day-core 0.6.2

// Copyright © The Daybrite Project
// SPDX-License-Identifier: MPL-2.0

//! day-core: the Piece model, realized tree, mounter, layout engine, and event routing
//! (DESIGN.md §5, §7). Build-once: pieces are constructed exactly once; all dynamism flows
//! through reactive bindings (day-reactive) writing to the thread-local tree.

/// The linked Day runtime version, for diagnostics from independently versioned extensions.
pub const VERSION: &str = env!("CARGO_PKG_VERSION");

day_reactive::tls_root! {
    layout: crate::layout::TlsGroupSlots,
    ambient: crate::ambient::TlsGroupSlots,
    anim: crate::anim::TlsGroupSlots,
    conformance: crate::conformance::TlsGroupSlots,
    frame: crate::frame::TlsGroupSlots,
    image: crate::image::TlsGroupSlots,
    list: crate::list::TlsGroupSlots,
    lifecycle: crate::lifecycle::TlsGroupSlots,
    menu: crate::menu::TlsGroupSlots,
    nav: crate::nav::TlsGroupSlots,
    present: crate::present::TlsGroupSlots,
    root: crate::TlsGroupSlots,
    shield: crate::shield::TlsGroupSlots,
    shortcuts: crate::shortcuts::TlsGroupSlots,
    status: crate::status::TlsGroupSlots,
    toolbar: crate::toolbar::TlsGroupSlots,
    documents: crate::documents::TlsGroupSlots,
    urls: crate::urls::TlsGroupSlots,
    tree: crate::tree::TlsGroupSlots,
    windows: crate::windows::TlsGroupSlots,
}

/// The blocking pool (docs/async.md): `spawn_blocking` runs a job on a worker thread and
/// resolves on the awaiting side; std-only, so it lives in day-async and is re-exported here.
pub use day_async::{
    Blocking, BlockingError, blocking_inline, blocking_limit, blocking_threads, spawn_blocking,
};
#[cfg(target_arch = "wasm32")]
#[doc(hidden)]
pub use day_macros::retain_element_id as __retain_element_id;
/// Re-exported so every crate above day-core can reach it without taking a direct
/// day-reactive dependency (see day-reactive for what it does and why Android needs it).
pub use day_reactive::tls_group;
#[cfg(target_arch = "wasm32")]
#[doc(hidden)]
pub use inventory;
/// The link-time registry crate `#[day::test]` expands through (docs/testing.md).
pub use linkme;

mod ambient;
mod anim;
mod build;
pub mod conformance;
pub mod element_id;
pub mod frame;
pub use element_id::{ElementId, IntoElementKey};
mod frames;
pub mod image;
mod layout;
pub mod lifecycle;
pub mod list;
mod memory_profile;
mod modal_sheet;
pub use modal_sheet::ComposedSheet;
pub mod menu;
mod nav;
mod present;
pub mod shield;
pub mod shortcuts;
pub mod status;
pub mod testing;
pub mod toolbar;
mod tree;
pub mod tree_driver;
pub mod tween;
pub mod windows;

pub use ambient::{
    Ambient, app_environment, environment, focused_environment, override_size_class, reset_ambient,
    restore_reported_size_class, safe_area, seed_safe_area, set_safe_area, set_size_class,
    set_window_safe_area, set_window_size_class, size_class, window_safe_area, window_size_class,
    window_size_class_untracked, with_environment,
};
pub use anim::{current_anim, with_animation};
pub use build::*;
pub use frame::{
    FrameConsumer, add_frame_consumer, frame_consumer_count, install_frame_requester,
    remove_frame_consumer,
};
pub use image::{
    Bitmap, CanvasExports, RenderBitmap, RenderPdf, canvas_exports, decode as decode_image,
    decode_async as decode_image_async, image_decode_support, image_encode_formats,
    image_encode_support, render_bitmap, render_bitmap_async, render_pdf, render_pdf_async,
    resolve_bitmap_render, resolve_image_decode, resolve_image_encode, resolve_pdf_render,
};
pub use layout::*;
pub use lifecycle::{dispatch_lifecycle, lifecycle_supported, on_lifecycle};
pub use list::{
    BuiltRow, ListDeleteDriver, ListDriver, ListReorderDriver, ListSectionsDriver, ListSwipeDriver,
    install_list, list_reload, list_scroll_to_end, list_scroll_to_row, list_set_editing,
    list_set_selected, list_splice, list_try_delete, list_try_reorder, list_try_swipe,
    set_scripted_reorder,
};
pub use menu::{
    dispatch_menu_action, has_app_menu, register_menu_action, register_scoped_menu_action,
    set_app_menu, set_dock_menu,
};
pub use nav::*;
pub use shortcuts::set_launcher_shortcuts;
pub use status::{
    KeepRunning, remove_status_item, set_app_progress, set_dock_visible, set_keep_running,
    set_status_item, status_items,
};
pub mod documents;
#[doc(hidden)]
pub use documents::{forward_launch_args, launch_link, take_launch_files};
mod urls;
pub use documents::{on_open_files, request_open_files};
pub use present::*;
pub use toolbar::{
    Chrome, chrome_changed, current_chrome, current_page_column, current_page_gate,
    current_page_window, dispatch_toolbar_value, patch_chrome, patch_toolbar,
    register_contribution, register_contribution_gated, register_toolbar_value,
    relabel_contribution, toggle_sidebar, unregister_contribution, update_contribution,
    window_being_built, with_page, with_page_gated, with_page_in,
};
pub use tree_driver::{
    TreeBuiltRow, TreeDriver, TreeMovesDriver, install_tree, tree_driver, tree_reload, tree_reveal,
    tree_set_expanded, tree_set_selected, tree_try_move, tree_visible_rows,
};
pub use urls::{on_open_url, request_open_url};
// The resource types live in day-spec (backends depend only on day-spec); re-exported for the
// facade.
pub use day_spec::resource::{
    AssetDir, AssetName, FontFamily, ImageName, Resource, ResourceOpener, VectorName, resource,
    set_resource_opener,
};
pub use tree::*;
pub use windows::{
    WindowHandle, about_action_id, check_for_updates, check_for_updates_action_id, current_window,
    finish_window_open, focused_scope, focused_window, initial_window, launch_options, monitors,
    open_about, open_new_window, open_new_window_for_group, open_preferences, open_window, quit,
    register_about, register_about_window, register_check_for_updates, register_new_window,
    register_new_window_for, register_preferences, register_preferences_with, window_by_key,
    window_title,
};

/// The app-wide layout direction (docs/localization): mirrors every horizontal placement in
/// the place pass when [`day_geometry::LayoutDirection::Rtl`]. Resolved lazily from the
/// `DAY_LOCALE` launch environment (so toolkits can read it before any UI exists);
/// `set_layout_direction` (called by `install_locales` for the resolved locale) overrides.
/// Fixed for the life of the process; switching locale at runtime does not re-mirror.
pub fn layout_direction() -> day_geometry::LayoutDirection {
    DIRECTION.with(|d| {
        if let Some(dir) = d.get() {
            return dir;
        }
        let dir = std::env::var("DAY_LOCALE")
            .map(|l| direction_of_locale(&l))
            .unwrap_or_default();
        d.set(Some(dir));
        dir
    })
}

/// Override the layout direction (normally from `install_locales`). Must be called before the
/// first layout pass to take effect everywhere.
pub fn set_layout_direction(dir: day_geometry::LayoutDirection) {
    DIRECTION.with(|d| d.set(Some(dir)));
}

/// Whether the app is being rendered right-to-left (docs/localization), a convenience over
/// [`layout_direction`]. The layout engine already mirrors widget *placement* under an RTL locale,
/// but a `canvas` draws in its own coordinate space, so a custom drawing that has a reading
/// direction (a battery that drains one way, an arrow, a progress sweep) can call this to mirror
/// itself. Fixed for the life of the process, like [`layout_direction`].
pub fn is_rtl() -> bool {
    layout_direction() == day_geometry::LayoutDirection::Rtl
}

/// The writing direction a locale implies (language subtag match).
pub fn direction_of_locale(locale: &str) -> day_geometry::LayoutDirection {
    let lang = locale
        .split(['-', '_'])
        .next()
        .unwrap_or("")
        .to_ascii_lowercase();
    match lang.as_str() {
        "ar" | "he" | "iw" | "fa" | "ur" | "ps" | "sd" | "ug" | "yi" | "dv" | "ku" => {
            day_geometry::LayoutDirection::Rtl
        }
        _ => day_geometry::LayoutDirection::Ltr,
    }
}

day_reactive::tls_slots! {
    root;
    static SEARCH_HOSTS: std::cell::RefCell<Vec<(RNode, RNode)>> =
        const { std::cell::RefCell::new(Vec::new()) };
    static DIRECTION: std::cell::Cell<Option<day_geometry::LayoutDirection>> =
        const { std::cell::Cell::new(None) };

    static UNDO_INVOKE: std::cell::RefCell<Option<UndoInvoke>> =
        const { std::cell::RefCell::new(None) };

    /// The dayscript executor's stand-in for held modifiers (a synthetic tap cannot hold a
    /// real shift key); `None` = ask the toolkit.
    static MODIFIER_OVERRIDE: std::cell::Cell<Option<day_spec::Modifiers>> =
        const { std::cell::Cell::new(None) };

    static EDIT_INVOKE: std::cell::RefCell<Option<EditInvoke>> =
        const { std::cell::RefCell::new(None) };

    static UNDO_ACTION_IDS: std::cell::Cell<(u64, u64)> = const { std::cell::Cell::new((0, 0)) };
    static EDIT_ACTION_IDS: std::cell::Cell<(u64, u64, u64, u64)> =
        const { std::cell::Cell::new((0, 0, 0, 0)) };

    static DARK_SIGNAL: std::cell::OnceCell<day_reactive::Signal<bool>> =
        const { std::cell::OnceCell::new() };

    /// The system's reduce-motion answer as last read (docs/accessibility.md), kept beside the
    /// signal so the animation gates can ask without a tracked read or a tree borrow: they run
    /// inside the tree.
    static REDUCE_MOTION: std::cell::Cell<bool> = const { std::cell::Cell::new(false) };
    static REDUCE_MOTION_SIGNAL: std::cell::OnceCell<day_reactive::Signal<bool>> =
        const { std::cell::OnceCell::new() };

    static FONT_FAMILIES: std::cell::OnceCell<std::rc::Rc<[day_spec::FontFamilyInfo]>> =
        const { std::cell::OnceCell::new() };

    static TEXT_METRICS: std::cell::RefCell<TextMetricsCache> =
        std::cell::RefCell::new(TextMetricsCache::new());
}

use day_spec::{Platform, WindowOptions};

// ---- crash observation (§8.5) --------------------------------------------------------------

/// Observer called after day-core contains a panic at one of its trampoline boundaries
/// (`contain_posted_panic` here, `tree::pump_events`), on the panicking thread, after the
/// reactive-runtime reset. A crash reporter (day-piece-crash-reporter, docs/break.md) registers one to
/// downgrade the report its panic hook just wrote: the panic was caught, the process is not
/// dying. A plain `fn` (no closure) so the containment path allocates nothing.
static CONTAINED_PANIC_OBSERVER: std::sync::OnceLock<fn()> = std::sync::OnceLock::new();

/// Register the contained-panic observer. First registration wins; later calls are no-ops
/// (there is one crash reporter per process).
pub fn set_contained_panic_observer(f: fn()) {
    let _ = CONTAINED_PANIC_OBSERVER.set(f);
}

pub(crate) fn notify_contained_panic() {
    if let Some(f) = CONTAINED_PANIC_OBSERVER.get() {
        f();
    }
}

/// The compile-time target key of the running backend (`"macos-appkit"`, `"ios-uikit"`, …,
/// the `Platform::TARGET` string), recorded by [`launch_with`]. `None` before launch.
pub fn backend_name() -> Option<&'static str> {
    BACKEND_NAME.get().copied()
}

static BACKEND_NAME: std::sync::OnceLock<&'static str> = std::sync::OnceLock::new();

/// The toolkit key of the running backend (`"appkit"`, `"gtk"`, …, the `Platform::TOOLKIT`
/// string), recorded by [`launch_with`]. `None` before launch.
pub fn toolkit_key() -> Option<&'static str> {
    TOOLKIT_KEY.get().copied()
}

static TOOLKIT_KEY: std::sync::OnceLock<&'static str> = std::sync::OnceLock::new();

/// The development tag every window title carries in a debug build:
/// `(<version>/<toolkit>[/<script>])`, for example `(1.1.0/appkit)`, or
/// `(1.1.0/gtk/walkthrough.yaml)` while a dayscript is driving. With several apps, toolkits and
/// scripted runs open at once, the title bar is the only place that says which window is which.
///
/// `None` in a release build (this is a development aid and must never ship), and before
/// [`launch_with`] has named the backend. The version and the script name come from
/// `DAY_APP_VERSION` and `DAY_SCRIPT`, which the `day` CLI sets on every launch; run the binary
/// some other way and the tag carries the parts it knows.
pub fn debug_title_tag() -> Option<String> {
    if !cfg!(debug_assertions) {
        return None;
    }
    let toolkit = toolkit_key()?;
    // The mock backend has no window and no title bar, so there is nothing for a tag to
    // disambiguate; it would only corrupt what a headless test asserts about a title.
    if toolkit == "mock" {
        return None;
    }
    let mut parts = Vec::new();
    if let Ok(v) = std::env::var("DAY_APP_VERSION")
        && !v.is_empty()
    {
        parts.push(v);
    }
    parts.push(toolkit.to_string());
    if let Ok(s) = std::env::var("DAY_SCRIPT")
        && !s.is_empty()
    {
        parts.push(s);
    }
    Some(format!("({})", parts.join("/")))
}

/// Append [`debug_title_tag`] to a window title. Every title day sets goes through here: the
/// primary window's, each secondary window's, and every [`crate::windows::WindowHandle::set_title`].
///
/// An empty title stays empty: a window the app left untitled should not grow a title bar full
/// of build metadata. An already-tagged title is left alone, since the same window can be
/// retitled repeatedly.
pub(crate) fn decorate_window_title(title: &str) -> String {
    tag_title(title, debug_title_tag().as_deref())
}

/// The join rule, split out from the environment so it can be tested.
fn tag_title(title: &str, tag: Option<&str>) -> String {
    match tag {
        Some(tag) if !title.is_empty() && !title.ends_with(tag) => format!("{title} {tag}"),
        _ => title.to_string(),
    }
}

#[cfg(test)]
mod title_tag_tests {
    use super::tag_title;

    #[test]
    fn the_tag_is_appended_once_and_never_to_an_empty_title() {
        assert_eq!(
            tag_title("Day Sheets", Some("(0.1.0/appkit)")),
            "Day Sheets (0.1.0/appkit)"
        );
        // A release build has no tag, so the title is the app's own, untouched.
        assert_eq!(tag_title("Day Sheets", None), "Day Sheets");
        // An untitled window stays untitled rather than growing a bar of build metadata.
        assert_eq!(tag_title("", Some("(0.1.0/appkit)")), "");
        // Retitling an already-tagged window must not stack tags.
        assert_eq!(
            tag_title("Day Sheets (0.1.0/appkit)", Some("(0.1.0/appkit)")),
            "Day Sheets (0.1.0/appkit)"
        );
    }
}

// ---- logging (docs/logging.md) ---------------------------------------------------------------

/// Where a formatted line goes. Unset means the process's own stderr, which is right on every
/// native target: a terminal on the desktop, Xcode's console on Apple, and logcat on Android
/// (day-toolkit-material `dup2`s fd 2 into it). `wasm32-unknown-unknown` is the one target where that is
/// wrong rather than merely different (std's stdio there accepts the bytes and drops them, so
/// every line vanishes), and day-toolkit-dom installs a sink reaching the browser console instead.
static LOG_SINK: std::sync::OnceLock<fn(log::Level, &str)> = std::sync::OnceLock::new();

/// Install the line sink. First registration wins (one host per process). Called before launch;
/// `day::web::start` does it for the browser.
pub fn set_log_sink(f: fn(log::Level, &str)) {
    let _ = LOG_SINK.set(f);
}

/// The width the level column is padded to. Shared by the formatter and the coloring below, which
/// splits the finished line at exactly this offset.
const LEVEL_WIDTH: usize = 5;

/// The terminal color for a level: `env_logger`'s palette, so a Day app's output reads like any
/// other Rust app's, and the same one `day-cli` uses when it re-emits these lines under
/// `day launch` (day-cli `src/term.rs`).
#[cfg(not(target_arch = "wasm32"))]
fn level_style(level: log::Level) -> anstyle::Style {
    let color = match level {
        log::Level::Error => anstyle::AnsiColor::Red,
        log::Level::Warn => anstyle::AnsiColor::Yellow,
        log::Level::Info => anstyle::AnsiColor::Green,
        log::Level::Debug => anstyle::AnsiColor::Blue,
        log::Level::Trace => anstyle::AnsiColor::Cyan,
    };
    anstyle::Style::new().fg_color(Some(anstyle::Color::Ansi(color)))
}

/// Write one already-formatted line to stderr, never panicking. That matters because the
/// `*println!` macros panic on a failed write, a closed stderr pipe is routine when `day launch`
/// tears the app down, and a panic raised inside a native trampoline aborts the process.
///
/// Only the level column is colored: a fully colored line is noise at `INFO`, and leaving the
/// message plain keeps it greppable. `anstream` decides whether the escapes survive: they are
/// stripped when stderr is not a color terminal, which covers the pipe `day launch` gives us
/// (day-cli re-colors from its own end), logcat, Xcode's console, `NO_COLOR`, and `TERM=dumb`.
#[cfg(not(target_arch = "wasm32"))]
fn write_line(level: log::Level, line: &str) {
    use std::io::Write as _;
    let style = level_style(level);
    // The column is ASCII and padded to `LEVEL_WIDTH` by the caller, so this never lands inside a
    // character; `split_at_checked` keeps even a malformed line from panicking.
    let (lvl, rest) = line.split_at_checked(LEVEL_WIDTH).unwrap_or((line, ""));
    let _ = writeln!(anstream::stderr(), "{style}{lvl}{style:#}{rest}");
}

/// The wasm fallback, used only if no sink was installed; `day::web::start` installs one before
/// an app's first line. std's stdio on `wasm32-unknown-unknown` accepts these bytes and drops
/// them, so this writes into nothing; it exists so the logger has a total function.
#[cfg(target_arch = "wasm32")]
fn write_line(_level: log::Level, line: &str) {
    use std::io::Write as _;
    let _ = writeln!(std::io::stderr(), "{line}");
}

/// Day's default [`log::Log`].
///
/// Not `env_logger` or another off-the-shelf backend, for the reason above: they print through
/// `*println!`, and a panic on that path is an abort rather than a lost line. An app that wants
/// one installs it itself; see [`init_logging`].
struct DayLogger;

impl log::Log for DayLogger {
    fn enabled(&self, metadata: &log::Metadata<'_>) -> bool {
        // The level filter is `log::set_max_level`'s job, checked by the macros before they even
        // format; answering `true` here keeps `log_enabled!` consistent with them for anything
        // that asks.
        metadata.level() <= log::max_level()
    }

    fn log(&self, record: &log::Record<'_>) {
        if !self.enabled(record.metadata()) {
            return;
        }
        // `WARN day_toolkit_gtk: could not register bundled font …`: the level first so a scan down
        // the column finds problems, then the emitting crate, which is what says which backend
        // or piece is unhappy. The old hand-written `day: ` prefixes said only that it was us.
        let sink = LOG_SINK.get();
        // Formatted plain: a sink is a structured destination (the browser console, a test
        // collector, an app's own transport) and wants the text, not terminal escapes. Color is
        // `write_line`'s business, applied only on the way to a real stderr.
        let line = format!(
            "{:<LEVEL_WIDTH$} {}: {}",
            record.level(),
            record.target(),
            record.args()
        );
        match sink {
            Some(f) => f(record.level(), &line),
            None => write_line(record.level(), &line),
        }
    }

    fn flush(&self) {}
}

/// Install Day's default logger and level, unless the app already installed its own.
///
/// Called from [`launch_with`], so logging works with no ceremony at all: an app writes
/// `log::info!(…)` (or `day::info!`, the same macro re-exported through the prelude) and the line
/// comes out on every platform. `set_logger` answers `Err` when a logger is already registered;
/// that is all the customization there is, and why the error is ignored: an app that
/// called `env_logger::init()` (or installed `tracing`, or its own `log::Log`) before
/// `day::launch` keeps it, and Day does not fight for the slot.
///
/// The default level is `Debug` in a debug build and `Info` in a release one; `DAY_LOG` overrides
/// it with a level name (`off`/`error`/`warn`/`info`/`debug`/`trace`). Reading an environment
/// variable is a native-only affordance; on the web there is no process environment, so the
/// launch path sets the level explicitly from the page's query string instead.
pub fn init_logging() {
    let level = std::env::var("DAY_LOG")
        .ok()
        .and_then(|v| v.parse::<log::LevelFilter>().ok())
        .unwrap_or(if cfg!(debug_assertions) {
            log::LevelFilter::Debug
        } else {
            log::LevelFilter::Info
        });
    set_log_level(level);
    // Err = the app got there first, which is exactly the intended escape hatch.
    let _ = log::set_logger(&DayLogger);
}

/// Set the maximum level that will be emitted. Separate from [`init_logging`] because the web
/// launch path has to supply the level from the page URL rather than the environment, and because
/// an app may want to raise or lower it at runtime.
pub fn set_log_level(level: log::LevelFilter) {
    log::set_max_level(level);
}

/// Run a posted main-thread task, containing any panic (the `pump_events` twin for the poster /
/// scheduler doors): log the cause and reset the reactive runtime so the app keeps running
/// (degraded) instead of aborting across the native trampoline's non-unwind boundary.
pub(crate) fn contain_posted_panic(f: Box<dyn FnOnce() + Send>) {
    if let Err(payload) = std::panic::catch_unwind(std::panic::AssertUnwindSafe(f)) {
        let msg = payload
            .downcast_ref::<&str>()
            .map(|s| (*s).to_string())
            .or_else(|| payload.downcast_ref::<String>().cloned())
            .unwrap_or_else(|| "unknown panic".to_string());
        log::warn!(
            "a posted main-thread task panicked and was contained — the app continues, but \
             reactive/UI state may be inconsistent until the next interaction. Cause: {msg}"
        );
        day_reactive::recover_from_panic();
        notify_contained_panic();
    }
}

/// The app's undo/redo entry point as the platform front calls it; `true` means redo.
/// `Rc` because the invocation clones it out of the cell before running, so the borrow is
/// released before app code (which may install a new bridge) gets control.
type UndoInvoke = std::rc::Rc<dyn Fn(bool)>;

/// Wire an undo history to the platform (docs/model.md): the four signals mirror into the
/// toolkit's native front where one exists (`Cap::UndoBridge`: the stock Edit menu retitles
/// and enables itself, the platform's gestures land), and every invocation the platform
/// delivers comes back through `on_invoke(redo)`. On a toolkit without a native undo system
/// the state goes nowhere and the app's own affordances call the stack directly; installing
/// the bridge is still harmless. Call once, after launch; installing again replaces the wiring.
pub fn install_undo_bridge(
    can_undo: day_reactive::Signal<bool>,
    can_redo: day_reactive::Signal<bool>,
    undo_label: day_reactive::Signal<String>,
    redo_label: day_reactive::Signal<String>,
    on_invoke: impl Fn(bool) + 'static,
) {
    UNDO_INVOKE.with(|u| *u.borrow_mut() = Some(std::rc::Rc::new(on_invoke)));
    day_reactive::bind(
        move || day_spec::UndoState {
            can_undo: can_undo.get(),
            can_redo: can_redo.get(),
            undo_label: undo_label.get(),
            redo_label: redo_label.get(),
        },
        |state: &day_spec::UndoState| {
            let state = state.clone();
            with_tree(|t| t.set_undo_state(&state));
        },
    );
}

/// The standing dispatch id behind a `MenuRole::Cut`/`Copy`/`Paste` item on a toolkit whose
/// role items come back as plain menu actions (docs/menus.md): the closure invokes the
/// installed edit bridge. Durable across menu installs, like the undo pair.
pub fn edit_action_id(op: day_spec::EditOp) -> u64 {
    use day_spec::EditOp as E;
    let (c, y, p, a) = EDIT_ACTION_IDS.with(|s| s.get());
    let have = match op {
        E::Cut => c,
        E::Copy => y,
        E::Paste => p,
        E::SelectAll => a,
    };
    if have != 0 {
        return have;
    }
    let id = menu::register_menu_action(std::rc::Rc::new(move || dispatch_edit_invoke(op)));
    EDIT_ACTION_IDS.with(|s| {
        let (c0, y0, p0, a0) = s.get();
        s.set(match op {
            E::Cut => (id, y0, p0, a0),
            E::Copy => (c0, id, p0, a0),
            E::Paste => (c0, y0, id, a0),
            E::SelectAll => (c0, y0, p0, id),
        });
    });
    id
}

fn dispatch_undo_invoke(redo: bool) {
    let _ = invoke_undo(redo);
}

/// Invoke the installed undo bridge (the same handler a native front reaches: ⌘Z, the Edit
/// menu, a three-finger swipe), returning whether one is installed. The dayscript
/// `undo:`/`redo:` steps drive history through this, so a walkthrough needs no app-provided
/// undo button on any target.
pub fn invoke_undo(redo: bool) -> bool {
    let f = UNDO_INVOKE.with(|u| u.borrow().clone());
    match f {
        Some(f) => {
            day_reactive::batch(|| f(redo));
            true
        }
        None => false,
    }
}

type EditInvoke = std::rc::Rc<dyn Fn(day_spec::EditOp)>;

/// The keyboard modifiers held right now, for interactions whose meaning they change
/// (shift-click adds to a selection). Touch backends answer all-false; a dayscript step's
/// declared modifiers take precedence while it dispatches.
pub fn modifiers() -> day_spec::Modifiers {
    if let Some(m) = MODIFIER_OVERRIDE.with(|o| o.get()) {
        return m;
    }
    with_tree(|t| t.modifiers())
}

/// Scoped modifier stand-in for the dayscript executor: `Some` while a step with declared
/// modifiers dispatches, back to `None` after.
pub fn set_modifier_override(m: Option<day_spec::Modifiers>) {
    MODIFIER_OVERRIDE.with(|o| o.set(m));
}

/// Wire the app's standard-edit handlers to the platform (docs/menus.md): `state` is a
/// tracked read whose value mirrors into the toolkit (`Cap::EditBridge`: native menu
/// validation enables the stock Cut/Copy/Paste exactly as it does for text widgets), and
/// every invocation a platform route delivers comes back through `on_invoke`. On a toolkit
/// with no native route, the `menu_role(Cut/Copy/Paste)` items dispatch here instead (the
/// same standing-id fallback the undo pair uses). Call once, after launch; installing again
/// replaces the wiring. Most apps want [`day::install_edit_commands`](/api/day/fn.install_edit_commands.html), which adds the
/// clipboard transport.
pub fn install_edit_bridge(
    state: impl Fn() -> day_spec::EditState + 'static,
    on_invoke: impl Fn(day_spec::EditOp) + 'static,
) {
    EDIT_INVOKE.with(|u| *u.borrow_mut() = Some(std::rc::Rc::new(on_invoke)));
    day_reactive::bind(state, |state: &day_spec::EditState| {
        let state = *state;
        with_tree(|t| t.set_edit_state(&state));
    });
}

fn dispatch_edit_invoke(op: day_spec::EditOp) {
    let _ = invoke_edit(op);
}

/// Invoke the installed edit bridge (the same handler the platform's Cut/Copy/Paste
/// route reaches, clipboard transport included), returning whether one is installed. An
/// app's own affordances (a context menu's Cut/Copy) call this rather than duplicating the
/// clipboard plumbing (docs/menus.md).
pub fn invoke_edit(op: day_spec::EditOp) -> bool {
    let f = EDIT_INVOKE.with(|u| u.borrow().clone());
    match f {
        Some(f) => {
            day_reactive::batch(|| f(op));
            true
        }
        None => false,
    }
}

/// The standing dispatch id behind a `MenuRole::Undo`/`Redo` item on a toolkit with no native
/// undo responder (docs/menus.md): activating the item comes back as a plain menu action, and
/// this id's closure invokes the installed undo bridge. Registered once and durable across
/// menu installs, like the preferences id. Toolkits with a native undo system (appkit) keep
/// their responder-chain selector instead, so a focused text field's own undo stays ahead of
/// the app stack there.
pub fn undo_action_id(redo: bool) -> u64 {
    let (u, r) = UNDO_ACTION_IDS.with(|c| c.get());
    let have = if redo { r } else { u };
    if have != 0 {
        return have;
    }
    let id = menu::register_menu_action(std::rc::Rc::new(move || dispatch_undo_invoke(redo)));
    UNDO_ACTION_IDS.with(|c| {
        let (u0, r0) = c.get();
        c.set(if redo { (u0, id) } else { (id, r0) });
    });
    id
}

/// A runtime route request from the backend (`Event::RouteRequested`: web-dom's URL hash
/// changing via browser back/forward or a hand-edited hash). Echoes of our own `set_route`
/// match the current route and are dropped.
fn handle_route_request(route: &str) {
    nav::apply_route_request(route);
}

/// Launch a Day app on the given platform backend: sets up the reactive scheduler and the
/// cross-thread poster, mounts the root piece into the window's content container, runs the
/// initial layout, and installs the turn-end layout callback (§3.3). The backend then owns
/// the native main loop.
pub fn launch_with<P: Platform>(
    backend: P,
    mut options: WindowOptions,
    root_piece: impl FnOnce() -> AnyPiece + 'static,
) {
    // Before anything else that might want to report: from here on `log::warn!` and friends come
    // out, on every backend, with no app-side setup. An app that installed its own logger first
    // keeps it (docs/logging.md).
    init_logging();
    // Record the backend identity for runtime introspection (crash reports, diagnostics).
    let _ = BACKEND_NAME.set(P::TARGET);
    let _ = TOOLKIT_KEY.set(P::TOOLKIT);
    // Tag the window title with version/toolkit/script in debug builds. Pin the app's display
    // name to the undecorated title first: backends fall back to `title` for the macOS App menu
    // and the About panel, which must keep reading "Day Sheets", not "Day Sheets (0.1.0/appkit)".
    if options.app_name.is_none() && !options.title.is_empty() {
        options.app_name = Some(options.title.clone());
    }
    options.title = decorate_window_title(&options.title);
    // `DAY_WINDOW=900x700` overrides the app's initial window size, for responsive-layout testing
    // (scripted runs can exercise a narrow window without a resize gesture). Desktop only in
    // effect; mobile/web backends size to the screen and ignore `options.size` anyway.
    if let Ok(v) = std::env::var("DAY_WINDOW")
        && let Some((w, h)) = v.split_once('x')
        && let (Ok(w), Ok(h)) = (w.trim().parse::<f64>(), h.trim().parse::<f64>())
        && w > 0.0
        && h > 0.0
    {
        options.size = day_spec::Size::new(w, h);
    }
    // Remember what the app asked for, so File ▸ New Window can open another window of the same
    // app rather than an untitled one the platform cannot list (docs/windows.md).
    windows::set_launch_options(&options);
    // Reactive plumbing rides the platform's main-loop poster. Both doors contain panics (the
    // `pump_events` rationale, tree.rs): posted closures run inside native main-loop trampolines
    // (a glib idle, a GCD block) that abort the process on unwind (`panic_cannot_unwind`), so a
    // panic in a `Setter` write's drain or a scheduled `flush_sync` would SIGABRT the app instead
    // of surfacing. Contain at this single backend-agnostic boundary and reset the runtime.
    // Backend FFI trampolines (JNI up-calls, C callbacks, posted closures) contain panics
    // through day-spec's `ffi_guard`; hand it the recovery hook here (the one layer that
    // knows day-reactive) so a contained panic can't strand the observer stack or leave a
    // half-open batch behind. Each door also defers its closure when the tree is borrowed
    // (`tree::run_posted_or_defer`): a backend pumping its native loop from inside a tree
    // method would otherwise run the closure on the borrowed tree.
    day_spec::ffi_guard::set_recovery(day_reactive::recover_from_panic);
    day_reactive::install_main_poster(|f| {
        P::post(Box::new(move || tree::run_posted_or_defer(f)));
    });
    // The timer door (docs/async.md `day::sleep`): same containment and deferral as the poster.
    day_reactive::install_delayed_poster(|ms, f| {
        P::post_delayed(ms, Box::new(move || tree::run_posted_or_defer(f)));
    });
    day_reactive::install_scheduler(|| {
        P::post(Box::new(|| {
            tree::run_posted_or_defer(Box::new(day_reactive::flush_sync));
        }))
    });
    // The async-spawn door (docs/async.md): day-reactive's `Resource` runs its fetch futures on
    // this executor; the returned closure aborts (a no-op once the task completed, the contract
    // Resource's eager-poll ordering relies on).
    day_reactive::install_spawner(|fut| {
        let handle = present::task(fut);
        Box::new(move || handle.abort())
    });
    // The frame clock (§8.4): the animation driver re-arms the platform's vsync callback while any
    // frame consumer (game loop / self-driven animation) is live.
    frame::install_frame_requester(|root, cb| tree::with_tree(|t| t.request_frame(root, cb)));

    // What the exit line names (docs/lifecycle.md), and when the clock started; and the exit
    // itself, for a loop that ends the process without returning.
    #[cfg(not(target_arch = "wasm32"))]
    if toolkit_key() != Some("mock") {
        deliver_exit_at_process_exit();
    }
    lifecycle::note_launch(
        options.app_name.clone().unwrap_or_default(),
        options.version.clone(),
    );

    // WillLaunch: before the window/UI exists (docs/lifecycle.md). Fired uniformly by day-core so
    // it is reliable on every backend; handlers must not touch the tree (there isn't one yet).
    lifecycle::dispatch_lifecycle(day_spec::Lifecycle::WillLaunch);

    P::run(
        backend,
        options,
        Box::new(move |mut toolkit, root_handle, size| {
            day_spec::Toolkit::set_event_sink(&mut toolkit, Box::new(tree::enqueue_native_event));
            let tree = Tree::new(toolkit, root_handle, size);
            let root = tree.root();
            tree::install_tree(Box::new(tree));
            // The launch read of the user's motion setting (docs/accessibility.md), before the
            // root piece builds: its first transitions already land where the user asked.
            note_motion_changed();

            // Seed the window's size class from the size the backend just reported
            // (docs/size-classes.md), before the root piece builds below: a nav host resolving
            // an automatic presentation reads it during its own build. Backends push later
            // changes themselves; one that never does keeps this launch value.
            ambient::set_window_size_class(
                root,
                day_spec::SizeClass::from_size(size.width, size.height),
            );

            // The backend is now known: warn about any lifecycle handlers already registered for
            // phases this platform doesn't deliver (docs/lifecycle.md).
            lifecycle::warn_unsupported_registrations();

            // Window resize → relayout. Route requests (runtime deep links: web-dom's URL
            // hash changing under the app via browser back/forward or a hand-edited hash) →
            // navigate, guarded against echo: the request the backend reflects back after
            // our own `set_route` matches the current route and is dropped here.
            with_tree(|t| {
                let rn = root;
                t.on_event(
                    rn,
                    std::rc::Rc::new(move |ev| match ev {
                        day_spec::Event::WindowResized(size) => {
                            let s = *size;
                            with_tree(|t| t.set_window_size(s));
                            // Re-bucket the window (docs/size-classes.md). Derived here, from the
                            // geometry every backend already reports, so there is one breakpoint
                            // table rather than one per toolkit, and only a class change
                            // notifies, so dragging an edge within a bucket costs nothing.
                            ambient::set_window_size_class(
                                rn,
                                day_spec::SizeClass::from_size(s.width, s.height),
                            );
                        }
                        day_spec::Event::RouteRequested(route) => handle_route_request(route),
                        // A native undo front's invocation (⌘Z through the stock Edit menu, a
                        // three-finger swipe): route to whatever stack the app installed.
                        day_spec::Event::Undo { redo } => dispatch_undo_invoke(*redo),
                        // A platform edit-command route (Edit ▸ Cut/Copy/Paste, the
                        // browser's clipboard events) with no text widget claiming it.
                        day_spec::Event::Edit(op) => dispatch_edit_invoke(*op),
                        _ => {}
                    }),
                );
            });

            // The first window is an ordinary primary window (docs/windows.md close policy):
            // it goes in the registry like any other, so closing it runs the same teardown and
            // the same "was that the last primary?" question. Its content builds in a child of
            // the root scope; disposing that on close takes this window's tree and nothing
            // else, while `Signal::global` state stays on the root scope above it.
            let win_scope = day_reactive::Scope::root().enter(day_reactive::Scope::child);
            windows::adopt_initial_window(root, win_scope);

            // Build the root piece under the window container.
            let piece = root_piece();
            win_scope.enter(|| {
                let mut cx = BuildCx::new(root);
                let _ = piece.build(&mut cx);
            });

            // Initial layout, then keep laying out at every turn boundary.
            with_tree(|t| {
                t.mark_layout_dirty();
                t.layout_if_needed();
            });
            day_reactive::on_turn_end(|| with_tree(|t| t.layout_if_needed()));
            // The Dock menu lists the app's launcher shortcuts from the start (docs/menus.md),
            // ahead of whatever `dock_menu(..)` the root just installed.
            menu::install_dock_menu();
            // A keep-running policy or a status item set before the tree existed reaches the
            // backend now (docs/windows.md "Keeping the app running").
            status::boot_sync();

            #[cfg(not(any(
                target_arch = "wasm32",
                target_os = "ios",
                target_os = "android",
                target_env = "ohos"
            )))]
            documents::request_open_files(documents::take_launch_files());
            day_reactive::on_main(documents::drain_open_files);
            if let Ok(url) = std::env::var("DAY_OPEN_URL") {
                urls::request_open_url(&url);
            }
            day_reactive::on_main(urls::drain);

            // DidLaunch: the UI is mounted and laid out, the app is about to run (docs/lifecycle.md).
            lifecycle::dispatch_lifecycle(day_spec::Lifecycle::DidLaunch);

            // Startup deep link (docs/navigation.md): uniform across platforms; desktop
            // sets `DAY_DEEPLINK` directly, mobile shells forward the launch URL/intent into
            // it, and web-dom (no process environment) records the URL hash via
            // `set_launch_deeplink`. Deferred one turn so the first frame mounts before the
            // destination pushes. The turn-end route sync (`Toolkit::set_route` on change;
            // web-dom mirrors it into the URL hash) installs in the same deferred closure,
            // after the deep link resolves, so the launch route is never clobbered by a sync
            // of the pre-navigation state.
            day_reactive::on_main(move || {
                if let Some(route) = nav::launch_deeplink()
                    && !nav::navigate(&route)
                {
                    log::warn!("launch deep link {route:?} did not match a route");
                }
                // Consume the `request_route` buffer the line above may have read: a tap that
                // cold-started the process has now been applied, and leaving it set would
                // re-navigate on the next request.
                let _ = nav::take_requested_route();
                // First reflection runs eagerly: a launch with no deep link ends no turn.
                let route = nav::current_route().unwrap_or_default();
                with_tree(|t| t.set_route(&route));
                let last = std::cell::RefCell::new(Some(route));
                day_reactive::on_turn_end(move || {
                    let route = nav::current_route().unwrap_or_default();
                    if last.borrow().as_deref() != Some(route.as_str()) {
                        *last.borrow_mut() = Some(route.clone());
                        with_tree(|t| t.set_route(&route));
                    }
                });
            });

            // Verification hook (headless CI / no-input environments): drive the app through
            // Day's own event path once the native loop starts (delayed past first allocation
            // so snapshots see a laid-out window). Precursor of dayscript (§14).
            if let Ok(spec) = std::env::var("DAY_AUTODRIVE") {
                day_reactive::on_main_delayed(800, move || autodrive(&spec));
            }
        }),
    );
    // DidExit: the native loop returned, which is the last thing the app runs on the desktop
    // backends whose loop returns (GTK, Qt, XAML). The backends whose process ends inside the
    // loop (AppKit's terminate exits) emit it themselves, after WillTerminate, and
    // `dispatch_lifecycle` delivers it once whichever comes first. Where the host owns the
    // loop and `run` hands back at once with the app still to come (ArkUI, Android, the web,
    // the mock's test), the return is no exit at all: DidExit there is the host's own last
    // callback, and delivering it here would run the exit handlers at launch and leave the
    // real exit undelivered (`RUN_ENDS_APP`, docs/lifecycle.md).
    if P::RUN_ENDS_APP {
        lifecycle::dispatch_lifecycle(day_spec::Lifecycle::DidExit);
    }
}

/// Deliver `DidExit` from the C runtime's exit, for the loop that never returns: Qt's macOS
/// Quit menu item ends the process through `exit()` from inside `exec()`, and any backend or
/// app that calls `std::process::exit` does the same. `atexit` runs on the exiting thread,
/// the main one for every such path, with the thread-locals still alive; delivery is once, so
/// a loop that did return costs nothing here. Registered by `launch_with` on every backend but
/// the mock and wasm, which have no process exit of their own.
#[cfg(not(target_arch = "wasm32"))]
fn deliver_exit_at_process_exit() {
    unsafe extern "C" {
        fn atexit(f: extern "C" fn()) -> i32;
    }
    extern "C" fn at_exit() {
        lifecycle::dispatch_lifecycle(day_spec::Lifecycle::DidExit);
    }
    // SAFETY: `atexit` takes a plain C function pointer and keeps it for the process's life.
    unsafe {
        atexit(at_exit);
    }
}

/// `DAY_AUTODRIVE="<id>:press;<id>:text:Ada;<id>:value:80;<id>:toggle:true;<id>:tap;
/// <id>:drag:40:60;shot:/tmp/x.png"`: synthesized Day events by element id, plus snapshots.
fn autodrive(spec: &str) {
    use day_spec::{DragPhase, Event, Point};
    for step in spec.split(';').filter(|s| !s.is_empty()) {
        let parts: Vec<&str> = step.splitn(3, ':').collect();
        if parts[0] == "shot" {
            let path = parts[1..].join(":");
            let png = with_tree(|t| t.snapshot());
            match png {
                Ok(bytes) => {
                    let _ = std::fs::write(&path, bytes);
                }
                Err(e) => log::warn!("day autodrive: snapshot failed: {e}"),
            }
            continue;
        }
        let node = with_tree(|t| t.find_by_id(parts[0]));
        let Some(node) = node else {
            log::warn!("day autodrive: id {:?} not found", parts[0]);
            continue;
        };
        // Gesture drivers (docs/shapes.md): tap fires at the node's local center; drag runs a
        // Began→Changed→Ended sequence translated by dx,dy, exercising `.on_tap`/`.on_drag`
        // hit-testing through Day's event path (the native recognizers deliver the same events).
        if parts.get(1) == Some(&"tap") {
            if let Some(f) = with_tree(|t| t.node_frame(node)) {
                let c = Point::new(f.size.width / 2.0, f.size.height / 2.0);
                tree::enqueue_event(tree::rnode_to_id(node), Event::Tap(c));
            }
            continue;
        }
        if parts.get(1) == Some(&"drag") {
            if let Some(f) = with_tree(|t| t.node_frame(node)) {
                let c = Point::new(f.size.width / 2.0, f.size.height / 2.0);
                let (dx, dy) = parts
                    .get(2)
                    .and_then(|s| s.split_once(':'))
                    .and_then(|(a, b)| Some((a.parse::<f64>().ok()?, b.parse::<f64>().ok()?)))
                    .unwrap_or((0.0, 0.0));
                let end = Point::new(c.x + dx, c.y + dy);
                let id = tree::rnode_to_id(node);
                tree::enqueue_event(
                    id,
                    Event::Drag {
                        phase: DragPhase::Began,
                        location: c,
                        translation: Point::ZERO,
                    },
                );
                tree::enqueue_event(
                    id,
                    Event::Drag {
                        phase: DragPhase::Changed,
                        location: end,
                        translation: Point::new(dx, dy),
                    },
                );
                tree::enqueue_event(
                    id,
                    Event::Drag {
                        phase: DragPhase::Ended,
                        location: end,
                        translation: Point::new(dx, dy),
                    },
                );
            }
            continue;
        }
        if let (Some("text"), Some(v)) = (parts.get(1).copied(), parts.get(2).copied()) {
            synthesize_text(node, v.to_string());
            continue;
        }
        let ev = match (parts.get(1).copied(), parts.get(2).copied()) {
            (Some("press"), _) => Event::Pressed,
            (Some("toggle"), Some(v)) => Event::ToggleChanged(v == "true"),
            (Some("value"), Some(v)) => Event::ValueChanged(v.parse().unwrap_or(0.0)),
            (Some("select"), Some(v)) => Event::SelectionChanged(v.parse().unwrap_or(-1)),
            _ => continue,
        };
        tree::enqueue_event(tree::rnode_to_id(node), ev);
    }
}

/// Whether the platform is rendering in dark appearance (see `Toolkit::dark_mode`): the
/// branch apps take when painting custom opaque surfaces so fills track the theme that the
/// default text colors already follow.
pub fn dark_mode() -> bool {
    dark_signal().get()
}

/// The platform's font families with their faces (docs/fonts.md), enumerated once per
/// process through `Toolkit::font_families` and cached: the answer is stable for the process
/// (bundled fonts register before the tree exists, and nothing tracks a system font install
/// under a running app; the OS font panels cache too), enumeration is expensive, and the
/// query is synchronous on the UI thread. Sorted case-insensitively by family, deduplicated,
/// and unioned with the bundled fonts (`day_spec::fonts::bundled_fonts`) the toolkit's own
/// database did not report, which is how a `res/font/` (Android), `ms-appx` (XAML) or
/// `registerFont` (HarmonyOS) family reaches a font menu without each shim parsing sfnt.
/// Empty where the toolkit answers `Cap::FontList` = `Unsupported`.
pub fn font_families() -> std::rc::Rc<[day_spec::FontFamilyInfo]> {
    // Headless (no tree on this thread, i.e. a model unit test): nothing to enumerate, and
    // nothing to cache, so the first call with a tree still fills the cache.
    if !tree::has_tree() {
        return std::rc::Rc::from(Vec::new());
    }
    FONT_FAMILIES.with(|c| {
        c.get_or_init(|| {
            let mut list = tree::with_tree(|t| t.font_families());
            for path in day_spec::fonts::bundled_fonts() {
                let Some(names) = std::fs::read(&path)
                    .ok()
                    .and_then(|bytes| day_spec::fonts::parse_font_names(&bytes))
                else {
                    continue;
                };
                if list
                    .iter()
                    .any(|f| f.family.eq_ignore_ascii_case(&names.family))
                {
                    continue;
                }
                list.push(day_spec::FontFamilyInfo {
                    family: names.family,
                    faces: vec![day_spec::FontFace {
                        name: "Regular".to_string(),
                        weight: day_spec::FontWeight::Regular,
                        italic: false,
                    }],
                });
            }
            list.sort_by_key(|f| f.family.to_lowercase());
            list.dedup_by(|a, b| a.family.eq_ignore_ascii_case(&b.family));
            list.into()
        })
        .clone()
    })
}

/// Distinct measurements held per generation. Two generations are live at once, so the cache
/// holds up to twice this before the older one is dropped whole.
///
/// 512 is far more than a drawing measures per frame (a chart with two axes, their titles and a
/// legend is around twenty) and small enough that the worst case (a text tool measuring a
/// different string every keystroke) costs tens of kilobytes rather than growing without end.
const TEXT_METRICS_CAP: usize = 512;

/// The `measure_text` cache: a **pure** memo, because every backend's measurement is a function
/// of `(text, size, font)` and nothing else. Nothing in a running app changes the answer: canvas
/// text takes absolute points, so it carries neither the reader's font-scale setting nor the
/// window's scale factor (docs/canvas.md "Text"), which is what makes caching it correct rather
/// than merely fast.
///
/// Two generations rather than an LRU list: a hit in `previous` is promoted into `current`, and
/// when `current` fills, `previous` is replaced by it wholesale. That approximates least-recently-
/// used closely enough for a working set that repeats every frame, is O(1) amortized with no
/// eviction scan, and needs no ordering structure beside the maps.
struct TextMetricsCache {
    /// Reused so a hit allocates nothing at all: the key is rebuilt into this buffer, compared,
    /// and only copied into the map on a miss.
    key: String,
    current: std::collections::HashMap<String, day_spec::TextMetrics>,
    previous: std::collections::HashMap<String, day_spec::TextMetrics>,
    hits: u64,
    misses: u64,
}

impl TextMetricsCache {
    fn new() -> Self {
        TextMetricsCache {
            key: String::new(),
            current: std::collections::HashMap::new(),
            previous: std::collections::HashMap::new(),
            hits: 0,
            misses: 0,
        }
    }

    /// Write the key for one measurement into the scratch buffer.
    ///
    /// The family's length is written before the family, so the encoding is injective whatever a
    /// family name contains; the alternative, a separator, is only injective while no font is
    /// ever named with it in it. `size` goes in as its bit pattern: exact, and two sizes that
    /// differ only in representation would measure the same anyway.
    fn build_key(&mut self, text: &str, size: f64, font: &day_spec::CanvasFont) {
        use std::fmt::Write;
        let family = font.family_str();
        self.key.clear();
        let _ = write!(
            self.key,
            "{:x}:{}:{}:{}:{family}{text}",
            size.to_bits(),
            font.css_weight(),
            u8::from(font.italic),
            family.len(),
        );
    }

    /// A hit promotes an older-generation entry into the current one, so a working set that
    /// survives a generation flip is not measured again.
    fn lookup(&mut self) -> Option<day_spec::TextMetrics> {
        if let Some(m) = self.current.get(&self.key) {
            self.hits += 1;
            return Some(*m);
        }
        let m = *self.previous.get(&self.key)?;
        self.hits += 1;
        self.current.insert(self.key.clone(), m);
        Some(m)
    }

    fn insert(&mut self, m: day_spec::TextMetrics) {
        self.misses += 1;
        if self.current.len() >= TEXT_METRICS_CAP {
            self.previous = std::mem::take(&mut self.current);
        }
        self.current.insert(self.key.clone(), m);
    }
}

/// What [`measure_text`]'s cache has done on this thread: a diagnostic, for an app that wants to
/// see whether its drawing is measuring the same strings over and over (it usually is).
#[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
pub struct TextMetricsCacheStats {
    /// Measurements answered without asking the toolkit.
    pub hits: u64,
    /// Measurements that had to cross into the toolkit.
    pub misses: u64,
    /// Distinct measurements currently held, across both generations.
    pub entries: usize,
}

/// Read the [`measure_text`] cache's counters (docs/fonts.md).
pub fn text_metrics_cache_stats() -> TextMetricsCacheStats {
    TEXT_METRICS.with(|c| {
        let c = c.borrow();
        TextMetricsCacheStats {
            hits: c.hits,
            misses: c.misses,
            entries: c.current.len() + c.previous.len(),
        }
    })
}

/// Drop every cached measurement (the counters keep running).
///
/// Nothing in day needs this: a measurement is a pure function of its key, and the font set is
/// fixed for the process (see [`font_families`]). It exists for a toolkit or app that registers a
/// face at runtime and so does change what a family name measures to.
pub fn clear_text_metrics_cache() {
    TEXT_METRICS.with(|c| {
        let mut c = c.borrow_mut();
        c.current.clear();
        c.previous.clear();
    });
}

/// Measure one line of canvas text at `size` points in `font` (docs/fonts.md), in the engine
/// that draws `DrawOp::Text`; a toolkit that cannot measure answers
/// `TextMetrics::approximate`, so a caller always gets a usable box.
///
/// **Memoized** (`TextMetricsCache`): measuring crosses into the toolkit and lays the text out
/// there (around 29 µs a call on AppKit, which builds an `NSString` and an attribute dictionary
/// every time), and a drawing measures the same handful of strings on every frame it records. A
/// repeat costs a hash of the key and nothing else.
pub fn measure_text(text: &str, size: f64, font: &day_spec::CanvasFont) -> day_spec::TextMetrics {
    if let Some(m) = TEXT_METRICS.with(|c| {
        let mut c = c.borrow_mut();
        c.build_key(text, size, font);
        c.lookup()
    }) {
        return m;
    }
    // The toolkit call happens with no borrow of the cache held: `replay` and a piece's own
    // measurement can reach back into day-core, and a borrow spanning the call would panic.
    // Headless (no tree on this thread) answers the approximation too, so a model unit test
    // that frames text runs without a toolkit.
    let Some(m) = tree::try_with_tree(|t| t.measure_text(text, size, font)).flatten() else {
        // Not cached. "The toolkit could not answer" is a fact about this moment (Android's
        // returns nothing until its VM is up), not about the text, and caching it would
        // pin a guess for the life of the process.
        return day_spec::TextMetrics::approximate(text, size);
    };
    TEXT_METRICS.with(|c| {
        let mut c = c.borrow_mut();
        c.build_key(text, size, font);
        c.insert(m);
    });
    m
}

/// Whether motion should be reduced (docs/accessibility.md): the user's system setting
/// (`Toolkit::reduce_motion`), or a launch that forced it (`DAY_REDUCE_MOTION=1`, and
/// `day launch --fast`, whose fast mode is this setting forced on). Day already applies every
/// finite animation at its destination under it and keeps only indeterminate motion; this read
/// is for the motion an app drives itself, such as a `day::frame` simulation or a decorative
/// loop, which the app opts down. Reactive: a closure reading it re-runs when the setting flips.
pub fn reduce_motion() -> bool {
    reduce_motion_signal().get()
}

/// [`reduce_motion`] untracked and borrow-free, for the animation gates inside the tree.
pub fn reduce_motion_now() -> bool {
    testing::forced_reduce_motion() || REDUCE_MOTION.with(|c| c.get())
}

/// The reactive backing for [`reduce_motion`], seeded from the current answer.
fn reduce_motion_signal() -> day_reactive::Signal<bool> {
    REDUCE_MOTION_SIGNAL.with(|c| {
        *c.get_or_init(|| {
            let seed = reduce_motion_now();
            day_reactive::Scope::detached().enter(|| day_reactive::Signal::new(seed))
        })
    })
}

/// Re-read the toolkit's reduce-motion setting into [`reduce_motion`]. Backends call this
/// when the user flips the setting under a running app; [`launch_with`] calls it once the tree
/// stands, which is the launch read. Like [`note_appearance_changed`], it may fire before the
/// tree is installed and then does nothing.
pub fn note_motion_changed() {
    if let Some(system) = tree::try_with_tree(|t| t.reduce_motion()) {
        REDUCE_MOTION.with(|c| c.set(system));
        let now = reduce_motion_now();
        REDUCE_MOTION_SIGNAL.with(|c| {
            if let Some(s) = c.get() {
                s.set(now);
            }
        });
    }
}

/// The reactive backing for [`dark_mode`], lazily seeded from the toolkit's answer.
fn dark_signal() -> day_reactive::Signal<bool> {
    DARK_SIGNAL.with(|c| {
        *c.get_or_init(|| {
            let seed = tree::with_tree(|t| t.dark_mode());
            day_reactive::Scope::detached().enter(|| day_reactive::Signal::new(seed))
        })
    })
}

/// Re-read the toolkit's appearance into the reactive [`dark_mode`] signal. Backends call
/// this when the system appearance changes under a running app (macOS theme switch, GTK
/// style-manager change), and [`set_appearance`] calls it after applying an override, so
/// closures reading `dark_mode()` recolor live instead of going stale until a rebuild.
pub fn note_appearance_changed() {
    // May fire before the tree is installed (GTK's StyleManager emits `dark` notifies while
    // `startup` applies a forced DAY_THEME scheme; AppKit's observer dispatches async): no
    // tree means no palette closures exist yet, and the signal seeds from the tree on first
    // use; skip rather than panic inside a native callback.
    if let Some(d) = tree::try_with_tree(|t| t.dark_mode()) {
        dark_signal().set(d);
    }
}

/// The system text size changed under a running app (iOS Dynamic Type, a desktop's text
/// scaling): every text measurement Day holds is stale. Backends call this from the platform's
/// notification, once the native text has taken the new size. Day re-measures every node and
/// re-lays every bound list row, so a content-sized row (docs/list.md "Row heights") takes its
/// new height through `ListPatch::RowSizeInvalidated`. Skipped when the tree is busy or not
/// installed yet: a tree built after the change measures at the new size anyway.
pub fn note_text_size_changed() {
    let _ = tree::try_with_tree(|t| {
        t.invalidate_all_measurements();
        t.layout_if_needed();
    });
}

// The `.searchable()` hosts whose field is INLINE (docs/search.md), newest last: each has no
// `day::search` bar item, so a caller holding that reserved id (dayscript's `toolbar:` step)
// resolves it here and delivers `Event::SearchChanged` to the host, as the native field would.
/// Record `host` as showing an inline search field. Registered by the host's builder for the
/// life of its scope; see [`inline_search_host`].
pub fn register_search_host(host: RNode) {
    let window = toolbar::current_page_window();
    SEARCH_HOSTS.with(|h| {
        let mut h = h.borrow_mut();
        h.retain(|(_, node)| *node != host);
        h.push((window, host));
    });
}

/// Forget `host`, once its scope is disposed.
pub fn unregister_search_host(host: RNode) {
    SEARCH_HOSTS.with(|h| h.borrow_mut().retain(|(_, node)| *node != host));
}

/// The inline search host the reserved `day::search` id names when no bar item carries it: the
/// most recently shown one in the focused window.
pub fn inline_search_host() -> Option<RNode> {
    let window = windows::focused_root();
    SEARCH_HOSTS.with(|h| {
        h.borrow()
            .iter()
            .rev()
            .find(|(root, _)| *root == window)
            .map(|(_, host)| *host)
    })
}

/// Activate the focused window's searchable navigation surface (for a Find command).
pub fn focus_search() {
    if toolbar::focus_toolbar_search() {
        return;
    }
    if let Some(host) = inline_search_host() {
        tree::with_tree(|t| t.patch(host, Box::new(day_spec::props::SearchPatch::Focus), false));
    }
}

/// Deliver synthetic typing to a text control (the dayscript `input` step and the autodrive
/// string commands both route here): paint the widget via the ordinary app-write patch, then
/// enqueue the `TextChanged` event. Both halves are needed: when a real user types, the text
/// is already in the native field by the time its change event fires, so the two-way
/// binding's echo guard suppresses the write-back (§4.4); a synthesized event
/// alone would drive the app's signal while the widget kept showing its old text.
pub fn synthesize_text(node: RNode, text: String) {
    tree::with_tree(|t| match t.node_kind(node) {
        Some(day_spec::kinds::TEXT_FIELD) => t.patch(
            node,
            Box::new(day_spec::props::TextFieldPatch::Text {
                text: text.clone(),
                from_native: false,
            }),
            false,
        ),
        Some(day_spec::kinds::TEXT_AREA) => t.patch(
            node,
            Box::new(day_spec::props::TextAreaPatch::SetText(text.clone())),
            false,
        ),
        // Other kinds (external pieces like the combobox) own their display, and hear about the
        // text first through [`SYNTHESIZED_TEXT`]: a piece that treats `TextChanged` as the
        // view's own report (the styled editor does) paints from that one instead, since the
        // view has not seen this text.
        _ => tree::enqueue_event(
            tree::rnode_to_id(node),
            day_spec::Event::custom(SYNTHESIZED_TEXT, text.clone()),
        ),
    });
    tree::enqueue_event(tree::rnode_to_id(node), day_spec::Event::TextChanged(text));
}

/// The tag of the `Event::Custom` [`synthesize_text`] sends to a piece of a kind day-core does not
/// paint itself, carrying the text, just before the `TextChanged` that follows it. A piece whose
/// native view owns the characters shows the text on this event and then sees the `TextChanged`
/// as the echo it already handles.
pub const SYNTHESIZED_TEXT: &str = "day:synthesized-text";

mod piece_ops;
pub use piece_ops::{
    PieceOperationDone, PieceOperationFn, piece_operation, register_piece_operation,
};

/// Override the app's appearance: `Some(true)` dark, `Some(false)` light, `None` follow the
/// system again. On backends reporting `Cap::Appearance` the native widgets restyle in place
/// and [`dark_mode`] answers the override; app-painted surfaces pick it up on their next
/// rebuild. Other backends ignore the call; probe before offering a theme picker.
pub fn set_appearance(dark: Option<bool>) {
    tree::with_tree(|t| t.set_appearance(dark));
    note_appearance_changed();
}

/// Put a badge on the app's icon: the Dock, launcher, home screen, or taskbar (docs/badge.md).
///
/// Fire-and-forget: a payload the running toolkit cannot render is ignored, so probe
/// `capability(Cap::AppBadgeCount | AppBadgeText | AppBadgeDot)` first and choose. Nothing is ever
/// substituted, because a wrong number on a user's icon is worse than no badge.
///
/// ```no_run
/// # use day_core::{set_app_badge};
/// # use day_spec::AppBadge;
/// set_app_badge(&AppBadge::Count(7));
/// set_app_badge(&AppBadge::None); // clear
/// ```
///
/// An iOS badge belongs to the installed app and survives termination, so an app that sets one
/// usually clears it from a `WillTerminate` handler (docs/lifecycle.md); a macOS Dock badge dies
/// with the process.
pub fn set_app_badge(badge: &day_spec::AppBadge) {
    tree::with_tree(|t| t.set_app_badge(badge));
}

#[cfg(test)]
mod posted_panic_tests {
    /// A panic inside a posted main-thread task must be contained (logged + runtime reset), never
    /// unwind into the native trampoline that posted it (`panic_cannot_unwind` → SIGABRT). This
    /// is the poster/scheduler twin of `pump_events`' containment.
    #[test]
    fn posted_panic_is_contained() {
        super::contain_posted_panic(Box::new(|| panic!("boom in a posted task")));
        // Reaching here is the assertion: the panic did not propagate. The runtime was reset, so
        // subsequent reactive work still runs.
        let s = day_reactive::Signal::new(1i32);
        s.set(2);
        assert_eq!(s.get_untracked(), 2);
    }
}