refactor: separate server client boundaries

1 files changed, 1430 insertions, 0 deletions

diff --git a/src/protocol/render_ansi.rs b/src/protocol/render_ansi.rs
new file mode 100644
index 00000000..8caa98d0
--- /dev/null
+++ b/src/protocol/render_ansi.rs
@@ -0,0 +1,1430 @@
+//! Frame blitting — renders FrameData to the terminal using diff-based updates.
+//!
+//! The blitting strategy:
+//! 1. On the first frame, write the entire buffer (full redraw).
+//! 2. On subsequent frames, diff against the last frame and only write
+//!    the cells that changed.
+//! 3. Wrap each frame in synchronized output so terminals that support it do
+//!    not expose intermediate cursor positions while the frame is painted.
+//! 4. Before writing any cells, hide the cursor to avoid stray cursor
+//!    artifacts on terminals that render the hardware cursor at intermediate
+//!    `CUP` positions during the frame stream.
+//! 5. After writing all changed cells, restore the final cursor visibility
+//!    and position from `frame.cursor`.
+//! 6. After ending synchronized output, repeat the final cursor anchor so
+//!    external IMEs can place candidate windows at the real input position.
+//!
+//! Escape sequences used:
+//! - `CSI H` (CUP) — move cursor to (row, col)
+//! - `CSI m` (SGR) — set graphic rendition (colors, bold, etc.)
+//! - `CSI ? 2026 h/l` — begin/end synchronized output
+//! - `CSI Ps SP q` — DECSCUSR cursor shape
+//! - `ESC ] 52 ; c ; <base64> BEL` — OSC 52 clipboard write
+//!
+//! The goal is minimal output: skip unchanged cells, batch adjacent changes,
+//! and minimize cursor movement.
+
+use std::cmp;
+use std::io::Write;
+
+use unicode_width::UnicodeWidthStr;
+
+use crate::protocol::{CellData, FrameData};
+
+/// Bytes produced by a [`BlitEncoder`] for one terminal frame.
+pub(crate) struct EncodedBlit {
+    /// Terminal escape bytes ready to write to the host terminal.
+    pub(crate) bytes: Vec<u8>,
+    /// Whether this frame was encoded as a full redraw.
+    pub(crate) full: bool,
+    next_last_visible_cursor: Option<(u16, u16)>,
+    next_last_cursor_shape: u8,
+}
+
+/// Stateful encoder that diffs semantic frames into terminal ANSI bytes.
+#[derive(Default)]
+pub(crate) struct BlitEncoder {
+    last_frame: Option<FrameData>,
+    last_visible_cursor: Option<(u16, u16)>,
+    last_cursor_shape: u8,
+}
+
+impl BlitEncoder {
+    pub(crate) fn new() -> Self {
+        Self::default()
+    }
+
+    pub(crate) fn encode(&self, frame: &FrameData, force_full: bool) -> EncodedBlit {
+        let prev = if force_full {
+            None
+        } else {
+            self.last_frame.as_ref()
+        };
+        let full = force_full
+            || prev.is_none()
+            || prev.is_some_and(|p| p.width != frame.width || p.height != frame.height);
+        let mut bytes = Vec::new();
+        let mut next_last_visible_cursor = self.last_visible_cursor;
+        let mut next_last_cursor_shape = self.last_cursor_shape;
+        blit_frame_to_with_cursor_memory(
+            &mut bytes,
+            frame,
+            prev,
+            &mut next_last_visible_cursor,
+            &mut next_last_cursor_shape,
+        );
+        EncodedBlit {
+            bytes,
+            full,
+            next_last_visible_cursor,
+            next_last_cursor_shape,
+        }
+    }
+
+    pub(crate) fn commit(&mut self, frame: FrameData, encoded: EncodedBlit) {
+        self.last_visible_cursor = encoded.next_last_visible_cursor;
+        self.last_cursor_shape = encoded.next_last_cursor_shape;
+        self.last_frame = Some(frame);
+    }
+
+    pub(crate) fn is_current(&self, frame: &FrameData) -> bool {
+        self.last_frame.as_ref() == Some(frame)
+    }
+}
+
+// ---------------------------------------------------------------------------
+// Color → escape sequence
+// ---------------------------------------------------------------------------
+
+/// Converts a packed u32 color to an SGR escape sequence fragment.
+///
+/// Returns a string like `38;5;123` (indexed) or `38;2;255;128;64` (RGB)
+/// or `39` (reset), without the leading `\x1b[` or trailing `m`.
+fn color_to_sgr_fg(val: u32) -> String {
+    match val >> 24 {
+        0x00 => match val & 0xFF {
+            0x00 => "39".to_owned(), // Reset
+            0x01 => "30".to_owned(), // Black
+            0x02 => "31".to_owned(), // Red
+            0x03 => "32".to_owned(), // Green
+            0x04 => "33".to_owned(), // Yellow
+            0x05 => "34".to_owned(), // Blue
+            0x06 => "35".to_owned(), // Magenta
+            0x07 => "36".to_owned(), // Cyan
+            0x08 => "37".to_owned(), // Gray (light gray)
+            0x09 => "90".to_owned(), // DarkGray
+            0x0A => "91".to_owned(), // LightRed
+            0x0B => "92".to_owned(), // LightGreen
+            0x0C => "93".to_owned(), // LightYellow
+            0x0D => "94".to_owned(), // LightBlue
+            0x0E => "95".to_owned(), // LightMagenta
+            0x0F => "96".to_owned(), // LightCyan
+            0x10 => "97".to_owned(), // White
+            _ => "39".to_owned(),    // Unknown → Reset
+        },
+        0x01 => format!("38;5;{}", val & 0xFF), // Indexed
+        0x02 => {
+            // RGB
+            let r = (val >> 16) & 0xFF;
+            let g = (val >> 8) & 0xFF;
+            let b = val & 0xFF;
+            format!("38;2;{r};{g};{b}")
+        }
+        _ => "39".to_owned(), // Unknown → Reset
+    }
+}
+
+/// Converts a packed u32 color to a background SGR fragment.
+fn color_to_sgr_bg(val: u32) -> String {
+    match val >> 24 {
+        0x00 => match val & 0xFF {
+            0x00 => "49".to_owned(),  // Reset
+            0x01 => "40".to_owned(),  // Black
+            0x02 => "41".to_owned(),  // Red
+            0x03 => "42".to_owned(),  // Green
+            0x04 => "43".to_owned(),  // Yellow
+            0x05 => "44".to_owned(),  // Blue
+            0x06 => "45".to_owned(),  // Magenta
+            0x07 => "46".to_owned(),  // Cyan
+            0x08 => "47".to_owned(),  // Gray (light gray)
+            0x09 => "100".to_owned(), // DarkGray
+            0x0A => "101".to_owned(), // LightRed
+            0x0B => "102".to_owned(), // LightGreen
+            0x0C => "103".to_owned(), // LightYellow
+            0x0D => "104".to_owned(), // LightBlue
+            0x0E => "105".to_owned(), // LightMagenta
+            0x0F => "106".to_owned(), // LightCyan
+            0x10 => "107".to_owned(), // White
+            _ => "49".to_owned(),     // Unknown → Reset
+        },
+        0x01 => format!("48;5;{}", val & 0xFF), // Indexed
+        0x02 => {
+            let r = (val >> 16) & 0xFF;
+            let g = (val >> 8) & 0xFF;
+            let b = val & 0xFF;
+            format!("48;2;{r};{g};{b}")
+        }
+        _ => "49".to_owned(),
+    }
+}
+
+// ---------------------------------------------------------------------------
+// Modifier → SGR
+// ---------------------------------------------------------------------------
+
+/// Converts a u16 modifier bitmask to SGR escape sequence fragments.
+///
+/// Returns a Vec of SGR parameter strings (e.g., "1" for bold, "3" for italic).
+fn modifier_to_sgr_parts(val: u16) -> Vec<&'static str> {
+    let mut parts = Vec::new();
+
+    // ratatui::Modifier bits (from bitflags)
+    const BOLD: u16 = 1 << 0; // 0x01
+    const DIM: u16 = 1 << 1; // 0x02
+    const ITALIC: u16 = 1 << 2; // 0x04
+    const UNDERLINED: u16 = 1 << 3; // 0x08
+    const SLOW_BLINK: u16 = 1 << 4; // 0x10
+    const RAPID_BLINK: u16 = 1 << 5; // 0x20
+    const REVERSED: u16 = 1 << 6; // 0x40
+    const HIDDEN: u16 = 1 << 7; // 0x80
+    const CROSSED_OUT: u16 = 1 << 8; // 0x100
+
+    if val & BOLD != 0 {
+        parts.push("1");
+    }
+    if val & DIM != 0 {
+        parts.push("2");
+    }
+    if val & ITALIC != 0 {
+        parts.push("3");
+    }
+    if val & UNDERLINED != 0 {
+        parts.push("4");
+    }
+    if val & SLOW_BLINK != 0 {
+        parts.push("5");
+    }
+    if val & RAPID_BLINK != 0 {
+        parts.push("6");
+    }
+    if val & REVERSED != 0 {
+        parts.push("7");
+    }
+    if val & HIDDEN != 0 {
+        parts.push("8");
+    }
+    if val & CROSSED_OUT != 0 {
+        parts.push("9");
+    }
+
+    parts
+}
+
+/// Builds a complete SGR escape sequence for a cell's style.
+fn build_sgr(fg: u32, bg: u32, modifier: u16) -> String {
+    let mut parts = vec!["0".to_owned()];
+    parts.extend(
+        modifier_to_sgr_parts(modifier)
+            .into_iter()
+            .map(str::to_owned),
+    );
+    parts.push(color_to_sgr_fg(fg));
+    parts.push(color_to_sgr_bg(bg));
+    format!("\x1b[{}m", parts.join(";"))
+}
+
+// ---------------------------------------------------------------------------
+// Cell comparison
+// ---------------------------------------------------------------------------
+
+/// Checks if two cells are visually identical.
+#[cfg(test)]
+fn cells_equal(a: &CellData, b: &CellData) -> bool {
+    a.symbol == b.symbol
+        && a.fg == b.fg
+        && a.bg == b.bg
+        && a.modifier == b.modifier
+        && a.hyperlink == b.hyperlink
+    // Skip flag is only for ratatui internal use, not visual.
+}
+
+// ---------------------------------------------------------------------------
+// Blitting
+// ---------------------------------------------------------------------------
+
+/// Blits a frame to a writer, diffing against the previous frame.
+#[cfg(test)]
+fn blit_frame_to(writer: impl Write, frame: &FrameData, prev: Option<&FrameData>) {
+    let mut last_visible_cursor = None;
+    let mut last_cursor_shape = 0;
+    blit_frame_to_with_cursor_memory(
+        writer,
+        frame,
+        prev,
+        &mut last_visible_cursor,
+        &mut last_cursor_shape,
+    );
+}
+
+fn blit_frame_to_with_cursor_memory(
+    mut writer: impl Write,
+    frame: &FrameData,
+    prev: Option<&FrameData>,
+    last_visible_cursor: &mut Option<(u16, u16)>,
+    last_cursor_shape: &mut u8,
+) {
+    // On first frame or size change, do a full redraw.
+    let full_redraw =
+        prev.is_none() || prev.is_some_and(|p| p.width != frame.width || p.height != frame.height);
+
+    // Ask terminals that support synchronized output to apply the whole frame
+    // atomically. This keeps IMEs and cursor trackers from observing the
+    // intermediate CUP positions used while painting changed cells.
+    let _ = writer.write_all(b"\x1b[?2026h");
+
+    // Hide cursor before any cell writes to avoid stray cursor artifacts
+    // on terminals that render the hardware cursor at intermediate CUP positions.
+    let _ = writer.write_all(b"\x1b[?25l");
+
+    // Start each frame from a known OSC 8 state. If a previous write was
+    // interrupted or the outer terminal had an active hyperlink, unlinked cells
+    // must not inherit it.
+    let _ = writer.write_all(b"\x1b]8;;\x1b\\");
+
+    if full_redraw {
+        // Clear the screen and write all cells.
+        let _ = writer.write_all(b"\x1b[2J\x1b[H");
+        write_all_cells(&mut writer, frame);
+    } else {
+        // Diff-based update: only write changed cells.
+        let prev = prev.unwrap();
+        write_changed_cells(&mut writer, frame, prev);
+    }
+
+    // Position the cursor while it is still hidden, then restore visibility.
+    // Showing before moving makes slow terminals and IMEs briefly observe the
+    // cursor at the last painted cell, which can be an animated sidebar/status
+    // cell rather than the focused pane's input position. When the focused pane
+    // hides its cursor, still park the host cursor intentionally so IMEs do not
+    // anchor to whichever cell happened to be painted last.
+    let host_cursor = resolve_host_cursor_state(frame, last_visible_cursor);
+    write_host_cursor_state(&mut writer, host_cursor, last_cursor_shape);
+
+    // End the synchronized output block immediately after the final cursor
+    // state is emitted so supporting terminals can present the frame atomically.
+    let _ = writer.write_all(b"\x1b[?2026l");
+
+    // Some native IMEs track candidate-window placement from normal terminal
+    // cursor updates and may not observe cursor moves emitted inside synchronized
+    // output. Re-emit only the resolved final cursor anchor after the sync block;
+    // intermediate paint cursor positions remain hidden and the focused pane's
+    // requested cursor visibility is preserved.
+    write_ime_anchor_cursor_state(&mut writer, host_cursor);
+    let _ = writer.flush();
+}
+
+/// Writes all cells in the frame (full redraw).
+fn cell_width(cell: &CellData) -> usize {
+    cell.symbol.width()
+}
+
+#[derive(Clone, Copy)]
+struct HostCursorState {
+    position: (u16, u16),
+    visible: bool,
+    /// DECSCUSR parameter (0–6). 0 means terminal default.
+    shape: u8,
+}
+
+fn resolve_host_cursor_state(
+    frame: &FrameData,
+    last_visible_cursor: &mut Option<(u16, u16)>,
+) -> HostCursorState {
+    if let Some(cursor) = &frame.cursor {
+        if cursor.visible {
+            let position = clamp_cursor_position(frame, cursor.x, cursor.y);
+            *last_visible_cursor = Some(position);
+            return HostCursorState {
+                position,
+                visible: true,
+                shape: normalize_cursor_shape(cursor.shape),
+            };
+        }
+
+        let position = clamp_cursor_position(frame, cursor.x, cursor.y);
+        return HostCursorState {
+            position,
+            visible: false,
+            shape: normalize_cursor_shape(cursor.shape),
+        };
+    }
+
+    let position = (*last_visible_cursor)
+        .map(|(x, y)| clamp_cursor_position(frame, x, y))
+        .unwrap_or_else(|| default_hidden_cursor_position(frame));
+    HostCursorState {
+        position,
+        visible: false,
+        shape: 0,
+    }
+}
+
+fn normalize_cursor_shape(shape: u8) -> u8 {
+    if shape <= 6 {
+        shape
+    } else {
+        0
+    }
+}
+
+fn default_hidden_cursor_position(frame: &FrameData) -> (u16, u16) {
+    (
+        frame.width.saturating_sub(1),
+        frame.height.saturating_sub(1),
+    )
+}
+
+fn clamp_cursor_position(frame: &FrameData, x: u16, y: u16) -> (u16, u16) {
+    (
+        x.min(frame.width.saturating_sub(1)),
+        y.min(frame.height.saturating_sub(1)),
+    )
+}
+
+fn write_cursor_position(writer: &mut impl Write, (x, y): (u16, u16)) {
+    // CUP: move cursor to (row+1, col+1) — 1-based.
+    let _ = write!(writer, "\x1b[{};{}H", y + 1, x + 1);
+}
+
+fn write_host_cursor_state(writer: &mut impl Write, cursor: HostCursorState, last_shape: &mut u8) {
+    write_cursor_position(writer, cursor.position);
+    if cursor.shape != *last_shape {
+        let _ = write!(writer, "\x1b[{} q", cursor.shape);
+        *last_shape = cursor.shape;
+    }
+    if cursor.visible {
+        // Show cursor only after it is already at the final position.
+        let _ = writer.write_all(b"\x1b[?25h");
+    } else {
+        let _ = writer.write_all(b"\x1b[?25l");
+    }
+}
+
+fn write_ime_anchor_cursor_state(writer: &mut impl Write, cursor: HostCursorState) {
+    write_cursor_position(writer, cursor.position);
+    if cursor.visible {
+        let _ = writer.write_all(b"\x1b[?25h");
+    } else {
+        let _ = writer.write_all(b"\x1b[?25l");
+    }
+}
+
+fn write_all_cells(writer: &mut impl Write, frame: &FrameData) {
+    let mut active_hyperlink = None;
+    for row in 0..frame.height {
+        let mut to_skip = 0usize;
+        for col in 0..frame.width {
+            if to_skip > 0 {
+                to_skip -= 1;
+                continue;
+            }
+
+            let idx = (row as usize) * (frame.width as usize) + (col as usize);
+            let cell = &frame.cells[idx];
+
+            if cell.skip {
+                continue;
+            }
+
+            // Move cursor to position (1-based).
+            let _ = write!(writer, "\x1b[{};{}H", row + 1, col + 1);
+
+            // Set style.
+            let sgr = build_sgr(cell.fg, cell.bg, cell.modifier);
+            let _ = writer.write_all(sgr.as_bytes());
+
+            write_hyperlink_if_changed(
+                writer,
+                &mut active_hyperlink,
+                cell_hyperlink_uri(frame, cell),
+            );
+
+            // Write the symbol.
+            let _ = writer.write_all(cell.symbol.as_bytes());
+            to_skip = cell_width(cell).saturating_sub(1);
+        }
+    }
+
+    close_hyperlink(writer, &mut active_hyperlink);
+
+    // Reset style at the end.
+    let _ = writer.write_all(b"\x1b[0m");
+}
+
+fn cell_hyperlink_uri<'a>(frame: &'a FrameData, cell: &CellData) -> Option<&'a str> {
+    let index = cell.hyperlink? as usize;
+    frame.hyperlinks.get(index).map(String::as_str)
+}
+
+fn sanitized_hyperlink_uri(uri: &str) -> Option<String> {
+    let sanitized: String = uri
+        .chars()
+        .filter(|ch| *ch != '\x1b' && *ch != '\x07' && !ch.is_control())
+        .collect();
+    (!sanitized.is_empty()).then_some(sanitized)
+}
+
+fn sanitized_frame_hyperlinks(frame: &FrameData) -> Vec<Option<String>> {
+    frame
+        .hyperlinks
+        .iter()
+        .map(|uri| sanitized_hyperlink_uri(uri))
+        .collect()
+}
+
+fn sanitized_cell_hyperlink_uri<'a>(
+    sanitized_hyperlinks: &'a [Option<String>],
+    cell: &CellData,
+) -> Option<&'a str> {
+    let index = cell.hyperlink? as usize;
+    sanitized_hyperlinks.get(index)?.as_deref()
+}
+
+fn write_hyperlink_if_changed(
+    writer: &mut impl Write,
+    active: &mut Option<String>,
+    requested: Option<&str>,
+) {
+    let requested = requested.and_then(sanitized_hyperlink_uri);
+    if active.as_deref() == requested.as_deref() {
+        return;
+    }
+
+    if active.is_some() {
+        let _ = writer.write_all(b"\x1b]8;;\x1b\\");
+    }
+    *active = requested;
+    if let Some(uri) = active.as_deref() {
+        let _ = write!(writer, "\x1b]8;;{uri}\x1b\\");
+    }
+}
+
+fn close_hyperlink(writer: &mut impl Write, active: &mut Option<String>) {
+    if active.take().is_some() {
+        let _ = writer.write_all(b"\x1b]8;;\x1b\\");
+    }
+}
+
+fn write_cell(
+    writer: &mut impl Write,
+    row: u16,
+    col: u16,
+    cell: &CellData,
+    last_sgr: &mut String,
+    active_hyperlink: &mut Option<String>,
+    frame: &FrameData,
+) {
+    if cell.skip {
+        return;
+    }
+
+    let _ = write!(writer, "\x1b[{};{}H", row + 1, col + 1);
+
+    let sgr = build_sgr(cell.fg, cell.bg, cell.modifier);
+    if sgr != *last_sgr {
+        let _ = writer.write_all(sgr.as_bytes());
+        *last_sgr = sgr;
+    }
+
+    write_hyperlink_if_changed(writer, active_hyperlink, cell_hyperlink_uri(frame, cell));
+    let _ = writer.write_all(cell.symbol.as_bytes());
+}
+
+/// Writes only the cells that changed between the previous and current frame.
+fn cells_visually_equal(
+    sanitized_hyperlinks: &[Option<String>],
+    cell: &CellData,
+    prev_sanitized_hyperlinks: &[Option<String>],
+    prev_cell: &CellData,
+) -> bool {
+    cell.symbol == prev_cell.symbol
+        && cell.fg == prev_cell.fg
+        && cell.bg == prev_cell.bg
+        && cell.modifier == prev_cell.modifier
+        && sanitized_cell_hyperlink_uri(sanitized_hyperlinks, cell)
+            == sanitized_cell_hyperlink_uri(prev_sanitized_hyperlinks, prev_cell)
+    // Skip flag is only for ratatui internal use, not visual.
+}
+
+fn write_changed_cells(writer: &mut impl Write, frame: &FrameData, prev: &FrameData) {
+    let mut last_sgr = String::new(); // Track last SGR to avoid redundant style changes.
+    let mut active_hyperlink = None;
+    let sanitized_hyperlinks = sanitized_frame_hyperlinks(frame);
+    let prev_sanitized_hyperlinks = sanitized_frame_hyperlinks(prev);
+
+    for row in 0..frame.height {
+        let mut invalidated = 0usize;
+        let mut to_skip = 0usize;
+
+        for col in 0..frame.width {
+            let idx = (row as usize) * (frame.width as usize) + (col as usize);
+            let cell = &frame.cells[idx];
+            let prev_cell = &prev.cells[idx];
+
+            if !cell.skip
+                && (!cells_visually_equal(
+                    &sanitized_hyperlinks,
+                    cell,
+                    &prev_sanitized_hyperlinks,
+                    prev_cell,
+                ) || invalidated > 0)
+                && to_skip == 0
+            {
+                write_cell(
+                    writer,
+                    row,
+                    col,
+                    cell,
+                    &mut last_sgr,
+                    &mut active_hyperlink,
+                    frame,
+                );
+            }
+
+            to_skip = cell_width(cell).saturating_sub(1);
+            let affected_width = cmp::max(cell_width(cell), cell_width(prev_cell));
+            invalidated = cmp::max(affected_width, invalidated).saturating_sub(1);
+        }
+    }
+
+    close_hyperlink(writer, &mut active_hyperlink);
+
+    // Reset style if we wrote anything.
+    if !last_sgr.is_empty() {
+        let _ = writer.write_all(b"\x1b[0m");
+    }
+}
+
+// ---------------------------------------------------------------------------
+// Tests
+// ---------------------------------------------------------------------------
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::protocol::{CellData, CursorState};
+
+    const WIDE_GRAPHEME: &str = "💡";
+
+    fn make_cell(symbol: &str, fg: u32, bg: u32, modifier: u16) -> CellData {
+        CellData {
+            symbol: symbol.to_owned(),
+            fg,
+            bg,
+            modifier,
+            skip: false,
+            hyperlink: None,
+        }
+    }
+
+    fn make_frame(width: u16, height: u16, cells: Vec<CellData>) -> FrameData {
+        FrameData {
+            cells,
+            width,
+            height,
+            cursor: None,
+            hyperlinks: Vec::new(),
+            graphics: Vec::new(),
+        }
+    }
+
+    fn linked_cell(symbol: &str, index: u32) -> CellData {
+        let mut cell = make_cell(symbol, 0, 0, 0);
+        cell.hyperlink = Some(index);
+        cell
+    }
+
+    #[test]
+    fn color_to_sgr_fg_named_colors() {
+        assert_eq!(color_to_sgr_fg(0x00_00_00_00), "39"); // Reset
+        assert_eq!(color_to_sgr_fg(0x00_00_00_01), "30"); // Black
+        assert_eq!(color_to_sgr_fg(0x00_00_00_02), "31"); // Red
+        assert_eq!(color_to_sgr_fg(0x00_00_00_10), "97"); // White
+    }
+
+    #[test]
+    fn color_to_sgr_fg_indexed() {
+        assert_eq!(color_to_sgr_fg(0x01_00_00_AB), "38;5;171");
+    }
+
+    #[test]
+    fn color_to_sgr_fg_rgb() {
+        assert_eq!(color_to_sgr_fg(0x02_FF_80_40), "38;2;255;128;64");
+    }
+
+    #[test]
+    fn color_to_sgr_bg_named_colors() {
+        assert_eq!(color_to_sgr_bg(0x00_00_00_00), "49"); // Reset
+        assert_eq!(color_to_sgr_bg(0x00_00_00_01), "40"); // Black
+        assert_eq!(color_to_sgr_bg(0x00_00_00_10), "107"); // White
+    }
+
+    #[test]
+    fn color_to_sgr_bg_rgb() {
+        assert_eq!(color_to_sgr_bg(0x02_FF_80_40), "48;2;255;128;64");
+    }
+
+    #[test]
+    fn modifier_to_sgr_parts_bold() {
+        let parts = modifier_to_sgr_parts(1); // BOLD
+        assert!(parts.contains(&"1"));
+    }
+
+    #[test]
+    fn modifier_to_sgr_parts_italic() {
+        let parts = modifier_to_sgr_parts(4); // ITALIC
+        assert!(parts.contains(&"3"));
+    }
+
+    #[test]
+    fn modifier_to_sgr_parts_empty() {
+        let parts = modifier_to_sgr_parts(0);
+        assert!(parts.is_empty());
+    }
+
+    #[test]
+    fn build_sgr_produces_valid_sequence() {
+        let sgr = build_sgr(0x00_00_00_02, 0x00_00_00_01, 1); // fg=Red, bg=Black, bold
+        assert!(sgr.starts_with("\x1b["));
+        assert!(sgr.ends_with("m"));
+        assert!(sgr.contains("0")); // reset existing style first
+        assert!(sgr.contains("1")); // bold
+        assert!(sgr.contains("31")); // fg red
+        assert!(sgr.contains("40")); // bg black
+    }
+
+    #[test]
+    fn build_sgr_resets_previous_modifiers_when_cell_is_plain() {
+        assert_eq!(build_sgr(0x00_00_00_00, 0x00_00_00_00, 0), "\x1b[0;39;49m");
+    }
+
+    #[test]
+    fn cells_equal_identical() {
+        let a = make_cell("A", 2, 1, 0);
+        let b = make_cell("A", 2, 1, 0);
+        assert!(cells_equal(&a, &b));
+    }
+
+    #[test]
+    fn cells_equal_different_symbol() {
+        let a = make_cell("A", 2, 1, 0);
+        let b = make_cell("B", 2, 1, 0);
+        assert!(!cells_equal(&a, &b));
+    }
+
+    #[test]
+    fn cells_equal_different_color() {
+        let a = make_cell("A", 2, 1, 0);
+        let b = make_cell("A", 3, 1, 0);
+        assert!(!cells_equal(&a, &b));
+    }
+
+    #[test]
+    fn blit_frame_hides_cursor_before_full_redraw_writes() {
+        let frame = make_frame(
+            2,
+            2,
+            vec![
+                make_cell("H", 0, 0, 0),
+                make_cell("i", 0, 0, 0),
+                make_cell("!", 0, 0, 0),
+                make_cell(" ", 0, 0, 0),
+            ],
+        );
+
+        let mut output = Vec::new();
+        blit_frame_to(&mut output, &frame, None);
+
+        let output_str = String::from_utf8(output).unwrap();
+        assert!(
+            output_str.starts_with("\x1b[?2026h\x1b[?25l"),
+            "should begin synchronized output and hide cursor before any cell writes during full redraw"
+        );
+    }
+
+    #[test]
+    fn blit_frame_hides_cursor_before_diff_writes() {
+        let prev = make_frame(
+            2,
+            2,
+            vec![
+                make_cell("H", 0, 0, 0),
+                make_cell("i", 0, 0, 0),
+                make_cell("!", 0, 0, 0),
+                make_cell(" ", 0, 0, 0),
+            ],
+        );
+
+        let curr = make_frame(
+            2,
+            2,
+            vec![
+                make_cell("X", 0, 0, 0), // Changed
+                make_cell("i", 0, 0, 0), // Same
+                make_cell("!", 0, 0, 0), // Same
+                make_cell(" ", 0, 0, 0), // Same
+            ],
+        );
+
+        let mut output = Vec::new();
+        blit_frame_to(&mut output, &curr, Some(&prev));
+
+        let output_str = String::from_utf8(output).unwrap();
+        assert!(
+            output_str.starts_with("\x1b[?2026h\x1b[?25l"),
+            "should begin synchronized output and hide cursor before any cell writes during diff"
+        );
+    }
+
+    #[test]
+    fn blit_frame_wraps_frame_in_synchronized_output() {
+        let frame = make_frame(1, 1, vec![make_cell("A", 0, 0, 0)]);
+
+        let mut output = Vec::new();
+        blit_frame_to(&mut output, &frame, None);
+
+        let output_str = String::from_utf8(output).unwrap();
+        assert!(
+            output_str.starts_with("\x1b[?2026h"),
+            "should begin synchronized output before frame writes"
+        );
+        let sync_end = output_str
+            .find("\x1b[?2026l")
+            .expect("should end synchronized output after frame writes");
+        assert!(
+            sync_end + "\x1b[?2026l".len() < output_str.len(),
+            "should end synchronized output before trailing IME cursor update"
+        );
+    }
+
+    #[test]
+    fn blit_frame_repeats_final_cursor_state_after_synchronized_output() {
+        let frame = FrameData {
+            cells: vec![make_cell("A", 0, 0, 0); 9],
+            width: 3,
+            height: 3,
+            cursor: Some(CursorState {
+                x: 2,
+                y: 1,
+                visible: true,
+                shape: 0,
+            }),
+            hyperlinks: Vec::new(),
+            graphics: Vec::new(),
+        };
+
+        let mut output = Vec::new();
+        blit_frame_to(&mut output, &frame, None);
+
+        let output_str = String::from_utf8(output).unwrap();
+        let sync_end = output_str
+            .find("\x1b[?2026l")
+            .expect("should end synchronized output");
+        let trailing_cursor = &output_str[sync_end + "\x1b[?2026l".len()..];
+        assert_eq!(
+            trailing_cursor, "\x1b[2;3H\x1b[?25h",
+            "should expose only the final cursor state after synchronized output"
+        );
+    }
+
+    #[test]
+    fn blit_frame_emits_cursor_shape_before_visibility_without_touching_ime_anchor() {
+        let frame = FrameData {
+            cells: vec![make_cell("A", 0, 0, 0)],
+            width: 1,
+            height: 1,
+            cursor: Some(CursorState {
+                x: 0,
+                y: 0,
+                visible: true,
+                shape: 6,
+            }),
+            hyperlinks: Vec::new(),
+            graphics: Vec::new(),
+        };
+
+        let mut output = Vec::new();
+        blit_frame_to(&mut output, &frame, None);
+
+        let output_str = String::from_utf8(output).unwrap();
+        let final_cursor = output_str
+            .find("\x1b[1;1H\x1b[6 q\x1b[?25h")
+            .expect("should set cursor shape before showing cursor");
+        let sync_end = output_str
+            .find("\x1b[?2026l")
+            .expect("should end synchronized output");
+        assert!(
+            final_cursor < sync_end,
+            "shape should be part of the synchronized final cursor state"
+        );
+        let trailing_cursor = &output_str[sync_end + "\x1b[?2026l".len()..];
+        assert_eq!(
+            trailing_cursor, "\x1b[1;1H\x1b[?25h",
+            "IME anchor update should preserve the existing position/visibility-only contract"
+        );
+    }
+
+    #[test]
+    fn blit_frame_repeats_explicit_hidden_cursor_anchor_after_synchronized_output() {
+        let visible = FrameData {
+            cells: vec![make_cell("A", 0, 0, 0); 9],
+            width: 3,
+            height: 3,
+            cursor: Some(CursorState {
+                x: 0,
+                y: 0,
+                visible: true,
+                shape: 0,
+            }),
+            hyperlinks: Vec::new(),
+            graphics: Vec::new(),
+        };
+        let hidden = FrameData {
+            cells: vec![make_cell("B", 0, 0, 0); 9],
+            width: 3,
+            height: 3,
+            cursor: Some(CursorState {
+                x: 2,
+                y: 1,
+                visible: false,
+                shape: 0,
+            }),
+            hyperlinks: Vec::new(),
+            graphics: Vec::new(),
+        };
+        let mut last_visible_cursor = None;
+        let mut last_cursor_shape = 0;
+        let mut output = Vec::new();
+
+        blit_frame_to_with_cursor_memory(
+            &mut output,
+            &visible,
+            None,
+            &mut last_visible_cursor,
+            &mut last_cursor_shape,
+        );
+        output.clear();
+        blit_frame_to_with_cursor_memory(
+            &mut output,
+            &hidden,
+            Some(&visible),
+            &mut last_visible_cursor,
+            &mut last_cursor_shape,
+        );
+
+        let output_str = String::from_utf8(output).unwrap();
+        let sync_end = output_str
+            .find("\x1b[?2026l")
+            .expect("should end synchronized output");
+        let trailing_cursor = &output_str[sync_end + "\x1b[?2026l".len()..];
+        assert_eq!(
+            trailing_cursor, "\x1b[2;3H\x1b[?25l",
+            "should repeat the explicit hidden cursor position while preserving visibility"
+        );
+    }
+
+    #[test]
+    fn blit_frame_emits_osc8_for_linked_cells() {
+        let mut frame = make_frame(
+            3,
+            1,
+            vec![
+                linked_cell("L", 0),
+                linked_cell("i", 0),
+                make_cell("!", 0, 0, 0),
+            ],
+        );
+        frame.hyperlinks.push("https://example.com".to_owned());
+
+        let mut output = Vec::new();
+        blit_frame_to(&mut output, &frame, None);
+
+        let output_str = String::from_utf8(output).unwrap();
+        assert!(output_str.contains("\x1b]8;;https://example.com\x1b\\L"));
+        assert!(output_str.contains('i'));
+        assert!(output_str.contains("\x1b]8;;\x1b\\"));
+    }
+
+    #[test]
+    fn blit_frame_sanitizes_hyperlink_uris() {
+        let mut frame = make_frame(1, 1, vec![linked_cell("L", 0)]);
+        frame
+            .hyperlinks
+            .push("https://exa\x1b\x07mple.com".to_owned());
+
+        let mut output = Vec::new();
+        blit_frame_to(&mut output, &frame, None);
+
+        let output_str = String::from_utf8(output).unwrap();
+        assert!(output_str.contains("\x1b]8;;https://example.com\x1b\\L"));
+    }
+
+    #[test]
+    fn blit_frame_first_frame_produces_output() {
+        let frame = make_frame(
+            2,
+            2,
+            vec![
+                make_cell("H", 0, 0, 0),
+                make_cell("i", 0, 0, 0),
+                make_cell("!", 0, 0, 0),
+                make_cell(" ", 0, 0, 0),
+            ],
+        );
+
+        let mut output = Vec::new();
+        blit_frame_to(&mut output, &frame, None);
+
+        let output_str = String::from_utf8(output).unwrap();
+        // Full redraw should start with clear screen.
+        assert!(
+            output_str.contains("\x1b[2J"),
+            "full redraw should clear screen"
+        );
+        assert!(
+            output_str.contains('H') || output_str.contains('i'),
+            "should contain cell content"
+        );
+    }
+
+    #[test]
+    fn blit_frame_diff_only_writes_changed_cells() {
+        let prev = make_frame(
+            2,
+            2,
+            vec![
+                make_cell("H", 0, 0, 0),
+                make_cell("i", 0, 0, 0),
+                make_cell("!", 0, 0, 0),
+                make_cell(" ", 0, 0, 0),
+            ],
+        );
+
+        // Only the first cell changed.
+        let curr = make_frame(
+            2,
+            2,
+            vec![
+                make_cell("X", 0, 0, 0), // Changed
+                make_cell("i", 0, 0, 0), // Same
+                make_cell("!", 0, 0, 0), // Same
+                make_cell(" ", 0, 0, 0), // Same
+            ],
+        );
+
+        let mut output = Vec::new();
+        blit_frame_to(&mut output, &curr, Some(&prev));
+
+        let output_str = String::from_utf8(output).unwrap();
+        // Diff should NOT clear the screen.
+        assert!(
+            !output_str.contains("\x1b[2J"),
+            "diff should not clear screen"
+        );
+        // Should contain the changed cell content.
+        assert!(output_str.contains('X'), "should contain changed cell 'X'");
+    }
+
+    #[test]
+    fn blit_frame_size_change_triggers_full_redraw() {
+        let prev = make_frame(2, 2, vec![make_cell("A", 0, 0, 0); 4]);
+
+        let curr = make_frame(3, 2, vec![make_cell("B", 0, 0, 0); 6]);
+
+        let mut output = Vec::new();
+        blit_frame_to(&mut output, &curr, Some(&prev));
+
+        let output_str = String::from_utf8(output).unwrap();
+        assert!(
+            output_str.contains("\x1b[2J"),
+            "size change should trigger full redraw"
+        );
+    }
+
+    #[test]
+    fn blit_frame_positions_cursor() {
+        let frame = FrameData {
+            cells: vec![make_cell("A", 0, 0, 0)],
+            width: 1,
+            height: 1,
+            cursor: Some(CursorState {
+                x: 0,
+                y: 0,
+                visible: true,
+                shape: 0,
+            }),
+            hyperlinks: Vec::new(),
+            graphics: Vec::new(),
+        };
+
+        let mut output = Vec::new();
+        blit_frame_to(&mut output, &frame, None);
+
+        let output_str = String::from_utf8(output).unwrap();
+        assert!(
+            output_str.contains("\x1b[1;1H"),
+            "should position cursor at (1,1)"
+        );
+    }
+
+    #[test]
+    fn blit_frame_hides_cursor_when_invisible() {
+        let frame = FrameData {
+            cells: vec![make_cell("A", 0, 0, 0)],
+            width: 1,
+            height: 1,
+            cursor: Some(CursorState {
+                x: 0,
+                y: 0,
+                visible: false,
+                shape: 0,
+            }),
+            hyperlinks: Vec::new(),
+            graphics: Vec::new(),
+        };
+
+        let mut output = Vec::new();
+        blit_frame_to(&mut output, &frame, None);
+
+        let output_str = String::from_utf8(output).unwrap();
+        assert!(
+            output_str.contains("\x1b[?25l"),
+            "should hide cursor when invisible"
+        );
+    }
+
+    #[test]
+    fn blit_frame_no_cursor_hides_cursor() {
+        let frame = FrameData {
+            cells: vec![make_cell("A", 0, 0, 0)],
+            width: 1,
+            height: 1,
+            cursor: None,
+            hyperlinks: Vec::new(),
+            graphics: Vec::new(),
+        };
+
+        let mut output = Vec::new();
+        blit_frame_to(&mut output, &frame, None);
+
+        let output_str = String::from_utf8(output).unwrap();
+        assert!(
+            output_str.contains("\x1b[?25l"),
+            "should hide cursor when no cursor state"
+        );
+    }
+
+    #[test]
+    fn blit_frame_restores_cursor_visibility() {
+        // First frame: cursor hidden.
+        let prev = FrameData {
+            cells: vec![make_cell("A", 0, 0, 0)],
+            width: 1,
+            height: 1,
+            cursor: Some(CursorState {
+                x: 0,
+                y: 0,
+                visible: false,
+                shape: 0,
+            }),
+            hyperlinks: Vec::new(),
+            graphics: Vec::new(),
+        };
+
+        let mut output = Vec::new();
+        blit_frame_to(&mut output, &prev, None);
+        assert!(
+            String::from_utf8(output).unwrap().contains("\x1b[?25l"),
+            "first frame should hide cursor"
+        );
+
+        // Second frame: cursor visible — should restore visibility.
+        let curr = FrameData {
+            cells: vec![make_cell("B", 0, 0, 0)],
+            width: 1,
+            height: 1,
+            cursor: Some(CursorState {
+                x: 0,
+                y: 0,
+                visible: true,
+                shape: 0,
+            }),
+            hyperlinks: Vec::new(),
+            graphics: Vec::new(),
+        };
+
+        let mut output = Vec::new();
+        blit_frame_to(&mut output, &curr, Some(&prev));
+        let output_str = String::from_utf8(output).unwrap();
+        assert!(
+            output_str.contains("\x1b[?25h"),
+            "second frame should restore cursor visibility with ?25h"
+        );
+        assert!(
+            output_str.contains("\x1b[1;1H"),
+            "should position cursor before showing it"
+        );
+    }
+
+    #[test]
+    fn blit_frame_positions_cursor_before_showing_it() {
+        let prev = FrameData {
+            cells: vec![make_cell("A", 0, 0, 0); 9],
+            width: 3,
+            height: 3,
+            cursor: Some(CursorState {
+                x: 0,
+                y: 0,
+                visible: true,
+                shape: 0,
+            }),
+            hyperlinks: Vec::new(),
+            graphics: Vec::new(),
+        };
+        let mut curr = prev.clone();
+        curr.cells[0] = make_cell("B", 0, 0, 0);
+        curr.cursor = Some(CursorState {
+            x: 2,
+            y: 2,
+            visible: true,
+            shape: 0,
+        });
+
+        let mut output = Vec::new();
+        blit_frame_to(&mut output, &curr, Some(&prev));
+        let output_str = String::from_utf8(output).unwrap();
+        let final_move = output_str
+            .rfind("\x1b[3;3H")
+            .expect("should move cursor to final position");
+        let show = output_str
+            .rfind("\x1b[?25h")
+            .expect("should show cursor after positioning it");
+
+        assert!(
+            final_move < show,
+            "should move cursor to final position before showing it"
+        );
+    }
+
+    #[test]
+    fn blit_frame_parks_hidden_cursor_at_last_visible_position() {
+        let visible = FrameData {
+            cells: vec![make_cell("A", 0, 0, 0); 9],
+            width: 3,
+            height: 3,
+            cursor: Some(CursorState {
+                x: 1,
+                y: 1,
+                visible: true,
+                shape: 0,
+            }),
+            hyperlinks: Vec::new(),
+            graphics: Vec::new(),
+        };
+        let hidden = FrameData {
+            cells: vec![make_cell("B", 0, 0, 0); 9],
+            width: 3,
+            height: 3,
+            cursor: None,
+            hyperlinks: Vec::new(),
+            graphics: Vec::new(),
+        };
+        let mut last_visible_cursor = None;
+        let mut last_cursor_shape = 0;
+        let mut output = Vec::new();
+
+        blit_frame_to_with_cursor_memory(
+            &mut output,
+            &visible,
+            None,
+            &mut last_visible_cursor,
+            &mut last_cursor_shape,
+        );
+        output.clear();
+        blit_frame_to_with_cursor_memory(
+            &mut output,
+            &hidden,
+            Some(&visible),
+            &mut last_visible_cursor,
+            &mut last_cursor_shape,
+        );
+
+        let output_str = String::from_utf8(output).unwrap();
+        let park = output_str
+            .rfind("\x1b[2;2H")
+            .expect("should park hidden cursor at last visible position");
+        let hide = output_str
+            .rfind("\x1b[?25l")
+            .expect("should keep hidden cursor hidden");
+        assert!(park < hide, "should park cursor before hiding it");
+    }
+
+    #[test]
+    fn blit_frame_parks_hidden_cursor_at_bottom_right_without_history() {
+        let frame = FrameData {
+            cells: vec![make_cell("A", 0, 0, 0); 6],
+            width: 3,
+            height: 2,
+            cursor: None,
+            hyperlinks: Vec::new(),
+            graphics: Vec::new(),
+        };
+        let mut last_visible_cursor = None;
+        let mut last_cursor_shape = 0;
+        let mut output = Vec::new();
+
+        blit_frame_to_with_cursor_memory(
+            &mut output,
+            &frame,
+            None,
+            &mut last_visible_cursor,
+            &mut last_cursor_shape,
+        );
+
+        let output_str = String::from_utf8(output).unwrap();
+        assert!(
+            output_str.contains("\x1b[2;3H\x1b[?25l"),
+            "should park hidden cursor at bottom-right before ending the frame"
+        );
+    }
+
+    #[test]
+    fn blit_frame_hides_previous_visible_cursor_when_next_frame_has_none() {
+        let prev = FrameData {
+            cells: vec![make_cell("A", 0, 0, 0)],
+            width: 1,
+            height: 1,
+            cursor: Some(CursorState {
+                x: 0,
+                y: 0,
+                visible: true,
+                shape: 0,
+            }),
+            hyperlinks: Vec::new(),
+            graphics: Vec::new(),
+        };
+        let curr = FrameData {
+            cells: vec![make_cell("B", 0, 0, 0)],
+            width: 1,
+            height: 1,
+            cursor: None,
+            hyperlinks: Vec::new(),
+            graphics: Vec::new(),
+        };
+
+        let mut output = Vec::new();
+        blit_frame_to(&mut output, &curr, Some(&prev));
+
+        assert!(
+            String::from_utf8(output).unwrap().contains("\x1b[?25l"),
+            "diff redraw should hide a previously visible cursor when the next frame has none"
+        );
+    }
+
+    #[test]
+    fn full_redraw_skips_trailing_cells_covered_by_wide_graphemes() {
+        let frame = FrameData {
+            cells: vec![
+                make_cell(WIDE_GRAPHEME, 0, 0, 0),
+                make_cell(" ", 0, 0, 0),
+                make_cell("Z", 0, 0, 0),
+            ],
+            width: 3,
+            height: 1,
+            cursor: None,
+            hyperlinks: Vec::new(),
+            graphics: Vec::new(),
+        };
+
+        let mut output = Vec::new();
+        blit_frame_to(&mut output, &frame, None);
+        let output_str = String::from_utf8(output).unwrap();
+
+        assert!(output_str.contains("\x1b[1;1H"));
+        assert!(!output_str.contains("\x1b[1;2H"));
+        assert!(output_str.contains("\x1b[1;3H"));
+    }
+
+    #[test]
+    fn diff_redraw_reveals_cells_hidden_by_previous_wide_graphemes() {
+        let prev = FrameData {
+            cells: vec![
+                make_cell(WIDE_GRAPHEME, 0, 0, 0),
+                make_cell(" ", 0, 0, 0),
+                make_cell("Z", 0, 0, 0),
+            ],
+            width: 3,
+            height: 1,
+            cursor: None,
+            hyperlinks: Vec::new(),
+            graphics: Vec::new(),
+        };
+        let curr = FrameData {
+            cells: vec![
+                make_cell("A", 0, 0, 0),
+                make_cell(" ", 0, 0, 0),
+                make_cell("Z", 0, 0, 0),
+            ],
+            width: 3,
+            height: 1,
+            cursor: None,
+            hyperlinks: Vec::new(),
+            graphics: Vec::new(),
+        };
+
+        let mut output = Vec::new();
+        blit_frame_to(&mut output, &curr, Some(&prev));
+        let output_str = String::from_utf8(output).unwrap();
+
+        assert!(output_str.contains("\x1b[1;1H"));
+        assert!(
+            output_str.contains("\x1b[1;2H"),
+            "cells hidden by a previous wide grapheme must be redrawn when they become visible"
+        );
+    }
+
+    #[test]
+    fn diff_redraw_skips_new_trailing_cells_covered_by_wide_graphemes() {
+        let prev = FrameData {
+            cells: vec![
+                make_cell("A", 0, 0, 0),
+                make_cell("B", 0, 0, 0),
+                make_cell("Z", 0, 0, 0),
+            ],
+            width: 3,
+            height: 1,
+            cursor: None,
+            hyperlinks: Vec::new(),
+            graphics: Vec::new(),
+        };
+        let curr = FrameData {
+            cells: vec![
+                make_cell(WIDE_GRAPHEME, 0, 0, 0),
+                make_cell(" ", 0, 0, 0),
+                make_cell("Z", 0, 0, 0),
+            ],
+            width: 3,
+            height: 1,
+            cursor: None,
+            hyperlinks: Vec::new(),
+            graphics: Vec::new(),
+        };
+
+        let mut output = Vec::new();
+        blit_frame_to(&mut output, &curr, Some(&prev));
+        let output_str = String::from_utf8(output).unwrap();
+
+        assert!(output_str.contains("\x1b[1;1H"));
+        assert!(!output_str.contains("\x1b[1;2H"));
+    }
+}