Refactor panes and filesystem; replace FUSE with 9P

Consolidate pane, layout, memory and host code. Serve 9P by default over Unix sockets, with runtime mounts and optional TCP/QUIC transports. Remove FUSE and obsolete proof-of-concept examples.

Fix highlighting and terminal-history performance, expand differential and stress-test infrastructure, sort navigation results while preserving the next occurrence, add syntax-colored Braille minimaps, remove SPC-k, and document 9P interaction as a repository skill.

1 files changed, 1706 insertions, 0 deletions

diff --git a/src/layout.zig b/src/layout.zig
new file mode 100644
index 00000000..51944856
--- /dev/null
+++ b/src/layout.zig
@@ -0,0 +1,1706 @@
+const std = @import("std");
+const pardes = @import("pardes.zig");
+const config = @import("config.zig");
+const panes = @import("panes.zig");
+const Pardes = pardes.Pardes;
+const Pane = pardes.Pane;
+pub const Rect = struct { x: u16 = 0, y: u16 = 0, w: u16 = 0, h: u16 = 0 };
+pub const Snapshot = struct { serial: u32, box: Box };
+const MAX_COLS = pardes.MAX_COLS;
+const MAX_PANES = pardes.MAX_PANES;
+const TOPBAR_H = pardes.TOPBAR_H;
+const BOX_H = pardes.BOX_H;
+
+pub const Presentation = struct {
+    // Only acknowledged draws advance shown geometry; committed and submitted may be ahead.
+    committed: [MAX_PANES]?Snapshot = @splat(null),
+    initialized: bool = false,
+    enabled: bool = false,
+    snap_once: bool = false,
+    tracks: [MAX_PANES]?Track = @splat(null),
+    closing: [MAX_PANES]Track = undefined,
+    closing_len: usize = 0,
+    submitted: [MAX_PANES]?Snapshot = @splat(null),
+    submitted_ready: bool = false,
+    shown: [MAX_PANES]?Snapshot = @splat(null),
+    shown_tracks: [MAX_PANES]?Track = @splat(null),
+    shown_closing: [MAX_PANES]Track = undefined,
+    shown_closing_len: usize = 0,
+    acknowledged: bool = false,
+    pending: bool = false,
+    previous_cells: []pardes.Cell = &.{},
+    previous_cols: u16 = 0,
+    previous_rows: u16 = 0,
+    previous_valid: bool = false,
+    previous_layout: [MAX_PANES]?Snapshot = @splat(null),
+    diffs: []pardes.PanelCellDiff = &.{},
+    diff_state: enum { none, requested, ready } = .none,
+
+    pub const CellPosition = struct { col: u16, row: u16 };
+
+    pub fn deinit(self: *Presentation, gpa: std.mem.Allocator) void {
+        if (self.previous_cells.len > 0) gpa.free(self.previous_cells);
+        if (self.diffs.len > 0) gpa.free(self.diffs);
+    }
+
+    pub fn sync(self: *Presentation, p: *Pardes) void {
+        const initializing = !self.initialized or !self.enabled;
+        if (initializing or self.snap_once) {
+            const had_layout = self.initialized;
+            self.committed = @splat(null);
+            self.tracks = @splat(null);
+            self.closing_len = 0;
+            self.diff_state = .none;
+            if (initializing) {
+                self.shown_tracks = @splat(null);
+                self.shown_closing_len = 0;
+            }
+            for (p.panes, 0..) |slot, id| {
+                const pane = slot orelse continue;
+                self.committed[id] = .{ .serial = pane.serial, .box = panelBox(p.rects[id]) };
+            }
+            self.initialized = true;
+            self.snap_once = false;
+            if (had_layout and self.acknowledged)
+                self.pending = true;
+            return;
+        }
+
+        const effect = p.settings.panel_transition;
+        if (effect.needsPreviousGrid() and self.diff_state != .none) {
+            var second_change = false;
+            for (p.panes, self.committed, 0..) |slot, snapshot, id| {
+                const pane = slot orelse {
+                    second_change = second_change or snapshot != null;
+                    continue;
+                };
+                const target = panelBox(p.rects[id]);
+                second_change = second_change or snapshot == null or
+                    snapshot.?.serial != pane.serial or !snapshot.?.box.eql(target);
+            }
+            if (second_change) {
+                p.abandonPanelAnimations();
+                self.committed = @splat(null);
+                for (p.panes, 0..) |slot, id| {
+                    const pane = slot orelse continue;
+                    self.committed[id] = .{ .serial = pane.serial, .box = panelBox(p.rects[id]) };
+                }
+                return;
+            }
+        }
+        var changed = false;
+        var animated_change = false;
+        for (p.panes, 0..) |slot, id| {
+            const pane = slot orelse {
+                if (self.committed[id]) |old| {
+                    changed = true;
+                    if (effect.lifecycleOnly()) {
+                        if (self.appendClosing(.{
+                            .serial = old.serial,
+                            .pane = @intCast(id),
+                            .phase = .closing,
+                            .effect = effect,
+                            .from = old.box,
+                            .to = closingBox(effect, old.box),
+                        })) animated_change = true;
+                    }
+                }
+                self.committed[id] = null;
+                self.tracks[id] = null;
+                // Never let pixels from a dead pane address a reused slot.
+                self.shown_tracks[id] = null;
+                continue;
+            };
+            const target = panelBox(p.rects[id]);
+            const previous = self.committed[id];
+            self.committed[id] = .{ .serial = pane.serial, .box = target };
+
+            if (effect == .off) {
+                changed = changed or previous == null or previous.?.serial != pane.serial or
+                    !previous.?.box.eql(target);
+                self.tracks[id] = null;
+                continue;
+            }
+            if (previous) |old| {
+                if (old.serial == pane.serial and old.box.eql(target)) continue;
+                const same_lifetime = old.serial == pane.serial;
+                const prior = if (self.tracks[id]) |track|
+                    if (track.serial == pane.serial and track.active()) track else null
+                else
+                    null;
+                if (effect.lifecycleOnly() and same_lifetime) {
+                    if (prior) |active| if (active.phase == .opening) {
+                        var next = active;
+                        next.from = openingBox(effect, target, p.screen_w);
+                        next.to = target;
+                        self.tracks[id] = next;
+                        changed = true;
+                        animated_change = true;
+                        continue;
+                    };
+                    self.tracks[id] = null;
+                    changed = true;
+                    continue;
+                }
+                if (effect.lifecycleOnly() and !same_lifetime) {
+                    _ = self.appendClosing(.{
+                        .serial = old.serial,
+                        .pane = @intCast(id),
+                        .phase = .closing,
+                        .effect = effect,
+                        .from = old.box,
+                        .to = closingBox(effect, old.box),
+                    });
+                }
+                const shown = if (self.shown[id]) |snapshot|
+                    if (snapshot.serial == pane.serial) snapshot.box else null
+                else
+                    null;
+                const from = if (!same_lifetime)
+                    openingBox(effect, target, p.screen_w)
+                else if (shown) |box|
+                    box
+                else if (self.acknowledged and prior != null)
+                    prior.?.from
+                else
+                    old.box;
+                const next: Track = .{
+                    .serial = pane.serial,
+                    .pane = @intCast(id),
+                    .phase = if (!same_lifetime or
+                        (prior != null and prior.?.phase == .opening)) .opening else .moving,
+                    .effect = effect,
+                    .from = from,
+                    .to = target,
+                };
+                self.tracks[id] = next;
+                changed = true;
+                animated_change = true;
+            } else {
+                const next: Track = .{
+                    .serial = pane.serial,
+                    .pane = @intCast(id),
+                    .phase = .opening,
+                    .effect = effect,
+                    .from = openingBox(effect, target, p.screen_w),
+                    .to = target,
+                };
+                self.tracks[id] = next;
+                changed = true;
+                animated_change = true;
+            }
+        }
+        if (animated_change and effect.needsPreviousGrid()) {
+            self.diff_state = .requested;
+        }
+        if (changed and self.acknowledged) self.pending = true;
+    }
+
+    fn appendClosing(self: *Presentation, track: Track) bool {
+        std.debug.assert(track.phase == .closing);
+        const baseline = self.previous_layout[track.pane] orelse return false;
+        if (!self.previous_valid or baseline.serial != track.serial or
+            !baseline.box.eql(track.from)) return false;
+        if (self.closing_len == self.closing.len) {
+            std.mem.copyForwards(
+                Track,
+                self.closing[0 .. self.closing.len - 1],
+                self.closing[1..],
+            );
+            self.closing_len -= 1;
+        }
+        self.closing[self.closing_len] = track;
+        self.closing_len += 1;
+        return true;
+    }
+
+    pub fn advance(self: *Presentation) void {
+        for (&self.tracks) |*slot| {
+            const track = if (slot.*) |*track| track else continue;
+            track.frame +|= 1;
+            if (!track.active()) slot.* = null;
+        }
+        var out: usize = 0;
+        for (self.closing[0..self.closing_len]) |value| {
+            var track = value;
+            track.frame +|= 1;
+            if (!track.active()) continue;
+            self.closing[out] = track;
+            out += 1;
+        }
+        self.closing_len = out;
+    }
+
+    pub fn acknowledge(self: *Presentation, p: *Pardes, tracks: []const Track) void {
+        var presented: [MAX_PANES]?Track = @splat(null);
+        var presented_closing: [MAX_PANES]Track = undefined;
+        var nclosing: usize = 0;
+        var presented_layout: [MAX_PANES]?Snapshot = if (self.submitted_ready)
+            self.submitted
+        else
+            @splat(null);
+        if (!self.submitted_ready) for (p.panes, 0..) |slot, id| {
+            const pane = slot orelse continue;
+            presented_layout[id] = .{ .serial = pane.serial, .box = panelBox(p.rects[id]) };
+        };
+        for (&presented_layout, 0..) |*snapshot, id| if (snapshot.*) |saved| {
+            const pane = p.panes[id] orelse {
+                snapshot.* = null;
+                continue;
+            };
+            if (pane.serial != saved.serial) snapshot.* = null;
+        };
+        for (tracks) |track| {
+            if (track.phase == .closing) {
+                const current = for (self.closing[0..self.closing_len]) |candidate| {
+                    if (candidate.serial == track.serial and candidate.pane == track.pane and
+                        candidate.effect == track.effect) break true;
+                } else false;
+                if (!current or !track.active() or nclosing == presented_closing.len) continue;
+                presented_closing[nclosing] = track;
+                nclosing += 1;
+                continue;
+            }
+            const id: usize = track.pane;
+            if (id >= p.panes.len or !track.active()) continue;
+            const pane = p.panes[id] orelse continue;
+            if (pane.serial != track.serial) continue;
+            presented[id] = track;
+            presented_layout[id] = .{ .serial = track.serial, .box = track.visualBox() };
+        }
+        self.shown_tracks = presented;
+        self.shown_closing = presented_closing;
+        self.shown_closing_len = nclosing;
+        self.shown = presented_layout;
+        self.acknowledged = true;
+        self.pending = false;
+        if (tracks.len == 0) {
+            self.tracks = @splat(null);
+            self.closing_len = 0;
+            self.diff_state = .none;
+            self.capturePrevious(p.gpa, &p.surface);
+        }
+    }
+
+    fn capturePrevious(self: *Presentation, gpa: std.mem.Allocator, surface: *const pardes.Surface) void {
+        self.diff_state = .none;
+        const cells = surface.cells;
+        if (cells.len == 0) {
+            self.previous_valid = false;
+            self.previous_layout = @splat(null);
+            return;
+        }
+        if (self.previous_cells.len != cells.len) {
+            const next = gpa.alloc(pardes.Cell, cells.len) catch {
+                self.previous_valid = false;
+                self.previous_layout = @splat(null);
+                return;
+            };
+            if (self.previous_cells.len > 0) gpa.free(self.previous_cells);
+            self.previous_cells = next;
+        }
+        @memcpy(self.previous_cells, cells);
+        self.previous_cols = surface.cols;
+        self.previous_rows = surface.rows;
+        self.previous_valid = true;
+        self.previous_layout = if (self.submitted_ready)
+            self.submitted
+        else
+            @splat(null);
+    }
+
+    pub fn cellDiff(self: *const Presentation, cols: u16, rows: u16, col: u16, row: u16) pardes.PanelCellDiff {
+        if (self.diff_state != .ready or col >= cols or row >= rows or
+            self.diffs.len != @as(usize, cols) * rows) return .unchanged;
+        return self.diffs[@as(usize, row) * cols + col];
+    }
+
+    pub fn pointer(self: *const Presentation, cols: u16, rows: u16, col: u16, row: u16) ?CellPosition {
+        if (self.pending) return null;
+        var closing = self.shown_closing_len;
+        while (closing > 0) {
+            closing -= 1;
+            const track = self.shown_closing[closing];
+            if (!track.active()) continue;
+            if (boxContainsCell(track.contentBox(), col, row) and
+                boxContainsCell(track.presented(), col, row)) return null;
+        }
+        const phases = [_]Phase{ .opening, .moving };
+        for (phases) |phase| {
+            // Backends paint pane slots forward within a phase. Probe them in
+            // reverse so overlapping transition quads address the top pixel.
+            var index = self.shown_tracks.len;
+            while (index > 0) {
+                index -= 1;
+                const track = self.shown_tracks[index] orelse continue;
+                if (!track.active() or track.phase != phase) continue;
+                switch (track.effect) {
+                    .slide, .zoom => {
+                        const shown = track.presented();
+                        if (shown.w <= 0 or shown.h <= 0 or !boxContainsCell(shown, col, row)) continue;
+                        const x = @as(f32, @floatFromInt(col)) + 0.5;
+                        const y = @as(f32, @floatFromInt(row)) + 0.5;
+                        const u = std.math.clamp((x - shown.x) / shown.w, 0, 0.999_999);
+                        const v = std.math.clamp((y - shown.y) / shown.h, 0, 0.999_999);
+                        const logical_x: i32 = @intFromFloat(@floor(track.to.x + u * track.to.w));
+                        const logical_y: i32 = @intFromFloat(@floor(track.to.y + v * track.to.h));
+                        return .{
+                            .col = @intCast(std.math.clamp(logical_x, 0, @as(i32, cols -| 1))),
+                            .row = @intCast(std.math.clamp(logical_y, 0, @as(i32, rows -| 1))),
+                        };
+                    },
+                    .vertical => {
+                        const clip = track.contentBox();
+                        if (!boxContainsCell(clip, col, row)) continue;
+                        const shown = track.presented();
+                        if (shown.w <= 0 or shown.h <= 0 or !boxContainsCell(shown, col, row))
+                            return null;
+                        const x = @as(f32, @floatFromInt(col)) + 0.5;
+                        const y = @as(f32, @floatFromInt(row)) + 0.5;
+                        const u = std.math.clamp((x - shown.x) / shown.w, 0, 0.999_999);
+                        const v = std.math.clamp((y - shown.y) / shown.h, 0, 0.999_999);
+                        return .{
+                            .col = @intFromFloat(@floor(clip.x + u * clip.w)),
+                            .row = @intFromFloat(@floor(clip.y + v * clip.h)),
+                        };
+                    },
+                    .dissolve, .ascii => {
+                        if (!boxContainsCell(track.to, col, row)) continue;
+                        const cell_diff = self.cellDiff(cols, rows, col, row);
+                        if (!cell_diff.changed())
+                            return .{ .col = col, .row = row };
+                        const relative_col: u16 = @intFromFloat(@floor(
+                            @as(f32, @floatFromInt(col)) + 0.5 - track.to.x,
+                        ));
+                        const relative_row: u16 = @intFromFloat(@floor(
+                            @as(f32, @floatFromInt(row)) + 0.5 - track.to.y,
+                        ));
+                        const visible = switch (track.effect) {
+                            .dissolve => dissolveRevealed(
+                                track.serial,
+                                relative_col,
+                                relative_row,
+                                track.amount(),
+                            ),
+                            .ascii => switch (cell_diff) {
+                                .ascii => |diff| diff.complete(track.frame),
+                                .unchanged, .visual => true,
+                            },
+                            else => unreachable,
+                        };
+                        return if (visible) .{ .col = col, .row = row } else null;
+                    },
+                    .edges, .fall, .wave, .curtain, .scramble, .typewriter => {
+                        if (!boxContainsCell(track.to, col, row)) continue;
+                        const area = CellArea.of(track.to);
+                        const settled = std.meta.eql(
+                            charSource(
+                                track,
+                                col -| area.x0,
+                                row -| area.y0,
+                                area,
+                            ),
+                            CharSource.settled,
+                        );
+                        return if (settled) .{ .col = col, .row = row } else null;
+                    },
+                    .off => {},
+                }
+            }
+        }
+        for (self.shown_tracks) |maybe| {
+            const track = maybe orelse continue;
+            if (!track.active() or (track.effect != .slide and track.effect != .zoom and
+                track.effect != .vertical)) continue;
+            if (boxContainsCell(track.to, col, row)) return null;
+        }
+        return .{ .col = col, .row = row };
+    }
+
+    fn boxContainsCell(box: Box, col: u16, row: u16) bool {
+        const x = @as(f32, @floatFromInt(col)) + 0.5;
+        const y = @as(f32, @floatFromInt(row)) + 0.5;
+        return x >= box.x and x < box.x + box.w and y >= box.y and y < box.y + box.h;
+    }
+
+    pub fn cancel(self: *Presentation) void {
+        self.tracks = @splat(null);
+        self.closing_len = 0;
+        self.shown_tracks = @splat(null);
+        self.shown_closing_len = 0;
+        self.diff_state = .none;
+        self.pending = self.acknowledged;
+    }
+
+    fn prepareDiff(self: *Presentation, gpa: std.mem.Allocator, surface: *const pardes.Surface) !bool {
+        const count = surface.cells.len;
+        if (!self.previous_valid or
+            self.previous_cols != surface.cols or
+            self.previous_rows != surface.rows or
+            self.previous_cells.len != count) return false;
+        if (self.diffs.len != count) {
+            const next = try gpa.alloc(pardes.PanelCellDiff, count);
+            if (self.diffs.len > 0) gpa.free(self.diffs);
+            self.diffs = next;
+        }
+        for (self.diffs, self.previous_cells, surface.cells) |*diff, *old, *new|
+            diff.* = pardes.PanelCellDiff.between(old, new);
+        for (&self.tracks) |*slot| {
+            const track = if (slot.*) |*track| track else continue;
+            if (track.effect != .ascii) continue;
+            var longest: u16 = 1;
+            var row: u16 = 0;
+            while (row < surface.rows) : (row += 1) {
+                var col: u16 = 0;
+                while (col < surface.cols) : (col += 1) {
+                    if (!boxContainsCell(track.to, col, row)) continue;
+                    const index = @as(usize, row) * surface.cols + col;
+                    switch (self.diffs[index]) {
+                        .ascii => |diff| longest = @max(longest, diff.frameCount()),
+                        .unchanged, .visual => {},
+                    }
+                }
+            }
+            track.frame_count = @max(track.frame_count, longest);
+        }
+        self.diff_state = .ready;
+        return true;
+    }
+
+    pub fn submit(self: *Presentation, p: *Pardes, s: *pardes.Surface) !void {
+        self.submitted = @splat(null);
+        for (p.panes, 0..) |slot, id| {
+            const pane = slot orelse continue;
+            self.submitted[id] = .{
+                .serial = pane.serial,
+                .box = panelBox(p.rects[id]),
+            };
+        }
+        self.submitted_ready = true;
+        if (self.diff_state != .none and !try self.prepareDiff(p.gpa, s)) {
+            for (&self.tracks) |*slot| {
+                const track = slot.* orelse continue;
+                if (track.effect.needsPreviousGrid()) slot.* = null;
+            }
+            self.closing_len = 0;
+            self.diff_state = .none;
+        }
+        if (self.diff_state == .ready) {
+            s.previous_cells = self.previous_cells;
+            s.cell_diffs = self.diffs;
+        }
+        s.npanel_tracks = paintOrder(
+            &self.tracks,
+            self.closing[0..self.closing_len],
+            &s.panel_tracks,
+        );
+    }
+
+    pub fn animating(self: *const Presentation) bool {
+        for (self.tracks) |track| if (track != null and track.?.active()) return true;
+        for (self.closing[0..self.closing_len]) |track|
+            if (track.active()) return true;
+        return false;
+    }
+
+    test "presentation cache allocation failures retain ownership and allow retry" {
+        var allocator: std.testing.FailingAllocator = .init(std.testing.allocator, .{});
+        const gpa = allocator.allocator();
+        var state: Presentation = .{};
+        defer state.deinit(gpa);
+        var cells: [3]pardes.Cell = @splat(.{});
+        cells[0].default = false;
+        cells[0].text[0] = 'a';
+        var surface: pardes.Surface = .{ .cols = 2, .rows = 1, .cells = cells[0..2] };
+        state.submitted[0] = .{ .serial = 7, .box = .{ .x = 0, .y = 0, .w = 2, .h = 1 } };
+        state.submitted_ready = true;
+        state.capturePrevious(gpa, &surface);
+        try std.testing.expect(state.previous_valid);
+        try std.testing.expectEqual(@as(u32, 7), state.previous_layout[0].?.serial);
+        const previous = state.previous_cells.ptr;
+        const allocated = allocator.allocations;
+        allocator.fail_index = allocator.alloc_index;
+        state.capturePrevious(gpa, &surface);
+        try std.testing.expect(state.previous_valid);
+        try std.testing.expectEqual(allocated, allocator.allocations);
+        try std.testing.expect(!allocator.has_induced_failure);
+
+        surface.cols = 3;
+        surface.cells = &cells;
+        state.submitted[0].?.box.w = 3;
+        state.diff_state = .ready;
+        state.capturePrevious(gpa, &surface);
+        try std.testing.expect(allocator.has_induced_failure);
+        try std.testing.expect(!state.previous_valid);
+        try std.testing.expectEqual(previous, state.previous_cells.ptr);
+        try std.testing.expectEqual(@as(usize, 2), state.previous_cells.len);
+        try std.testing.expectEqual(@as(u8, 'a'), state.previous_cells[0].text[0]);
+        try std.testing.expect(state.previous_layout[0] == null);
+        try std.testing.expectEqual(.none, state.diff_state);
+
+        allocator.fail_index = std.math.maxInt(usize);
+        state.capturePrevious(gpa, &surface);
+        try std.testing.expect(state.previous_valid);
+        try std.testing.expectEqual(@as(usize, 3), state.previous_cells.len);
+        try std.testing.expectEqual(@as(f32, 3), state.previous_layout[0].?.box.w);
+        state.diff_state = .requested;
+        allocator.fail_index = allocator.alloc_index;
+        try std.testing.expectError(error.OutOfMemory, state.prepareDiff(gpa, &surface));
+        try std.testing.expectEqual(.requested, state.diff_state);
+        try std.testing.expectEqual(@as(usize, 0), state.diffs.len);
+        allocator.fail_index = std.math.maxInt(usize);
+        try std.testing.expect(try state.prepareDiff(gpa, &surface));
+        try std.testing.expectEqual(.ready, state.diff_state);
+        try std.testing.expectEqual(@as(usize, 3), state.diffs.len);
+        for (state.diffs) |diff| try std.testing.expectEqual(.unchanged, diff);
+    }
+
+    test "presentation submission failure preserves shown geometry until retry is acknowledged" {
+        var allocator: std.testing.FailingAllocator = .init(std.testing.allocator, .{});
+        const p = try Pardes.init(allocator.allocator(), .{ .tty_only = true, .cols = 40, .rows = 12 });
+        defer p.deinit();
+        var frame: std.heap.ArenaAllocator = .init(std.testing.allocator);
+        defer frame.deinit();
+        _ = try p.render(frame.allocator());
+        p.acknowledgePanelPresentation(&.{});
+        const shown = p.presentation.shown;
+        const previous = p.presentation.previous_cells.ptr;
+        p.presentation.diff_state = .requested;
+        p.presentation.pending = true;
+        allocator.fail_index = allocator.alloc_index;
+        try std.testing.expectError(error.OutOfMemory, p.presentation.submit(p, &p.surface));
+        try std.testing.expectEqualDeep(shown, p.presentation.shown);
+        try std.testing.expectEqual(previous, p.presentation.previous_cells.ptr);
+        try std.testing.expectEqual(.requested, p.presentation.diff_state);
+        try std.testing.expect(p.presentation.pending);
+        try std.testing.expect(p.presentation.pointer(p.screen_w, p.screen_h, 5, 3) == null);
+
+        allocator.fail_index = std.math.maxInt(usize);
+        try p.presentation.submit(p, &p.surface);
+        try std.testing.expectEqualDeep(shown, p.presentation.shown);
+        try std.testing.expect(p.presentation.pending);
+        p.acknowledgePanelPresentation(&.{});
+        try std.testing.expect(!p.presentation.pending);
+        try std.testing.expectEqual(CellPosition{ .col = 5, .row = 3 }, p.presentation.pointer(p.screen_w, p.screen_h, 5, 3).?);
+    }
+};
+
+pub const column_weight_unit: u64 = 1 << 32;
+pub const max_column_weight: u64 = std.math.maxInt(u64) / MAX_COLS;
+
+pub const MovePlacement = struct {
+    preview_col: usize,
+    above_id: usize,
+    row: u16,
+    above_y: u16,
+    above_h: u16,
+};
+
+pub fn focusDir(p: *Pardes, from: usize, dir: enum { left, right, up, down }) void {
+    const a = p.rects[from];
+    var best: ?usize = null;
+    var best_d: i32 = 0;
+    for (p.panes, 0..) |slot, i| {
+        if (slot == null or i == from) continue;
+        const r = p.rects[i];
+        const vov = a.y < r.y + r.h and r.y < a.y + a.h;
+        const hov = a.x < r.x + r.w and r.x < a.x + a.w;
+        const ok = switch (dir) {
+            .left => r.x + r.w <= a.x and vov,
+            .right => r.x >= a.x + a.w and vov,
+            .up => r.y + r.h <= a.y and hov,
+            .down => r.y >= a.y + a.h and hov,
+        };
+        if (!ok) continue;
+        const d: i32 = switch (dir) {
+            .left => @as(i32, a.x) - @as(i32, r.x + r.w),
+            .right => @as(i32, r.x) - @as(i32, a.x + a.w),
+            .up => @as(i32, a.y) - @as(i32, r.y + r.h),
+            .down => @as(i32, r.y) - @as(i32, a.y + a.h),
+        };
+        if (best == null or d < best_d) {
+            best = i;
+            best_d = d;
+        }
+    }
+    if (best) |b| {
+        p.active = b;
+        // a count typed before the hop was meant for the pane you left
+        p.panes[b].?.normal.clear();
+    }
+}
+
+pub fn targetColumn(p: *Pardes, cur_x: u16) usize {
+    var tc: usize = if (p.ncol > 0) p.ncol - 1 else 0;
+    for (0..p.ncol) |c| {
+        if (cur_x >= p.col_x[c] and cur_x < p.col_x[c] + p.col_w[c]) {
+            tc = c;
+            break;
+        }
+    }
+    return tc;
+}
+
+pub fn splitRowForExtent(y: u16, h: u16, cur_y: u16) ?u16 {
+    if (h < 2) return null;
+    const min_each: u16 = if (h >= config.MINH * 2) config.MINH else 1;
+    const lo = y +| min_each;
+    const hi = y + h - min_each;
+    if (lo > hi) return y + h / 2;
+    return std.math.clamp(cur_y, lo, hi);
+}
+
+pub fn movePlacement(p: *Pardes, id: usize, cur_x: u16, cur_y: u16) ?MovePlacement {
+    const src = findPane(p, id) orelse return null;
+    const tc = targetColumn(p, cur_x);
+    if (tc == src.col and p.col_n[src.col] == 1) return null;
+    if (tc == src.col) {
+        const sr = p.rects[id];
+        if (cur_y >= sr.y and cur_y < sr.y + sr.h) return null;
+    }
+    var heights: [MAX_PANES]u16 = @splat(0);
+    for (0..p.ncol) |c| {
+        for (0..p.col_n[c]) |k| {
+            const pid = p.col_panes[c][k];
+            heights[pid] = p.rects[pid].h;
+        }
+    }
+    if (p.col_n[src.col] > 1) {
+        const sib = if (src.idx > 0) p.col_panes[src.col][src.idx - 1] else p.col_panes[src.col][src.idx + 1];
+        heights[sib] +|= p.rects[id].h;
+    }
+    var y: u16 = TOPBAR_H;
+    var last: ?MovePlacement = null;
+    for (0..p.col_n[tc]) |k| {
+        const pid = p.col_panes[tc][k];
+        if (pid == id) continue;
+        const h = heights[pid];
+        const row = splitRowForExtent(y, h, cur_y) orelse {
+            y +|= h;
+            continue;
+        };
+        const placement: MovePlacement = .{
+            .preview_col = tc,
+            .above_id = pid,
+            .row = row,
+            .above_y = y,
+            .above_h = h,
+        };
+        last = placement;
+        if (cur_y < y + h) return placement;
+        y +|= h;
+    }
+    return last;
+}
+
+pub fn movePane(p: *Pardes, id: usize, cur_x: u16, cur_y: u16) void {
+    const placement = movePlacement(p, id, cur_x, cur_y) orelse return;
+    const src = findPane(p, id) orelse return;
+    const source_multi = p.col_n[src.col] > 1;
+    var heights: [MAX_PANES]u16 = @splat(0);
+    for (0..p.ncol) |c| {
+        for (0..p.col_n[c]) |k| {
+            const pid = p.col_panes[c][k];
+            heights[pid] = p.rects[pid].h;
+        }
+    }
+    removePane(p, id);
+    if (source_multi and src.col < p.ncol and p.col_n[src.col] > 0) {
+        const sib = if (src.idx > 0) p.col_panes[src.col][src.idx - 1] else p.col_panes[src.col][src.idx];
+        heights[sib] +|= p.rects[id].h;
+    }
+    const af = findPane(p, placement.above_id) orelse return;
+    const upper_h = @max(1, placement.row -| placement.above_y);
+    const lower_h = @max(1, placement.above_h -| upper_h);
+    heights[placement.above_id] = upper_h;
+    heights[id] = lower_h;
+    insert(p, af.col, af.idx + 1, id);
+    setColumnWeights(p, af.col, &heights);
+    if (source_multi and src.col < p.ncol and src.col != af.col) setColumnWeights(p, src.col, &heights);
+}
+
+pub fn setColumnWeights(p: *Pardes, col: usize, heights: *const [MAX_PANES]u16) void {
+    if (col >= p.ncol) return;
+    for (0..p.col_n[col]) |k| {
+        const pid = p.col_panes[col][k];
+        if (p.panes[pid]) |pane| pane.vweight = @floatFromInt(@max(1, heights[pid]));
+    }
+}
+
+pub fn applyRowSplit(p: *Pardes, cc: usize, k: usize, cur_y: u16) void {
+    if (k + 1 >= p.col_n[cc]) return;
+    const a = p.panes[p.col_panes[cc][k]] orelse return;
+    const b = p.panes[p.col_panes[cc][k + 1]] orelse return;
+    const ra = p.rects[p.col_panes[cc][k]];
+    const rb = p.rects[p.col_panes[cc][k + 1]];
+    const combined: f32 = @floatFromInt(ra.h + rb.h);
+    var nt: f32 = @floatFromInt(if (p.settings.tag_bottom) cur_y -| ra.y else (cur_y + 1) -| ra.y);
+    nt = std.math.clamp(nt, @as(f32, BOX_H), @max(@as(f32, BOX_H), combined - BOX_H));
+    const pair = a.vweight + b.vweight;
+    a.vweight = pair * (nt / combined);
+    b.vweight = pair - a.vweight;
+}
+
+pub fn findPane(p: *Pardes, id: usize) ?struct { col: usize, idx: usize } {
+    for (0..p.ncol) |c| {
+        for (0..p.col_n[c]) |k| {
+            if (p.col_panes[c][k] == id) return .{ .col = c, .idx = k };
+        }
+    }
+    return null;
+}
+
+pub fn insert(p: *Pardes, c: usize, idx: usize, id: usize) void {
+    var k = p.col_n[c];
+    while (k > idx) : (k -= 1) p.col_panes[c][k] = p.col_panes[c][k - 1];
+    p.col_panes[c][idx] = id;
+    p.col_n[c] += 1;
+}
+
+pub fn removePane(p: *Pardes, id: usize) void {
+    const f = findPane(p, id) orelse return;
+    const c = f.col;
+    var k = f.idx;
+    while (k + 1 < p.col_n[c]) : (k += 1) p.col_panes[c][k] = p.col_panes[c][k + 1];
+    p.col_n[c] -= 1;
+    if (p.col_n[c] == 0) {
+        if (p.ncol > 1) p.col_weight[if (c > 0) c - 1 else c + 1] +|= p.col_weight[c];
+        var j = c;
+        while (j + 1 < p.ncol) : (j += 1) {
+            p.col_panes[j] = p.col_panes[j + 1];
+            p.col_n[j] = p.col_n[j + 1];
+            p.col_weight[j] = p.col_weight[j + 1];
+        }
+        p.ncol -= 1;
+    }
+}
+
+pub fn joinCol(p: *Pardes) void {
+    const f = findPane(p, p.active) orelse return;
+    const c = f.col;
+    if (c + 1 >= p.ncol) return;
+    const dst = c + 1;
+    p.col_weight[dst] +|= p.col_weight[c];
+    for (0..p.col_n[c]) |k| p.col_panes[dst][p.col_n[dst] + k] = p.col_panes[c][k];
+    p.col_n[dst] += p.col_n[c];
+    var j = c;
+    while (j + 1 < p.ncol) : (j += 1) {
+        p.col_panes[j] = p.col_panes[j + 1];
+        p.col_n[j] = p.col_n[j + 1];
+        p.col_weight[j] = p.col_weight[j + 1];
+    }
+    p.ncol -= 1;
+}
+
+pub fn canSplitColumn(p: *Pardes, source_id: usize) bool {
+    if (p.ncol >= MAX_COLS or source_id >= MAX_PANES or p.panes[source_id] == null) return false;
+    const source = findPane(p, source_id) orelse return false;
+    // Refresh derived widths: public layout surgery may be chained between
+    // syncs, and a cached width must never admit a now-too-narrow split.
+    compute(p);
+    if (p.col_w[source.col] < config.MINW * 2) return false;
+
+    const weight = p.col_weight[source.col];
+    if (weight >= 2 and weight % 2 == 0) return true;
+    for (0..p.ncol) |column| if (p.col_weight[column] > std.math.maxInt(u64) / 2)
+        return false;
+    return weight > 0;
+}
+
+pub fn splitColumn(p: *Pardes, source_id: usize, id: usize, before: bool) bool {
+    if (id >= MAX_PANES or p.panes[id] == null) return false;
+    if (!canSplitColumn(p, source_id)) return false;
+    const source = findPane(p, source_id) orelse return false;
+    const source_col = source.col;
+    var old_weight = p.col_weight[source_col];
+    const needs_rebase = old_weight < 2 or old_weight % 2 != 0;
+    if (needs_rebase) old_weight *= 2;
+    if (id == source_id) {
+        if (p.col_n[source_col] <= 1) return false;
+        absorbVWeight(p, id);
+        removePane(p, id);
+    } else if (findPane(p, id) != null) return false;
+
+    if (needs_rebase) {
+        for (0..p.ncol) |column| p.col_weight[column] *= 2;
+    }
+    const source_weight = old_weight / 2;
+    const new_weight = old_weight - source_weight;
+    p.col_weight[source_col] = source_weight;
+    const c = source_col + @intFromBool(!before);
+    var j = p.ncol;
+    while (j > c) : (j -= 1) {
+        p.col_panes[j] = p.col_panes[j - 1];
+        p.col_n[j] = p.col_n[j - 1];
+        p.col_weight[j] = p.col_weight[j - 1];
+    }
+    p.col_weight[c] = new_weight;
+    p.col_panes[c][0] = id;
+    p.col_n[c] = 1;
+    p.ncol += 1;
+    return true;
+}
+
+pub fn snapColWeights(p: *Pardes, c: usize) void {
+    for (0..p.col_n[c]) |k| {
+        const pid = p.col_panes[c][k];
+        if (p.panes[pid]) |pp| pp.vweight = @floatFromInt(@max(1, p.rects[pid].h));
+    }
+}
+
+pub fn absorbVWeight(p: *Pardes, id: usize) void {
+    const f = findPane(p, id) orelse return;
+    if (p.col_n[f.col] <= 1) return;
+    snapColWeights(p, f.col);
+    var sib = if (f.idx > 0) p.col_panes[f.col][f.idx - 1] else p.col_panes[f.col][f.idx + 1];
+    var k = f.idx;
+    while (k > 0) : (k -= 1) {
+        sib = p.col_panes[f.col][k - 1];
+        if (p.panes[sib]) |pp| if (if (pp.file) |ff| panes.Output.fileTraits(ff.output).doc else true) break;
+    }
+    if (p.panes[sib]) |s| s.vweight += @as(f32, @floatFromInt(@max(1, p.rects[id].h)));
+}
+
+pub fn splitParent(p: *Pardes, want: usize) usize {
+    const need = 2 * BOX_H + 3;
+    if (p.rects[want].h >= need) return want;
+    if (findPane(p, want)) |f| for (0..p.col_n[f.col]) |k| {
+        if (p.rects[p.col_panes[f.col][k]].h >= need) return p.col_panes[f.col][k];
+    };
+    var tallest = want;
+    for (0..p.ncol) |c| for (0..p.col_n[c]) |k| {
+        const pid = p.col_panes[c][k];
+        if (p.rects[pid].h >= need) return pid;
+        if (p.rects[pid].h > p.rects[tallest].h) tallest = pid;
+    };
+    return tallest;
+}
+
+pub fn splitBelow(p: *Pardes, src_id: usize, nw: *Pane) void {
+    const src = p.panes[src_id] orelse return;
+    const src_h = p.rects[src_id].h;
+    const body: u16 = if (src_h > BOX_H) src_h - BOX_H else 1;
+    const cur: u16 = if (!src.isTerminal()) body / 2 else panes.Terminal.gridCursor(src).y + 1;
+    // cap keep so a content-full source still leaves the new pane a tag +
+    // a few body rows (an Alt-n from a full shell was born 0 rows tall)
+    const keep = std.math.clamp(cur, 1, @max(1, body -| (BOX_H + 3)));
+    if (findPane(p, src_id)) |f| for (0..p.col_n[f.col]) |k| {
+        const pid = p.col_panes[f.col][k];
+        if (p.panes[pid]) |pp| if (pp != nw) {
+            pp.vweight = @floatFromInt(@max(1, p.rects[pid].h));
+        };
+    };
+    src.vweight = @floatFromInt(BOX_H + keep);
+    nw.vweight = @floatFromInt(@max(1, src_h -| (BOX_H + keep)));
+    if (nw.file) |f| if (!panes.Output.fileTraits(f.output).doc) {
+        // trimmed: every row ends in a newline, and the empty line after
+        // the last one is not a result
+        const want: f32 = @floatFromInt(BOX_H + panes.File.lineCount(std.mem.trimEnd(u8, f.content, "\n")));
+        if (want < nw.vweight) {
+            src.vweight += nw.vweight - want;
+            nw.vweight = want;
+        }
+    };
+}
+
+pub fn columnFitsHalves(p: *Pardes, source_id: usize, min_cells: u16) bool {
+    const f = findPane(p, source_id) orelse return false;
+    compute(p);
+    return p.col_w[f.col] >= min_cells * 2;
+}
+
+pub fn panelBox(rect: Rect) Box {
+    return .{
+        .x = @floatFromInt(rect.x),
+        .y = @floatFromInt(rect.y),
+        .w = @floatFromInt(rect.w),
+        .h = @floatFromInt(rect.h),
+    };
+}
+
+pub fn columnBoundary(width: u16, prefix: u128, total: u128) u16 {
+    if (total == 0) return 0;
+    const pixels = (@as(u128, width) * prefix + total / 2) / total;
+    return @intCast(@min(@as(u128, width), pixels));
+}
+
+pub fn compute(p: *Pardes) void {
+    p.rects = @splat(.{});
+    if (p.ncol == 0) return;
+    var wsum: u128 = 0;
+    for (0..p.ncol) |c| wsum += p.col_weight[c];
+    if (wsum == 0) wsum = 1;
+
+    // Round cumulative boundaries so widths still sum to the available screen.
+    var prefix: u128 = 0;
+    for (0..p.ncol) |c| {
+        const last = c + 1 == p.ncol;
+        const x = columnBoundary(p.screen_w, prefix, wsum);
+        prefix += p.col_weight[c];
+        const end: u16 = if (last)
+            p.screen_w
+        else
+            columnBoundary(p.screen_w, prefix, wsum);
+        const cw = end -| x;
+        p.col_x[c] = x;
+        p.col_w[c] = cw;
+
+        var vsum: f32 = 0;
+        for (0..p.col_n[c]) |k| {
+            if (p.panes[p.col_panes[c][k]]) |pane| vsum += pane.vweight;
+        }
+        if (vsum <= 0) vsum = 1;
+
+        var y: u16 = TOPBAR_H;
+        const avail_h = p.screen_h -| TOPBAR_H;
+        for (0..p.col_n[c]) |k| {
+            const id = p.col_panes[c][k];
+            const pane = p.panes[id] orelse continue;
+            const lastk = k + 1 == p.col_n[c];
+            const fh = @as(f32, @floatFromInt(avail_h)) * pane.vweight / vsum;
+            const room = p.screen_h -| y;
+            const ch: u16 = if (lastk) room else @min(room, @max(1, @as(u16, @intFromFloat(@round(fh)))));
+            p.rects[id] = .{ .x = x, .y = y, .w = cw, .h = ch };
+            y +|= ch;
+        }
+    }
+}
+
+pub const ascii_max_movement_frames: u16 = 12;
+
+pub const Easing = enum(u8) {
+    linear,
+    smooth,
+    /// Quintic ease-in-out. It creeps at both ends and crosses the middle of
+    /// the distance fast, inside the same frame count a linear walk would use.
+    smoother,
+    in_cubic,
+    out_cubic,
+    out_back,
+};
+
+pub const Transition = enum(u8) {
+    // Numeric values are shared with the GUI shader ABI.
+    off = 0,
+    slide = 1,
+    zoom = 2,
+    dissolve = 3,
+    ascii = 4,
+    vertical = 5,
+    edges = 6,
+    fall = 7,
+    wave = 8,
+    curtain = 9,
+    scramble = 10,
+    typewriter = 11,
+
+    pub fn easing(effect: Transition) Easing {
+        return switch (effect) {
+            .off, .dissolve, .wave => .smooth,
+            .slide, .vertical, .edges => .out_cubic,
+            .zoom => .out_back,
+            // Character walks and per-cell locks read best with a slow start,
+            // a fast middle, and a slow settle over their fixed frame count.
+            .ascii, .fall, .scramble => .smoother,
+            // A sweep and a typewriter are constant-rate by definition: easing
+            // their head would make the pass visibly hesitate mid-pane.
+            .curtain, .typewriter => .linear,
+        };
+    }
+
+    pub fn frames(effect: Transition) u16 {
+        return switch (effect) {
+            .off => 0,
+            .slide => 12,
+            .zoom => 14,
+            .dissolve => 10,
+            .ascii => ascii_max_movement_frames + 1,
+            .vertical => 12,
+            .edges, .curtain, .scramble => 12,
+            // Travelling motion needs a couple more samples than a lock or a
+            // rigid slide before it stops reading as a jump.
+            .fall, .wave, .typewriter => 14,
+        };
+    }
+
+    pub fn composedByCore(effect: Transition) bool {
+        return switch (effect) {
+            .ascii, .edges, .fall, .wave, .curtain, .scramble, .typewriter => true,
+            .off, .slide, .zoom, .dissolve, .vertical => false,
+        };
+    }
+
+    pub fn needsPreviousGrid(effect: Transition) bool {
+        return effect == .dissolve or effect == .vertical or effect.composedByCore();
+    }
+
+    pub fn lifecycleOnly(effect: Transition) bool {
+        return effect == .vertical;
+    }
+};
+
+pub const SceneEffect = struct {
+    crt: bool = false,
+    ripple: bool = false,
+    glitch: bool = false,
+};
+
+pub const Phase = enum(u8) {
+    opening = 0,
+    moving = 1,
+    /// Presentation-only content whose pane lifetime has already ended.
+    /// It is never a valid input target.
+    closing = 2,
+};
+
+pub const Box = extern struct {
+    x: f32 = 0,
+    y: f32 = 0,
+    w: f32 = 0,
+    h: f32 = 0,
+
+    pub fn eql(a: Box, b: Box) bool {
+        return a.x == b.x and a.y == b.y and a.w == b.w and a.h == b.h;
+    }
+
+    // A cell belongs to a fractional box when its center lies inside.
+    pub fn contains(box: Box, col: u16, row: u16) bool {
+        const x: f32 = @floatFromInt(col);
+        const y: f32 = @floatFromInt(row);
+        return x >= box.x and x < box.x + box.w and y >= box.y and y < box.y + box.h;
+    }
+};
+
+// Drawing and hit testing share this moving/opening/closing order.
+pub fn paintOrder(live: []const ?Track, closing: []const Track, out: []Track) usize {
+    var len: usize = 0;
+    for ([_]Phase{ .moving, .opening }) |phase| for (live) |maybe| {
+        const track = maybe orelse continue;
+        if (!track.active() or track.phase != phase) continue;
+        if (len == out.len) return len;
+        out[len] = track;
+        len += 1;
+    };
+    for (closing) |track| {
+        if (!track.active()) continue;
+        if (len == out.len) return len;
+        out[len] = track;
+        len += 1;
+    }
+    return len;
+}
+
+/// One POD record is enough for every backend. `from` and `to` are logical
+/// cell boxes; frontends convert them to pixels only at their render edge.
+pub const Track = extern struct {
+    serial: u32 = 0,
+    pane: u8 = 0,
+    phase: Phase = .moving,
+    effect: Transition = .off,
+    _padding: u8 = 0,
+    frame: u16 = 0,
+    frame_count: u16 = 0,
+    from: Box = .{},
+    to: Box = .{},
+
+    pub fn active(track: Track) bool {
+        return track.effect != .off and track.frame < track.frames();
+    }
+
+    pub fn frames(track: Track) u16 {
+        return if (track.frame_count != 0) track.frame_count else track.effect.frames();
+    }
+
+    pub fn amount(track: Track) f32 {
+        // Opening rises quickly and settles; closing reverses that motion and
+        // accelerates down out of the fixed clip.
+        if (track.phase == .closing and track.effect == .vertical)
+            return progressEased(.in_cubic, track.frames(), track.frame);
+        return progressEased(track.effect.easing(), track.frames(), track.frame);
+    }
+
+    pub fn presented(track: Track) Box {
+        return lerpBox(track.from, track.to, track.amount());
+    }
+
+    pub fn visualBox(track: Track) Box {
+        return switch (track.effect) {
+            .slide, .zoom, .vertical => track.presented(),
+            .off, .dissolve => track.to,
+            // Every character effect stays inside the pane's final rectangle.
+            .ascii, .edges, .fall, .wave, .curtain, .scramble, .typewriter => track.to,
+        };
+    }
+
+    pub fn contentBox(track: Track) Box {
+        return if (track.phase == .closing) track.from else track.to;
+    }
+};
+
+pub fn openingBox(effect: Transition, target: Box, screen_width: u16) Box {
+    return switch (effect) {
+        .slide => blk: {
+            var from = target;
+            const middle = target.x + target.w * 0.5;
+            from.x = if (middle < @as(f32, @floatFromInt(screen_width)) * 0.5)
+                -target.w
+            else
+                @floatFromInt(screen_width);
+            break :blk from;
+        },
+        .zoom => .{
+            .x = target.x + target.w * 0.5,
+            .y = target.y + target.h * 0.5,
+            .w = 0,
+            .h = 0,
+        },
+        .vertical => blk: {
+            var from = target;
+            from.y += target.h;
+            break :blk from;
+        },
+        .off, .dissolve => target,
+        // Character effects own the glyphs inside a fixed rectangle, so their
+        // panel opens at exactly its final geometry.
+        .ascii, .edges, .fall, .wave, .curtain, .scramble, .typewriter => target,
+    };
+}
+
+pub fn closingBox(effect: Transition, source: Box) Box {
+    return switch (effect) {
+        .vertical => blk: {
+            var to = source;
+            to.y += source.h;
+            break :blk to;
+        },
+        else => source,
+    };
+}
+
+pub fn sample(easing: Easing, raw: f32) f32 {
+    const t = std.math.clamp(raw, 0.0, 1.0);
+    return switch (easing) {
+        .linear => t,
+        .smooth => t * t * (3.0 - 2.0 * t),
+        .smoother => t * t * t * (t * (6.0 * t - 15.0) + 10.0),
+        .in_cubic => t * t * t,
+        .out_cubic => 1.0 - (1.0 - t) * (1.0 - t) * (1.0 - t),
+        // Robert Penner's ease-out-back polynomial. It intentionally travels
+        // a little past one before settling exactly on the endpoint.
+        .out_back => blk: {
+            const c1: f32 = 1.70158;
+            const c3 = c1 + 1.0;
+            const u = t - 1.0;
+            break :blk 1.0 + c3 * u * u * u + c1 * u * u;
+        },
+    };
+}
+
+pub fn progress(effect: Transition, frame: u16) f32 {
+    return progressEased(effect.easing(), effect.frames(), frame);
+}
+
+fn progressEased(easing: Easing, frames: u16, frame: u16) f32 {
+    if (frames <= 1 or frame >= frames - 1) return 1.0;
+    return sample(easing, @as(f32, @floatFromInt(frame)) / @as(f32, @floatFromInt(frames - 1)));
+}
+
+pub fn lerpBox(from: Box, to: Box, t: f32) Box {
+    const u = @max(0.0, t);
+    return .{
+        .x = from.x + (to.x - from.x) * u,
+        .y = from.y + (to.y - from.y) * u,
+        .w = @max(0.0, from.w + (to.w - from.w) * u),
+        .h = @max(0.0, from.h + (to.h - from.h) * u),
+    };
+}
+
+/// Stable cell noise shared by the TTY reveal and shader ports. Integer-only
+/// hashing means resizing or repainting a frame does not make cells flicker.
+pub fn cellNoise(serial: u32, col: u16, row: u16) f32 {
+    var x = serial ^ (@as(u32, col) *% 0x9e37_79b9) ^ (@as(u32, row) *% 0x85eb_ca6b);
+    x ^= x >> 16;
+    x *%= 0x7feb_352d;
+    x ^= x >> 15;
+    x *%= 0x846c_a68b;
+    x ^= x >> 16;
+    return @as(f32, @floatFromInt(x & 0xffff)) / 65535.0;
+}
+
+/// Whether a changed dissolve cell has crossed from the frozen old grid to
+/// the new one. Exact endpoints are part of the presentation contract.
+pub fn dissolveRevealed(serial: u32, col: u16, row: u16, raw_progress: f32) bool {
+    const t = std.math.clamp(raw_progress, 0.0, 1.0);
+    if (t <= 0) return false;
+    if (t >= 1) return true;
+    return cellNoise(serial, col, row) < t;
+}
+
+pub const CellArea = struct {
+    x0: u16 = 0,
+    y0: u16 = 0,
+    cols: u16 = 1,
+    rows: u16 = 1,
+
+    pub fn of(box: Box) CellArea {
+        return .{
+            .x0 = floorCell(box.x),
+            .y0 = floorCell(box.y),
+            .cols = ceilCell(box.w),
+            .rows = ceilCell(box.h),
+        };
+    }
+};
+
+fn floorCell(value: f32) u16 {
+    return @intFromFloat(std.math.clamp(@floor(value), 0.0, @as(f32, std.math.maxInt(u16))));
+}
+
+fn ceilCell(value: f32) u16 {
+    return @intFromFloat(std.math.clamp(@ceil(value), 1.0, @as(f32, std.math.maxInt(u16))));
+}
+
+pub const CharSource = union(enum) {
+    /// Nothing has arrived here yet: keep the frozen old cell.
+    old,
+    at: Offset,
+    /// Paint this printable byte in the destination cell's own style, whatever
+    /// that cell holds — a caret marching over empty space is still a caret.
+    byte: u8,
+    churn: u8,
+
+    pub const Offset = struct { cols: i32 = 0, rows: i32 = 0 };
+
+    pub const settled: CharSource = .{ .at = .{} };
+};
+
+pub fn charSource(track: Track, col: u16, row: u16, area: CellArea) CharSource {
+    const t = track.amount();
+    if (t >= 1.0) return .settled;
+    const w: f32 = @floatFromInt(area.cols);
+    const h: f32 = @floatFromInt(area.rows);
+    const c: f32 = @floatFromInt(col);
+    const r: f32 = @floatFromInt(row);
+    const remaining = 1.0 - t;
+    return switch (track.effect) {
+        .edges => blk: {
+            const travel = cellsOf(remaining * (w + 1.0));
+            break :blk .{ .at = .{ .cols = if (row % 2 == 0) travel else -travel } };
+        },
+        // Columns rain down, each with its own stable head start, so the pane
+        // fills from the top and the last glyphs land at the bottom.
+        .fall => blk: {
+            const local = staggered(t, cellNoise(track.serial, col, 0) * 0.4);
+            if (local <= 0.0) break :blk .old;
+            break :blk .{ .at = .{ .rows = cellsOf((1.0 - local) * (h + 1.0)) } };
+        },
+        // A vertical ripple travels left to right and its amplitude decays, so
+        // the pane settles out of a wave instead of a fade.
+        .wave => .{ .at = .{
+            .rows = cellsOf(remaining * @min(4.0, h) * @sin(c * 0.55 - t * 9.0)),
+        } },
+        // A curtain of glyphs marches in from the right, column by column, left
+        // to right; each column still has a short slide of its own.
+        .curtain => blk: {
+            const lead = t * (w + 1.0) - c;
+            if (lead <= 0.0) break :blk .old;
+            break :blk .{ .at = .{ .cols = -cellsOf(@max(0.0, 3.0 - lead)) } };
+        },
+        // Every cell churns through printable ASCII and locks onto its final
+        // glyph at its own stable threshold: the pane resolves out of noise.
+        .scramble => blk: {
+            if (t >= cellNoise(track.serial, col, row) * 0.8) break :blk .settled;
+            const churn = cellNoise(
+                track.serial ^ (@as(u32, track.frame) *% 0x27d4_eb2f),
+                col,
+                row,
+            );
+            break :blk .{ .churn = @intCast(33 + @min(93, @as(u32, @intFromFloat(churn * 94.0)))) };
+        },
+        // Reading-order reveal with a caret sitting on the write head.
+        .typewriter => blk: {
+            const head = t * w * h;
+            const index = r * w + c;
+            if (index + 1.0 <= head) break :blk .settled;
+            if (index <= head) break :blk .{ .byte = '_' };
+            break :blk .old;
+        },
+        // PanelAscii walks its own byte distance per cell, and the geometry
+        // effects never reach this path at all.
+        .off, .slide, .zoom, .dissolve, .vertical, .ascii => .settled,
+    };
+}
+
+fn cellsOf(distance: f32) i32 {
+    return @intFromFloat(@round(std.math.clamp(distance, -65535.0, 65535.0)));
+}
+
+/// Remap track progress into one cell's own window. A stagger delays a glyph
+/// without making the effect as a whole end after its last frame.
+fn staggered(t: f32, delay: f32) f32 {
+    if (delay >= 1.0) return t;
+    return (t - delay) / (1.0 - delay);
+}
+
+test "easing presets have exact endpoints and intended shapes" {
+    inline for (std.enums.values(Easing)) |easing| {
+        try std.testing.expectEqual(@as(f32, 0), sample(easing, 0));
+        try std.testing.expectEqual(@as(f32, 1), sample(easing, 1));
+    }
+    try std.testing.expectEqual(@as(f32, 0.5), sample(.linear, 0.5));
+    try std.testing.expect(sample(.in_cubic, 0.5) < sample(.linear, 0.5));
+    try std.testing.expect(sample(.out_cubic, 0.5) > sample(.linear, 0.5));
+    try std.testing.expect(sample(.out_back, 0.8) > 1.0);
+    // Slow at both ends, fast through the middle, and symmetric about the
+    // halfway point: the same curve the integer byte walk reproduces.
+    try std.testing.expectEqual(@as(f32, 0.5), sample(.smoother, 0.5));
+    try std.testing.expect(sample(.smoother, 0.15) < sample(.smooth, 0.15));
+    try std.testing.expect(sample(.smoother, 0.85) > sample(.smooth, 0.85));
+    try std.testing.expect(sample(.smoother, 0.6) - sample(.smoother, 0.4) >
+        sample(.linear, 0.6) - sample(.linear, 0.4));
+}
+
+test "transition progress completes exactly" {
+    inline for (std.enums.values(Transition)) |effect| {
+        try std.testing.expectEqual(@as(f32, 1), progress(effect, effect.frames()));
+        if (effect.frames() > 0)
+            try std.testing.expectEqual(@as(f32, 1), progress(effect, effect.frames() - 1));
+        try std.testing.expectEqual(@as(f32, 1), progress(effect, std.math.maxInt(u16)));
+    }
+    try std.testing.expectEqual(@as(f32, 1), progress(.off, 0));
+
+    const shrinking = lerpBox(.{ .w = 100, .h = 40 }, .{}, sample(.out_back, 0.8));
+    try std.testing.expectEqual(@as(f32, 0), shrinking.w);
+    try std.testing.expectEqual(@as(f32, 0), shrinking.h);
+    const opening = lerpBox(.{}, .{ .w = 100, .h = 40 }, sample(.out_back, 0.8));
+    try std.testing.expect(opening.w > 100);
+    try std.testing.expect(opening.h > 40);
+}
+
+test "character effects are core-composed and settle on the canonical glyph" {
+    const box: Box = .{ .x = 4, .y = 2, .w = 20, .h = 6 };
+    const area: CellArea = .of(box);
+    try std.testing.expectEqual(@as(u16, 4), area.x0);
+    try std.testing.expectEqual(@as(u16, 2), area.y0);
+    try std.testing.expectEqual(@as(u16, 20), area.cols);
+    try std.testing.expectEqual(@as(u16, 6), area.rows);
+
+    inline for (std.enums.values(Transition)) |effect| {
+        if (comptime !effect.composedByCore()) continue;
+        // Core composition needs the frozen old grid for every glyph which has
+        // not arrived, so no character effect may animate without it.
+        try std.testing.expect(effect.needsPreviousGrid());
+        if (comptime effect == .ascii) continue; // owns its own per-cell byte walk
+
+        const last: Track = .{ .effect = effect, .frame = effect.frames() - 1, .to = box };
+        const first: Track = .{ .effect = effect, .frame = 0, .to = box };
+        var moving = false;
+        var row: u16 = 0;
+        while (row < area.rows) : (row += 1) {
+            var col: u16 = 0;
+            while (col < area.cols) : (col += 1) {
+                // The last active sample is the exact canonical grid: no cell
+                // is displaced, churning, or still frozen.
+                try std.testing.expectEqual(CharSource.settled, charSource(last, col, row, area));
+                if (!std.meta.eql(CharSource.settled, charSource(first, col, row, area)))
+                    moving = true;
+            }
+        }
+        try std.testing.expect(moving);
+    }
+}
+
+test "each character effect moves glyphs along its own axis" {
+    const box: Box = .{ .w = 30, .h = 8 };
+    const area: CellArea = .of(box);
+
+    // Rows alternate which screen edge they come from, and every glyph in a row
+    // travels as one rigid slide: one offset, no vertical component.
+    var edges: Track = .{ .effect = .edges, .frame = 2, .to = box };
+    const even = charSource(edges, 5, 0, area).at;
+    const odd = charSource(edges, 5, 1, area).at;
+    try std.testing.expect(even.cols > 0);
+    try std.testing.expectEqual(-even.cols, odd.cols);
+    try std.testing.expectEqual(@as(i32, 0), even.rows);
+    try std.testing.expectEqual(even, charSource(edges, 17, 0, area).at);
+    edges.frame = 5;
+    try std.testing.expect(charSource(edges, 5, 0, area).at.cols < even.cols);
+
+    // Falling columns are vertical only, staggered, and sample from below the
+    // destination because the new text is still above the pane.
+    const fall: Track = .{ .effect = .fall, .frame = 4, .to = box };
+    var falling = false;
+    var col: u16 = 0;
+    while (col < area.cols) : (col += 1) switch (charSource(fall, col, 0, area)) {
+        .old => {},
+        .byte, .churn => return error.FallShouldNotChurn,
+        .at => |offset| {
+            try std.testing.expectEqual(@as(i32, 0), offset.cols);
+            try std.testing.expect(offset.rows >= 0);
+            if (offset.rows > 0) falling = true;
+        },
+    };
+    try std.testing.expect(falling);
+
+    // The wave displaces rows both ways as it travels, and only rows.
+    const wave: Track = .{ .effect = .wave, .frame = 1, .to = box };
+    var above = false;
+    var below = false;
+    col = 0;
+    while (col < area.cols) : (col += 1) {
+        const offset = charSource(wave, col, 3, area).at;
+        try std.testing.expectEqual(@as(i32, 0), offset.cols);
+        if (offset.rows < 0) above = true;
+        if (offset.rows > 0) below = true;
+    }
+    try std.testing.expect(above and below);
+
+    // The curtain has a head: columns behind it hold the old grid, columns the
+    // head has passed are settled, and the head itself is still sliding.
+    const curtain: Track = .{ .effect = .curtain, .frame = 5, .to = box };
+    try std.testing.expectEqual(CharSource.settled, charSource(curtain, 0, 0, area));
+    try std.testing.expectEqual(CharSource{ .old = {} }, charSource(curtain, 29, 0, area));
+    var sliding = false;
+    col = 0;
+    while (col < area.cols) : (col += 1) switch (charSource(curtain, col, 0, area)) {
+        .at => |offset| if (offset.cols < 0) {
+            sliding = true;
+        },
+        .old, .byte, .churn => {},
+    };
+    try std.testing.expect(sliding);
+
+    var scramble: Track = .{ .effect = .scramble, .frame = 3, .to = box };
+    var churning: usize = 0;
+    var locked: usize = 0;
+    var changed = false;
+    col = 0;
+    while (col < area.cols) : (col += 1) switch (charSource(scramble, col, 0, area)) {
+        .churn => |byte| {
+            try std.testing.expect(byte >= ' ' and byte <= '~');
+            churning += 1;
+            scramble.frame = 4;
+            switch (charSource(scramble, col, 0, area)) {
+                .churn => |next| changed = changed or next != byte,
+                .old, .at, .byte => {},
+            }
+            scramble.frame = 3;
+        },
+        .at => locked += 1,
+        .old, .byte => return error.ScrambleShouldNotFreeze,
+    };
+    try std.testing.expect(churning > 0 and locked > 0 and changed);
+
+    // The typewriter writes in reading order with a caret on its head.
+    const typewriter: Track = .{ .effect = .typewriter, .frame = 7, .to = box };
+    try std.testing.expectEqual(CharSource.settled, charSource(typewriter, 0, 0, area));
+    try std.testing.expectEqual(
+        CharSource{ .old = {} },
+        charSource(typewriter, area.cols - 1, area.rows - 1, area),
+    );
+    var carets: usize = 0;
+    var row: u16 = 0;
+    while (row < area.rows) : (row += 1) {
+        col = 0;
+        while (col < area.cols) : (col += 1) switch (charSource(typewriter, col, row, area)) {
+            .byte => |byte| {
+                try std.testing.expectEqual(@as(u8, '_'), byte);
+                carets += 1;
+            },
+            .old, .at, .churn => {},
+        };
+    }
+    try std.testing.expectEqual(@as(usize, 1), carets);
+}
+
+test "opening presets separate geometry and content transitions" {
+    const target: Box = .{ .x = 30, .y = 2, .w = 20, .h = 8 };
+    try std.testing.expectEqual(target, openingBox(.ascii, target, 80));
+    try std.testing.expectEqual(@as(f32, 0), openingBox(.zoom, target, 80).w);
+    try std.testing.expectEqual(@as(f32, 80), openingBox(.slide, target, 80).x);
+    try std.testing.expectEqual(@as(f32, target.y + target.h), openingBox(.vertical, target, 80).y);
+    try std.testing.expectEqual(@as(f32, target.y + target.h), closingBox(.vertical, target).y);
+
+    var track: Track = .{ .effect = .slide, .from = target, .to = target };
+    try std.testing.expect(track.active());
+    track.frame = track.effect.frames();
+    try std.testing.expect(!track.active());
+
+    track = .{ .effect = .dissolve, .frame = 3, .from = .{}, .to = target };
+    try std.testing.expectEqual(target, track.visualBox());
+
+    var closing: Track = .{
+        .phase = .closing,
+        .effect = .vertical,
+        .from = target,
+        .to = closingBox(.vertical, target),
+    };
+    try std.testing.expectEqual(target, closing.contentBox());
+    closing.frame = 2;
+    try std.testing.expect(closing.amount() < progress(.vertical, closing.frame));
+}
+
+test "dissolve has exact stable endpoints" {
+    for (0..64) |col| {
+        const x: u16 = @intCast(col);
+        try std.testing.expect(!dissolveRevealed(42, x, 7, 0));
+        try std.testing.expect(dissolveRevealed(42, x, 7, 1));
+        if (dissolveRevealed(42, x, 7, 0.25))
+            try std.testing.expect(dissolveRevealed(42, x, 7, 0.75));
+    }
+}
+
+pub const Animation = struct {
+    pub const frame_ms: u32 = 16;
+    pub const frame_ns: u64 = frame_ms * std.time.ns_per_ms;
+    pub const transition_steps: u16 = 10;
+
+    pub fn Transition(comptime Value: type) type {
+        return struct {
+            const Self = @This();
+
+            from: Value,
+            to: Value,
+            displayed: Value,
+            step: u16 = transition_steps,
+
+            pub fn init(value: Value) Self {
+                return .{ .from = value, .to = value, .displayed = value };
+            }
+
+            pub fn isActive(a: *const Self) bool {
+                return a.step < transition_steps;
+            }
+
+            pub fn retarget(a: *Self, target: Value) void {
+                a.from = a.displayed;
+                a.to = target;
+                a.step = if (std.meta.eql(a.from, target)) transition_steps else 0;
+                if (a.step == transition_steps) a.displayed = target;
+            }
+
+            pub fn advance(a: *Self) void {
+                if (!a.isActive()) return;
+                a.step += 1;
+                a.displayed = if (a.step == transition_steps)
+                    a.to
+                else
+                    Value.interpolate(a.from, a.to, a.step, transition_steps);
+            }
+
+            pub fn snap(a: *Self, value: Value) void {
+                a.* = init(value);
+            }
+        };
+    }
+
+    pub fn Immediate(comptime Value: type) type {
+        return struct {
+            const Self = @This();
+
+            displayed: Value,
+
+            pub fn init(value: Value) Self {
+                return .{ .displayed = value };
+            }
+
+            pub fn isActive(_: *const Self) bool {
+                return false;
+            }
+
+            pub fn retarget(a: *Self, target: Value) void {
+                a.displayed = target;
+            }
+
+            pub fn advance(_: *Self) void {}
+
+            pub fn snap(a: *Self, value: Value) void {
+                a.displayed = value;
+            }
+        };
+    }
+
+    pub fn interpolateRgb(from: [3]u8, to: [3]u8, step: u16, steps: u16) [3]u8 {
+        if (step == 0) return from;
+        if (step >= steps) return to;
+        var out: [3]u8 = undefined;
+        for (&out, from, to) |*dst, a, b| {
+            const numerator = @as(u32, a) * (steps - step) + @as(u32, b) * step;
+            dst.* = @intCast((numerator + steps / 2) / steps);
+        }
+        return out;
+    }
+
+    const TestColor = struct {
+        rgb: [3]u8,
+
+        pub fn interpolate(from: TestColor, to: TestColor, step: u16, steps: u16) TestColor {
+            return .{ .rgb = interpolateRgb(from.rgb, to.rgb, step, steps) };
+        }
+    };
+
+    test "Immediate lands where a completed Transition lands" {
+        const from: TestColor = .{ .rgb = .{ 240, 3, 90 } };
+        const to: TestColor = .{ .rgb = .{ 5, 222, 90 } };
+
+        var faded = Animation.Transition(TestColor).init(from);
+        faded.retarget(to);
+        for (0..transition_steps) |_| faded.advance();
+
+        var instant = Immediate(TestColor).init(from);
+        try std.testing.expect(!instant.isActive());
+        instant.retarget(to);
+        try std.testing.expectEqual(faded.displayed, instant.displayed);
+
+        // Never active, so a frontend that renders only while something is animating stops immediately
+        // rather than spending ten frames discovering there is nothing to draw.
+        try std.testing.expect(!instant.isActive());
+        instant.advance();
+        try std.testing.expectEqual(to, instant.displayed);
+
+        instant.snap(from);
+        try std.testing.expectEqual(from, instant.displayed);
+    }
+
+    test "fixed-step interpolation has exact monotonic endpoints" {
+        const Tween = Animation.Transition(TestColor);
+        const from: TestColor = .{ .rgb = .{ 240, 3, 90 } };
+        const to: TestColor = .{ .rgb = .{ 5, 222, 90 } };
+        var tween = Tween.init(from);
+        tween.retarget(to);
+        try std.testing.expectEqual(from, tween.displayed);
+
+        var previous = tween.displayed;
+        for (0..transition_steps) |_| {
+            tween.advance();
+            try std.testing.expect(tween.displayed.rgb[0] <= previous.rgb[0]);
+            try std.testing.expect(tween.displayed.rgb[1] >= previous.rgb[1]);
+            try std.testing.expectEqual(@as(u8, 90), tween.displayed.rgb[2]);
+            previous = tween.displayed;
+        }
+        try std.testing.expect(!tween.isActive());
+        try std.testing.expectEqual(to, tween.displayed);
+        tween.advance();
+        try std.testing.expectEqual(to, tween.displayed);
+    }
+
+    test "retarget starts at the currently displayed value" {
+        const Tween = Animation.Transition(TestColor);
+        const first: TestColor = .{ .rgb = .{ 0, 40, 200 } };
+        const second: TestColor = .{ .rgb = .{ 200, 140, 0 } };
+        const third: TestColor = .{ .rgb = .{ 20, 10, 250 } };
+        var tween = Tween.init(first);
+        tween.retarget(second);
+        tween.advance();
+        tween.advance();
+        tween.advance();
+        const on_screen = tween.displayed;
+
+        tween.retarget(third);
+        try std.testing.expectEqual(on_screen, tween.from);
+        try std.testing.expectEqual(on_screen, tween.displayed);
+        try std.testing.expect(tween.isActive());
+        for (0..transition_steps) |_| tween.advance();
+        try std.testing.expectEqual(third, tween.displayed);
+    }
+};