break point: the last whitespace cell whose index is
        // < width, scanning backwards from width - 1. A filler cell ('\0')
        // is skipped as a candidate break (it is never whitespace) but does
        // not stop the scan.
        let mut break_at = None;
        for i in (0..width).rev() {
            if rest[i].ch.is_whitespace() {
                break_at = Some(i);
                break;
            }
        }
        if let Some(i) = break_at {
            rows.push(rest[..i].to_vec());
            rest = &rest[i + 1..]; // drop the whitespace cell itself
        } else {
            // A plain character-break cut at `width` could land between a
            // double-width character's leading cell and its filler ('\0');
            // if so, pull the cut back one column so the whole glyph moves
            // to the next row instead of splitting it.
            let mut cut = width;
            if cut > 1 && rest.get(cut).is_some_and(|c| c.ch == '\0') {
                cut -= 1;
            }
            rows.push(rest[..cut].to_vec());
            rest = &rest[cut..];
        }
    }
    rows.push(rest.to_vec());
    rows
}

/// A scrollback of styled lines, with autoscroll that releases when the user
/// scrolls back and re-arms at the bottom.
#[derive(Debug)]
pub struct StreamView {
    bounds: Rect,
    /// How this view follows its parent when the terminal is resized.
    ///
    /// `View`'s default is 0, meaning fixed, and a fixed view is skipped by
    /// the desktop's resize cascade: the window frame would resize around a
    /// scrollback still wrapped for the old width. `HI_X | HI_Y` pins the
    /// top-left and moves the bottom-right edge, which is what a view that
    /// fills its window wants.
    grow_mode: GrowFlags,
    /// Completed lines, oldest first. This is the source of truth: the log
    /// text as the producer sent it, one entry per logical line, never
    /// baked with this window's current wrap points. `plain_text()` reads
    /// from here, not from `wrapped`.
    lines: Vec<Vec<Cell>>,
    /// The line currently streaming in, not yet terminated by a newline.
    partial: Option<Vec<Cell>>,
    /// Display rows for `lines`, in order, each logical line's rows
    /// contiguous. `draw` and all scroll arithmetic read only from here (and
    /// from `partial_wrapped` below), never from `lines` directly.
    wrapped: Vec<Vec<Cell>>,
    /// How many display rows in `wrapped` each entry of `lines` currently
    /// occupies, parallel to `lines`. Lets `trim` drop exactly the rows a
    /// dropped logical line contributed without re-wrapping everything.
    row_counts: Vec<usize>,
    /// Display rows for the in-progress `partial` line, wrapped the same
    /// way; kept separate from `wrapped` because `set_partial` replaces
    /// rather than appends.
    partial_wrapped: Vec<Vec<Cell>>,
    /// Bounds by logical lines, not display rows: a narrower window wraps
    /// the same history into more rows, and bounding by rows would make a
    /// narrow window silently forget more history than a wide one for the
    /// same underlying stream. Logical-line count is the stable, resize-
    /// independent budget.
    max_lines: usize,
    /// Index of the topmost displayed row, in `wrapped`.
    top: usize,
    /// True while the view follows the tail.
    follow: bool,
    fill: Attr,
    /// The active text selection, if any. Positions are in absolute
    /// wrapped-row coordinates (see [`SelPos`]). Dropped whenever the buffer
    /// mutates, since row indices would otherwise dangle.
    selection: Option<Selection>,
}

impl StreamView {
    #[must_use]
    pub fn new(bounds: Rect) -> Self {
        Self {
            bounds,
            grow_mode: GF_GROW_HI_X | GF_GROW_HI_Y,
            lines: Vec::new(),
            partial: None,
            wrapped: Vec::new(),
            row_counts: Vec::new(),
            partial_wrapped: Vec::new(),
            max_lines: DEFAULT_MAX_LINES,
            top: 0,
            follow: true,
            fill: Attr::new(TvColor::LightGray, TvColor::Black),
            selection: None,
        }
    }

    fn width(&self) -> usize {
        usize::try_from(self.bounds.width()).unwrap_or(0)
    }

    pub fn set_max_lines(&mut self, n: usize) {
        self.max_lines = n.max(1);
        self.trim();
    }

    /// Appends a completed line.
    pub fn push_line(&mut self, cells: &[Cell]) {
        // Row indices shift when the buffer grows/trims, so a held selection
        // would dangle; drop it.
        self.selection = None;
        let normalized = normalize_line(cells);
        let rows = wrap_cells(&normalized, self.width());
        self.row_counts.push(rows.len());
        self.wrapped.extend(rows);
        self.lines.push(normalized);
        self.trim();
        if self.follow {
            self.scroll_to_bottom();
        }
    }

    /// Replaces the in-progress line. Called on every repaint while a line is
    /// still streaming, so it must overwrite rather than append.
    pub fn set_partial(&mut self, cells: &[Cell]) {
        let cells = normalize_line(cells);
        if cells.is_empty() {
            self.partial = None;
            self.partial_wrapped.clear();
        } else {
            self.partial_wrapped = wrap_cells(&cells, self.width());
            self.partial = Some(cells);
        }
        if self.follow {
            self.scroll_to_bottom();
        }
    }

    pub fn clear(&mut self) {
        self.lines.clear();
        self.partial = None;
        self.wrapped.clear();
        self.row_counts.clear();
        self.partial_wrapped.clear();
        self.top = 0;
        self.follow = true;
        self.selection = None;
    }

    /// Total displayed lines, including the in-progress one.
    #[must_use]
    pub fn line_count(&self) -> usize {
        self.lines.len() + usize::from(self.partial.is_some())
    }

    /// Total display rows currently shown, including the in-progress line's
    /// wrapped rows. This is what scroll arithmetic (`page`, `max_top`, and
    /// the keyboard handlers) counts, so scrolling lands correctly wherever
    /// a wrapped long line pushes rows out of alignment with logical lines.
    #[must_use]
    pub fn row_count(&self) -> usize {
        self.wrapped.len() + self.partial_wrapped.len()
    }

    /// Visible rows, i.e. the view height.
    fn page(&self) -> usize {
        usize::try_from(self.bounds.height()).unwrap_or(0).max(1)
    }

    fn max_top(&self) -> usize {
        self.row_count().saturating_sub(self.page())
    }

    /// Rewraps every logical line and the in-progress partial at the current
    /// width, rebuilding `wrapped`, `row_counts` and `partial_wrapped` from
    /// scratch. Needed whenever the width itself changes (a resize), since
    /// every existing wrap point can be stale in either direction.
    fn rewrap(&mut self) {
        let width = self.width();
        self.wrapped.clear();
        self.row_counts.clear();
        for line in &self.lines {
            let rows = wrap_cells(line, width);
            self.row_counts.push(rows.len());
            self.wrapped.extend(rows);
        }
        self.partial_wrapped = match &self.partial {
            Some(cells) => wrap_cells(cells, width),
            None => Vec::new(),
        };
    }

    pub fn scroll_to_bottom(&mut self) {
        self.top = self.max_top();
        self.follow = true;
    }

    pub fn scroll_to_top(&mut self) {
        self.top = 0;
        self.follow = false;
    }

    pub fn scroll_up(&mut self, n: usize) {
        self.top = self.top.saturating_sub(n);
        self.follow = false;
    }

    pub fn scroll_down(&mut self, n: usize) {
        self.top = (self.top + n).min(self.max_top());
        self.follow = self.top == self.max_top();
    }

    #[must_use]
    pub fn is_at_bottom(&self) -> bool {
        self.follow
    }

    // ---- selection ----

    /// Selects the entire scrollback in stream mode. Leaves no selection if
    /// the buffer is empty.
    pub fn select_all(&mut self) {
        let rows = self.row_count();
        if rows == 0 {
            self.selection = None;
            return;
        }
        let last = rows - 1;
        let last_len = self.row_at(last).map_or(0, Vec::len);
        self.selection = Some(Selection {
            anchor: SelPos { row: 0, col: 0 },
            head: SelPos {
                row: last,
                col: last_len,
            },
        });
    }

    /// Sets a selection between two carets `(row, col)`. Order-independent:
    /// anchor and head may be given in either order.
    pub fn set_selection(&mut self, anchor: (usize, usize), head: (usize, usize)) {
        self.selection = Some(Selection {
            anchor: SelPos {
                row: anchor.0,
                col: anchor.1,
            },
            head: SelPos {
                row: head.0,
                col: head.1,
            },
        });
    }

    pub fn clear_selection(&mut self) {
        self.selection = None;
    }

    #[must_use]
    pub fn has_selection(&self) -> bool {
        self.selection.is_some()
    }

    /// The selected text, or `None` when there is no selection. Logical lines
    /// are reconstructed: a soft wrap within a line does not become a newline.
    #[must_use]
    pub fn selected_text(&self) -> Option<String> {
        let sel = self.selection?;
        let (start, end) = order(sel.anchor, sel.head);
        let mut out = String::new();
        for row in start.row..=end.row {
            let Some(cells) = self.row_at(row) else {
                continue;
            };
            let from = if row == start.row { start.col } else { 0 }.min(cells.len());
            let to = if row == end.row { end.col } else { cells.len() }.min(cells.len());
            if row > start.row && self.row_is_logical_start(row) {
                out.push('\n');
            }
            out.extend(cells[from..to.max(from)].iter().map(|c| c.ch).filter(|&ch| ch != '\0'));
        }
        Some(out)
    }

    fn row_at(&self, idx: usize) -> Option<&Vec<Cell>> {
        self.iter_rows().nth(idx)
    }

    /// Maps a screen position to a caret in the scrollback, or `None` if it
    /// falls outside the view or below the last row. The column is clamped to
    /// the hit row's length, so dragging past a line's end caps at its end.
    fn hit(&self, pos: Point) -> Option<SelPos> {
        let x = usize::try_from(pos.x - self.bounds.a.x).ok()?;
        let y = usize::try_from(pos.y - self.bounds.a.y).ok()?;
        if y >= self.page() {
            return None;
        }
        let abs_row = self.top + y;
        let len = self.row_at(abs_row)?.len();
        Some(SelPos {
            row: abs_row,
            col: x.min(len),
        })
    }

    /// Whether the cell at absolute wrapped-row `abs_row`, column `col` (a cell
    /// index) lies inside the current selection.
    fn is_selected(&self, abs_row: usize, col: usize) -> bool {
        let Some(sel) = self.selection else {
            return false;
        };
        let (s, e) = order(sel.anchor, sel.head);
        (abs_row, col) >= (s.row, s.col) && (abs_row, col) < (e.row, e.col)
    }

    /// Whether absolute wrapped-row `abs_row` is the first row of a logical
    /// line (as opposed to a soft-wrap continuation of the one above).
    fn row_is_logical_start(&self, abs_row: usize) -> bool {
        let mut offset = 0;
        for &count in &self.row_counts {
            if abs_row == offset {
                return true;
            }
            offset += count;
        }
        // `offset` now equals `wrapped.len()`, where the partial line begins.
        abs_row == offset
    }

    /// The whole scrollback with attributes stripped, for File > Save As.
    #[must_use]
    pub fn plain_text(&self) -> String {
        let mut out = String::new();
        for (i, line) in self.iter_lines().enumerate() {
            if i > 0 {
                out.push('\n');
            }
            // Spacer cells (the second column of a wide char) carry no
            // text of their own; skip them so the saved text round-trips
            // the original characters with no padding artifacts.
            out.extend(line.iter().map(|c| c.ch).filter(|&ch| ch != '\0'));
        }
        out
    }

    /// The whole scrollback with attributes intact, for tests and golden files.
    #[must_use]
    pub fn styled_lines(&self) -> Vec<Vec<Cell>> {
        self.iter_lines().cloned().collect()
    }

    fn iter_lines(&self) -> impl Iterator<Item = &Vec<Cell>> {
        self.lines.iter().chain(self.partial.iter())
    }

    /// Display rows currently on screen or scrolled to, in order: the wrapped
    /// completed lines followed by the wrapped in-progress line.
    fn iter_rows(&self) -> impl Iterator<Item = &Vec<Cell>> {
        self.wrapped.iter().chain(self.partial_wrapped.iter())
    }

    /// Bounds the scrollback by logical lines (see `max_lines`'s doc
    /// comment), dropping the oldest ones and exactly the display rows they
    /// contributed to `wrapped`.
    fn trim(&mut self) {
        if self.lines.len() > self.max_lines {
            let drop = self.lines.len() - self.max_lines;
            self.lines.drain(..drop);
            let dropped_rows: usize = self.row_counts.drain(..drop).sum();
            self.wrapped.drain(..dropped_rows);
            self.top = self.top.saturating_sub(dropped_rows);
        }
    }
}

impl View for StreamView {
    fn bounds(&self) -> Rect {
        self.bounds
    }

    fn set_bounds(&mut self, bounds: Rect) {
        let width_changed = self.width() != usize::try_from(bounds.width()).unwrap_or(0);
        self.bounds = bounds;
        if width_changed {
            self.rewrap();
        }
        if self.follow {
            self.scroll_to_bottom();
        } else {
            self.top = self.top.min(self.max_top());
        }
    }

    fn draw(&mut self, terminal: &mut Terminal) {
        if self.bounds.height() <= 0 {
            return;
        }
        let width = usize::try_from(self.bounds.width()).unwrap_or(0);
        let page = self.page();
        let rows: Vec<&Vec<Cell>> = self.iter_rows().skip(self.top).take(page).collect();

        for row in 0..page {
            let mut buf = DrawBuffer::new(width);
            for i in 0..width {
                buf.put_char(i, ' ', self.fill);
            }
            if let Some(line) = rows.get(row) {
                let abs_row = self.top + row;
                for (i, cell) in line.iter().take(width).enumerate() {
                    let attr = if self.is_selected(abs_row, i) {
                        reverse(cell.attr)
                    } else {
                        cell.attr
                    };
                    buf.put_char(i, cell.ch, attr);
                }
            }
            let y = self.bounds.a.y + i16::try_from(row).unwrap_or(i16::MAX);
            write_line_to_terminal(terminal, self.bounds.a.x, y, &buf);
        }
    }

    fn handle_event(&mut self, event: &mut Event) {
        match event.what {
            EventType::Keyboard => {
                let page = self.page();
                match event.key_code {
                    KB_UP => self.scroll_up(1),
                    KB_DOWN => self.scroll_down(1),
                    KB_PGUP => self.scroll_up(page),
                    KB_PGDN => self.scroll_down(page),
                    KB_HOME => self.scroll_to_top(),
                    KB_END => self.scroll_to_bottom(),
                    KB_ESC if self.selection.is_some() => self.clear_selection(),
                    _ => return,
                }
                event.clear();
            }
            EventType::MouseDown if event.mouse.buttons & MB_LEFT_BUTTON != 0 => {
                let Some(pos) = self.hit(event.mouse.pos) else {
                    return;
                };
                self.selection = Some(Selection {
                    anchor: pos,
                    head: pos,
                });
                event.clear();
            }
            EventType::MouseMove | EventType::MouseAuto
                if event.mouse.buttons & MB_LEFT_BUTTON != 0 =>
            {
                if let (Some(mut sel), Some(pos)) = (self.selection, self.hit(event.mouse.pos)) {
                    sel.head = pos;
                    self.selection = Some(sel);
                    event.clear();
                }
            }
            EventType::MouseUp => {
                // A press with no drag (anchor == head) is a plain click: it
                // selects nothing, so drop the empty selection.
                if let Some(sel) = self.selection
                    && sel.anchor == sel.head
                {
                    self.selection = None;
                }
                event.clear();
            }
            _ => {}
        }
    }

    fn grow_mode(&self) -> GrowFlags {
        self.grow_mode
    }

    fn set_grow_mode(&mut self, grow_mode: GrowFlags) {
        self.grow_mode = grow_mode;
    }

    fn can_focus(&self) -> bool {
        true
    }

    fn get_palette(&self) -> Option<turbo_vision::core::palette::Palette> {
        // Cells already carry resolved `Attr`s (from `AnsiLineAssembler`), so
        // there is no logical-color index for a palette to remap.
        None
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use std::io;
    use std::time::Duration;
    use turbo_vision::core::palette::TvColor;
    use turbo_vision::terminal::Backend;

    fn line(s: &str) -> Vec<Cell> {
        s.chars()
            .map(|c| Cell::new(c, Attr::new(TvColor::LightGray, TvColor::Black)))
            .collect()
    }

    fn view() -> StreamView {
        StreamView::new(Rect::new(0, 0, 40, 10))
    }

    #[test]
    fn select_all_extracts_logical_lines_without_soft_wrap_newlines() {
        let mut v = StreamView::new(Rect::new(0, 0, 10, 10));
        v.push_line(&line("hello"));
        v.push_line(&line("abcdefghijABCDEFGHIJ")); // 20 cols wraps at width 10
        v.select_all();
        assert_eq!(
            v.selected_text().unwrap(),
            "hello\nabcdefghijABCDEFGHIJ",
            "soft wraps within a logical line must not become newlines"
        );
    }

    #[test]
    fn stream_selection_spans_from_anchor_to_head_across_a_line_break() {
        let mut v = view();
        v.push_line(&line("hello"));
        v.push_line(&line("world"));
        // caret before col 2 of row 0 to caret before col 3 of row 1
        v.set_selection((0, 2), (1, 3));
        assert_eq!(v.selected_text().unwrap(), "llo\nwor");
    }

    #[test]
    fn stream_selection_is_order_independent() {
        let mut v = view();
        v.push_line(&line("hello"));
        v.push_line(&line("world"));
        v.set_selection((1, 3), (0, 2)); // reversed
        assert_eq!(v.selected_text().unwrap(), "llo\nwor");
    }

    #[test]
    fn no_selection_yields_no_text() {
        let mut v = view();
        v.push_line(&line("hello"));
        assert!(v.selected_text().is_none());
        assert!(!v.has_selection());
    }

    #[test]
    fn selected_cells_render_reverse_video() {
        let mut v = StreamView::new(Rect::new(0, 0, 8, 4));
        v.push_line(&line("abcd"));
        v.set_selection((0, 1), (0, 3)); // 'b','c'
        let mut terminal = fake_terminal(20, 10);
        v.draw(&mut terminal);
        let a = terminal.read_cell(0, 0).unwrap(); // unselected 'a'
        let b = terminal.read_cell(1, 0).unwrap(); // selected 'b'
        assert_eq!(b.ch, 'b');
        assert_eq!(b.attr.fg, a.attr.bg, "selected fg is the normal bg");
        assert_eq!(b.attr.bg, a.attr.fg, "selected bg is the normal fg");
        // The cell just past the selection ('d' region, col 3) is normal.
        let d = terminal.read_cell(3, 0).unwrap();
        assert_eq!(d.attr.fg, a.attr.fg, "col 3 is outside [1,3), so normal");
    }

    #[test]
    fn mouse_drag_creates_a_stream_selection() {
        let mut v = view();
        v.push_line(&line("hello"));
        v.push_line(&line("world"));
        let mut down = Event::mouse(EventType::MouseDown, Point::new(2, 0), MB_LEFT_BUTTON, false);
        v.handle_event(&mut down);
        let mut mv = Event::mouse(EventType::MouseMove, Point::new(3, 1), MB_LEFT_BUTTON, false);
        v.handle_event(&mut mv);
        let mut up = Event::mouse(EventType::MouseUp, Point::new(3, 1), 0, false);
        v.handle_event(&mut up);
        assert_eq!(v.selected_text().unwrap(), "llo\nwor");
    }

    #[test]
    fn a_plain_click_clears_any_selection() {
        let mut v = view();
        v.push_line(&line("hello"));
        v.select_all();
        assert!(v.has_selection());
        let mut down = Event::mouse(EventType::MouseDown, Point::new(2, 0), MB_LEFT_BUTTON, false);
        v.handle_event(&mut down);
        let mut up = Event::mouse(EventType::MouseUp, Point::new(2, 0), 0, false);
        v.handle_event(&mut up);
        assert!(!v.has_selection(), "click without drag deselects");
    }

    #[test]
    fn esc_clears_the_selection() {
        let mut v = view();
        v.push_line(&line("hello"));
        v.select_all();
        let mut esc = Event::keyboard(KB_ESC);
        v.handle_event(&mut esc);
        assert!(!v.has_selection());
    }

    #[test]
    fn mutating_the_buffer_clears_the_selection() {
        let mut v = view();
        v.push_line(&line("hello"));
        v.select_all();
        assert!(v.has_selection());
        v.push_line(&line("more"));
        assert!(!v.has_selection(), "new content must drop a stale selection");
    }

    /// An in-memory `Backend` for tests: no real TTY, fixed size, no I/O.
    /// `Terminal::write_line`/`write_cell` write straight into `Terminal`'s
    /// own in-memory buffer, so this stub only needs to satisfy
    /// initialization and size queries for `Terminal::with_backend`.
    struct FakeBackend {
        width: u16,
        height: u16,
    }

    impl Backend for FakeBackend {
        fn as_any_mut(&mut self) -> &mut dyn std::any::Any {
            self
        }

        fn init(&mut self) -> io::Result<()> {
            Ok(())
        }

        fn cleanup(&mut self) -> io::Result<()> {
            Ok(())
        }

        fn size(&self) -> io::Result<(u16, u16)> {
            Ok((self.width, self.height))
        }

        fn poll_event(&mut self, _timeout: Duration) -> io::Result<Option<Event>> {
            Ok(None)
        }

        fn write_raw(&mut self, _data: &[u8]) -> io::Result<()> {
            Ok(())
        }

        fn flush(&mut self) -> io::Result<()> {
            Ok(())
        }

        fn show_cursor(&mut self, _x: u16, _y: u16) -> io::Result<()> {
            Ok(())
        }

        fn hide_cursor(&mut self) -> io::Result<()> {
            Ok(())
        }
    }

    fn fake_terminal(width: u16, height: u16) -> Terminal {
        Terminal::with_backend(Box::new(FakeBackend { width, height }))
            .expect("fake backend never fails to init")
    }

    /// A `Backend` that records every byte `Terminal::flush` actually sends
    /// downstream, via a shared buffer -- the write-through path
    /// `FakeBackend` above stubs out. `Terminal::flush` is the one place
    /// that decides what physically reaches a real terminal (it does a
    /// diffed, escape-coded re-encode of the cell buffer, and knowingly
    /// skips `'\0'` filler cells), so a bug specific to *that* encoding is
    /// invisible to any test that only inspects `Terminal::read_cell`,
    /// which reflects the in-memory cell buffer `write_line` always
    /// updates unconditionally.
    #[derive(Clone, Default)]
    struct RecordingBackend {
        width: u16,
        height: u16,
        output: std::sync::Arc<std::sync::Mutex<Vec<u8>>>,
    }

    impl Backend for RecordingBackend {
        fn as_any_mut(&mut self) -> &mut dyn std::any::Any {
            self
        }

        fn init(&mut self) -> io::Result<()> {
            Ok(())
        }

        fn cleanup(&mut self) -> io::Result<()> {
            Ok(())
        }

        fn size(&self) -> io::Result<(u16, u16)> {
            Ok((self.width, self.height))
        }

        fn poll_event(&mut self, _timeout: Duration) -> io::Result<Option<Event>> {
            Ok(None)
        }

        fn write_raw(&mut self, data: &[u8]) -> io::Result<()> {
            self.output.lock().unwrap().extend_from_slice(data);
            Ok(())
        }

        fn flush(&mut self) -> io::Result<()> {
            Ok(())
        }

        fn show_cursor(&mut self, _x: u16, _y: u16) -> io::Result<()> {
            Ok(())
        }

        fn hide_cursor(&mut self) -> io::Result<()> {
            Ok(())
        }
    }

    /// Builds a `Terminal` whose every `flush`-emitted byte lands in the
    /// returned buffer, so a test can inspect what actually reaches a real
    /// terminal rather than only the in-memory cell buffer.
    fn recording_terminal(
        width: u16,
        height: u16,
    ) -> (Terminal, std::sync::Arc<std::sync::Mutex<Vec<u8>>>) {
        let output = std::sync::Arc::new(std::sync::Mutex::new(Vec::new()));
        let backend = RecordingBackend {
            width,
            height,
            output: output.clone(),
        };
        let terminal =
            Terminal::with_backend(Box::new(backend)).expect("fake backend never fails to init");
        (terminal, output)
    }

    /// Replays `flush`'s escape-coded byte stream onto a plain grid the way
    /// a real terminal would: `ESC[row;colH` repositions the cursor
    /// (1-indexed), an SGR color sequence is consumed and ignored, and every
    /// other character is placed at the cursor and advances it by its own
    /// display width -- 2 for a double-width glyph, 0 for a combining or
    /// selector character, exactly as a real terminal renders it (not by
    /// our internal one-`Cell`-per-logical-column bookkeeping, which is
    /// precisely what could drift from physical reality). Bytes from
    /// successive flushes are replayed in order onto the same grid, since a
    /// real terminal's screen persists across flushes the same way.
    fn replay_onto_grid(bytes: &[u8], grid: &mut [Vec<char>]) {
        let text = std::str::from_utf8(bytes).expect("flush emits valid UTF-8");
        let mut chars = text.chars().peekable();
        let mut row = 0usize;
        let mut col = 0usize;
        while let Some(c) = chars.next() {
            if c == '\u{1b}' && chars.peek() == Some(&'[') {
                chars.next(); // consume '['
                let mut params = String::new();
                let mut final_byte = ' ';
                for pc in chars.by_ref() {
                    if pc.is_ascii_digit() || pc == ';' {
                        params.push(pc);
                    } else {
                        final_byte = pc;
                        break;
                    }
                }
                if final_byte == 'H' {
                    let mut parts = params.split(';');
                    let r: usize = parts.next().and_then(|p| p.parse().ok()).unwrap_or(1);
                    let cix: usize = parts.next().and_then(|p| p.parse().ok()).unwrap_or(1);
                    row = r.saturating_sub(1);
                    col = cix.saturating_sub(1);
                }
                // An SGR ('m') sequence carries no cursor movement.
                continue;
            }
            let width = c.width().unwrap_or(0);
            if row < grid.len() && col < grid[row].len() {
                grid[row][col] = c;
            }
            col += width;
        }
    }

    #[test]
    fn scrollback_cap_drops_oldest_lines() {
        let mut v = view();
        v.set_max_lines(3);
        for i in 0..5 {
            v.push_line(&line(&i.to_string()));
        }
        assert_eq!(v.line_count(), 3);
        assert_eq!(v.plain_text(), "2\n3\n4");
    }

    #[test]
    fn autoscroll_holds_at_bottom_while_lines_arrive() {
        let mut v = view();
        for i in 0..50 {
            v.push_line(&line(&i.to_string()));
        }
        assert!(v.is_at_bottom());
    }

    #[test]
    fn scrolling_up_releases_autoscroll_and_end_rearms_it() {
        let mut v = view();
        for i in 0..50 {
            v.push_line(&line(&i.to_string()));
        }
        v.scroll_up(5);
        assert!(!v.is_at_bottom());
        v.push_line(&line("new"));
        assert!(
            !v.is_at_bottom(),
            "a new line must not yank a scrolled-back reader to the bottom"
        );
        v.scroll_to_bottom();
        assert!(v.is_at_bottom());
    }

    #[test]
    fn partial_line_is_replaced_not_appended() {
        let mut v = view();
        v.set_partial(&line("par"));
        v.set_partial(&line("part"));
        assert_eq!(v.plain_text(), "part");
        assert_eq!(v.line_count(), 1);
    }

    #[test]
    fn plain_text_strips_attributes() {
        let mut v = view();
        v.push_line(&[Cell::new('x', Attr::new(TvColor::LightRed, TvColor::Blue))]);
        assert_eq!(v.plain_text(), "x");
    }

    #[test]
    fn resize_larger_while_scrolled_back_reclamps_top_to_show_a_full_page() {
        let mut v = StreamView::new(Rect::new(0, 0, 40, 5));
        for i in 0..50 {
            v.push_line(&line(&i.to_string()));
        }
        // Scroll back so `top` sits well below the current max_top()
        // (line_count 50, page 5 -> max_top 45).
        v.scroll_to_top();
        v.scroll_down(40);
        assert!(!v.is_at_bottom());
        let old_top = v.top;
        assert!(old_top < v.max_top());

        // Grow the view a lot: max_top() shrinks to line_count - new_page
        // (50 - 48 = 2), which is now well below the old `top` (40). Left
        // unclamped, that would leave blank rows at the bottom of the
        // viewport even though unshown history sits above.
        v.set_bounds(Rect::new(0, 0, 40, 48));

        assert!(
            v.top <= v.max_top(),
            "top ({}) must not exceed max_top ({}) after growing",
            v.top,
            v.max_top()
        );
        let rows: Vec<&Vec<Cell>> = v.iter_rows().skip(v.top).take(v.page()).collect();
        assert_eq!(
            rows.len(),
            v.page().min(v.row_count()),
            "a full page of content should be visible after growing"
        );
    }

    #[test]
    fn draw_clips_to_bounds_width() {
        let mut v = StreamView::new(Rect::new(2, 1, 8, 4));
        v.push_line(&line("short")); // shorter than the 6-wide view

        let mut terminal = fake_terminal(20, 10);
        v.draw(&mut terminal);

        // Row 0 (bounds.a.y == 1): "short", padded with the fill space for
        // the remaining column.
        for (i, expected) in "short ".chars().enumerate() {
            let cell = terminal
                .read_cell(2 + i16::try_from(i).unwrap_or(i16::MAX), 1)
                .expect("cell within terminal bounds");
            assert_eq!(cell.ch, expected);
        }
        // Nothing is drawn past the view's width (x == 8 is out of bounds).
        assert_eq!(terminal.read_cell(8, 1).unwrap().ch, ' ');

        // Nothing above the view's rows was touched.
        assert_eq!(terminal.read_cell(2, 0).unwrap().ch, ' ');
    }

    /// The exact banner glyph plank emits: U+1F6E0 HAMMER AND WRENCH followed
    /// by U+FE0F VARIATION SELECTOR-16 (the emoji presentation selector).
    /// The base character alone is East-Asian-Width `Neutral` (width 1 per
    /// `unicode-width`'s plain per-`char` rule) -- it is only the
    /// *emoji presentation sequence* (base + U+FE0F) that is double-width,
    /// which is exactly the sequence a real tool-call banner sends and the
    /// case this fix targets. `line()` builds one `Cell` per `char` here
    /// too, since `.chars()` splits the base and the selector into two
    /// separate `char`s -- the same shape `tracefmt`'s `cells()` produces.
    const WRENCH: &str = "\u{1F6E0}\u{FE0F}";

    #[test]
    fn wide_character_row_paints_the_correct_total_number_of_columns() {
        // wrench (2 columns) + space + x = 4 columns total.
        let mut v = StreamView::new(Rect::new(0, 0, 10, 4));
        v.push_line(&line(&format!("{WRENCH} x")));
        let mut terminal = fake_terminal(20, 10);
        v.draw(&mut terminal);

        // Column 0 holds the wrench glyph itself.
        assert_eq!(terminal.read_cell(0, 0).unwrap().ch, '\u{1F6E0}');
        // Column 1 is the wrench's second column: the trailing presentation
        // selector itself, kept (not an invented '\0') because it is a real
        // character.
        assert_eq!(terminal.read_cell(1, 0).unwrap().ch, '\u{FE0F}');
        // The rest of the row lands at its true, width-aware columns.
        assert_eq!(terminal.read_cell(2, 0).unwrap().ch, ' ');
        assert_eq!(terminal.read_cell(3, 0).unwrap().ch, 'x');
        // And the row is blank-padded for the remaining columns of the view.
        for x in 4..10 {
            assert_eq!(terminal.read_cell(x, 0).unwrap().ch, ' ');
        }
    }

    #[test]
    fn text_after_a_wide_character_lands_at_the_right_column() {
        let mut v = StreamView::new(Rect::new(0, 0, 30, 4));
        v.push_line(&line(&format!("{WRENCH} Reading src/dsml.rs")));
        let mut terminal = fake_terminal(30, 10);
        v.draw(&mut terminal);

        let expected = "\u{1F6E0}\u{FE0F} Reading src/dsml.rs";
        for (i, expected_ch) in expected.chars().enumerate() {
            let cell = terminal
                .read_cell(i16::try_from(i).unwrap(), 0)
                .expect("cell within terminal bounds");
            assert_eq!(cell.ch, expected_ch, "column {i} mismatch");
        }
    }

    #[test]
    fn short_row_is_blank_padded_so_nothing_shows_through_from_beneath() {
        let mut v = StreamView::new(Rect::new(0, 0, 10, 4));
        // First paint a row that fills the whole width...
        v.push_line(&line("XXXXXXXXXX"));
        let mut terminal = fake_terminal(20, 10);
        v.draw(&mut terminal);
        // ...then a shorter, width-shrinking row should overwrite every
        // column the first row touched, leaving nothing behind.
        v.clear();
        v.push_line(&line(&format!("{WRENCH}hi")));
        v.draw(&mut terminal);

        assert_eq!(terminal.read_cell(0, 0).unwrap().ch, '\u{1F6E0}');
        assert_eq!(terminal.read_cell(1, 0).unwrap().ch, '\u{FE0F}');
        assert_eq!(terminal.read_cell(2, 0).unwrap().ch, 'h');
        assert_eq!(terminal.read_cell(3, 0).unwrap().ch, 'i');
        for x in 4..10 {
            assert_eq!(
                terminal.read_cell(x, 0).unwrap().ch,
                ' ',
                "column {x} must be blanked, not left over from the previous row"
            );
        }
    }

    #[test]
    fn a_double_width_character_straddling_a_wrap_boundary_is_never_split() {
        // Columns: a b [中 col0] [中 col1: a '\0' filler cell] c d -- 6
        // columns, wrapped at width 3. A naive character-break cut at column
        // 3 would land squarely on the filler cell, splitting the glyph in
        // half; the wrap must instead push the whole character to the next
        // row.
        let mut v = StreamView::new(Rect::new(0, 0, 3, 4));
        v.push_line(&line("ab中cd"));

        assert_eq!(v.row_count(), 3, "the 6-column line wraps to three rows");

        let mut terminal = fake_terminal(20, 10);
        v.draw(&mut terminal);

        // Row 0 holds only "ab": the wide character was pushed whole to the
        // next row rather than being split across the boundary.
        assert_eq!(terminal.read_cell(0, 0).unwrap().ch, 'a');
        assert_eq!(terminal.read_cell(1, 0).unwrap().ch, 'b');

        // Row 1 holds the wide character (both its columns) followed by "c".
        assert_eq!(terminal.read_cell(0, 1).unwrap().ch, '中');
        assert_eq!(terminal.read_cell(1, 1).unwrap().ch, '\0');
        assert_eq!(terminal.read_cell(2, 1).unwrap().ch, 'c');

        // Row 2 holds the remaining "d".
        assert_eq!(terminal.read_cell(0, 2).unwrap().ch, 'd');
    }

    #[test]
    fn plain_text_round_trips_a_wide_character_with_no_padding_artifacts() {
        let mut v = view();
        v.push_line(&line(&format!("{WRENCH} Reading src/dsml.rs")));
        assert_eq!(v.plain_text(), format!("{WRENCH} Reading src/dsml.rs"));
    }

    /// Reproduces the real, two-window bug: a lower window paints a row
    /// containing plank's real tool-call banner glyph and *flushes* it (not
    /// just `draw`s it -- the defect lives in what `Terminal::flush` sends
    /// downstream, invisible to any test that only checks
    /// `Terminal::read_cell`, since `write_line` updates the in-memory cell
    /// buffer unconditionally regardless of what flush later encodes). A
    /// second, unrelated window then opens on top with the same bounds and
    /// paints an all-blank row over the identical region, and flushes too.
    /// A real terminal's screen must show nothing left over from the first
    /// window afterwards.
    #[test]
    fn a_covering_window_s_flush_fully_blanks_a_row_that_held_a_wide_character() {
        let (mut terminal, output) = recording_terminal(30, 4);
        let mut grid = vec![vec![' '; 30]; 4];

        // Lower window: the real banner line at row 0, drawn and flushed.
        let mut lower = StreamView::new(Rect::new(0, 0, 30, 4));
        lower.push_line(&line(&format!("{WRENCH} Reading src/dsml.rs 1:500...")));
        lower.draw(&mut terminal);
        terminal
            .flush()
            .expect("flush never fails against a fake backend");
        replay_onto_grid(&output.lock().unwrap(), &mut grid);
        output.lock().unwrap().clear();

        // Upper window: same bounds, no content of its own at all -- opens
        // on top and must blank every column of row 0 that the lower
        // window's banner occupied.
        let mut upper = StreamView::new(Rect::new(0, 0, 30, 4));
        upper.draw(&mut terminal);
        terminal
            .flush()
            .expect("flush never fails against a fake backend");
        replay_onto_grid(&output.lock().unwrap(), &mut grid);

        // Row 0 must now be fully blank -- nothing from the lower window's
        // banner may still show through.
        for (col, &ch) in grid[0].iter().enumerate() {
            assert_eq!(
                ch, ' ',
                "row 0 column {col} still shows a leftover character from \
                 the window underneath: {grid:?}"
            );
        }
    }

    #[test]
    fn a_line_longer_than_the_width_wraps_across_the_right_number_of_rows_with_complete_content() {
        let mut v = StreamView::new(Rect::new(0, 0, 10, 20));
        // 25 non-space characters at width 10 -> ceil(25/10) = 3 rows.
        let text = "abcdefghijklmnopqrstuvwxy";
        v.push_line(&line(text));

        assert_eq!(v.row_count(), 3);
        assert_eq!(
            v.plain_text(),
            text,
            "wrapping must not drop or duplicate any character"
        );

        // Also verify via the rendered rows that content is complete and in
        // order across them.
        let mut terminal = fake_terminal(20, 20);
        v.draw(&mut terminal);
        let mut rendered = String::new();
        for row in 0..3 {
            for col in 0..10 {
                rendered.push(terminal.read_cell(col, row).unwrap().ch);
            }
        }
        assert_eq!(rendered, "abcdefghijklmnopqrstuvwxy     ");
    }

    #[test]
    fn a_wrap_breaks_at_a_space_rather_than_mid_word_when_one_is_available() {
        let mut v = StreamView::new(Rect::new(0, 0, 10, 20));
        v.push_line(&line("hello world"));

        // "hello world" is 11 columns wide; wrapping at 10 without a
        // space-aware break would cut mid-word ("hello worl" / "d"). The
        // break must instead land on the space, dropping it, and produce
        // "hello" / "world".
        assert_eq!(v.row_count(), 2);
        let mut terminal = fake_terminal(20, 20);
        v.draw(&mut terminal);
        for (i, expected) in "hello     ".chars().enumerate() {
            assert_eq!(
                terminal.read_cell(i16::try_from(i).unwrap(), 0).unwrap().ch,
                expected
            );
        }
        for (i, expected) in "world     ".chars().enumerate() {
            assert_eq!(
                terminal.read_cell(i16::try_from(i).unwrap(), 1).unwrap().ch,
                expected
            );
        }
    }

    #[test]
    fn a_single_token_longer_than_the_width_is_broken_rather_than_truncated() {
        let mut v = StreamView::new(Rect::new(0, 0, 5, 20));
        // A 12-character token with no whitespace at all -- a long path,
        // say -- must still be fully visible, broken mid-token instead of
        // truncated.
        v.push_line(&line("abcdefghijkl"));

        assert_eq!(v.row_count(), 3); // ceil(12/5) = 3
        assert_eq!(
            v.plain_text(),
            "abcdefghijkl",
            "the logical text is preserved even though it had to be broken mid-token"
        );
    }

    #[test]
    fn plain_text_returns_the_original_unwrapped_logical_lines() {
        let mut v = StreamView::new(Rect::new(0, 0, 5, 20));
        v.push_line(&line("a much longer line than the five-column view"));
        v.push_line(&line("short"));

        assert_eq!(
            v.plain_text(),
            "a much longer line than the five-column view\nshort",
            "Save As must get the original logical lines, not this window's wrap points"
        );
    }

    #[test]
    fn resizing_narrower_then_wider_rewraps_and_content_survives_both() {
        let mut v = StreamView::new(Rect::new(0, 0, 20, 20));
        let text = "abcdefghijklmnopqrstuvwxyz";
        v.push_line(&line(text));
        assert_eq!(v.row_count(), 2); // ceil(26/20)

        v.set_bounds(Rect::new(0, 0, 5, 20));
        assert_eq!(v.row_count(), 6); // ceil(26/5)
        assert_eq!(v.plain_text(), text);

        v.set_bounds(Rect::new(0, 0, 30, 20));
        assert_eq!(v.row_count(), 1); // fits on one row now
        assert_eq!(v.plain_text(), text);
    }

    #[test]
    fn scrolling_by_page_lands_correctly_when_wrapped_rows_are_present() {
        // One long line that wraps to 20 rows, in a 5-row-tall view.
        let mut v = StreamView::new(Rect::new(0, 0, 4, 5));
        let text: String = (0..80).map(|i| char::from(b'a' + (i % 26))).collect();
        v.push_line(&line(&text));
        assert_eq!(v.row_count(), 20);

        v.scroll_to_top();
        assert_eq!(v.top, 0);
        v.scroll_down(v.page()); // one page down: page() == 5
        assert_eq!(
            v.top, 5,
            "paging must move by display rows, not logical lines"
        );

        v.scroll_to_bottom();
        assert_eq!(v.top, v.row_count() - v.page());
    }

    #[test]
    fn draw_on_zero_height_view_writes_nothing() {
        let mut v = StreamView::new(Rect::new(0, 0, 10, 0));
        v.push_line(&line("hello"));
        let mut terminal = fake_terminal(20, 10);
        v.draw(&mut terminal);
        for y in 0..10 {
            for x in 0..20 {
                assert_eq!(
                    terminal.read_cell(x, y).unwrap().ch,
                    ' ',
                    "zero-height view must not write any cell"
                );
            }
        }
    }
}