path: root/src/tty/panel_compositor.zig

//! TTY implementation of panel transitions.
//!
//! The core publishes presentation cells plus POD tracks. It has already
//! composed PanelAscii because byte stepping is semantic, while this backend
//! copies the grid for geometry and dissolve operations. No terminal escape
//! trick or pixel assumption leaks into the core.

const std = @import("std");
const pardes = @import("../pardes.zig");
const panel_animation = @import("../panel_animation.zig");

const Box = panel_animation.Box;
const Track = panel_animation.Track;

/// Kitty placements cannot be resampled through the character-grid transform.
/// Keep their transmitted pixels cached, but omit the placement while its pane
/// geometry moves. Data effects animate changed grid cells in place and have
/// no honest old Kitty bitmap, so leaving an unchanged attachment canonical is
/// better than making it disappear for the whole reveal and pop back.
pub fn hidesAttachment(tracks: []const Track, serial: u32) bool {
    for (tracks) |track| {
        if (track.serial != serial or !track.active()) continue;
        if (track.effect != .slide and track.effect != .zoom and track.effect != .vertical) continue;
        // The last active sample is already the exact canonical frame. Place
        // the image there too, rather than making it pop in one tick later.
        if (track.frame + 1 < track.frames()) return true;
    }
    return false;
}

/// Return `source` unchanged when idle, otherwise an arena-owned Surface whose
/// cells contain this frame's TTY presentation. Geometry exposed by a moving
/// panel uses the theme's page background when it has one; null deliberately
/// leaves the host terminal's default background showing.
pub fn compose(
    arena: std.mem.Allocator,
    source: *pardes.Surface,
    tracks: []const Track,
    theme_bg: ?[3]u8,
) !*pardes.Surface {
    var active = false;
    var noncanonical = false;
    for (tracks) |track| {
        if (!drawable(source, track)) continue;
        active = true;
        noncanonical = noncanonical or track.amount() < 1.0;
    }
    if (!active) return source;
    // Every effect's last active sample is its exact canonical endpoint. Keep
    // the real cursor and attachments in that sample instead of suppressing
    // them for one extra frame that is otherwise pixel-identical.
    if (!noncanonical) return source;

    const out = try arena.create(pardes.Surface);
    out.* = source.*;
    out.cells = try arena.dupe(pardes.Cell, source.cells);
    // A cursor has no pane/phase record of its own. All native backends hide
    // it for the short transition rather than disagreeing about whether it
    // should move, stay static, or cover an overlapping opening pane.
    out.cursor = null;

    // Clear every final geometry before drawing any presented geometry. If
    // clear and draw are interleaved, a new panel can erase the old full-width
    // panel that was deliberately drawn underneath it at frame zero.
    for (tracks) |track|
        if (drawable(source, track) and (track.effect == .slide or track.effect == .zoom))
            clearBox(out, track.to, theme_bg);

    // A vertical opener starts over the exact old pixels in its fixed target
    // clip. Its new content then rises into that clip from below. Restore all
    // bases before painting anything so an opener cannot erase an earlier
    // moving panel merely because its pane slot sorts later.
    for (tracks) |track|
        if (drawable(source, track) and track.effect == .vertical and track.phase == .opening)
            restorePrevious(out, source, track.contentBox());

    // Stable layout motion first, newly opening panels above it, and closing
    // tombstones last. A closing pane no longer owns input or canonical cells,
    // but its frozen old content remains the top visual until it slides out.
    for ([_]panel_animation.Phase{ .moving, .opening, .closing }) |phase| {
        for (tracks) |track| {
            if (!drawable(source, track) or track.phase != phase) continue;
            switch (track.effect) {
                .off => {},
                .slide, .zoom => transform(out, source, track),
                .dissolve => dissolve(out, source, track),
                // Pardes core has already composed every character effect
                // into source.cells — byte walks, glyph motion, and per-cell
                // locks are semantic. TTY only owns cursor suppression here.
                .ascii, .edges, .fall, .wave, .curtain, .scramble, .typewriter => {},
                .vertical => vertical(out, source, track),
            }
        }
    }
    return out;
}

/// Content effects have no honest frame without the frozen old grid. In
/// production the core drops such tracks before publishing Surface; retain
/// this guard at the backend boundary so a partial/custom caller also snaps
/// to canonical content instead of manufacturing blanks.
fn drawable(source: *const pardes.Surface, track: Track) bool {
    return track.active() and
        (!track.effect.needsPreviousGrid() or source.hasPanelDiff());
}

fn transform(out: *pardes.Surface, source: *const pardes.Surface, track: Track) void {
    const shown = track.presented();
    if (shown.w <= 0 or shown.h <= 0) return;
    const dst = clipped(shown, out.cols, out.rows) orelse return;
    var y = dst.y0;
    while (y < dst.y1) : (y += 1) {
        var x = dst.x0;
        while (x < dst.x1) : (x += 1) {
            // The core hit-tests cell CENTRES against this same half-open box.
            // floor/ceil clipping includes boundary cells for iteration, but
            // they are not visible unless their centre is actually inside.
            // Test that containment before division: at large u16 grids f32
            // can round an inside right-edge ratio to exactly 1.0.
            const cx = @as(f32, @floatFromInt(x)) + 0.5;
            const cy = @as(f32, @floatFromInt(y)) + 0.5;
            if (cx < shown.x or cx >= shown.x + shown.w or
                cy < shown.y or cy >= shown.y + shown.h) continue;
            const u = std.math.clamp((cx - shown.x) / shown.w, 0, 0.999_999);
            const v = std.math.clamp((cy - shown.y) / shown.h, 0, 0.999_999);
            const sx: i32 = @intFromFloat(@floor(track.to.x + u * track.to.w));
            const sy: i32 = @intFromFloat(@floor(track.to.y + v * track.to.h));
            if (sx < 0 or sy < 0 or sx >= source.cols or sy >= source.rows) continue;
            out.at(x, y).* = source.cells[@as(usize, @intCast(sy)) * source.cols + @as(usize, @intCast(sx))];
        }
    }
}

fn dissolve(out: *pardes.Surface, source: *const pardes.Surface, track: Track) void {
    const area = clipped(track.to, out.cols, out.rows) orelse return;
    const amount = track.amount();
    var y = area.y0;
    while (y < area.y1) : (y += 1) {
        var x = area.x0;
        while (x < area.x1) : (x += 1) {
            if (!source.panelCellChanged(x, y)) continue;
            if (panel_animation.dissolveRevealed(
                track.serial,
                x - area.x0,
                y - area.y0,
                amount,
            )) continue;
            out.at(x, y).* = previousCell(source, x, y).*;
        }
    }
}

fn previousCell(source: *const pardes.Surface, x: u16, y: u16) *const pardes.Cell {
    std.debug.assert(source.hasPanelDiff());
    return &source.previous_cells[@as(usize, y) * source.cols + x];
}

fn restorePrevious(out: *pardes.Surface, source: *const pardes.Surface, box: Box) void {
    const area = clipped(box, out.cols, out.rows) orelse return;
    var y = area.y0;
    while (y < area.y1) : (y += 1) {
        var x = area.x0;
        while (x < area.x1) : (x += 1)
            out.at(x, y).* = previousCell(source, x, y).*;
    }
}

fn vertical(out: *pardes.Surface, source: *const pardes.Surface, track: Track) void {
    const cells = if (track.phase == .closing) source.previous_cells else source.cells;
    transformClipped(out, source.cols, source.rows, cells, track, track.contentBox());
}

/// Translate one old/new canonical panel through its fixed lifecycle clip.
/// Sampling is always relative to contentBox: an opener reads its new target,
/// while a closer reads the old box retained after that pane was destroyed.
fn transformClipped(
    out: *pardes.Surface,
    source_cols: u16,
    source_rows: u16,
    cells: []const pardes.Cell,
    track: Track,
    clip: Box,
) void {
    const shown = track.presented();
    if (shown.w <= 0 or shown.h <= 0) return;
    const dst = clipped(shown, out.cols, out.rows) orelse return;
    const content = track.contentBox();
    var y = dst.y0;
    while (y < dst.y1) : (y += 1) {
        var x = dst.x0;
        while (x < dst.x1) : (x += 1) {
            const cx = @as(f32, @floatFromInt(x)) + 0.5;
            const cy = @as(f32, @floatFromInt(y)) + 0.5;
            if (cx < shown.x or cx >= shown.x + shown.w or
                cy < shown.y or cy >= shown.y + shown.h or
                cx < clip.x or cx >= clip.x + clip.w or
                cy < clip.y or cy >= clip.y + clip.h) continue;
            const u = std.math.clamp((cx - shown.x) / shown.w, 0, 0.999_999);
            const v = std.math.clamp((cy - shown.y) / shown.h, 0, 0.999_999);
            const sx: i32 = @intFromFloat(@floor(content.x + u * content.w));
            const sy: i32 = @intFromFloat(@floor(content.y + v * content.h));
            if (sx < 0 or sy < 0 or sx >= source_cols or sy >= source_rows) continue;
            out.at(x, y).* = cells[@as(usize, @intCast(sy)) * source_cols + @as(usize, @intCast(sx))];
        }
    }
}

fn clearBox(surface: *pardes.Surface, box: Box, theme_bg: ?[3]u8) void {
    const area = clipped(box, surface.cols, surface.rows) orelse return;
    var blank: pardes.Cell = .{};
    if (theme_bg) |rgb| {
        blank.default = false;
        blank.style.bg = .{ .rgb = rgb };
    }
    var y = area.y0;
    while (y < area.y1) : (y += 1) {
        var x = area.x0;
        while (x < area.x1) : (x += 1) surface.at(x, y).* = blank;
    }
}

const GridBox = struct { x0: u16, y0: u16, x1: u16, y1: u16 };

fn clipped(box: Box, cols: u16, rows: u16) ?GridBox {
    const x0: i32 = @intFromFloat(@floor(box.x));
    const y0: i32 = @intFromFloat(@floor(box.y));
    const x1: i32 = @intFromFloat(@ceil(box.x + box.w));
    const y1: i32 = @intFromFloat(@ceil(box.y + box.h));
    const area: GridBox = .{
        .x0 = @intCast(std.math.clamp(x0, 0, @as(i32, cols))),
        .y0 = @intCast(std.math.clamp(y0, 0, @as(i32, rows))),
        .x1 = @intCast(std.math.clamp(x1, 0, @as(i32, cols))),
        .y1 = @intCast(std.math.clamp(y1, 0, @as(i32, rows))),
    };
    return if (area.x0 < area.x1 and area.y0 < area.y1) area else null;
}

test "TTY consumes core-composed ASCII cells without reinterpreting glyphs" {
    var cells: [8]pardes.Cell = @splat(.{});
    var previous: [8]pardes.Cell = @splat(.{});
    const diffs: [8]pardes.PanelCellDiff = @splat(.visual);
    for (&cells) |*cell| {
        cell.text[0] = 'X';
        cell.len = 1;
        cell.default = false;
    }
    for (&previous) |*cell| {
        cell.text[0] = 'O';
        cell.len = 1;
        cell.default = false;
    }
    var surface: pardes.Surface = .{
        .cols = 4,
        .rows = 2,
        .cells = &cells,
        .previous_cells = &previous,
        .cell_diffs = &diffs,
    };
    var arena = std.heap.ArenaAllocator.init(std.testing.allocator);
    defer arena.deinit();
    const track: Track = .{
        .serial = 9,
        .effect = .ascii,
        .frame = 0,
        .from = .{ .w = 4, .h = 2 },
        .to = .{ .w = 4, .h = 2 },
    };
    const animated = try compose(arena.allocator(), &surface, &.{track}, null);
    try std.testing.expect(animated != &surface);
    try std.testing.expectEqualStrings("X", surface.at(0, 0).grapheme());
    for (animated.cells) |cell| try std.testing.expectEqualStrings("X", cell.grapheme());
}

test "dissolve changes only diff cells from old to new" {
    var cells: [2]pardes.Cell = @splat(.{});
    var previous: [2]pardes.Cell = @splat(.{});
    for (&cells, "NU") |*cell, glyph| {
        cell.text[0] = glyph;
        cell.len = 1;
        cell.default = false;
    }
    for (&previous, "OU") |*cell, glyph| {
        cell.text[0] = glyph;
        cell.len = 1;
        cell.default = false;
    }
    const diffs = [_]pardes.PanelCellDiff{ .{ .ascii = .{ .from = 'O', .to = 'N' } }, .unchanged };
    var surface: pardes.Surface = .{
        .cols = 2,
        .rows = 1,
        .cells = &cells,
        .previous_cells = &previous,
        .cell_diffs = &diffs,
    };
    var arena = std.heap.ArenaAllocator.init(std.testing.allocator);
    defer arena.deinit();
    var track: Track = .{
        .serial = 1,
        .effect = .dissolve,
        .from = .{ .w = 2, .h = 1 },
        .to = .{ .w = 2, .h = 1 },
    };

    const first = try compose(arena.allocator(), &surface, &.{track}, null);
    try std.testing.expectEqualStrings("O", first.at(0, 0).grapheme());
    try std.testing.expectEqual(cells[1], first.at(1, 0).*);

    _ = arena.reset(.retain_capacity);
    track.frame = track.effect.frames() - 1;
    const final = try compose(arena.allocator(), &surface, &.{track}, null);
    try std.testing.expectEqual(cells[0], final.at(0, 0).*);
    try std.testing.expectEqual(cells[1], final.at(1, 0).*);
}

test "TTY content effects never touch cells outside the published diff" {
    var cells: [4]pardes.Cell = @splat(.{});
    var previous: [4]pardes.Cell = @splat(.{});
    for (&cells, "NUQV") |*cell, glyph| {
        cell.text[0] = glyph;
        cell.len = 1;
        cell.default = false;
    }
    // Deliberately disagree even where the mask says unchanged. This makes
    // the test prove that the compositor obeys the core cell diff rather than
    // merely appearing correct because old and new happened to match there.
    for (&previous, "OXRY") |*cell, glyph| {
        cell.text[0] = glyph;
        cell.len = 1;
        cell.default = false;
    }
    const diffs = [_]pardes.PanelCellDiff{ .visual, .unchanged, .visual, .unchanged };
    var surface: pardes.Surface = .{
        .cols = 4,
        .rows = 1,
        .cells = &cells,
        .previous_cells = &previous,
        .cell_diffs = &diffs,
    };
    var arena = std.heap.ArenaAllocator.init(std.testing.allocator);
    defer arena.deinit();

    for ([_]panel_animation.Transition{ .ascii, .dissolve }) |effect| {
        const track: Track = .{
            .serial = 17,
            .effect = effect,
            .from = .{ .w = 4, .h = 1 },
            .to = .{ .w = 4, .h = 1 },
        };
        const animated = try compose(arena.allocator(), &surface, &.{track}, null);
        try std.testing.expectEqual(cells[1], animated.at(1, 0).*);
        try std.testing.expectEqual(cells[3], animated.at(3, 0).*);
        _ = arena.reset(.retain_capacity);
    }
}

test "content transition without a diff snaps to canonical surface" {
    var cells = [_]pardes.Cell{.{}};
    cells[0].text[0] = 'N';
    cells[0].default = false;
    var surface: pardes.Surface = .{
        .cols = 1,
        .rows = 1,
        .cells = &cells,
        .cursor = .{ .x = 0, .y = 0 },
    };
    var arena = std.heap.ArenaAllocator.init(std.testing.allocator);
    defer arena.deinit();

    for ([_]panel_animation.Transition{ .dissolve, .ascii, .vertical }) |effect| {
        const track: Track = .{
            .effect = effect,
            .phase = .opening,
            .from = .{ .y = 1, .w = 1, .h = 1 },
            .to = .{ .w = 1, .h = 1 },
        };
        try std.testing.expectEqual(&surface, try compose(arena.allocator(), &surface, &.{track}, null));
    }
}

test "exact transition endpoint preserves the canonical cursor" {
    var cells = [_]pardes.Cell{.{}};
    var previous = cells;
    const diffs = [_]pardes.PanelCellDiff{.visual};
    var surface: pardes.Surface = .{
        .cols = 1,
        .rows = 1,
        .cells = &cells,
        .cursor = .{ .x = 0, .y = 0 },
        .previous_cells = &previous,
        .cell_diffs = &diffs,
    };
    const track: Track = .{
        .effect = .ascii,
        .frame = panel_animation.Transition.ascii.frames() - 1,
        .from = .{ .w = 1, .h = 1 },
        .to = .{ .w = 1, .h = 1 },
    };
    var arena = std.heap.ArenaAllocator.init(std.testing.allocator);
    defer arena.deinit();

    const result = try compose(arena.allocator(), &surface, &.{track}, null);
    try std.testing.expectEqual(&surface, result);
    try std.testing.expect(result.cursor != null);
}

test "vertical opening rises through a fixed old-grid clip" {
    var cells: [12]pardes.Cell = @splat(.{});
    var previous: [12]pardes.Cell = @splat(.{});
    for (&cells, "aaabbbcccddd") |*cell, glyph| {
        cell.text[0] = glyph;
        cell.len = 1;
        cell.default = false;
    }
    for (&previous, "000111222333") |*cell, glyph| {
        cell.text[0] = glyph;
        cell.len = 1;
        cell.default = false;
    }
    const diffs: [12]pardes.PanelCellDiff = @splat(.visual);
    var surface: pardes.Surface = .{
        .cols = 3,
        .rows = 4,
        .cells = &cells,
        .previous_cells = &previous,
        .cell_diffs = &diffs,
    };
    var arena = std.heap.ArenaAllocator.init(std.testing.allocator);
    defer arena.deinit();
    const target: Box = .{ .y = 1, .w = 3, .h = 2 };
    var track: Track = .{
        .serial = 5,
        .phase = .opening,
        .effect = .vertical,
        .from = panel_animation.openingBox(.vertical, target, 3),
        .to = target,
    };

    const first = try compose(arena.allocator(), &surface, &.{track}, null);
    try std.testing.expectEqualStrings("1", first.at(0, 1).grapheme());
    try std.testing.expectEqualStrings("2", first.at(0, 2).grapheme());
    try std.testing.expectEqualStrings("a", first.at(0, 0).grapheme());
    try std.testing.expectEqualStrings("d", first.at(0, 3).grapheme());

    _ = arena.reset(.retain_capacity);
    track.frame = 2;
    const entering = try compose(arena.allocator(), &surface, &.{track}, null);
    try std.testing.expectEqualStrings("1", entering.at(0, 1).grapheme());
    try std.testing.expectEqualStrings("b", entering.at(0, 2).grapheme());
    try std.testing.expectEqualStrings("a", entering.at(0, 0).grapheme());
    try std.testing.expectEqualStrings("d", entering.at(0, 3).grapheme());

    _ = arena.reset(.retain_capacity);
    track.frame = track.effect.frames() - 1;
    const final = try compose(arena.allocator(), &surface, &.{track}, null);
    try std.testing.expectEqualStrings("b", final.at(0, 1).grapheme());
    try std.testing.expectEqualStrings("c", final.at(0, 2).grapheme());
}

test "vertical closing drops frozen content over canonical cells" {
    var cells: [12]pardes.Cell = @splat(.{});
    var previous: [12]pardes.Cell = @splat(.{});
    for (&cells, "aaabbbcccddd") |*cell, glyph| {
        cell.text[0] = glyph;
        cell.len = 1;
        cell.default = false;
    }
    for (&previous, "000111222333") |*cell, glyph| {
        cell.text[0] = glyph;
        cell.len = 1;
        cell.default = false;
    }
    const diffs: [12]pardes.PanelCellDiff = @splat(.visual);
    var surface: pardes.Surface = .{
        .cols = 3,
        .rows = 4,
        .cells = &cells,
        .previous_cells = &previous,
        .cell_diffs = &diffs,
    };
    var arena = std.heap.ArenaAllocator.init(std.testing.allocator);
    defer arena.deinit();
    const old_box: Box = .{ .y = 1, .w = 3, .h = 2 };
    var track: Track = .{
        .serial = 5,
        .phase = .closing,
        .effect = .vertical,
        .from = old_box,
        .to = panel_animation.closingBox(.vertical, old_box),
    };

    const first = try compose(arena.allocator(), &surface, &.{track}, null);
    try std.testing.expectEqualStrings("1", first.at(0, 1).grapheme());
    try std.testing.expectEqualStrings("2", first.at(0, 2).grapheme());

    _ = arena.reset(.retain_capacity);
    track.frame = 8;
    const leaving = try compose(arena.allocator(), &surface, &.{track}, null);
    try std.testing.expectEqualStrings("b", leaving.at(0, 1).grapheme());
    try std.testing.expectEqualStrings("1", leaving.at(0, 2).grapheme());
    try std.testing.expectEqualStrings("a", leaving.at(0, 0).grapheme());
    try std.testing.expectEqualStrings("d", leaving.at(0, 3).grapheme());

    _ = arena.reset(.retain_capacity);
    track.frame = track.effect.frames() - 1;
    const final = try compose(arena.allocator(), &surface, &.{track}, null);
    try std.testing.expectEqual(cells[3], final.at(0, 1).*);
    try std.testing.expectEqual(cells[6], final.at(0, 2).*);
}

test "closing content paints after opening content regardless of track order" {
    var cells = [_]pardes.Cell{.{}};
    var previous = [_]pardes.Cell{.{}};
    cells[0].text[0] = 'N';
    cells[0].default = false;
    previous[0].text[0] = 'O';
    previous[0].default = false;
    const diffs = [_]pardes.PanelCellDiff{.visual};
    var surface: pardes.Surface = .{
        .cols = 1,
        .rows = 1,
        .cells = &cells,
        .previous_cells = &previous,
        .cell_diffs = &diffs,
    };
    var arena = std.heap.ArenaAllocator.init(std.testing.allocator);
    defer arena.deinit();
    const box: Box = .{ .w = 1, .h = 1 };
    const closing: Track = .{
        .serial = 1,
        .phase = .closing,
        .effect = .vertical,
        .from = box,
        .to = panel_animation.closingBox(.vertical, box),
    };
    const opening: Track = .{
        .serial = 2,
        .phase = .opening,
        .effect = .ascii,
        .frame = panel_animation.Transition.ascii.frames() - 1,
        .from = box,
        .to = box,
    };

    const animated = try compose(arena.allocator(), &surface, &.{ closing, opening }, null);
    try std.testing.expectEqualStrings("O", animated.at(0, 0).grapheme());
}

test "opening panel cannot erase an existing panel drawn underneath" {
    var cells: [8]pardes.Cell = @splat(.{});
    for (&cells, 0..) |*cell, x| {
        cell.text[0] = if (x < 4) 'A' else 'B';
        cell.len = 1;
        cell.default = false;
    }
    var surface: pardes.Surface = .{ .cols = 8, .rows = 1, .cells = &cells };
    var arena = std.heap.ArenaAllocator.init(std.testing.allocator);
    defer arena.deinit();
    const tracks = [_]Track{
        .{
            .serial = 1,
            .phase = .moving,
            .effect = .slide,
            .from = .{ .w = 8, .h = 1 },
            .to = .{ .w = 4, .h = 1 },
        },
        .{
            .serial = 2,
            .phase = .opening,
            .effect = .slide,
            .from = .{ .x = 8, .w = 4, .h = 1 },
            .to = .{ .x = 4, .w = 4, .h = 1 },
        },
    };
    const animated = try compose(arena.allocator(), &surface, &tracks, null);
    try std.testing.expectEqualStrings("A", animated.at(7, 0).grapheme());
}

test "geometry gap is a blank painted in an explicit theme background" {
    var cells: [4]pardes.Cell = @splat(.{});
    for (&cells, "ABCD") |*cell, glyph| {
        cell.text[0] = glyph;
        cell.len = 1;
        cell.default = false;
        cell.style = .{
            .fg = .{ .rgb = .{ 220, 210, 200 } },
            .bg = .{ .rgb = .{ 30, 20, 10 } },
            .bold = true,
            .font_role = .tagline,
        };
    }
    var surface: pardes.Surface = .{ .cols = 4, .rows = 1, .cells = &cells };
    var arena = std.heap.ArenaAllocator.init(std.testing.allocator);
    defer arena.deinit();
    const track: Track = .{
        .serial = 1,
        .effect = .slide,
        .from = .{ .x = 2, .w = 2, .h = 1 },
        .to = .{ .w = 4, .h = 1 },
    };
    const page_bg = [3]u8{ 248, 244, 232 };
    const animated = try compose(arena.allocator(), &surface, &.{track}, page_bg);

    const gap = animated.at(0, 0);
    try std.testing.expect(!gap.default);
    try std.testing.expectEqualStrings(" ", gap.grapheme());
    try std.testing.expectEqual(pardes.Color{ .rgb = page_bg }, gap.style.bg);
    try std.testing.expectEqual(pardes.Color.default, gap.style.fg);
    try std.testing.expect(!gap.style.bold);
    try std.testing.expectEqual(pardes.FontRole.body, gap.style.font_role);
    // The clear supplies only exposed geometry. Cells sampled from the moving
    // panel retain all canonical content and style fields byte-for-byte.
    try std.testing.expectEqual(cells[1], animated.at(2, 0).*);
}

test "geometry gap stays terminal-default for a transparent theme" {
    var cells: [2]pardes.Cell = @splat(.{});
    for (&cells, "AB") |*cell, glyph| {
        cell.text[0] = glyph;
        cell.len = 1;
        cell.default = false;
    }
    var surface: pardes.Surface = .{ .cols = 2, .rows = 1, .cells = &cells };
    var arena = std.heap.ArenaAllocator.init(std.testing.allocator);
    defer arena.deinit();
    const track: Track = .{
        .serial = 1,
        .effect = .slide,
        .from = .{ .x = 1, .w = 1, .h = 1 },
        .to = .{ .w = 2, .h = 1 },
    };
    const animated = try compose(arena.allocator(), &surface, &.{track}, null);

    const gap = animated.at(0, 0);
    try std.testing.expect(gap.default);
    try std.testing.expectEqualStrings(" ", gap.grapheme());
    try std.testing.expectEqual(pardes.Color.default, gap.style.bg);
}

test "fractional geometry draws exactly the cell centres core can hit" {
    var cells: [3]pardes.Cell = @splat(.{});
    for (&cells, "ABC") |*cell, glyph| {
        cell.text[0] = glyph;
        cell.len = 1;
        cell.default = false;
    }
    var surface: pardes.Surface = .{ .cols = 3, .rows = 1, .cells = &cells };
    var arena = std.heap.ArenaAllocator.init(std.testing.allocator);
    defer arena.deinit();
    const track: Track = .{
        .serial = 1,
        .effect = .slide,
        .from = .{ .x = 0.9, .w = 1, .h = 1 },
        .to = .{ .x = 0, .w = 1, .h = 1 },
    };
    const animated = try compose(arena.allocator(), &surface, &.{track}, null);
    // Cell 0 lies in floor/ceil iteration bounds, but its centre (0.5) is
    // outside [0.9, 1.9); cell 1 is the only presented/hit-testable cell.
    try std.testing.expectEqualStrings(" ", animated.at(0, 0).grapheme());
    try std.testing.expectEqualStrings("A", animated.at(1, 0).grapheme());
}

test "large-grid right edge uses containment before rounded ratio" {
    const cols = std.math.maxInt(u16);
    const source_cells = try std.testing.allocator.alloc(pardes.Cell, cols);
    defer std.testing.allocator.free(source_cells);
    const output_cells = try std.testing.allocator.alloc(pardes.Cell, cols);
    defer std.testing.allocator.free(output_cells);
    @memset(source_cells, .{});
    @memset(output_cells, .{});
    for (source_cells) |*cell| {
        cell.text[0] = 'X';
        cell.len = 1;
        cell.default = false;
    }
    var source: pardes.Surface = .{ .cols = cols, .rows = 1, .cells = source_cells };
    var output: pardes.Surface = .{ .cols = cols, .rows = 1, .cells = output_cells };
    const track: Track = .{
        .serial = 1,
        .effect = .zoom,
        .frame = 8,
        .from = .{ .x = 56_796, .w = 8_299, .h = 1 },
        .to = .{ .x = 556, .w = 63_483, .h = 1 },
    };
    const shown = track.presented();
    try std.testing.expect(@as(f32, 63_935.5) < shown.x + shown.w);
    transform(&output, &source, track);
    try std.testing.expectEqualStrings("X", output.at(63_935, 0).grapheme());
}

test "active panel transition hides only its own native attachment" {
    const tracks = [_]Track{
        .{ .serial = 41, .effect = .slide, .frame = 0 },
        .{ .serial = 42, .effect = .ascii, .frame = 0 },
        .{ .serial = 43, .effect = .zoom, .frame = panel_animation.Transition.zoom.frames() - 1 },
        .{ .serial = 44, .effect = .zoom, .frame = panel_animation.Transition.zoom.frames() },
        .{ .serial = 45, .phase = .opening, .effect = .vertical, .frame = 0 },
    };

    try std.testing.expect(hidesAttachment(&tracks, 41));
    try std.testing.expect(!hidesAttachment(&tracks, 42));
    try std.testing.expect(!hidesAttachment(&tracks, 43));
    try std.testing.expect(!hidesAttachment(&tracks, 44));
    try std.testing.expect(hidesAttachment(&tracks, 45));
    try std.testing.expect(!hidesAttachment(&tracks, 46));
}