retroglyph_window/winit/

run.rs

//! The winit event loop and the windowed app drivers.
//!
//! [`run_windowed`] drives a raw `FnMut(&mut Terminal<..>)` closure;
//! [`run_app`] drives an [`App`](retroglyph_core::app::App). This is the inverted
//! driver: winit owns the loop and calls back into the app on each redraw,
//! so it cannot be core's generic
//! [`run_on`](retroglyph_core::app::run_on), which owns its own
//! `while` loop.

use super::translate::{
    translate_ime, translate_key, translate_modifiers, translate_mouse_button,
    translate_physical_pos,
};
#[cfg(target_arch = "wasm32")]
use super::web;
use crate::backend::WindowBackend;
use crate::presenter::Presenter;
use retroglyph_core::backend::{Input, Output};
use retroglyph_core::event::{
    Event, KeyModifiers, MouseButton, MouseEvent, MouseEventKind, PhysicalPos,
};
use retroglyph_core::grid::HasSize;
use retroglyph_core::terminal::Terminal;
use std::cell::Cell;
use std::fmt;
use std::marker::PhantomData;
use std::rc::Rc;
use std::sync::Arc;
use std::time::Duration;
use winit::application::ApplicationHandler;
use winit::event::WindowEvent;
use winit::event_loop::{ActiveEventLoop, EventLoop};
use winit::window::{Window, WindowId};

/// A thread-safe handle for injecting application-defined events into a running windowed event
/// loop from another thread (network, audio, timer, ...).
///
/// Obtained via the `on_proxy` callback passed to [`run_windowed_with_proxy`]/
/// [`run_app_with_proxy`] (payload fixed to `u64`, delivered as [`Event::Custom`]) or
/// [`run_windowed_with_typed_proxy`]/[`run_app_with_typed_proxy`] (any `T: Send + 'static`,
/// delivered to a caller-supplied handler), invoked synchronously right after the event loop
/// (and this proxy) is created, before the loop starts blocking the calling thread. Clone it
/// freely to hand a copy to each worker thread that needs to wake the loop; wraps winit's own
/// [`EventLoopProxy`](winit::event_loop::EventLoopProxy), which is `Send + Sync` for any
/// `T: Send + 'static` payload.
///
/// `T` defaults to `u64` (the payload [`Event::Custom`] itself carries), so existing code
/// naming the bare `EventProxy` type (from before this type became generic) keeps compiling
/// unchanged.
pub struct EventProxy<T: Send + 'static = u64>(winit::event_loop::EventLoopProxy<T>);

// Hand-written rather than `#[derive(Clone, Debug)]`: a derive would add `T: Clone`/`T: Debug`
// bounds to the impl, but `winit::event_loop::EventLoopProxy<T>` itself needs neither: cloning
// or formatting the proxy handle never touches a buffered `T` value (there isn't one; `T` is
// only ever a transient argument to `send_event`).
impl<T: Send + 'static> Clone for EventProxy<T> {
    fn clone(&self) -> Self {
        Self(self.0.clone())
    }
}

impl<T: Send + 'static> fmt::Debug for EventProxy<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_tuple("EventProxy").field(&self.0).finish()
    }
}

impl<T: Send + 'static> EventProxy<T> {
    /// Injects `payload` into the event loop's queue, waking it if it's asleep.
    ///
    /// With the default `T = u64` (via [`run_windowed_with_proxy`]/[`run_app_with_proxy`]), the
    /// payload surfaces through the app's normal `poll_event`/frame loop as
    /// [`Event::Custom(payload)`](Event::Custom), like any other [`Event`]. With a custom `T`
    /// (via [`run_windowed_with_typed_proxy`]/[`run_app_with_typed_proxy`]), the payload is
    /// handed directly to that call's `on_custom_event` handler instead: it never becomes an
    /// [`Event`], since [`Event::Custom`] is fixed to `u64`.
    ///
    /// # Errors
    ///
    /// Returns [`EventProxyClosed`] if the event loop has already exited.
    pub fn send_event(&self, payload: T) -> Result<(), EventProxyClosed<T>> {
        self.0
            .send_event(payload)
            .map_err(|e| EventProxyClosed(e.0))
    }
}

/// Error returned by [`EventProxy::send_event`] when the event loop it targets has already
/// exited.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub struct EventProxyClosed<T = u64>(T);

impl<T> EventProxyClosed<T> {
    /// The payload that could not be delivered.
    #[must_use]
    pub fn into_inner(self) -> T {
        self.0
    }
}

impl<T> fmt::Display for EventProxyClosed<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "event loop closed")
    }
}

impl<T: fmt::Debug> std::error::Error for EventProxyClosed<T> {}

/// Window configuration for [`run_windowed`] / [`run_app`].
///
/// Renderer-agnostic: pixel dimensions, not grid/font/scale.
/// Use [`fit`](Self::fit) to derive the pixel size from a presenter's own
/// cell geometry.
///
/// Several builder methods below ([`resizable`](Self::resizable), [`decorations`](Self::decorations),
/// [`transparency`](Self::transparency), [`fullscreen`](Self::fullscreen)) target an OS-level
/// window control that a `wasm32` canvas doesn't have; on that target winit's web backend either
/// ignores the value outright or can't reliably apply it (see each method for which, and why).
/// The value is still applied for source-level parity with native either way, so the same call
/// chain compiles and runs on both targets, it just may not visibly do anything in the browser.
// Five independent window attribute toggles (`fill_viewport`, `resizable`, `decorations`,
// `fullscreen`, `transparency`), not a state machine in disguise: each maps to one winit
// `WindowAttributes` builder call and is meaningful on its own.
#[allow(clippy::struct_excessive_bools)]
pub struct WindowConfig {
    title: String,
    width: u32,
    height: u32,
    target_fps: Option<u32>,
    event_driven: bool,
    fill_viewport: bool,
    resizable: bool,
    decorations: bool,
    min_size: Option<(u32, u32)>,
    max_size: Option<(u32, u32)>,
    initial_position: Option<(i32, i32)>,
    fullscreen: bool,
    transparency: bool,
}

impl WindowConfig {
    /// Size the window to exactly fit `presenter`'s grid:
    /// `cols x cell_w` by `rows x cell_h` physical pixels.
    ///
    /// This is why renderer crates don't need their own windowing code: the
    /// grid/cell geometry already lives behind
    /// [`Output::size`] and
    /// [`Presenter::cell_size`].
    ///
    /// `target_fps` and `event_driven` are independent controls, on native and `wasm32` alike:
    ///
    /// - `target_fps` is the frame-rate cap applied whenever a frame is actually rendered: `None`
    ///   is uncapped (render as fast as the loop reaches a redraw), `Some(fps)` paces redraws to
    ///   no more than `fps` per second.
    /// - `event_driven` picks between the two redraw-triggering modes:
    ///   - `true` is **redraw-on-demand**: a frame is rendered only after something happened (an
    ///     input or window event, an injected [`Event::Custom`], window creation), and the loop
    ///     sleeps otherwise. Right for event-driven retro/terminal UIs, which are idle most of
    ///     the time; wrong for anything that animates from
    ///     [`Frame::delta`](retroglyph_core::app::Frame::delta), which will render one frame and then
    ///     sit still until the next stray event.
    ///   - `false` is **continuous**: a frame is rendered every tick whether or not anything
    ///     happened, which is what a `retroglyph_ui::Tween`/
    ///     [`FrameClock`](retroglyph_core::frames::FrameClock)-driven app needs.
    ///
    /// The two combine independently: `(Some(fps), false)` is the common capped-animation shape
    /// (see [`Self::animated`] for a shorthand), `(None, true)` is the common idle-UI shape, and
    /// `(None, false)` (render every tick, uncapped) is the one combination that was
    /// previously inexpressible, useful for e.g. measuring a render loop's raw throughput.
    ///
    /// On `wasm32` the browser owns frame pacing: winit's web backend delivers each requested
    /// redraw on the next `requestAnimationFrame`, so an uncapped or `event_driven: false` loop
    /// still runs at the display refresh rate and `target_fps`'s specific number is advisory
    /// (there is no way to render faster than `requestAnimationFrame`, and rendering slower would
    /// mean discarding frames the browser already scheduled). Only the `event_driven` choice
    /// carries across unaffected.
    #[must_use]
    pub fn fit<P: Presenter>(
        presenter: &P,
        title: impl Into<String>,
        target_fps: Option<u32>,
        event_driven: bool,
    ) -> Self {
        let grid = presenter.size();
        let (cell_w, cell_h) = presenter.cell_size();
        Self {
            title: title.into(),
            width: u32::from(grid.width()) * cell_w,
            height: u32::from(grid.height()) * cell_h,
            target_fps,
            event_driven,
            fill_viewport: false,
            resizable: true,
            decorations: true,
            min_size: None,
            max_size: None,
            initial_position: None,
            fullscreen: false,
            transparency: false,
        }
    }

    /// The window title, as set by [`fit`](Self::fit).
    #[must_use]
    pub fn title(&self) -> &str {
        &self.title
    }

    /// Initial inner width in physical pixels, as computed by [`fit`](Self::fit).
    #[must_use]
    pub const fn width(&self) -> u32 {
        self.width
    }

    /// Initial inner height in physical pixels, as computed by [`fit`](Self::fit).
    #[must_use]
    pub const fn height(&self) -> u32 {
        self.height
    }

    /// Shorthand for [`fit`](Self::fit) with continuous, non-event-driven, `fps`-capped
    /// redraws: the shape most animated apps want. Equivalent to
    /// `Self::fit(presenter, title, Some(fps), false)`.
    #[must_use]
    pub fn animated<P: Presenter>(presenter: &P, title: impl Into<String>, fps: u32) -> Self {
        Self::fit(presenter, title, Some(fps), false)
    }

    /// The frame-rate cap passed to [`fit`](Self::fit); see its doc comment for what `None` vs.
    /// `Some(fps)` means and how it combines with [`event_driven`](Self::event_driven).
    #[must_use]
    pub const fn target_fps(&self) -> Option<u32> {
        self.target_fps
    }

    /// The redraw-triggering mode passed to [`fit`](Self::fit); see its doc comment for what
    /// `true` vs. `false` means and how it combines with [`target_fps`](Self::target_fps).
    #[must_use]
    pub const fn event_driven(&self) -> bool {
        self.event_driven
    }

    /// Overwrites [`target_fps`](Self::target_fps) and [`event_driven`](Self::event_driven) with
    /// `options`' own [`target_fps`](retroglyph_core::app::RunOptions::target_fps) and
    /// [`is_event_driven`](retroglyph_core::app::RunOptions::is_event_driven), so pacing can be
    /// configured the same way as the blocking driver's
    /// [`run_on_with`](retroglyph_core::app::run_on_with).
    ///
    /// Like any other builder call, applying this after [`fit`](Self::fit)/[`animated`](Self::animated)
    /// makes `options` win over whatever those constructors set; call it last if both a `target_fps`/
    /// `event_driven` pair and a [`RunOptions`](retroglyph_core::app::RunOptions) are in play, so
    /// there is exactly one answer for "how do I pace this" rather than two competing ones.
    ///
    /// [`RunOptions::idle_wake`](retroglyph_core::app::RunOptions::idle_wake) has no windowed
    /// meaning and is ignored: winit's redraw-on-demand mode (`event_driven: true`) already
    /// parks the loop until the next event instead of a periodic wake, so there is no idle-poll
    /// interval for it to configure here.
    #[must_use]
    pub const fn with_run_options(mut self, options: retroglyph_core::app::RunOptions) -> Self {
        self.target_fps = options.target_fps();
        self.event_driven = options.is_event_driven();
        self
    }

    /// Sets whether to size (and keep resizing) the canvas to fill the browser viewport on
    /// `wasm32`, instead of the pixel size [`fit`](Self::fit) computed: a full-screen,
    /// mobile-web-app feel for games that want it. Has no effect on native, where the OS window
    /// is already sized by [`fit`](Self::fit) and the window manager owns further resizing
    /// either way.
    ///
    /// Defaults to `false`: most demos/examples should render at their natural grid size
    /// (`cols x cell_w` by `rows x cell_h`) wherever they land on the page, not stretch to fill
    /// whatever viewport happens to be hosting them. Opt in explicitly for an app-like,
    /// full-screen game.
    #[must_use]
    pub const fn fill_viewport(mut self, fill_viewport: bool) -> Self {
        self.fill_viewport = fill_viewport;
        self
    }

    /// Sets whether the window can be resized by the user/window manager after creation.
    ///
    /// Defaults to `true` (winit's own default). Set to `false` for fixed-size retro windows
    /// where the grid is meant to stay put: resizing a pseudo-graphic UI usually means picking
    /// a new grid size, not stretching cells, and most callers that care already size the window
    /// to their content via [`fit`](Self::fit).
    ///
    /// On `wasm32`, winit's web backend ignores this: there is no OS-level resize grip on a
    /// canvas.
    #[must_use]
    pub const fn resizable(mut self, resizable: bool) -> Self {
        self.resizable = resizable;
        self
    }

    /// Sets whether the window has OS chrome: title bar, borders, close/minimize/maximize
    /// buttons.
    ///
    /// Defaults to `true` (winit's own default). Set to `false` for a borderless window
    /// (custom-drawn title bars, retro full-bleed layouts).
    ///
    /// On `wasm32`, winit's web backend ignores this: a canvas has no OS chrome to begin with.
    #[must_use]
    pub const fn decorations(mut self, decorations: bool) -> Self {
        self.decorations = decorations;
        self
    }

    /// Sets the minimum inner (content) size in physical pixels.
    ///
    /// Defaults to no minimum.
    #[must_use]
    pub const fn min_size(mut self, width: u32, height: u32) -> Self {
        self.min_size = Some((width, height));
        self
    }

    /// Sets the maximum inner (content) size in physical pixels.
    ///
    /// Defaults to no maximum.
    #[must_use]
    pub const fn max_size(mut self, width: u32, height: u32) -> Self {
        self.max_size = Some((width, height));
        self
    }

    /// Sets the desired initial outer window position in physical pixels.
    ///
    /// Defaults to letting the platform choose.
    ///
    /// On `wasm32`, winit's web backend maps this to the canvas's `position: absolute`
    /// left/top, which only does anything if the page's CSS has already opted the canvas into
    /// absolute/relative positioning; otherwise normal document flow overrides it.
    #[must_use]
    pub const fn initial_position(mut self, x: i32, y: i32) -> Self {
        self.initial_position = Some((x, y));
        self
    }

    /// Sets whether to request borderless fullscreen (on the window's current monitor) at
    /// creation.
    ///
    /// Defaults to `false`. This only exposes borderless fullscreen, not winit's
    /// exclusive-fullscreen video-mode API: retro/terminal-style apps render a fixed cell grid,
    /// not a resolution-dependent 3D scene, so there is no benefit to an exclusive video-mode
    /// switch, only extra platform-specific complexity (enumerating
    /// [`VideoModeHandle`](winit::monitor::VideoModeHandle)s) for a mode real games would rarely
    /// want here.
    ///
    /// On `wasm32`, winit's web backend maps this to the browser's Fullscreen API
    /// (`Element.requestFullscreen`), which most browsers refuse to grant without a user
    /// gesture; requesting it unconditionally at window-creation time (before any gesture) is
    /// liable to silently fail there.
    #[must_use]
    pub const fn fullscreen(mut self, fullscreen: bool) -> Self {
        self.fullscreen = fullscreen;
        self
    }

    /// Sets whether the window's background supports transparency (alpha blending with whatever
    /// is behind it).
    ///
    /// Defaults to `false` (winit's own default).
    ///
    /// On `wasm32`, winit's web backend ignores this: a canvas is already alpha-blended with the
    /// page behind it via normal CSS compositing.
    #[must_use]
    pub const fn transparency(mut self, transparency: bool) -> Self {
        self.transparency = transparency;
        self
    }
}

/// Open a window and drive `app_loop` from the winit event loop.
///
/// On native this blocks the calling thread until the loop exits; on wasm it
/// returns immediately and the loop continues on `requestAnimationFrame`.
///
/// The closure receives `&mut Terminal<WindowBackend<P>>` and is called on
/// every frame tick. Window close pushes [`Event::Close`] into the event
/// queue rather than exiting: the game decides when to terminate.
///
/// Unlike [`run_on`](retroglyph_core::app::run_on), this driver presents automatically: see
/// <https://main.retroglyph.dev/book/explanation/architecture.html#presenting-is-automatic>.
///
/// # Errors
///
/// Returns [`winit::error::EventLoopError`] if the event loop cannot be
/// created or fails while running.
pub fn run_windowed<P, F>(
    config: WindowConfig,
    presenter: P,
    app_loop: F,
) -> Result<(), winit::error::EventLoopError>
where
    P: Presenter + 'static,
    F: FnMut(&mut Terminal<WindowBackend<P>>) + 'static,
{
    run_windowed_with_proxy(config, presenter, app_loop, |_proxy| {})
}

/// Same as [`run_windowed`], but also hands `on_proxy` an [`EventProxy`] for injecting
/// cross-thread events.
///
/// `on_proxy` is called synchronously right after the event loop (and the proxy) is created,
/// before this function starts blocking the calling thread on native. Use this over
/// [`run_windowed`] whenever another thread (network, audio, timer, ...) needs to wake the event
/// loop and deliver an [`Event::Custom`] to the app; `on_proxy` is the hook to hand a clone of the
/// proxy off to that thread before the loop takes over the calling thread.
///
/// The injected payload is always a `u64`, delivered as [`Event::Custom`] through the app's
/// normal `poll_event`/frame loop; see [`run_windowed_with_typed_proxy`] if a worker thread
/// needs to hand back a real payload (a loaded asset, a network response) instead of a
/// correlation id into a side table.
///
/// See <https://main.retroglyph.dev/book/explanation/architecture.html#presenting-is-automatic>:
/// this function shares the same automatic-present behavior.
///
/// # Examples
///
/// ```no_run
/// use retroglyph_core::event::Event;
/// use retroglyph_software::SoftwareBackendBuilder;
/// use retroglyph_window::winit::{WindowConfig, run_windowed_with_proxy};
/// use std::time::Duration;
///
/// let renderer = SoftwareBackendBuilder::new()
///     .grid_size(80, 25)
///     .scale(2)
///     .build()
///     .expect("backend init failed")
///     .into_renderer()
///     .expect("renderer init failed");
/// let config = WindowConfig::fit(&renderer, "My Game", None, true);
///
/// run_windowed_with_proxy(
///     config,
///     renderer,
///     move |term| {
///         if let Some(Event::Custom(id)) = term.poll(Duration::from_millis(16)) {
///             // Handle the tick/network/audio result tagged `id`.
///             println!("got custom event {id}");
///         }
///     },
///     |proxy| {
///         // Runs before the blocking call below starts, so the proxy can be
///         // handed off to a worker thread up front.
///         std::thread::spawn(move || loop {
///             std::thread::sleep(Duration::from_secs(1));
///             if proxy.send_event(1).is_err() {
///                 break; // The window closed; stop ticking.
///             }
///         });
///     },
/// )
/// .expect("event loop failed");
/// ```
///
/// # Errors
///
/// Returns [`winit::error::EventLoopError`] if the event loop cannot be
/// created or fails while running.
pub fn run_windowed_with_proxy<P, F, O>(
    config: WindowConfig,
    presenter: P,
    app_loop: F,
    on_proxy: O,
) -> Result<(), winit::error::EventLoopError>
where
    P: Presenter + 'static,
    F: FnMut(&mut Terminal<WindowBackend<P>>) + 'static,
    O: FnOnce(EventProxy),
{
    run_windowed_with_typed_proxy_and_exit_flag(
        config,
        Terminal::new(WindowBackend::new(presenter)),
        app_loop,
        on_proxy,
        push_custom_event,
        Rc::new(Cell::new(false)),
        Rc::new(Cell::new(false)),
    )
}

/// Same as [`run_windowed_with_proxy`], but the injected payload can be any `T: Send + 'static`
/// instead of a fixed `u64`.
///
/// A `T` payload never becomes a [`retroglyph_core::event::Event`]: [`Event::Custom`] is fixed to
/// `u64` (see its doc comment for why), so genericizing it would be a breaking change to
/// [`retroglyph_core`] far larger than this API needs. Instead, each injected `T` is handed
/// directly to `on_custom_event`, called synchronously from winit's `user_event` callback with
/// the same `&mut Terminal<WindowBackend<P>>` `app_loop` receives on redraw, so a handler that
/// wants the result to affect the next frame just needs to record it in state the closures
/// share, or push its own backend-agnostic event/marker for `app_loop` to notice.
///
/// See <https://main.retroglyph.dev/book/explanation/architecture.html#presenting-is-automatic>:
/// this function shares the same automatic-present behavior.
///
/// This delivery is a side channel, not a queued [`Event`]: `on_custom_event` runs as soon as
/// winit dispatches the `user_event`, which can be before `app_loop` next drains earlier-queued
/// window/input events via [`poll`](retroglyph_core::terminal::Terminal::poll). Don't assume a `T` arrives
/// interleaved with the `poll()` stream in send order relative to those events; if that matters,
/// use [`run_windowed_with_proxy`]'s plain `u64`/[`Event::Custom`] path instead, which does
/// interleave on the backend's own FIFO.
///
/// # Examples
///
/// ```no_run
/// use retroglyph_software::SoftwareBackendBuilder;
/// use retroglyph_window::winit::{WindowConfig, run_windowed_with_typed_proxy};
/// use std::time::Duration;
///
/// enum WorkerResult {
///     AssetLoaded { name: String, bytes: Vec<u8> },
/// }
///
/// let renderer = SoftwareBackendBuilder::new()
///     .grid_size(80, 25)
///     .scale(2)
///     .build()
///     .expect("backend init failed")
///     .into_renderer()
///     .expect("renderer init failed");
/// let config = WindowConfig::fit(&renderer, "My Game", None, true);
///
/// run_windowed_with_typed_proxy(
///     config,
///     renderer,
///     move |term| {
///         let _ = term.poll(Duration::from_millis(16));
///     },
///     |proxy| {
///         std::thread::spawn(move || {
///             let bytes = std::fs::read("asset.bin").unwrap_or_default();
///             let _ = proxy.send_event(WorkerResult::AssetLoaded {
///                 name: "asset.bin".into(),
///                 bytes,
///             });
///         });
///     },
///     |result: WorkerResult, _term| match result {
///         WorkerResult::AssetLoaded { name, bytes } => {
///             println!("loaded {name}: {} bytes", bytes.len());
///         }
///     },
/// )
/// .expect("event loop failed");
/// ```
///
/// # Errors
///
/// Returns [`winit::error::EventLoopError`] if the event loop cannot be
/// created or fails while running.
pub fn run_windowed_with_typed_proxy<T, P, F, O, D>(
    config: WindowConfig,
    presenter: P,
    app_loop: F,
    on_proxy: O,
    on_custom_event: D,
) -> Result<(), winit::error::EventLoopError>
where
    T: Send + 'static,
    P: Presenter + 'static,
    F: FnMut(&mut Terminal<WindowBackend<P>>) + 'static,
    O: FnOnce(EventProxy<T>),
    D: FnMut(T, &mut Terminal<WindowBackend<P>>) + 'static,
{
    run_windowed_with_typed_proxy_and_exit_flag(
        config,
        Terminal::new(WindowBackend::new(presenter)),
        app_loop,
        on_proxy,
        on_custom_event,
        Rc::new(Cell::new(false)),
        Rc::new(Cell::new(false)),
    )
}

/// Delivers a `u64` payload injected through [`EventProxy::send_event`] as
/// [`Event::Custom`]: the fixed `on_custom_event` behind [`run_windowed_with_proxy`]/
/// [`run_app_with_proxy`], preserving the pre-generic behavior exactly.
fn push_custom_event<P: Presenter>(id: u64, term: &mut Terminal<WindowBackend<P>>) {
    term.backend_mut().push_event(Event::Custom(id));
}

/// Shared implementation behind [`run_windowed_with_proxy`], [`run_windowed_with_typed_proxy`],
/// [`run_app_with_proxy`], [`run_app_with_typed_proxy`], and (via `run_app_on_with_typed_proxy`)
/// [`run_app_on`]. Takes an already-built `Terminal` rather than a bare `presenter`: every caller
/// but [`run_app_on`] itself builds one with `Terminal::new(WindowBackend::new(presenter))` first.
///
/// `exit_requested` is checked after every [`WindowEvent::RedrawRequested`] and, when set, drives
/// [`ActiveEventLoop::exit`] so the loop unwinds normally (see [`WindowApp::exit_requested`]'s doc
/// comment for why this can't be plumbed through `app_loop`'s return value instead).
/// [`run_windowed_with_proxy`]/[`run_windowed_with_typed_proxy`] pass flags nobody ever sets (a
/// plain `FnMut(&mut Terminal<..>)` closure has no way to reach them); [`run_app_with_proxy`]/
/// [`run_app_with_typed_proxy`] share both with the closure they build around `app_loop`: it sets
/// `exit_requested` on [`Flow::Exit`](retroglyph_core::app::Flow::Exit) and `skip_present` on
/// [`Flow::Idle`](retroglyph_core::app::Flow::Idle).
fn run_windowed_with_typed_proxy_and_exit_flag<T, P, F, O, D>(
    config: WindowConfig,
    terminal: Terminal<WindowBackend<P>>,
    app_loop: F,
    on_proxy: O,
    on_custom_event: D,
    exit_requested: Rc<Cell<bool>>,
    skip_present: Rc<Cell<bool>>,
) -> Result<(), winit::error::EventLoopError>
where
    T: Send + 'static,
    P: Presenter + 'static,
    F: FnMut(&mut Terminal<WindowBackend<P>>) + 'static,
    O: FnOnce(EventProxy<T>),
    D: FnMut(T, &mut Terminal<WindowBackend<P>>) + 'static,
{
    let event_loop = EventLoop::<T>::with_user_event().build()?;
    on_proxy(EventProxy(event_loop.create_proxy()));

    // `Some(0)` has no finite pacing interval to express, so it falls back to uncapped rather
    // than computing `Duration::from_secs_f64(f64::INFINITY)` (which panics).
    let frame_interval = config
        .target_fps
        .filter(|&fps| fps != 0)
        .map(|fps| Duration::from_secs_f64(1.0 / f64::from(fps)));

    let attrs = WindowAttrs::from(&config);
    let app = WindowApp {
        terminal: Some(terminal),
        app_loop,
        on_custom_event,
        window: None,
        title: config.title,
        init_size: InitWindowSize {
            width: config.width,
            height: config.height,
        },
        attrs,
        #[cfg(target_arch = "wasm32")]
        fill_viewport: config.fill_viewport,
        current_modifiers: KeyModifiers::NONE,
        cursor_px: (0.0, 0.0),
        active_touch: None,
        held_buttons: 0,
        frame_interval,
        event_driven: config.event_driven,
        #[cfg(not(target_arch = "wasm32"))]
        next_frame: std::time::Instant::now(),
        exit_requested,
        skip_present,
        needs_redraw: true,
        consecutive_present_errors: 0,
        _user_event: PhantomData,
    };

    #[cfg(not(target_arch = "wasm32"))]
    {
        let mut app = app;
        event_loop.run_app(&mut app)
    }

    #[cfg(target_arch = "wasm32")]
    {
        use winit::platform::web::EventLoopExtWebSys;
        event_loop.spawn_app(app);
        Ok(())
    }
}

/// Drive an [`App`](retroglyph_core::app::App) from the windowed event loop.
///
/// This is the inverted driver: winit owns the event loop and calls back
/// into the app on each redraw, rather than the app owning a `while` loop.
///
/// Each frame builds a [`Frame`](retroglyph_core::app::Frame) with a wall-clock
/// `dt` measured via [`web_time::Instant`]: a plain [`std::time::Instant`]
/// re-export on native, backed by the browser's `Performance.now()` on
/// `wasm32` (where `std::time::Instant` itself is unavailable). Calls
/// [`App::update`](retroglyph_core::app::App::update).
///
/// On [`Flow::Exit`](retroglyph_core::app::Flow) the event loop exits gracefully
/// (via [`ActiveEventLoop::exit`]) instead of force-exiting the process, so
/// the stack unwinds normally and `Drop` impls up the call chain (unflushed
/// writes, GPU/surface teardown, app-level RAII) run before the process
/// exits. This works the same on wasm: winit's web backend implements
/// `ActiveEventLoop::exit` by stopping its `requestAnimationFrame`-driven
/// runner rather than leaving it a no-op.
///
/// See <https://main.retroglyph.dev/book/explanation/architecture.html#presenting-is-automatic>:
/// this driver presents automatically after each [`App::update`](retroglyph_core::app::App::update)
/// call, except on [`Flow::Idle`](retroglyph_core::app::Flow::Idle).
///
/// # Resizing is not automatic
///
/// This driver does not resize the [`Terminal`] itself. On every window resize it pushes
/// [`Event::Resize`] with the new cell dimensions; the app must poll that event and call
/// [`Terminal::resize`] to resize the terminal's own grid buffers.
///
/// # Return contract differs on `wasm32`
///
/// On native this call blocks and only returns once the loop reaches
/// [`Flow::Exit`](retroglyph_core::app::Flow::Exit). On `wasm32` it returns `Ok(())` immediately
/// after handing the app to the browser's `requestAnimationFrame`-driven runner: the app keeps
/// running after this call returns, so code that follows it executes concurrently with a live
/// app rather than after it exits. [`retroglyph_core::app::run_on_with`] has no such split;
/// it always blocks until the app exits, on every target.
///
/// # Errors
///
/// Returns [`winit::error::EventLoopError`] if the event loop cannot be
/// created or fails while running.
pub fn run_app<P, A>(
    config: WindowConfig,
    presenter: P,
    app: A,
) -> Result<(), winit::error::EventLoopError>
where
    P: Presenter + 'static,
    A: retroglyph_core::app::App<WindowBackend<P>> + 'static,
{
    run_app_with_proxy(config, presenter, app, |_proxy| {})
}

/// Same as [`run_app`], but takes an already-built [`Terminal`] instead of a bare `presenter`.
///
/// [`run_app`] builds its `Terminal` internally (`Terminal::new(WindowBackend::new(presenter))`);
/// this is the lower-level entry for a caller that needs to configure the `Terminal` (or reach
/// into its [`WindowBackend`]) before the loop starts, which is impossible through `run_app`
/// alone.
///
/// See [`run_app`]'s "Resizing is not automatic" and "Return contract differs on `wasm32`"
/// sections, and <https://main.retroglyph.dev/book/explanation/architecture.html#presenting-is-automatic>:
/// this function shares all three behaviors.
///
/// # Errors
///
/// Returns [`winit::error::EventLoopError`] if the event loop cannot be
/// created or fails while running.
pub fn run_app_on<P, A>(
    config: WindowConfig,
    terminal: Terminal<WindowBackend<P>>,
    app: A,
) -> Result<(), winit::error::EventLoopError>
where
    P: Presenter + 'static,
    A: retroglyph_core::app::App<WindowBackend<P>> + 'static,
{
    run_app_on_with_typed_proxy(
        config,
        terminal,
        app,
        |_proxy: EventProxy| {},
        push_custom_event,
    )
}

/// Same as [`run_app`], but also hands `on_proxy` an [`EventProxy`] for injecting cross-thread
/// events.
///
/// See [`run_windowed_with_proxy`] for when/why to use the `_with_proxy` variant over the plain
/// one. The injected payload is always a `u64`, delivered as [`Event::Custom`]; see
/// [`run_app_with_typed_proxy`] for injecting any `T: Send + 'static`.
///
/// See <https://main.retroglyph.dev/book/explanation/architecture.html#presenting-is-automatic>:
/// this function shares the same automatic-present behavior.
///
/// # Errors
///
/// Returns [`winit::error::EventLoopError`] if the event loop cannot be
/// created or fails while running.
pub fn run_app_with_proxy<P, A, O>(
    config: WindowConfig,
    presenter: P,
    app: A,
    on_proxy: O,
) -> Result<(), winit::error::EventLoopError>
where
    P: Presenter + 'static,
    A: retroglyph_core::app::App<WindowBackend<P>> + 'static,
    O: FnOnce(EventProxy),
{
    run_app_with_typed_proxy(config, presenter, app, on_proxy, push_custom_event)
}

/// Same as [`run_app_with_proxy`], but the injected payload can be any `T: Send + 'static`
/// instead of a fixed `u64`.
///
/// See [`run_windowed_with_typed_proxy`] for the same generalization on the raw closure-based
/// driver, including why a non-`u64` payload bypasses [`retroglyph_core::event::Event`] entirely
/// and goes straight to `on_custom_event`.
///
/// See <https://main.retroglyph.dev/book/explanation/architecture.html#presenting-is-automatic>:
/// this function shares the same automatic-present behavior.
///
/// # Errors
///
/// Returns [`winit::error::EventLoopError`] if the event loop cannot be
/// created or fails while running.
pub fn run_app_with_typed_proxy<T, P, A, O, D>(
    config: WindowConfig,
    presenter: P,
    app: A,
    on_proxy: O,
    on_custom_event: D,
) -> Result<(), winit::error::EventLoopError>
where
    T: Send + 'static,
    P: Presenter + 'static,
    A: retroglyph_core::app::App<WindowBackend<P>> + 'static,
    O: FnOnce(EventProxy<T>),
    D: FnMut(T, &mut Terminal<WindowBackend<P>>) + 'static,
{
    run_app_on_with_typed_proxy(
        config,
        Terminal::new(WindowBackend::new(presenter)),
        app,
        on_proxy,
        on_custom_event,
    )
}

/// Shared implementation behind [`run_app_with_typed_proxy`] and [`run_app_on`], taking an
/// already-built `Terminal` the way [`run_app_on`] does; [`run_app_with_typed_proxy`] builds one
/// from a bare `presenter` and delegates here.
fn run_app_on_with_typed_proxy<T, P, A, O, D>(
    config: WindowConfig,
    terminal: Terminal<WindowBackend<P>>,
    mut app: A,
    on_proxy: O,
    on_custom_event: D,
) -> Result<(), winit::error::EventLoopError>
where
    T: Send + 'static,
    P: Presenter + 'static,
    A: retroglyph_core::app::App<WindowBackend<P>> + 'static,
    O: FnOnce(EventProxy<T>),
    D: FnMut(T, &mut Terminal<WindowBackend<P>>) + 'static,
{
    let mut frame_count = 0u64;
    let mut last = web_time::Instant::now();
    let exit_requested = Rc::new(Cell::new(false));
    let exit_requested_in_loop = exit_requested.clone();
    let skip_present = Rc::new(Cell::new(false));
    let skip_present_in_loop = skip_present.clone();
    run_windowed_with_typed_proxy_and_exit_flag(
        config,
        terminal,
        move |term| {
            let now = web_time::Instant::now();
            let delta = now.duration_since(last);
            last = now;
            let frame = retroglyph_core::app::Frame {
                delta,
                frame: frame_count,
            };
            frame_count = frame_count.wrapping_add(1);
            match app.update(term, &frame) {
                retroglyph_core::app::Flow::Exit => exit_requested_in_loop.set(true),
                // Nothing changed: tell `handle_redraw_requested` to skip its automatic present
                // for this frame. `Terminal::present` always presents unconditionally, so this
                // flag is the only thing standing between an idle frame and an unwanted redraw.
                retroglyph_core::app::Flow::Idle => skip_present_in_loop.set(true),
                // `Flow` is `#[non_exhaustive]`; any other variant (including `Continue`) presents
                // as usual via `handle_redraw_requested`'s automatic present.
                _ => {}
            }
        },
        on_proxy,
        on_custom_event,
        exit_requested,
        skip_present,
    )
}

/// Initial window dimensions used before the first Resized event.
struct InitWindowSize {
    width: u32,
    height: u32,
}

/// The subset of [`WindowConfig`]'s builder attributes applied once, up front, to
/// `Window::default_attributes()` in [`create_window_and_surface`](WindowApp::create_window_and_surface).
///
/// Grouped into its own type (rather than six more fields directly on [`WindowApp`]) since
/// they're only ever read in that one place, unlike `fill_viewport`, which also gates per-resize
/// behavior elsewhere.
// See `WindowConfig`'s matching `#[allow]` for why these bools are independent toggles, not a
// state machine.
#[allow(clippy::struct_excessive_bools)]
struct WindowAttrs {
    resizable: bool,
    decorations: bool,
    min_size: Option<(u32, u32)>,
    max_size: Option<(u32, u32)>,
    initial_position: Option<(i32, i32)>,
    fullscreen: bool,
    transparency: bool,
}

impl From<&WindowConfig> for WindowAttrs {
    fn from(config: &WindowConfig) -> Self {
        Self {
            resizable: config.resizable,
            decorations: config.decorations,
            min_size: config.min_size,
            max_size: config.max_size,
            initial_position: config.initial_position,
            fullscreen: config.fullscreen,
            transparency: config.transparency,
        }
    }
}

impl Default for WindowAttrs {
    /// Mirrors [`WindowConfig::fit`]'s defaults, for tests that construct a [`WindowApp`]
    /// directly without going through a [`WindowConfig`].
    fn default() -> Self {
        Self {
            resizable: true,
            decorations: true,
            min_size: None,
            max_size: None,
            initial_position: None,
            fullscreen: false,
            transparency: false,
        }
    }
}

/// Bitmask for [`MouseButton::Left`] in [`WindowApp::held_buttons`].
const BUTTON_MASK_LEFT: u8 = 1 << 0;
/// Bitmask for [`MouseButton::Right`] in [`WindowApp::held_buttons`].
const BUTTON_MASK_RIGHT: u8 = 1 << 1;
/// Bitmask for [`MouseButton::Middle`] in [`WindowApp::held_buttons`].
const BUTTON_MASK_MIDDLE: u8 = 1 << 2;

/// Maps a [`MouseButton`] to its bit in [`WindowApp::held_buttons`].
const fn button_mask(button: MouseButton) -> u8 {
    match button {
        MouseButton::Left => BUTTON_MASK_LEFT,
        MouseButton::Right => BUTTON_MASK_RIGHT,
        MouseButton::Middle => BUTTON_MASK_MIDDLE,
        // `MouseButton` is `#[non_exhaustive]`; treat any future variant as unmasked (never
        // drives a `Drag`) rather than failing to compile when one is added upstream.
        _ => 0,
    }
}

/// The winit `ApplicationHandler`: owns the window, the terminal, and the
/// per-frame closure.
///
/// Generic over the injected user-event payload `T` and its delivery handler `D`, so the same
/// type backs both the `u64`/[`Event::Custom`] path ([`run_windowed_with_proxy`]/
/// [`run_app_with_proxy`], where `T = u64` and `D` is [`push_custom_event`]) and the typed-`T`
/// path ([`run_windowed_with_typed_proxy`]/[`run_app_with_typed_proxy`], where `D` is the
/// caller-supplied `on_custom_event`).
struct WindowApp<P: Presenter, F, T, D> {
    terminal: Option<Terminal<WindowBackend<P>>>,
    app_loop: F,
    /// Delivers one injected `T` payload to the app; see [`handle_user_event`](Self::handle_user_event).
    on_custom_event: D,
    /// `T` only ever appears as `D`'s argument, never stored directly: see [`ApplicationHandler`]
    /// for why `WindowApp` still needs to name it (winit dispatches `user_event` generically over
    /// the event-loop's payload type).
    _user_event: PhantomData<fn(T)>,
    window: Option<Arc<Window>>,
    title: String,
    init_size: InitWindowSize,
    /// See [`WindowConfig`]'s `resizable`/`decorations`/`min_size`/`max_size`/
    /// `initial_position`/`fullscreen`/`transparency` fields; applied once at window creation.
    attrs: WindowAttrs,
    /// See [`WindowConfig::fill_viewport`]. Only meaningful on `wasm32`; not
    /// even stored on native, where it would do nothing.
    #[cfg(target_arch = "wasm32")]
    fill_viewport: bool,
    /// Current modifier key state, updated by `ModifiersChanged` events.
    current_modifiers: KeyModifiers,
    /// Last known cursor position in physical pixels.
    cursor_px: (f64, f64),
    /// The finger currently treated as the pointer, if any.
    ///
    /// Touch input (mobile browsers, touchscreens) arrives as
    /// [`WindowEvent::Touch`], not as `CursorMoved`/`MouseInput`. The first
    /// finger down is adopted as "the pointer" and synthesized into the same
    /// left-button mouse events games already handle; other fingers are
    /// ignored until it lifts, so a stray second finger can't teleport the
    /// cursor mid-drag.
    active_touch: Option<u64>,
    /// Bitmask of currently held mouse buttons, built from [`button_mask`]. Updated by
    /// [`on_mouse_input`](Self::on_mouse_input) and consulted by
    /// [`on_cursor_moved`](Self::on_cursor_moved) to decide between [`MouseEventKind::Moved`] and
    /// [`MouseEventKind::Drag`]. A bitmask (rather than tracking only the most recent button)
    /// because more than one button can be held at once, and each needs its own accurate
    /// press/release accounting.
    held_buttons: u8,
    /// Frame-rate cap derived from [`WindowConfig::target_fps`]: `Some(interval)` paces redraws
    /// to no more than one per `interval`, `None` leaves them uncapped. Independent of
    /// [`event_driven`](Self::event_driven); see [`WindowConfig::fit`].
    ///
    /// Stored on `wasm32` too, where only the `Some`/`None` distinction is used: the browser's
    /// `requestAnimationFrame` already paces the loop, so there is no deadline to sleep until.
    frame_interval: Option<Duration>,
    /// Deadline for the next frame when `frame_interval` is set. Native only: `wasm32` has no
    /// sleeping event loop to schedule against.
    #[cfg(not(target_arch = "wasm32"))]
    next_frame: std::time::Instant,
    /// Whether [`about_to_wait`](ApplicationHandler::about_to_wait) gates redraws on
    /// [`needs_redraw`](Self::needs_redraw) (`true`) or always redraws every tick (`false`),
    /// as passed to [`WindowConfig::fit`]. Independent of
    /// [`frame_interval`](Self::frame_interval): this controls *whether* a tick redraws at all,
    /// the frame-rate cap controls *how often* once it does.
    event_driven: bool,
    /// Set by `app_loop` (specifically [`run_app_with_proxy`]'s closure) to request the event
    /// loop stop, instead of calling `std::process::exit` directly.
    ///
    /// `app_loop` is a plain `FnMut(&mut Terminal<..>)` with no return value and no
    /// [`ActiveEventLoop`] handle, so it can't call `event_loop.exit()` itself; it can only flip
    /// this shared flag. [`handle_window_event`](Self::handle_window_event) (which runs
    /// `app_loop` on [`WindowEvent::RedrawRequested`]) also takes no
    /// [`ActiveEventLoop`], so unit tests can drive it without a live winit loop (see its
    /// doc comment). `ApplicationHandler::window_event`, which does have the `ActiveEventLoop`,
    /// checks this flag right after `handle_window_event` returns and calls `event_loop.exit()`
    /// if it's set, letting the stack unwind normally (`Drop` impls run) instead of
    /// force-terminating the process.
    exit_requested: Rc<Cell<bool>>,
    /// Set by `app_loop` (specifically [`run_app_with_proxy`]'s closure) on
    /// [`Flow::Idle`](retroglyph_core::app::Flow::Idle) to tell
    /// [`handle_redraw_requested`](Self::handle_redraw_requested) to skip its automatic present
    /// for this frame. Cleared at the start of every `handle_redraw_requested` call, so it only
    /// ever reflects the outcome of the `app_loop` call about to run.
    ///
    /// A plain `FnMut(&mut Terminal<..>)` closure (`run_windowed`/`run_windowed_with_proxy`) has
    /// no `Flow` concept and never sets this, the same way it never sets `exit_requested`.
    skip_present: Rc<Cell<bool>>,
    /// Set whenever something happened that the app loop should get a chance to react to:
    /// window creation, an input/window event, or an injected [`Event::Custom`]. Cleared once
    /// [`about_to_wait`](ApplicationHandler::about_to_wait) turns it into a `request_redraw()`
    /// call.
    ///
    /// Retro/terminal-style apps are event-driven, not animation-driven, so "nothing happened"
    /// should mean "render nothing new": see this field's use in `about_to_wait` for why that
    /// keeps the loop asleep (`ControlFlow::Wait`) instead of spinning at ~100% CPU redrawing an
    /// unchanged frame forever.
    ///
    /// Only consulted when [`event_driven`](Self::event_driven) is `true`, i.e. redraw-on-demand
    /// mode. An app that animates over time has no event to point at and would freeze under this
    /// gate, which is what `event_driven: false` (continuous mode) is for; see
    /// [`WindowConfig::fit`].
    needs_redraw: bool,
    /// Count of consecutive `present()` failures, reset to 0 on the next success. Drives
    /// [`present_failure_action`]'s logging-verbosity and surface-recovery decisions in the
    /// `RedrawRequested` arm of [`handle_window_event`](Self::handle_window_event).
    consecutive_present_errors: u32,
}

impl<P: Presenter, F, T, D> WindowApp<P, F, T, D> {
    /// Create the window and initialize the surface.
    ///
    /// Returns `Some(window)` on success, logs and returns `None` on failure.
    fn create_window_and_surface(&mut self, event_loop: &ActiveEventLoop) -> Option<Arc<Window>> {
        // On native, size the window to fit the grid (`WindowConfig::fit`)
        // and let the OS window manager own further resizing. On wasm, if
        // `fill_viewport` is set, there's no OS window to fit into (the
        // canvas *is* the page), so size it to the browser viewport
        // instead, for a full-screen, mobile-web-app feel; otherwise it's
        // sized the same as native (`init_size`, the natural grid size),
        // which is what most demos/examples want; see
        // `WindowConfig::fill_viewport`'s doc comment. winit sets an inline
        // `width`/`height` style on the canvas matching whatever size we
        // request here; it does not derive that size from page CSS, so this
        // has to happen in Rust.
        //
        // Crucially, the viewport-filling size *must* be the viewport size
        // at the real (uncapped) device pixel ratio, not the DPR-capped size
        // used for the software backing store below. winit's wasm backend
        // converts whatever `PhysicalSize` we pass here back to a logical
        // (CSS pixel) size using `window.devicePixelRatio()` (the actual,
        // uncapped ratio) to set the canvas's inline `style.width`/
        // `style.height`. Handing it a DPR-capped physical size makes it
        // divide by a *larger* real DPR than the one used to compute that
        // size, so the resulting CSS size comes out smaller than the
        // viewport (the higher the real DPR above the cap, the more the
        // canvas visibly shrinks, on a phone with DPR 3 and our 1.5 cap,
        // that's 50% of the screen). See `web::web_viewport_surface_physical_size`
        // for the separate, capped size used for the raster backing store.
        // On native, `init_size` is already expressed in true physical
        // pixels; `WindowConfig::fit` derives it from
        // `Presenter::cell_size()`, which is documented to return physical
        // (not logical/DPI-scaled) pixels. Requesting that count directly
        // as a `PhysicalSize` is therefore already correct on a HiDPI
        // display; scaling it again by the monitor's `scale_factor` would
        // double the window size (see retroglyph#701).
        #[cfg(not(target_arch = "wasm32"))]
        let physical_size =
            winit::dpi::PhysicalSize::new(self.init_size.width, self.init_size.height);
        #[cfg(target_arch = "wasm32")]
        let physical_size = if self.fill_viewport {
            web::web_viewport_layout_physical_size().unwrap_or_else(|| {
                winit::dpi::PhysicalSize::new(self.init_size.width, self.init_size.height)
            })
        } else {
            winit::dpi::PhysicalSize::new(self.init_size.width, self.init_size.height)
        };
        #[cfg(target_arch = "wasm32")]
        let surface_physical_size = if self.fill_viewport {
            web::web_viewport_surface_physical_size().unwrap_or(physical_size)
        } else {
            physical_size
        };
        #[cfg(not(target_arch = "wasm32"))]
        let surface_physical_size = physical_size;

        let attrs = Window::default_attributes()
            .with_title(&self.title)
            .with_inner_size(physical_size)
            .with_resizable(self.attrs.resizable)
            .with_decorations(self.attrs.decorations)
            .with_transparent(self.attrs.transparency);
        let attrs = match self.attrs.min_size {
            Some((w, h)) => attrs.with_min_inner_size(winit::dpi::PhysicalSize::new(w, h)),
            None => attrs,
        };
        let attrs = match self.attrs.max_size {
            Some((w, h)) => attrs.with_max_inner_size(winit::dpi::PhysicalSize::new(w, h)),
            None => attrs,
        };
        let attrs = match self.attrs.initial_position {
            Some((x, y)) => attrs.with_position(winit::dpi::PhysicalPosition::new(x, y)),
            None => attrs,
        };
        let attrs = if self.attrs.fullscreen {
            attrs.with_fullscreen(Some(winit::window::Fullscreen::Borderless(None)))
        } else {
            attrs
        };

        #[cfg(target_family = "wasm")]
        let attrs = {
            use winit::platform::web::WindowAttributesExtWebSys;
            attrs.with_append(true)
        };

        let window = Arc::new(match event_loop.create_window(attrs) {
            Ok(w) => w,
            Err(e) => {
                log::error!("window creation failed: {e}");
                event_loop.exit();
                return None;
            }
        });

        // IME composition (`WindowEvent::Ime`) is opt-in per winit's own doc comment on that
        // variant: without this, platform input methods (Pinyin, Kana, dead-key accents, ...)
        // never surface composed text at all, silently limiting windowed-app text input to
        // whatever a bare `KeyboardInput` logical key can express. See `translate::translate_ime`
        // for how a committed composition is turned into an `Event`.
        window.set_ime_allowed(true);

        if let Some(term) = self.terminal.as_mut() {
            // Hand the presenter a windowing-library-agnostic handle (see
            // `Presenter::init_surface`); the winit window stays owned here.
            let handle: Arc<dyn crate::presenter::WindowHandle> = window.clone();
            if let Err(e) = term.backend_mut().presenter_mut().init_surface(handle) {
                log::error!("surface init failed: {e}");
                event_loop.exit();
                return None;
            }
            // Set the initial surface size (required on WASM before first present), using
            // `surface_physical_size`, not `physical_size`: the
            // raster backing store stays DPR-capped for present() cost even
            // though the canvas's CSS size (driven by `physical_size` via
            // winit above) matches the full, uncapped viewport.
            term.backend_mut()
                .presenter_mut()
                .resize_surface(surface_physical_size.width, surface_physical_size.height);
        }

        // Keep the canvas matching the browser viewport as it changes
        // (device rotation, browser window resize, address-bar
        // show/hide): winit only reacts to size changes we ask for
        // ourselves (`request_inner_size`), so a `resize` listener is
        // required to make this genuinely responsive rather than a
        // one-shot fit at startup. Only installed when `fill_viewport` is
        // set, otherwise the canvas should stay at its natural grid size
        // regardless of viewport changes.
        #[cfg(target_arch = "wasm32")]
        if self.fill_viewport {
            web::install_viewport_resize_listener(&window);
        }

        // `WindowEvent::ThemeChanged` (handled in `handle_window_event`)
        // only fires on a *change*, so an app that never sees a system
        // theme change would otherwise never learn the starting one.
        // `Window::theme()` reflects the current system theme both on
        // native and on winit's web target (backed by the
        // `prefers-color-scheme` media query there), so query it once
        // up-front and synthesize the same event a live change would send.
        if let Some(theme) = window.theme()
            && let Some(term) = self.terminal.as_mut()
        {
            term.backend_mut().push_event(system_theme_event(theme));
        }

        Some(window)
    }
}

/// Number of consecutive `present()` failures after which
/// [`handle_window_event`](WindowApp::handle_window_event)'s `RedrawRequested` arm attempts to
/// recover by re-initializing the surface (see [`PresentFailureAction::Recover`]).
///
/// Roughly half a second at 60 FPS: long enough that a single dropped frame (a transient `VSync`
/// hiccup, a momentarily occluded window) never triggers a surface rebuild, but short enough that
/// a genuinely broken surface (context loss, invalidated swapchain) doesn't sit unrecovered for
/// many seconds.
const PRESENT_FAILURE_RECOVERY_THRESHOLD: u32 = 30;

/// What [`handle_window_event`](WindowApp::handle_window_event)'s `RedrawRequested` arm should do
/// in response to the outcome of one `present()` call, given the running count of consecutive
/// failures *before* this call.
///
/// [`Presenter::SurfaceError`] is a generic associated type: the software backend's
/// `SurfaceError` just wraps `softbuffer::SoftBufferError`, a plain `#[non_exhaustive]` enum with
/// no `Lost`/`Outdated`/`Timeout` discrimination the way `wgpu::SurfaceError` has, so most
/// backends can't pattern-match on *why* a present failed to decide whether it's recoverable the
/// way a wgpu-based app would. All they can generally observe is a bare `Display`able error and
/// whether the failure is a one-off or persistent (via the consecutive-failure count), so the
/// recovery strategy here is generic for that case: rate-limit logging so a
/// persistent failure doesn't spam every frame, and after a run of failures long enough to rule
/// out a one-off glitch, attempt the one backend-agnostic recovery available: re-running
/// [`Presenter::init_surface`] to rebuild the surface from scratch, the same call
/// [`create_window_and_surface`](WindowApp::create_window_and_surface) makes at startup.
///
/// [`RecoverableError::is_recoverable`](crate::presenter::RecoverableError::is_recoverable) is
/// the escape hatch for a presenter that *can* categorize its errors: when a failed `present()`
/// reports `is_recoverable() == false`, that decision table is skipped entirely in favor of
/// [`PresentFailureAction::Fatal`]: retrying a failure the presenter itself already knows is
/// unrecoverable can't help, so there's no reason to wait out the consecutive-failure threshold
/// first.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum PresentFailureAction {
    /// Presenting succeeded; if `was_failing` is `true` the caller should log recovery at `info`
    /// or `warn` level (a prior failure streak just ended).
    Ok { was_failing: bool },
    /// Presenting failed; log at `error!` (first failure in a streak, or the very first ever)
    /// or suppress (a already-logged, ongoing streak below the recovery threshold).
    Log { at_error_level: bool },
    /// Presenting failed and the consecutive-failure count just crossed the recovery threshold:
    /// log at `warn!` and attempt to reinitialize the surface.
    Recover,
    /// Presenting failed with an error the presenter reports as unrecoverable (see
    /// [`RecoverableError::is_recoverable`](crate::presenter::RecoverableError::is_recoverable)):
    /// log at `error!` immediately and skip the consecutive-failure/recovery bookkeeping
    /// entirely: rebuilding the surface via [`Presenter::init_surface`] cannot help a failure
    /// already classified as fatal.
    Fatal,
}

/// Decides the action for one `present()` outcome, given `consecutive_failures` *before* this
/// call (0 if the previous call succeeded or this is the first call) and, for a failed call,
/// whether the presenter reports the error as recoverable (see
/// [`RecoverableError::is_recoverable`](crate::presenter::RecoverableError::is_recoverable);
/// ignored when `succeeded` is `true`).
///
/// Pure decision table, kept separate from the live `RedrawRequested` handling (which needs a
/// real `Terminal`/`Presenter`/`Window`) so the threshold and logging-level logic is unit
/// -testable without any of those, the same reasoning as [`web::dpr_pointer_scale`] above.
const fn present_failure_action(
    consecutive_failures: u32,
    succeeded: bool,
    recoverable: bool,
) -> PresentFailureAction {
    if succeeded {
        return PresentFailureAction::Ok {
            was_failing: consecutive_failures > 0,
        };
    }
    if !recoverable {
        return PresentFailureAction::Fatal;
    }
    // `consecutive_failures` is the count *before* this failure, so the count *including* this
    // one is `consecutive_failures + 1`; recover exactly when that reaches the threshold, and
    // again every full threshold-worth of failures after that (so a failed recovery attempt
    // doesn't get retried on literally the next frame, hot-looping surface rebuilds).
    if (consecutive_failures + 1).is_multiple_of(PRESENT_FAILURE_RECOVERY_THRESHOLD) {
        return PresentFailureAction::Recover;
    }
    PresentFailureAction::Log {
        at_error_level: consecutive_failures == 0,
    }
}

/// Continuous mode's next-frame decision on native: `None` while `now` is still short of
/// `next_frame` (the caller parks the loop on `ControlFlow::WaitUntil(next_frame)`), or
/// `Some(advanced)` once the deadline has passed, where `advanced` is the deadline for the frame
/// after this one.
///
/// `advanced` is `next_frame + interval` clamped to `now`, so a frame that overran its budget (a
/// stalled GPU, a descheduled thread) resumes from the present rather than firing a burst of
/// catch-up renders to "make up" the lost time: there is nothing to make up when every frame
/// renders the current state.
///
/// Pure function of the two instants and the interval, kept separate from the live `about_to_wait`
/// handling (which needs an [`ActiveEventLoop`] no unit test can construct) for the same reason as
/// [`present_failure_action`] above. `wasm32` has no sleeping event loop
/// to schedule against and never calls this; see `about_to_wait`.
#[cfg(not(target_arch = "wasm32"))]
fn next_frame_deadline(
    now: std::time::Instant,
    next_frame: std::time::Instant,
    interval: Duration,
) -> Option<std::time::Instant> {
    if next_frame > now {
        return None;
    }
    Some((next_frame + interval).max(now))
}

/// Maps winit's [`Theme`](winit::window::Theme) to the backend-agnostic
/// [`Event::ThemeChanged`], the only place that conversion needs to happen.
const fn system_theme_event(theme: winit::window::Theme) -> Event {
    use retroglyph_core::event::SystemTheme;
    match theme {
        winit::window::Theme::Light => Event::ThemeChanged(SystemTheme::Light),
        winit::window::Theme::Dark => Event::ThemeChanged(SystemTheme::Dark),
    }
}

impl<P, F, T, D> ApplicationHandler<T> for WindowApp<P, F, T, D>
where
    P: Presenter,
    F: FnMut(&mut Terminal<WindowBackend<P>>) + 'static,
    T: 'static,
    D: FnMut(T, &mut Terminal<WindowBackend<P>>) + 'static,
{
    fn resumed(&mut self, event_loop: &ActiveEventLoop) {
        if let Some(window) = self.create_window_and_surface(event_loop) {
            self.window = Some(window);
        }
        // First frame: nothing has "happened" yet in the input-event sense, but the app still
        // needs an initial render once the window/surface exists.
        self.needs_redraw = true;
    }

    fn window_event(
        &mut self,
        event_loop: &ActiveEventLoop,
        _window_id: WindowId,
        event: WindowEvent,
    ) {
        self.handle_window_event(event);
        // `app_loop` (run on `RedrawRequested`, inside `handle_window_event`) can only signal
        // exit by setting `exit_requested`; see its doc comment for why. Check it here, where
        // an `ActiveEventLoop` is actually available, and ask winit to exit gracefully instead of
        // the caller force-exiting the process.
        if self.exit_requested.get() {
            event_loop.exit();
        }
    }

    fn user_event(&mut self, _event_loop: &ActiveEventLoop, event: T) {
        self.handle_user_event(event);
    }

    fn about_to_wait(&mut self, event_loop: &ActiveEventLoop) {
        // `event_driven` (redraw-on-demand): only proceed if something actually happened since
        // the last redraw. Otherwise park the loop at `ControlFlow::Wait` so it sleeps instead of
        // spinning at ~100% CPU re-rendering an unchanged frame every iteration -- retro/terminal-
        // style apps are idle most of the time and event-driven, so "nothing happened" should mean
        // "render nothing new". The reset must be explicit: winit's `ControlFlow` is sticky (a
        // `Cell` that persists across iterations; "Defaults to `Wait`" describes only the value
        // before the loop's first iteration, not a per-iteration reset), so once the paced branch
        // below has parked it at a `WaitUntil` deadline, that deadline stays live -- once it
        // elapses with `ControlFlow` never reset, the loop wakes again immediately, every
        // iteration, forever. See `needs_redraw`'s doc comment. Not `event_driven` (continuous):
        // always proceed, regardless of `needs_redraw`: an app driving a tween off `Frame::delta`
        // has something new to show every tick even though no input event arrived, which is
        // precisely what the `needs_redraw` gate cannot express.
        if self.event_driven && !self.needs_redraw {
            event_loop.set_control_flow(winit::event_loop::ControlFlow::Wait);
            return;
        }

        let Some(interval) = self.frame_interval else {
            // Uncapped: render every tick this point is reached.
            self.needs_redraw = false;
            self.request_redraw();
            return;
        };

        // Capped: pace to `interval`. The two platforms do that differently. Native sleeps until
        // the deadline and then renders, since `request_redraw` is serviced within the same loop
        // iteration. On `wasm32` there is nothing to sleep in: winit's web backend services
        // `request_redraw` on the browser's next `requestAnimationFrame`, roughly one display
        // frame later, so sleeping out a full interval *before* asking would pay that latency on
        // top of it and halve the achieved frame rate. Ask on every iteration instead and let
        // `requestAnimationFrame` do the pacing, which is also what the browser wants, since it
        // already throttles background tabs and matches the compositor's cadence.
        #[cfg(not(target_arch = "wasm32"))]
        match next_frame_deadline(std::time::Instant::now(), self.next_frame, interval) {
            None => {
                event_loop
                    .set_control_flow(winit::event_loop::ControlFlow::WaitUntil(self.next_frame));
                return;
            }
            Some(advanced) => self.next_frame = advanced,
        }
        #[cfg(target_arch = "wasm32")]
        let _ = interval;
        self.needs_redraw = false;
        self.request_redraw();
    }
}

impl<P, F, T, D> WindowApp<P, F, T, D>
where
    P: Presenter,
    F: FnMut(&mut Terminal<WindowBackend<P>>) + 'static,
    D: FnMut(T, &mut Terminal<WindowBackend<P>>) + 'static,
{
    /// Ask winit for a `RedrawRequested`, if the window exists yet.
    ///
    /// Both [`about_to_wait`](ApplicationHandler::about_to_wait) branches end here; the window is
    /// `None` only before `resumed` has run.
    fn request_redraw(&self) {
        if let Some(window) = &self.window {
            window.request_redraw();
        }
    }

    /// Drain one injected user event into `on_custom_event`.
    ///
    /// Extracted from the `ApplicationHandler::user_event` impl for the same reason as
    /// [`handle_window_event`](Self::handle_window_event): so the drain logic can be exercised in
    /// unit tests without a live [`ActiveEventLoop`]. There is only ever one event to drain per
    /// call (winit calls `user_event` once per [`EventProxy::send_event`]), so "drain" here
    /// means "push the one event this call carries", not draining a whole queue at once. For the
    /// `u64`/[`Event::Custom`] path, `on_custom_event` is [`push_custom_event`]; for a typed `T`,
    /// it's the caller-supplied `on_custom_event` handler passed to
    /// [`run_windowed_with_typed_proxy`]/[`run_app_with_typed_proxy`].
    fn handle_user_event(&mut self, event: T) {
        if let Some(term) = self.terminal.as_mut() {
            (self.on_custom_event)(event, term);
        }
        self.needs_redraw = true;
    }

    /// Dispatch a [`WindowEvent`] without requiring an [`ActiveEventLoop`].
    ///
    /// Extracted from the `ApplicationHandler` impl so the translation and
    /// event-buffer logic can be called directly in unit tests, where
    /// [`ActiveEventLoop`] is not constructable.
    fn handle_window_event(&mut self, event: WindowEvent) {
        // Every branch below (other than `RedrawRequested`, which *is* the render this flag
        // exists to gate) represents something the app loop should get a chance to react to on
        // the next frame; see `needs_redraw`'s doc comment for why that matters for idle CPU.
        // Set unconditionally up front rather than per-arm: simpler, and the only event that must
        // *not* set it (`RedrawRequested`) already clears it again in `about_to_wait` right before
        // requesting this same redraw, so a same-tick `RedrawRequested` can't retrigger itself.
        if !matches!(event, WindowEvent::RedrawRequested) {
            self.needs_redraw = true;
        }
        match event {
            WindowEvent::CloseRequested => {
                // Push the event so the game loop can process it (save game,
                // confirm dialog, etc.).  Do not call event_loop.exit() here;
                // the game decides when to terminate.
                if let Some(term) = self.terminal.as_mut() {
                    term.backend_mut().push_event(Event::Close);
                }
            }
            WindowEvent::Resized(size) => self.on_resized(size),
            WindowEvent::CursorMoved { position, .. } => self.on_cursor_moved(position),
            WindowEvent::MouseInput { state, button, .. } => self.on_mouse_input(state, button),
            WindowEvent::MouseWheel { delta, .. } => self.on_mouse_wheel(delta),
            WindowEvent::Touch(touch) => self.on_touch(touch),
            WindowEvent::ModifiersChanged(mods) => {
                self.current_modifiers = translate_modifiers(mods.state());
            }
            WindowEvent::ThemeChanged(theme) => {
                if let Some(term) = self.terminal.as_mut() {
                    term.backend_mut().push_event(system_theme_event(theme));
                }
            }
            WindowEvent::Focused(gained) => self.on_focus_changed(gained),
            WindowEvent::KeyboardInput { event, .. } => {
                if let Some(term) = self.terminal.as_mut()
                    && let Some(e) = translate_key(event, self.current_modifiers)
                {
                    term.backend_mut().push_event(e);
                }
            }
            WindowEvent::Ime(ime) => {
                if let Some(term) = self.terminal.as_mut()
                    && let Some(e) = translate_ime(ime)
                {
                    term.backend_mut().push_event(e);
                }
            }
            WindowEvent::ScaleFactorChanged { scale_factor, .. } => {
                self.on_scale_factor_changed(scale_factor);
            }

            WindowEvent::RedrawRequested => self.handle_redraw_requested(),

            _ => {}
        }
    }

    /// Runs the app closure, automatically presents the `Terminal` if the app didn't already (and
    /// didn't return [`Flow::Idle`](retroglyph_core::app::Flow::Idle)), and presents the frame to the
    /// surface, tracking consecutive `present()` failures to rate-limit logging and trigger
    /// surface recovery.
    ///
    /// See [`present_failure_action`] for the decision table; this method just runs the `Terminal`
    /// -/`Presenter`-dependent side effects (`app_loop`, `present`, `init_surface`, logging) that
    /// function can't perform itself since it's a pure function of the failure count alone.
    ///
    /// # Automatic `Terminal::present`
    ///
    /// Windowed apps no longer need to call [`Terminal::present`] themselves: this method calls it
    /// once, right after `app_loop` returns, unless [`skip_present`](Self::skip_present) was set
    /// (an [`App`](retroglyph_core::app::App) returned `Flow::Idle`) or
    /// [`Terminal::present_count`] shows `app_loop` already called it. A [`Terminal::present`]
    /// error is logged and does not stop the surface-level present below from running (matching
    /// this function's existing keep-going-on-failure philosophy); it uses a different error type
    /// (`<B as Output>::Error`) than [`Presenter::SurfaceError`], so it is tracked and logged
    /// independently of the consecutive-failure counter below, which is scoped to the surface
    /// present.
    fn handle_redraw_requested(&mut self) {
        let Some(term) = self.terminal.as_mut() else {
            return;
        };
        self.skip_present.set(false);
        let present_count_before = term.present_count();
        (self.app_loop)(term);
        if !self.skip_present.get()
            && term.present_count() == present_count_before
            && let Err(e) = term.present()
        {
            log::error!("automatic terminal present failed: {e}");
        }
        let result = term.backend_mut().presenter_mut().present();
        let succeeded = result.is_ok();
        let recoverable = result
            .as_ref()
            .err()
            .is_none_or(crate::presenter::RecoverableError::is_recoverable);
        match present_failure_action(self.consecutive_present_errors, succeeded, recoverable) {
            PresentFailureAction::Ok { was_failing } => {
                if was_failing {
                    log::info!(
                        "frame present recovered after {} consecutive failures",
                        self.consecutive_present_errors
                    );
                }
                self.consecutive_present_errors = 0;
            }
            PresentFailureAction::Log { at_error_level } => {
                self.consecutive_present_errors += 1;
                let e = result.unwrap_err();
                if at_error_level {
                    log::error!("frame present failed: {e}");
                } else {
                    // Ongoing failure streak below the recovery threshold: already logged at
                    // `error!` when the streak started, so avoid re-logging every single frame
                    // (the log-spam this issue exists to fix) while still keeping the detail
                    // available at `debug!` for anyone investigating a live failure.
                    log::debug!("frame present still failing: {e}");
                }
            }
            PresentFailureAction::Recover => {
                self.consecutive_present_errors += 1;
                let e = result.unwrap_err();
                log::warn!(
                    "frame present failed {} times consecutively ({e}); attempting surface recovery",
                    self.consecutive_present_errors
                );
                self.try_recover_surface();
            }
            PresentFailureAction::Fatal => {
                self.consecutive_present_errors += 1;
                let e = result.unwrap_err();
                log::error!("frame present failed with an unrecoverable error: {e}");
            }
        }
    }

    /// Attempts to recover from a persistent `present()` failure by re-running
    /// [`Presenter::init_surface`], the same call
    /// [`create_window_and_surface`](Self::create_window_and_surface) makes at startup.
    ///
    /// This is the only recovery available generically: [`Presenter::SurfaceError`] carries no
    /// structured "is this recoverable" signal (see [`present_failure_action`]'s doc comment), so
    /// rebuilding the surface from scratch is the one action that's meaningful across every
    /// backend. A no-op if there is no window to rebuild the surface from (headless/pre-`resumed`
    /// states), or if the terminal has already been torn down.
    fn try_recover_surface(&mut self) {
        let Some(window) = self.window.clone() else {
            return;
        };
        let Some(term) = self.terminal.as_mut() else {
            return;
        };
        let handle: Arc<dyn crate::presenter::WindowHandle> = window;
        if let Err(e) = term.backend_mut().presenter_mut().init_surface(handle) {
            log::error!("surface recovery failed: {e}");
        }
    }

    fn on_resized(&mut self, size: winit::dpi::PhysicalSize<u32>) {
        // On wasm with `fill_viewport` set, `size` is whatever (uncapped)
        // physical size we last handed winit for CSS layout purposes, not
        // the backing store size. Recompute the DPR-capped surface size
        // independently so the raster buffer doesn't silently lose its cap
        // on every resize. Without `fill_viewport`, the canvas never resizes
        // on its own (no listener installed above), so `size` here is
        // already the natural grid size and needs no such override.
        #[cfg(target_arch = "wasm32")]
        let size = if self.fill_viewport {
            web::web_viewport_surface_physical_size().unwrap_or(size)
        } else {
            size
        };
        self.resize_to(size);
    }

    /// React to a scale-factor (DPI) change: notify the presenter, then
    /// realign the surface and grid to the window's new physical size.
    ///
    /// Every modern `HiDPI` display is scaled, so without this the surface
    /// silently keeps rendering at the old (pre-change) physical size --
    /// e.g. half the true resolution after moving to a 2x-scale display --
    /// until (if ever) an independent `Resized` event happens to arrive.
    /// Reusing [`resize_to`](Self::resize_to) here mirrors
    /// [`on_resized`](Self::on_resized), so both paths clamp/align the
    /// surface to whole cells the same way.
    fn on_scale_factor_changed(&mut self, scale_factor: f64) {
        if let Some(term) = self.terminal.as_mut() {
            term.backend_mut()
                .presenter_mut()
                .scale_factor_changed(scale_factor);
        }
        let Some(window) = self.window.clone() else {
            return;
        };
        self.resize_to(window.inner_size());
    }

    /// Recompute the grid size (in cells) from a physical pixel size, resize
    /// the presenter's surface to the whole-cell-aligned pixel size, update
    /// the backend's own reported [`Output::size`], and push [`Event::Resize`] with the new
    /// cell dimensions.
    ///
    /// This keeps `backend.size()` in sync with the surface immediately, but it does not
    /// resize the [`Terminal`]'s own grid buffers: that stays the app's responsibility,
    /// done by calling [`Terminal::resize`] in response to the pushed [`Event::Resize`].
    ///
    /// Shared by [`on_resized`](Self::on_resized) and
    /// [`on_scale_factor_changed`](Self::on_scale_factor_changed): both need
    /// the same clamp-to-cell-grid math, just triggered by different winit
    /// events.
    fn resize_to(&mut self, size: winit::dpi::PhysicalSize<u32>) {
        let Some(term) = self.terminal.as_mut() else {
            return;
        };
        let (cell_w, cell_h) = term.backend().presenter().cell_size();
        // Clamp to at least one cell: a window smaller than one cell in
        // either dimension would otherwise divide down to 0 cols/rows,
        // which in turn asks `resize_surface` for a zero-size surface --
        // softbuffer (and likely other presenters) can't handle that and
        // panics. `Event::Resize` must report the same clamped grid the
        // surface was actually sized to, or callers reading `Event::Resize`
        // and querying the presenter's surface size would disagree.
        //
        // Integer division here also truncates any sub-cell remainder: when
        // `size` isn't an exact multiple of the cell size, `cols`/`rows`
        // round down and the surface below is sized to exactly
        // `cols * cell_w` x `rows * cell_h`, which can be smaller than
        // `size` itself. The OS window stays at the full physical `size`
        // the window manager gave it (retroglyph never resizes the OS
        // window to match), so a non-exact-multiple resize leaves a thin
        // strip at the window's trailing (right/bottom) edge outside the
        // surface entirely. That strip is not cleared or painted by
        // retroglyph; whatever the OS/windowing backend leaves there (old
        // frame content, backdrop color) shows through until the window is
        // resized again to a size the presenter does cover. See
        // `Presenter::resize_surface` for the documented contract.
        let cols = (size.width / cell_w).max(1);
        let rows = (size.height / cell_h).max(1);
        term.backend_mut()
            .presenter_mut()
            .resize_surface(cols * cell_w, rows * cell_h);
        #[allow(clippy::cast_possible_truncation)]
        let (cols, rows) = (cols as u16, rows as u16);
        // Update the backend's own reported size immediately so `backend.size()` agrees with
        // the surface without waiting for the app to react to `Event::Resize` below. This does
        // not touch the `Terminal`'s grid content (see `Terminal::resize`, which additionally
        // resizes/clears both grids): that remains the app's job in response to the event.
        term.backend_mut()
            .resize(retroglyph_core::grid::Size::new(cols, rows));
        term.backend_mut().push_event(Event::Resize(cols, rows));
    }

    fn on_cursor_moved(&mut self, position: winit::dpi::PhysicalPosition<f64>) {
        // winit always reports pointer positions in real-DPR physical
        // pixels; rescale to the (possibly DPR-capped, on wasm) backing-store
        // pixel space that `Presenter::geometry`/`pixel_to_cell` use, so taps land on
        // the cell actually under the finger/cursor instead of drifting
        // south-east of it as the real DPR grows past the cap. `1.0` on
        // native (no such cap exists there) *and* on wasm when
        // `fill_viewport` is off: `create_window_and_surface` only computes
        // a DPR-capped `surface_physical_size` when `fill_viewport` is set
        // (see its branch above); without it, the backing store already
        // matches the real, uncapped DPR 1:1, so applying the cap
        // correction anyway scales every reported position *down* toward
        // the origin for no reason, biasing every tap/click up-and-left of
        // where it actually landed on any real_dpr > 1.5 device (most
        // phones, and Retina/HiDPI desktops).
        #[cfg(target_arch = "wasm32")]
        let scale = if self.fill_viewport {
            web::wasm_pointer_scale()
        } else {
            1.0
        };
        #[cfg(not(target_arch = "wasm32"))]
        let scale = 1.0;
        let (x, y) = (position.x * scale, position.y * scale);
        self.cursor_px = (x, y);
        let px = translate_physical_pos(x, y);
        let Some(term) = self.terminal.as_mut() else {
            return;
        };
        let pos = term.backend().presenter().geometry().pixel_to_cell(x, y);
        // Report a drag (rather than a plain move) while any button is held. Left takes
        // priority over Right over Middle when more than one is held at once: an arbitrary but
        // deterministic choice, matching the order the buttons are declared in `MouseButton`.
        let kind = if self.held_buttons & BUTTON_MASK_LEFT != 0 {
            MouseEventKind::Drag(MouseButton::Left)
        } else if self.held_buttons & BUTTON_MASK_RIGHT != 0 {
            MouseEventKind::Drag(MouseButton::Right)
        } else if self.held_buttons & BUTTON_MASK_MIDDLE != 0 {
            MouseEventKind::Drag(MouseButton::Middle)
        } else {
            MouseEventKind::Moved
        };
        term.backend_mut()
            .push_event(Event::Mouse(MouseEvent::with_pixel_position(
                kind,
                pos,
                self.current_modifiers,
                px,
            )));
    }

    fn on_mouse_input(
        &mut self,
        state: winit::event::ElementState,
        button: winit::event::MouseButton,
    ) {
        let Some(btn) = translate_mouse_button(button) else {
            return;
        };
        let px = self.cursor_physical_pos();
        let Some(term) = self.terminal.as_mut() else {
            return;
        };
        let pos = term
            .backend()
            .presenter()
            .geometry()
            .pixel_to_cell(self.cursor_px.0, self.cursor_px.1);
        let kind = if state.is_pressed() {
            self.held_buttons |= button_mask(btn);
            MouseEventKind::Down(btn)
        } else {
            self.held_buttons &= !button_mask(btn);
            MouseEventKind::Up(btn)
        };
        term.backend_mut()
            .push_event(Event::Mouse(MouseEvent::with_pixel_position(
                kind,
                pos,
                self.current_modifiers,
                px,
            )));
    }

    fn on_mouse_wheel(&mut self, delta: winit::event::MouseScrollDelta) {
        let px = self.cursor_physical_pos();
        let Some(term) = self.terminal.as_mut() else {
            return;
        };
        let pos = term
            .backend()
            .presenter()
            .geometry()
            .pixel_to_cell(self.cursor_px.0, self.cursor_px.1);
        let (scroll_x, scroll_y) = match delta {
            winit::event::MouseScrollDelta::LineDelta(x, y) => (f64::from(x), f64::from(y)),
            winit::event::MouseScrollDelta::PixelDelta(p) => (p.x, p.y),
        };
        // A delta of exactly zero on both axes emits nothing (retroglyph#293's original
        // reasoning for not synthesizing a spurious event still applies).
        if scroll_x == 0.0 && scroll_y == 0.0 {
            return;
        }
        #[allow(clippy::cast_possible_truncation)]
        let kind = MouseEventKind::Scroll {
            dx: scroll_x as f32,
            dy: scroll_y as f32,
        };
        term.backend_mut()
            .push_event(Event::Mouse(MouseEvent::with_pixel_position(
                kind,
                pos,
                self.current_modifiers,
                px,
            )));
    }

    /// Synthesize mouse events from a touch so tap/drag work out of the box.
    ///
    /// Mobile browsers (and native touchscreens) deliver touch input as
    /// [`WindowEvent::Touch`], which has no `CursorMoved`/`MouseInput`
    /// counterpart. Games shouldn't need a second input path for it, so the
    /// first finger down becomes the pointer: its start is a `Moved` +
    /// left-button `Down`, its motion is `Moved` (a drag), and its lift is
    /// `Up`. Additional simultaneous fingers are ignored.
    fn on_touch(&mut self, touch: winit::event::Touch) {
        use winit::event::TouchPhase;

        match touch.phase {
            TouchPhase::Started => {
                if self.active_touch.is_some() {
                    return; // a second finger; keep tracking the first
                }
                self.active_touch = Some(touch.id);
                self.on_cursor_moved(touch.location);
                self.on_mouse_input(
                    winit::event::ElementState::Pressed,
                    winit::event::MouseButton::Left,
                );
            }
            TouchPhase::Moved => {
                if self.active_touch == Some(touch.id) {
                    self.on_cursor_moved(touch.location);
                }
            }
            TouchPhase::Ended | TouchPhase::Cancelled => {
                if self.active_touch != Some(touch.id) {
                    return;
                }
                self.active_touch = None;
                self.on_cursor_moved(touch.location);
                self.on_mouse_input(
                    winit::event::ElementState::Released,
                    winit::event::MouseButton::Left,
                );
            }
        }
    }

    /// Convert the cached cursor pixel position to [`PhysicalPos`].
    const fn cursor_physical_pos(&self) -> PhysicalPos {
        translate_physical_pos(self.cursor_px.0, self.cursor_px.1)
    }

    /// Push [`Event::FocusGained`]/[`Event::FocusLost`], and on loss, reset state that only makes
    /// sense while the window is focused.
    ///
    /// Winit keeps delivering `ModifiersChanged` only while focused, so a modifier key held down
    /// when focus is lost (e.g. alt-tabbing away while holding Shift) never generates the release
    /// that would normally clear it: without this, `current_modifiers` stays stuck "held" for
    /// every event after focus returns. Similarly, a finger lifted while the window is
    /// unfocused/backgrounded never delivers `TouchPhase::Ended`/`Cancelled`, so `active_touch`
    /// would otherwise stay set forever, permanently ignoring the next finger down. The stuck
    /// touch is released the same way a real lift is (see [`on_touch`](Self::on_touch)'s
    /// `Ended`/`Cancelled` arm): a left-button `Up` at the last known cursor position, so the app
    /// sees a normal, balanced Down/Up pair instead of a Down with no matching Up. No `Moved` is
    /// synthesized first, unlike a real lift: blur carries no new pointer location, and
    /// `cursor_px` already holds the touch's last reported position from the `Started`/`Moved`
    /// arms that got it there.
    ///
    /// The same problem applies to `held_buttons`: a mouse button released while the window is
    /// unfocused never delivers `MouseInput`, so without this it would stay marked "held" and
    /// every move after refocus would keep reporting a stale `Drag` instead of `Moved`. It's
    /// force-cleared directly (not via a synthesized `Up`, since there's no single button, or
    /// combination of buttons, that unambiguously round-trips through `on_mouse_input`).
    fn on_focus_changed(&mut self, gained: bool) {
        if let Some(term) = self.terminal.as_mut() {
            let event = if gained {
                Event::FocusGained
            } else {
                Event::FocusLost
            };
            term.backend_mut().push_event(event);
        }
        if !gained {
            self.current_modifiers = KeyModifiers::NONE;
            if self.active_touch.take().is_some() {
                self.on_mouse_input(
                    winit::event::ElementState::Released,
                    winit::event::MouseButton::Left,
                );
            }
            self.held_buttons = 0;
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use retroglyph_core::backend::DrawCell;
    use retroglyph_core::backend::Output;
    use retroglyph_core::event::{MouseButton, MouseEvent, MouseEventKind};
    use retroglyph_core::grid::{Pos, Size};
    use std::cell::RefCell;
    use std::time::Duration;

    // ── WindowConfig builder chain ───────────────────────────────────────────

    #[test]
    fn fit_defaults_match_winit_defaults() {
        // `fit` should start from the same defaults winit itself uses for a plain
        // `Window::default_attributes()`, so a caller that never touches the new builder
        // methods gets identical behavior to before this API existed.
        let presenter = MockPresenter::default();
        let config = WindowConfig::fit(&presenter, "test", None, true);
        assert!(config.resizable);
        assert!(config.decorations);
        assert_eq!(config.min_size, None);
        assert_eq!(config.max_size, None);
        assert_eq!(config.initial_position, None);
        assert!(!config.fullscreen);
        assert!(!config.transparency);
        assert!(!config.fill_viewport);
    }

    #[test]
    fn fit_width_height_are_physical_pixels_not_rescaled() {
        // Regression test for retroglyph#701: `Presenter::cell_size()` is documented as
        // physical pixels, so `fit()`'s width/height must be exactly `grid * cell_size`,
        // with nothing scaling that by a monitor's DPI factor before it reaches
        // `WindowApp::init_size` and, from there, `create_window_and_surface`.
        let mut presenter = MockPresenter::default();
        presenter.resize(Size::new(80, 25));
        let config = WindowConfig::fit(&presenter, "test", None, true);
        assert_eq!(config.width, 80 * 8);
        assert_eq!(config.height, 25 * 16);
    }

    #[test]
    fn builder_chain_sets_each_attribute() {
        let presenter = MockPresenter::default();
        let config = WindowConfig::fit(&presenter, "test", None, true)
            .resizable(false)
            .decorations(false)
            .min_size(320, 240)
            .max_size(1920, 1080)
            .initial_position(10, 20)
            .fullscreen(true)
            .transparency(true);
        assert!(!config.resizable);
        assert!(!config.decorations);
        assert_eq!(config.min_size, Some((320, 240)));
        assert_eq!(config.max_size, Some((1920, 1080)));
        assert_eq!(config.initial_position, Some((10, 20)));
        assert!(config.fullscreen);
        assert!(config.transparency);
    }

    #[test]
    fn window_attrs_from_config_copies_all_fields() {
        let presenter = MockPresenter::default();
        let config = WindowConfig::fit(&presenter, "test", None, true)
            .resizable(false)
            .decorations(false)
            .min_size(1, 2)
            .max_size(3, 4)
            .initial_position(5, 6)
            .fullscreen(true)
            .transparency(true);
        let attrs = WindowAttrs::from(&config);
        assert!(!attrs.resizable);
        assert!(!attrs.decorations);
        assert_eq!(attrs.min_size, Some((1, 2)));
        assert_eq!(attrs.max_size, Some((3, 4)));
        assert_eq!(attrs.initial_position, Some((5, 6)));
        assert!(attrs.fullscreen);
        assert!(attrs.transparency);
    }

    // ── present_failure_action ───────────────────────────────────────────────

    #[test]
    fn present_success_with_no_prior_failures_is_plain_ok() {
        assert_eq!(
            present_failure_action(0, true, true),
            PresentFailureAction::Ok { was_failing: false }
        );
    }

    #[test]
    fn present_success_after_a_failure_streak_reports_recovery() {
        assert_eq!(
            present_failure_action(5, true, true),
            PresentFailureAction::Ok { was_failing: true }
        );
    }

    #[test]
    fn first_failure_in_a_streak_logs_at_error_level() {
        assert_eq!(
            present_failure_action(0, false, true),
            PresentFailureAction::Log {
                at_error_level: true
            }
        );
    }

    #[test]
    fn subsequent_failures_below_threshold_log_below_error_level() {
        for count in 1..PRESENT_FAILURE_RECOVERY_THRESHOLD - 1 {
            assert_eq!(
                present_failure_action(count, false, true),
                PresentFailureAction::Log {
                    at_error_level: false
                },
                "consecutive_failures = {count}"
            );
        }
    }

    #[test]
    fn failure_crossing_the_threshold_triggers_recovery() {
        // consecutive_failures is the count *before* this call, so
        // `PRESENT_FAILURE_RECOVERY_THRESHOLD - 1` failures already happened; this call is the
        // one that reaches the threshold.
        assert_eq!(
            present_failure_action(PRESENT_FAILURE_RECOVERY_THRESHOLD - 1, false, true),
            PresentFailureAction::Recover
        );
    }

    #[test]
    fn failure_recovers_again_every_full_threshold_after_the_first() {
        // A failed recovery attempt must not be retried on literally the next frame: the next
        // `Recover` only fires after another full threshold's worth of failures.
        assert_eq!(
            present_failure_action(2 * PRESENT_FAILURE_RECOVERY_THRESHOLD - 1, false, true),
            PresentFailureAction::Recover
        );
        for count in
            PRESENT_FAILURE_RECOVERY_THRESHOLD..(2 * PRESENT_FAILURE_RECOVERY_THRESHOLD - 1)
        {
            assert_eq!(
                present_failure_action(count, false, true),
                PresentFailureAction::Log {
                    at_error_level: false
                },
                "consecutive_failures = {count}"
            );
        }
    }

    #[test]
    fn unrecoverable_failure_is_fatal_immediately_regardless_of_streak_length() {
        // A presenter reporting `is_recoverable() == false` should skip straight to `Fatal` on
        // the very first failure, not wait for the consecutive-failure threshold the way the
        // generic (`recoverable == true`) path does.
        assert_eq!(
            present_failure_action(0, false, false),
            PresentFailureAction::Fatal
        );
    }

    #[test]
    fn unrecoverable_failure_stays_fatal_mid_streak() {
        // Whatever the running consecutive-failure count, an unrecoverable error always takes
        // the fatal path rather than the count-dependent `Log`/`Recover` decision.
        assert_eq!(
            present_failure_action(5, false, false),
            PresentFailureAction::Fatal
        );
        assert_eq!(
            present_failure_action(PRESENT_FAILURE_RECOVERY_THRESHOLD - 1, false, false),
            PresentFailureAction::Fatal
        );
    }

    #[test]
    fn recoverable_flag_is_ignored_on_success() {
        // `recoverable` only matters for a failed present; passing `false` alongside
        // `succeeded == true` must not change the outcome.
        assert_eq!(
            present_failure_action(3, true, false),
            PresentFailureAction::Ok { was_failing: true }
        );
    }

    /// A dependency-free [`Presenter`] with fixed 8x16 cells.
    ///
    /// The `WindowApp` tests only exercise event translation, cell math, and the `WindowBackend`
    /// queue: no rasterization or surface is needed.
    struct MockPresenter {
        /// Records the last [`Presenter::scale_factor_changed`] argument, if any.
        last_scale_factor: Cell<Option<f64>>,
        /// The size last reported by [`Output::size`], updated by [`Output::resize`] so tests
        /// can assert that `resize_to` keeps it in sync with the surface immediately, rather
        /// than only via a separate `Terminal::resize` call in response to `Event::Resize`.
        size: Cell<Size>,
    }

    impl Default for MockPresenter {
        fn default() -> Self {
            Self {
                last_scale_factor: Cell::new(None),
                size: Cell::new(Size::new(10, 5)),
            }
        }
    }

    impl Output for MockPresenter {
        type Error = core::convert::Infallible;

        fn draw<'a, I>(&mut self, _content: I) -> Result<(), Self::Error>
        where
            I: Iterator<Item = DrawCell<'a>>,
        {
            Ok(())
        }

        fn draw_layers<'a, I>(&mut self, _content: I) -> Result<(), Self::Error>
        where
            I: Iterator<Item = DrawCell<'a>>,
        {
            Ok(())
        }

        fn flush(&mut self) -> Result<(), Self::Error> {
            Ok(())
        }

        fn size(&self) -> Size {
            self.size.get()
        }

        fn clear(&mut self) -> Result<(), Self::Error> {
            Ok(())
        }

        fn resize(&mut self, size: Size) {
            self.size.set(size);
        }
    }

    impl Presenter for MockPresenter {
        type SurfaceError = core::convert::Infallible;

        fn init_surface(
            &mut self,
            _window: Arc<dyn crate::presenter::WindowHandle>,
        ) -> Result<(), Self::SurfaceError> {
            Ok(())
        }

        fn resize_surface(&mut self, _width: u32, _height: u32) {}

        fn present(&mut self) -> Result<(), Self::SurfaceError> {
            Ok(())
        }

        fn cell_size(&self) -> (u32, u32) {
            (8, 16)
        }

        fn scale_factor_changed(&mut self, scale_factor: f64) {
            self.last_scale_factor.set(Some(scale_factor));
        }
    }

    /// A [`Presenter`] that records every `resize_surface` call, so tests
    /// can assert on the pixel dimensions `on_resized` actually requests.
    #[derive(Default)]
    struct RecordingPresenter {
        resize_calls: Rc<RefCell<Vec<(u32, u32)>>>,
    }

    impl Output for RecordingPresenter {
        type Error = core::convert::Infallible;

        fn draw<'a, I>(&mut self, _content: I) -> Result<(), Self::Error>
        where
            I: Iterator<Item = DrawCell<'a>>,
        {
            Ok(())
        }

        fn draw_layers<'a, I>(&mut self, _content: I) -> Result<(), Self::Error>
        where
            I: Iterator<Item = DrawCell<'a>>,
        {
            Ok(())
        }

        fn flush(&mut self) -> Result<(), Self::Error> {
            Ok(())
        }

        fn size(&self) -> Size {
            Size::new(10, 5)
        }

        fn clear(&mut self) -> Result<(), Self::Error> {
            Ok(())
        }

        fn resize(&mut self, _size: Size) {}
    }

    impl Presenter for RecordingPresenter {
        type SurfaceError = core::convert::Infallible;

        fn init_surface(
            &mut self,
            _window: Arc<dyn crate::presenter::WindowHandle>,
        ) -> Result<(), Self::SurfaceError> {
            Ok(())
        }

        fn resize_surface(&mut self, width: u32, height: u32) {
            self.resize_calls.borrow_mut().push((width, height));
        }

        fn present(&mut self) -> Result<(), Self::SurfaceError> {
            Ok(())
        }

        fn cell_size(&self) -> (u32, u32) {
            (8, 16)
        }
    }

    /// A [`Presenter`] whose `present()` fails on demand, and which counts `init_surface` calls
    /// so tests can assert whether [`WindowApp::try_recover_surface`] actually ran.
    #[derive(Default)]
    struct FailingPresenter {
        /// `present()` returns `Err` while this is `true`.
        failing: Rc<Cell<bool>>,
        /// Number of `init_surface` calls observed (1 at construction time in real use; extra
        /// calls here are surface-recovery attempts).
        init_surface_calls: Rc<Cell<u32>>,
    }

    impl Output for FailingPresenter {
        type Error = core::convert::Infallible;

        fn draw<'a, I>(&mut self, _content: I) -> Result<(), Self::Error>
        where
            I: Iterator<Item = DrawCell<'a>>,
        {
            Ok(())
        }

        fn draw_layers<'a, I>(&mut self, _content: I) -> Result<(), Self::Error>
        where
            I: Iterator<Item = DrawCell<'a>>,
        {
            Ok(())
        }

        fn flush(&mut self) -> Result<(), Self::Error> {
            Ok(())
        }

        fn size(&self) -> Size {
            Size::new(10, 5)
        }

        fn clear(&mut self) -> Result<(), Self::Error> {
            Ok(())
        }

        fn resize(&mut self, _size: Size) {}
    }

    impl Presenter for FailingPresenter {
        type SurfaceError = &'static str;

        fn init_surface(
            &mut self,
            _window: Arc<dyn crate::presenter::WindowHandle>,
        ) -> Result<(), Self::SurfaceError> {
            self.init_surface_calls
                .set(self.init_surface_calls.get() + 1);
            Ok(())
        }

        fn resize_surface(&mut self, _width: u32, _height: u32) {}

        fn present(&mut self) -> Result<(), Self::SurfaceError> {
            if self.failing.get() {
                Err("simulated present failure")
            } else {
                Ok(())
            }
        }

        fn cell_size(&self) -> (u32, u32) {
            (8, 16)
        }
    }

    // `&'static str` inherits the default `is_recoverable() -> true`: `FailingPresenter`'s tests
    // exercise the existing (pre-`RecoverableError`) `Log`/`Recover` behavior, which must stay
    // unchanged now that `Presenter::SurfaceError` is bounded by `RecoverableError` instead of
    // plain `Debug + Display`.
    impl crate::presenter::RecoverableError for &'static str {}

    /// A `present()` error that always reports itself as unrecoverable (overrides
    /// [`RecoverableError::is_recoverable`](crate::presenter::RecoverableError::is_recoverable) to
    /// return `false`), so tests can exercise [`PresentFailureAction::Fatal`] end to end through
    /// [`WindowApp::handle_redraw_requested`].
    #[derive(Debug)]
    struct UnrecoverableError(&'static str);

    impl core::fmt::Display for UnrecoverableError {
        fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
            write!(f, "{}", self.0)
        }
    }

    impl crate::presenter::RecoverableError for UnrecoverableError {
        fn is_recoverable(&self) -> bool {
            false
        }
    }

    /// A [`Presenter`] whose `present()` always fails with an [`UnrecoverableError`] on demand,
    /// otherwise identical to [`FailingPresenter`].
    #[derive(Default)]
    struct FatalPresenter {
        /// `present()` returns `Err` while this is `true`.
        failing: Rc<Cell<bool>>,
        /// Number of `init_surface` calls observed.
        init_surface_calls: Rc<Cell<u32>>,
    }

    impl Output for FatalPresenter {
        type Error = core::convert::Infallible;

        fn draw<'a, I>(&mut self, _content: I) -> Result<(), Self::Error>
        where
            I: Iterator<Item = DrawCell<'a>>,
        {
            Ok(())
        }

        fn draw_layers<'a, I>(&mut self, _content: I) -> Result<(), Self::Error>
        where
            I: Iterator<Item = DrawCell<'a>>,
        {
            Ok(())
        }

        fn flush(&mut self) -> Result<(), Self::Error> {
            Ok(())
        }

        fn size(&self) -> Size {
            Size::new(10, 5)
        }

        fn clear(&mut self) -> Result<(), Self::Error> {
            Ok(())
        }

        fn resize(&mut self, _size: Size) {}
    }

    impl Presenter for FatalPresenter {
        type SurfaceError = UnrecoverableError;

        fn init_surface(
            &mut self,
            _window: Arc<dyn crate::presenter::WindowHandle>,
        ) -> Result<(), Self::SurfaceError> {
            self.init_surface_calls
                .set(self.init_surface_calls.get() + 1);
            Ok(())
        }

        fn resize_surface(&mut self, _width: u32, _height: u32) {}

        fn present(&mut self) -> Result<(), Self::SurfaceError> {
            if self.failing.get() {
                Err(UnrecoverableError(
                    "simulated unrecoverable present failure",
                ))
            } else {
                Ok(())
            }
        }

        fn cell_size(&self) -> (u32, u32) {
            (8, 16)
        }
    }

    type MockApp = WindowApp<
        MockPresenter,
        fn(&mut Terminal<WindowBackend<MockPresenter>>),
        u64,
        fn(u64, &mut Terminal<WindowBackend<MockPresenter>>),
    >;

    fn test_window_app() -> MockApp {
        let terminal = Terminal::new(WindowBackend::new(MockPresenter::default()));
        WindowApp {
            terminal: Some(terminal),
            app_loop: |_| {},
            on_custom_event: push_custom_event,
            _user_event: PhantomData,
            window: None,
            title: String::new(),
            init_size: InitWindowSize {
                width: 80,
                height: 80,
            },
            attrs: WindowAttrs::default(),
            current_modifiers: KeyModifiers::NONE,
            cursor_px: (0.0, 0.0),
            active_touch: None,
            held_buttons: 0,
            frame_interval: None,
            event_driven: true,
            #[cfg(not(target_arch = "wasm32"))]
            next_frame: std::time::Instant::now(),
            exit_requested: Rc::new(Cell::new(false)),
            skip_present: Rc::new(Cell::new(false)),
            needs_redraw: false,
            consecutive_present_errors: 0,
        }
    }

    fn poll(app: &mut MockApp) -> Option<Event> {
        app.terminal
            .as_mut()
            .unwrap()
            .backend_mut()
            .poll_event(Duration::ZERO)
    }

    // ── WindowBackend queue ───────────────────────────────────────────────────

    #[test]
    fn mouse_event_round_trips_through_event_buffer() {
        let mut backend = WindowBackend::new(MockPresenter::default());
        let ev = Event::Mouse(MouseEvent::new(
            MouseEventKind::Down(MouseButton::Left),
            Pos { x: 3, y: 1 },
            KeyModifiers::NONE,
        ));
        backend.push_event(ev.clone());
        assert_eq!(backend.poll_event(Duration::ZERO), Some(ev));
        assert_eq!(backend.poll_event(Duration::ZERO), None);
    }

    #[test]
    fn multiple_mouse_events_preserve_fifo_order() {
        let mut backend = WindowBackend::new(MockPresenter::default());
        let moved = Event::Mouse(MouseEvent::new(
            MouseEventKind::Moved,
            Pos { x: 1, y: 2 },
            KeyModifiers::NONE,
        ));
        let clicked = Event::Mouse(MouseEvent::new(
            MouseEventKind::Down(MouseButton::Left),
            Pos { x: 1, y: 2 },
            KeyModifiers::NONE,
        ));
        backend.push_event(moved.clone());
        backend.push_event(clicked.clone());
        assert_eq!(backend.poll_event(Duration::ZERO), Some(moved));
        assert_eq!(backend.poll_event(Duration::ZERO), Some(clicked));
    }

    // ── handle_window_event ──────────────────────────────────────────────────

    #[test]
    fn cursor_moved_pushes_moved_event_at_correct_cell() {
        // 8-wide × 16-tall cells; cursor at pixel (20, 32) → col 2, row 2.
        let mut app = test_window_app();
        app.handle_window_event(WindowEvent::CursorMoved {
            device_id: winit::event::DeviceId::dummy(),
            position: winit::dpi::PhysicalPosition::new(20.0_f64, 32.0_f64),
        });
        assert_eq!(
            poll(&mut app),
            Some(Event::Mouse(MouseEvent::with_pixel_position(
                MouseEventKind::Moved,
                Pos { x: 2, y: 2 },
                KeyModifiers::NONE,
                PhysicalPos { x: 20, y: 32 },
            )))
        );
    }

    #[test]
    fn cursor_moved_caches_position_for_subsequent_click() {
        // Move to pixel (16, 16) = col 2, row 1, then click; button event
        // must reuse the cached position.
        let mut app = test_window_app();
        app.handle_window_event(WindowEvent::CursorMoved {
            device_id: winit::event::DeviceId::dummy(),
            position: winit::dpi::PhysicalPosition::new(16.0_f64, 16.0_f64),
        });
        let _ = poll(&mut app); // discard the Moved event
        app.handle_window_event(WindowEvent::MouseInput {
            device_id: winit::event::DeviceId::dummy(),
            state: winit::event::ElementState::Pressed,
            button: winit::event::MouseButton::Left,
        });
        assert_eq!(
            poll(&mut app),
            Some(Event::Mouse(MouseEvent::with_pixel_position(
                MouseEventKind::Down(MouseButton::Left),
                Pos { x: 2, y: 1 },
                KeyModifiers::NONE,
                PhysicalPos { x: 16, y: 16 },
            )))
        );
    }

    #[test]
    fn mouse_button_release_produces_up_event() {
        let mut app = test_window_app();
        app.handle_window_event(WindowEvent::MouseInput {
            device_id: winit::event::DeviceId::dummy(),
            state: winit::event::ElementState::Released,
            button: winit::event::MouseButton::Right,
        });
        assert_eq!(
            poll(&mut app),
            Some(Event::Mouse(MouseEvent::with_pixel_position(
                MouseEventKind::Up(MouseButton::Right),
                Pos { x: 0, y: 0 },
                KeyModifiers::NONE,
                PhysicalPos { x: 0, y: 0 },
            )))
        );
    }

    #[test]
    fn unknown_mouse_button_produces_no_event() {
        let mut app = test_window_app();
        app.handle_window_event(WindowEvent::MouseInput {
            device_id: winit::event::DeviceId::dummy(),
            state: winit::event::ElementState::Pressed,
            button: winit::event::MouseButton::Other(99),
        });
        assert_eq!(poll(&mut app), None);
    }

    fn touch(id: u64, phase: winit::event::TouchPhase, x: f64, y: f64) -> WindowEvent {
        WindowEvent::Touch(winit::event::Touch {
            device_id: winit::event::DeviceId::dummy(),
            phase,
            location: winit::dpi::PhysicalPosition::new(x, y),
            force: None,
            id,
        })
    }

    #[test]
    fn touch_tap_synthesizes_left_click() {
        use winit::event::TouchPhase;
        let mut app = test_window_app();
        // MockPresenter cells are 8x16 px; a tap at (20, 18) lands on cell (2, 1).
        app.handle_window_event(touch(7, TouchPhase::Started, 20.0, 18.0));
        // Moved (from the synthesized cursor move) then Down.
        assert!(matches!(
            poll(&mut app),
            Some(Event::Mouse(MouseEvent {
                kind: MouseEventKind::Moved,
                position: Pos { x: 2, y: 1 },
                ..
            }))
        ));
        assert!(matches!(
            poll(&mut app),
            Some(Event::Mouse(MouseEvent {
                kind: MouseEventKind::Down(MouseButton::Left),
                position: Pos { x: 2, y: 1 },
                ..
            }))
        ));

        app.handle_window_event(touch(7, TouchPhase::Ended, 20.0, 18.0));
        // The synthesized move fires while the touch's `Left` button is still held (the release
        // hasn't been synthesized yet), so it's reported as a drag, not a plain move.
        assert!(matches!(
            poll(&mut app),
            Some(Event::Mouse(MouseEvent {
                kind: MouseEventKind::Drag(MouseButton::Left),
                ..
            }))
        ));
        assert!(matches!(
            poll(&mut app),
            Some(Event::Mouse(MouseEvent {
                kind: MouseEventKind::Up(MouseButton::Left),
                position: Pos { x: 2, y: 1 },
                ..
            }))
        ));
        assert_eq!(poll(&mut app), None);
    }

    #[test]
    fn touch_drag_synthesizes_moves_between_down_and_up() {
        use winit::event::TouchPhase;
        let mut app = test_window_app();
        app.handle_window_event(touch(1, TouchPhase::Started, 0.0, 0.0));
        poll(&mut app); // Moved
        poll(&mut app); // Down

        app.handle_window_event(touch(1, TouchPhase::Moved, 40.0, 32.0));
        // Held Left button since Started: this is a drag, not a plain move.
        assert!(matches!(
            poll(&mut app),
            Some(Event::Mouse(MouseEvent {
                kind: MouseEventKind::Drag(MouseButton::Left),
                position: Pos { x: 5, y: 2 },
                ..
            }))
        ));

        app.handle_window_event(touch(1, TouchPhase::Cancelled, 40.0, 32.0));
        poll(&mut app); // Drag (button still held until the synthesized Up just below)
        assert!(matches!(
            poll(&mut app),
            Some(Event::Mouse(MouseEvent {
                kind: MouseEventKind::Up(MouseButton::Left),
                ..
            }))
        ));
    }

    #[test]
    fn second_finger_is_ignored_while_first_is_down() {
        use winit::event::TouchPhase;
        let mut app = test_window_app();
        app.handle_window_event(touch(1, TouchPhase::Started, 0.0, 0.0));
        poll(&mut app); // Moved
        poll(&mut app); // Down

        // A second finger goes down, moves, and lifts: all ignored.
        app.handle_window_event(touch(2, TouchPhase::Started, 80.0, 80.0));
        app.handle_window_event(touch(2, TouchPhase::Moved, 88.0, 80.0));
        app.handle_window_event(touch(2, TouchPhase::Ended, 88.0, 80.0));
        assert_eq!(poll(&mut app), None);

        // The first finger still completes its gesture.
        app.handle_window_event(touch(1, TouchPhase::Ended, 8.0, 0.0));
        poll(&mut app); // Moved
        assert!(matches!(
            poll(&mut app),
            Some(Event::Mouse(MouseEvent {
                kind: MouseEventKind::Up(MouseButton::Left),
                position: Pos { x: 1, y: 0 },
                ..
            }))
        ));
    }

    #[test]
    fn scroll_up_line_delta() {
        let mut app = test_window_app();
        app.handle_window_event(WindowEvent::MouseWheel {
            device_id: winit::event::DeviceId::dummy(),
            delta: winit::event::MouseScrollDelta::LineDelta(0.0, 1.0),
            phase: winit::event::TouchPhase::Moved,
        });
        let ev = poll(&mut app).unwrap();
        assert!(matches!(
            ev,
            Event::Mouse(MouseEvent {
                kind: MouseEventKind::Scroll { dx: 0.0, dy },
                ..
            }) if dy > 0.0
        ));
    }

    #[test]
    fn scroll_down_line_delta() {
        let mut app = test_window_app();
        app.handle_window_event(WindowEvent::MouseWheel {
            device_id: winit::event::DeviceId::dummy(),
            delta: winit::event::MouseScrollDelta::LineDelta(0.0, -1.0),
            phase: winit::event::TouchPhase::Moved,
        });
        let ev = poll(&mut app).unwrap();
        assert!(matches!(
            ev,
            Event::Mouse(MouseEvent {
                kind: MouseEventKind::Scroll { dx: 0.0, dy },
                ..
            }) if dy < 0.0
        ));
    }

    #[test]
    fn scroll_up_pixel_delta() {
        let mut app = test_window_app();
        app.handle_window_event(WindowEvent::MouseWheel {
            device_id: winit::event::DeviceId::dummy(),
            delta: winit::event::MouseScrollDelta::PixelDelta(winit::dpi::PhysicalPosition::new(
                0.0_f64, 15.0_f64,
            )),
            phase: winit::event::TouchPhase::Moved,
        });
        let ev = poll(&mut app).unwrap();
        assert!(matches!(
            ev,
            Event::Mouse(MouseEvent {
                kind: MouseEventKind::Scroll { dx: 0.0, dy },
                ..
            }) if dy > 0.0
        ));
    }

    #[test]
    fn scroll_right_line_delta() {
        // A pure horizontal LineDelta (trackpad swipe, tilt wheel): scroll_y == 0.0.
        let mut app = test_window_app();
        app.handle_window_event(WindowEvent::MouseWheel {
            device_id: winit::event::DeviceId::dummy(),
            delta: winit::event::MouseScrollDelta::LineDelta(1.0, 0.0),
            phase: winit::event::TouchPhase::Moved,
        });
        let ev = poll(&mut app).unwrap();
        assert!(matches!(
            ev,
            Event::Mouse(MouseEvent {
                kind: MouseEventKind::Scroll { dx, dy: 0.0 },
                ..
            }) if dx > 0.0
        ));
    }

    #[test]
    fn scroll_left_pixel_delta() {
        // Regression test for retroglyph#293: before the fix, a pure-horizontal `PixelDelta`
        // (scroll_y == 0.0) spuriously fell through to a spurious vertical scroll instead of
        // being reported as (or, before horizontal scroll was wired up, dropped as) a
        // horizontal scroll.
        let mut app = test_window_app();
        app.handle_window_event(WindowEvent::MouseWheel {
            device_id: winit::event::DeviceId::dummy(),
            delta: winit::event::MouseScrollDelta::PixelDelta(winit::dpi::PhysicalPosition::new(
                -15.0_f64, 0.0_f64,
            )),
            phase: winit::event::TouchPhase::Moved,
        });
        let ev = poll(&mut app).unwrap();
        assert!(matches!(
            ev,
            Event::Mouse(MouseEvent {
                kind: MouseEventKind::Scroll { dx, dy: 0.0 },
                ..
            }) if dx < 0.0
        ));
    }

    #[test]
    fn scroll_with_zero_delta_on_both_axes_pushes_no_event() {
        let mut app = test_window_app();
        app.handle_window_event(WindowEvent::MouseWheel {
            device_id: winit::event::DeviceId::dummy(),
            delta: winit::event::MouseScrollDelta::LineDelta(0.0, 0.0),
            phase: winit::event::TouchPhase::Moved,
        });
        assert_eq!(poll(&mut app), None);
    }

    #[test]
    fn modifiers_propagate_to_mouse_event() {
        let mut app = test_window_app();
        // Simulate a ModifiersChanged before the click.
        app.handle_window_event(WindowEvent::ModifiersChanged(
            winit::event::Modifiers::from(winit::keyboard::ModifiersState::SHIFT),
        ));
        let _ = poll(&mut app); // no event emitted for modifiers
        app.handle_window_event(WindowEvent::MouseInput {
            device_id: winit::event::DeviceId::dummy(),
            state: winit::event::ElementState::Pressed,
            button: winit::event::MouseButton::Left,
        });
        let ev = poll(&mut app).unwrap();
        assert!(matches!(
            ev,
            Event::Mouse(MouseEvent {
                modifiers,
                ..
            }) if modifiers.contains(KeyModifiers::SHIFT)
        ));
    }

    // ── mouse drag (retroglyph#554) ───────────────────────────────────────────

    #[test]
    fn cursor_moved_with_no_button_held_emits_moved() {
        let mut app = test_window_app();
        app.handle_window_event(WindowEvent::CursorMoved {
            device_id: winit::event::DeviceId::dummy(),
            position: winit::dpi::PhysicalPosition::new(8.0_f64, 16.0_f64),
        });
        assert!(matches!(
            poll(&mut app),
            Some(Event::Mouse(MouseEvent {
                kind: MouseEventKind::Moved,
                ..
            }))
        ));
    }

    #[test]
    fn cursor_moved_while_button_held_emits_drag_not_moved() {
        let mut app = test_window_app();
        app.handle_window_event(WindowEvent::MouseInput {
            device_id: winit::event::DeviceId::dummy(),
            state: winit::event::ElementState::Pressed,
            button: winit::event::MouseButton::Left,
        });
        let _ = poll(&mut app); // Down

        app.handle_window_event(WindowEvent::CursorMoved {
            device_id: winit::event::DeviceId::dummy(),
            position: winit::dpi::PhysicalPosition::new(40.0_f64, 32.0_f64),
        });
        assert!(matches!(
            poll(&mut app),
            Some(Event::Mouse(MouseEvent {
                kind: MouseEventKind::Drag(MouseButton::Left),
                ..
            }))
        ));
    }

    #[test]
    fn cursor_moved_after_button_release_goes_back_to_moved() {
        let mut app = test_window_app();
        app.handle_window_event(WindowEvent::MouseInput {
            device_id: winit::event::DeviceId::dummy(),
            state: winit::event::ElementState::Pressed,
            button: winit::event::MouseButton::Left,
        });
        let _ = poll(&mut app); // Down
        app.handle_window_event(WindowEvent::MouseInput {
            device_id: winit::event::DeviceId::dummy(),
            state: winit::event::ElementState::Released,
            button: winit::event::MouseButton::Left,
        });
        let _ = poll(&mut app); // Up

        app.handle_window_event(WindowEvent::CursorMoved {
            device_id: winit::event::DeviceId::dummy(),
            position: winit::dpi::PhysicalPosition::new(40.0_f64, 32.0_f64),
        });
        assert!(matches!(
            poll(&mut app),
            Some(Event::Mouse(MouseEvent {
                kind: MouseEventKind::Moved,
                ..
            }))
        ));
    }

    #[test]
    fn right_button_drag_reports_right_not_left() {
        let mut app = test_window_app();
        app.handle_window_event(WindowEvent::MouseInput {
            device_id: winit::event::DeviceId::dummy(),
            state: winit::event::ElementState::Pressed,
            button: winit::event::MouseButton::Right,
        });
        let _ = poll(&mut app); // Down

        app.handle_window_event(WindowEvent::CursorMoved {
            device_id: winit::event::DeviceId::dummy(),
            position: winit::dpi::PhysicalPosition::new(40.0_f64, 32.0_f64),
        });
        assert!(matches!(
            poll(&mut app),
            Some(Event::Mouse(MouseEvent {
                kind: MouseEventKind::Drag(MouseButton::Right),
                ..
            }))
        ));
    }

    #[test]
    fn left_button_takes_priority_over_right_when_both_are_held() {
        // Deterministic tie-break documented on `on_cursor_moved`: Left wins when more than one
        // button is held at once.
        let mut app = test_window_app();
        app.handle_window_event(WindowEvent::MouseInput {
            device_id: winit::event::DeviceId::dummy(),
            state: winit::event::ElementState::Pressed,
            button: winit::event::MouseButton::Right,
        });
        let _ = poll(&mut app); // Down
        app.handle_window_event(WindowEvent::MouseInput {
            device_id: winit::event::DeviceId::dummy(),
            state: winit::event::ElementState::Pressed,
            button: winit::event::MouseButton::Left,
        });
        let _ = poll(&mut app); // Down

        app.handle_window_event(WindowEvent::CursorMoved {
            device_id: winit::event::DeviceId::dummy(),
            position: winit::dpi::PhysicalPosition::new(40.0_f64, 32.0_f64),
        });
        assert!(matches!(
            poll(&mut app),
            Some(Event::Mouse(MouseEvent {
                kind: MouseEventKind::Drag(MouseButton::Left),
                ..
            }))
        ));
    }

    #[test]
    fn touch_drag_produces_drag_left_not_moved() {
        // Regression test for retroglyph#554: `on_touch` synthesizes a left-button `Down` before
        // its `Moved` phase forwards to `on_cursor_moved`, so a touch drag must fall out of the
        // same `held_buttons` tracking a real mouse drag uses, with no touch-specific code.
        use winit::event::TouchPhase;
        let mut app = test_window_app();
        app.handle_window_event(touch(1, TouchPhase::Started, 0.0, 0.0));
        poll(&mut app); // Moved
        poll(&mut app); // Down

        app.handle_window_event(touch(1, TouchPhase::Moved, 40.0, 32.0));
        assert!(matches!(
            poll(&mut app),
            Some(Event::Mouse(MouseEvent {
                kind: MouseEventKind::Drag(MouseButton::Left),
                ..
            }))
        ));
    }

    #[test]
    fn focus_lost_clears_held_button_so_refocus_move_is_not_a_stale_drag() {
        // Regression test for retroglyph#554: a button released while the window is unfocused
        // never delivers `MouseInput`, so `held_buttons` must be force-cleared on blur or every
        // move after refocus keeps reporting a `Drag` for a button that's actually up.
        let mut app = test_window_app();
        app.handle_window_event(WindowEvent::MouseInput {
            device_id: winit::event::DeviceId::dummy(),
            state: winit::event::ElementState::Pressed,
            button: winit::event::MouseButton::Left,
        });
        let _ = poll(&mut app); // Down

        app.handle_window_event(WindowEvent::Focused(false));
        assert_eq!(poll(&mut app), Some(Event::FocusLost));
        assert_eq!(app.held_buttons, 0);

        app.handle_window_event(WindowEvent::Focused(true));
        assert_eq!(poll(&mut app), Some(Event::FocusGained));
        app.handle_window_event(WindowEvent::CursorMoved {
            device_id: winit::event::DeviceId::dummy(),
            position: winit::dpi::PhysicalPosition::new(40.0_f64, 32.0_f64),
        });
        assert!(matches!(
            poll(&mut app),
            Some(Event::Mouse(MouseEvent {
                kind: MouseEventKind::Moved,
                ..
            }))
        ));
    }

    // ── user events (EventProxy) ─────────────────────────────────────────────

    #[test]
    fn user_event_pushes_custom_event() {
        let mut app = test_window_app();
        app.handle_user_event(42);
        assert_eq!(poll(&mut app), Some(Event::Custom(42)));
    }

    #[test]
    fn multiple_user_events_preserve_fifo_order() {
        let mut app = test_window_app();
        app.handle_user_event(1);
        app.handle_user_event(2);
        assert_eq!(poll(&mut app), Some(Event::Custom(1)));
        assert_eq!(poll(&mut app), Some(Event::Custom(2)));
        assert_eq!(poll(&mut app), None);
    }

    #[test]
    fn user_events_interleave_with_window_events_in_arrival_order() {
        let mut app = test_window_app();
        app.handle_user_event(7);
        app.handle_window_event(WindowEvent::CloseRequested);
        assert_eq!(poll(&mut app), Some(Event::Custom(7)));
        assert_eq!(poll(&mut app), Some(Event::Close));
    }

    #[test]
    fn event_proxy_closed_reports_the_undelivered_id() {
        let err = EventProxyClosed(42);
        assert_eq!(err.into_inner(), 42);
        assert_eq!(err.to_string(), "event loop closed");
    }

    #[test]
    fn event_proxy_closed_round_trips_a_non_u64_payload() {
        // `EventProxyClosed<T>` carries whatever `T` `EventProxy<T>::send_event` was called
        // with, not just the `u64` default.
        let err = EventProxyClosed(String::from("asset.bin"));
        assert_eq!(err.to_string(), "event loop closed");
        assert_eq!(err.into_inner(), "asset.bin");
    }

    // ── typed EventProxy<T> (non-`u64` custom payload) ────────────────────────

    /// A payload that is emphatically not `u64`, to prove the typed path never funnels through
    /// [`Event::Custom`] (which is fixed to `u64` in `retroglyph_core`).
    #[derive(Debug, Clone, PartialEq, Eq)]
    struct AssetLoaded {
        name: String,
        bytes: usize,
    }

    type TypedAppLoop = fn(&mut Terminal<WindowBackend<MockPresenter>>);
    type TypedHandler = Box<dyn FnMut(AssetLoaded, &mut Terminal<WindowBackend<MockPresenter>>)>;
    type TypedApp = WindowApp<MockPresenter, TypedAppLoop, AssetLoaded, TypedHandler>;

    fn test_typed_window_app(on_custom_event: TypedHandler) -> TypedApp {
        let terminal = Terminal::new(WindowBackend::new(MockPresenter::default()));
        WindowApp {
            terminal: Some(terminal),
            app_loop: |_| {},
            on_custom_event,
            _user_event: PhantomData,
            window: None,
            title: String::new(),
            init_size: InitWindowSize {
                width: 80,
                height: 80,
            },
            attrs: WindowAttrs::default(),
            current_modifiers: KeyModifiers::NONE,
            cursor_px: (0.0, 0.0),
            active_touch: None,
            held_buttons: 0,
            frame_interval: None,
            event_driven: true,
            #[cfg(not(target_arch = "wasm32"))]
            next_frame: std::time::Instant::now(),
            exit_requested: Rc::new(Cell::new(false)),
            skip_present: Rc::new(Cell::new(false)),
            needs_redraw: false,
            consecutive_present_errors: 0,
        }
    }

    #[test]
    fn typed_user_event_reaches_the_custom_handler_not_event_custom() {
        let received: Rc<RefCell<Vec<AssetLoaded>>> = Rc::new(RefCell::new(Vec::new()));
        let received_in_handler = received.clone();
        let handler: TypedHandler = Box::new(move |payload, _term| {
            received_in_handler.borrow_mut().push(payload);
        });
        let mut app = test_typed_window_app(handler);

        let payload = AssetLoaded {
            name: "asset.bin".to_string(),
            bytes: 4096,
        };
        app.handle_user_event(payload.clone());

        // Delivered to the handler directly...
        assert_eq!(received.borrow().as_slice(), &[payload]);
        // ...and never pushed onto the `WindowBackend` event queue as an `Event` at all: there is
        // no `Event` variant a non-`u64` payload could become.
        assert_eq!(
            app.terminal
                .as_mut()
                .unwrap()
                .backend_mut()
                .poll_event(Duration::ZERO),
            None
        );
    }

    #[test]
    fn typed_user_event_still_sets_needs_redraw() {
        // Same wake-the-idle-loop behavior as the `u64`/`Event::Custom` path.
        let handler: TypedHandler = Box::new(|_payload, _term| {});
        let mut app = test_typed_window_app(handler);
        assert!(!app.needs_redraw);
        app.handle_user_event(AssetLoaded {
            name: "asset.bin".to_string(),
            bytes: 4096,
        });
        assert!(app.needs_redraw);
    }

    #[test]
    fn close_requested_pushes_close_event() {
        let mut app = test_window_app();
        app.handle_window_event(WindowEvent::CloseRequested);
        assert_eq!(poll(&mut app), Some(Event::Close));
    }

    // ── IME (issue #296) ──────────────────────────────────────────────────────

    #[test]
    fn ime_commit_pushes_paste_event() {
        let mut app = test_window_app();
        app.handle_window_event(WindowEvent::Ime(winit::event::Ime::Commit(
            "pasted".to_string(),
        )));
        assert_eq!(poll(&mut app), Some(Event::Paste("pasted".to_string())));
    }

    #[test]
    fn ime_preedit_and_enabled_push_no_event() {
        let mut app = test_window_app();
        app.handle_window_event(WindowEvent::Ime(winit::event::Ime::Enabled));
        app.handle_window_event(WindowEvent::Ime(winit::event::Ime::Preedit(
            "nihon".to_string(),
            Some((0, 5)),
        )));
        assert_eq!(poll(&mut app), None);
    }

    // ── graceful exit (issue #157) ────────────────────────────────────────────

    /// A `WindowApp` whose `app_loop` is a boxed closure, so a test can capture and flip a
    /// shared flag from inside it, mirroring how `run_app_with_proxy`'s real closure sets
    /// `exit_requested` on `Flow::Exit` (it can't return a value or reach `ActiveEventLoop`
    /// itself; see `exit_requested`'s doc comment).
    type BoxedAppLoop = Box<dyn FnMut(&mut Terminal<WindowBackend<MockPresenter>>)>;
    type BoxedApp = WindowApp<
        MockPresenter,
        BoxedAppLoop,
        u64,
        fn(u64, &mut Terminal<WindowBackend<MockPresenter>>),
    >;

    #[test]
    fn redraw_requested_runs_app_loop_and_does_not_set_exit_by_default() {
        let mut app = test_window_app();
        app.handle_window_event(WindowEvent::RedrawRequested);
        assert!(!app.exit_requested.get());
    }

    #[test]
    fn app_loop_setting_exit_requested_is_observed_after_redraw() {
        // Simulates `run_app_with_proxy`'s closure: on `Flow::Exit` it sets the shared flag
        // instead of calling `std::process::exit`. `handle_window_event` itself never calls
        // `event_loop.exit()` (it can't: no `ActiveEventLoop`, see its doc comment); that
        // happens in `ApplicationHandler::window_event`, which this flag lets the test assert
        // on without a live winit event loop.
        let terminal = Terminal::new(WindowBackend::new(MockPresenter::default()));
        let exit_requested = Rc::new(Cell::new(false));
        let exit_requested_in_loop = exit_requested.clone();
        let app_loop: BoxedAppLoop = Box::new(move |_term| exit_requested_in_loop.set(true));
        let mut app: BoxedApp = WindowApp {
            terminal: Some(terminal),
            app_loop,
            on_custom_event: push_custom_event,
            _user_event: PhantomData,
            window: None,
            title: String::new(),
            init_size: InitWindowSize {
                width: 80,
                height: 80,
            },
            attrs: WindowAttrs::default(),
            current_modifiers: KeyModifiers::NONE,
            cursor_px: (0.0, 0.0),
            active_touch: None,
            held_buttons: 0,
            frame_interval: None,
            event_driven: true,
            #[cfg(not(target_arch = "wasm32"))]
            next_frame: std::time::Instant::now(),
            exit_requested,
            skip_present: Rc::new(Cell::new(false)),
            needs_redraw: false,
            consecutive_present_errors: 0,
        };

        assert!(!app.exit_requested.get());
        app.handle_window_event(WindowEvent::RedrawRequested);
        assert!(app.exit_requested.get());
    }

    #[test]
    fn theme_changed_pushes_mapped_system_theme_event() {
        let mut app = test_window_app();
        app.handle_window_event(WindowEvent::ThemeChanged(winit::window::Theme::Light));
        assert_eq!(
            poll(&mut app),
            Some(Event::ThemeChanged(
                retroglyph_core::event::SystemTheme::Light
            ))
        );

        app.handle_window_event(WindowEvent::ThemeChanged(winit::window::Theme::Dark));
        assert_eq!(
            poll(&mut app),
            Some(Event::ThemeChanged(
                retroglyph_core::event::SystemTheme::Dark
            ))
        );
    }

    #[test]
    fn focused_pushes_focus_gained_and_lost_events() {
        let mut app = test_window_app();
        app.handle_window_event(WindowEvent::Focused(true));
        assert_eq!(poll(&mut app), Some(Event::FocusGained));

        app.handle_window_event(WindowEvent::Focused(false));
        assert_eq!(poll(&mut app), Some(Event::FocusLost));
    }

    #[test]
    fn focus_lost_resets_stuck_modifiers() {
        // Regression test for #153: a modifier held down when focus is lost
        // (e.g. alt-tabbing away while holding Shift) must not stay "held"
        // for events delivered after focus returns.
        let mut app = test_window_app();
        app.handle_window_event(WindowEvent::ModifiersChanged(
            winit::event::Modifiers::from(winit::keyboard::ModifiersState::SHIFT),
        ));
        let _ = poll(&mut app); // no event emitted for modifiers
        assert_eq!(app.current_modifiers, KeyModifiers::SHIFT);

        app.handle_window_event(WindowEvent::Focused(false));
        assert_eq!(poll(&mut app), Some(Event::FocusLost));
        assert_eq!(app.current_modifiers, KeyModifiers::NONE);

        // A click after refocusing must not still carry the stale Shift.
        app.handle_window_event(WindowEvent::Focused(true));
        assert_eq!(poll(&mut app), Some(Event::FocusGained));
        app.handle_window_event(WindowEvent::MouseInput {
            device_id: winit::event::DeviceId::dummy(),
            state: winit::event::ElementState::Pressed,
            button: winit::event::MouseButton::Left,
        });
        let ev = poll(&mut app).unwrap();
        assert!(matches!(
            ev,
            Event::Mouse(MouseEvent { modifiers, .. }) if modifiers == KeyModifiers::NONE
        ));
    }

    #[test]
    fn focus_lost_releases_stuck_active_touch() {
        // Regression test for #153: a finger lifted while the window is
        // unfocused/backgrounded never delivers `TouchPhase::Ended` or
        // `Cancelled`, so `active_touch` must be released on blur instead of
        // silently ignoring every subsequent finger down.
        use winit::event::TouchPhase;
        let mut app = test_window_app();
        app.handle_window_event(touch(3, TouchPhase::Started, 20.0, 18.0));
        poll(&mut app); // Moved
        poll(&mut app); // Down
        assert_eq!(app.active_touch, Some(3));

        app.handle_window_event(WindowEvent::Focused(false));
        assert_eq!(poll(&mut app), Some(Event::FocusLost));
        // Synthesized Up releasing the stuck touch at its last known
        // position; no new Moved, since blur carries no fresh location.
        assert!(matches!(
            poll(&mut app),
            Some(Event::Mouse(MouseEvent {
                kind: MouseEventKind::Up(MouseButton::Left),
                ..
            }))
        ));
        assert_eq!(poll(&mut app), None);
        assert_eq!(app.active_touch, None);

        // A new finger down after refocusing must be tracked, not ignored.
        app.handle_window_event(WindowEvent::Focused(true));
        assert_eq!(poll(&mut app), Some(Event::FocusGained));
        app.handle_window_event(touch(4, TouchPhase::Started, 40.0, 32.0));
        assert!(matches!(
            poll(&mut app),
            Some(Event::Mouse(MouseEvent {
                kind: MouseEventKind::Moved,
                ..
            }))
        ));
        assert!(matches!(
            poll(&mut app),
            Some(Event::Mouse(MouseEvent {
                kind: MouseEventKind::Down(MouseButton::Left),
                ..
            }))
        ));
        assert_eq!(app.active_touch, Some(4));
    }

    #[test]
    fn focus_lost_without_active_touch_pushes_no_extra_events() {
        // No touch in progress: blur should push exactly one FocusLost, no
        // synthesized mouse events.
        let mut app = test_window_app();
        app.handle_window_event(WindowEvent::Focused(false));
        assert_eq!(poll(&mut app), Some(Event::FocusLost));
        assert_eq!(poll(&mut app), None);
    }

    #[test]
    fn resized_pushes_resize_event_in_cells() {
        // 8x16 cells: 88x80 px -> 11 cols, 5 rows.
        let mut app = test_window_app();
        app.handle_window_event(WindowEvent::Resized(winit::dpi::PhysicalSize::new(88, 80)));
        assert_eq!(poll(&mut app), Some(Event::Resize(11, 5)));
    }

    // ── scale factor changes ─────────────────────────────────────────────────

    #[test]
    fn scale_factor_changed_notifies_presenter() {
        // `handle_window_event` can't be exercised directly here: winit's
        // `InnerSizeWriter::new` is `pub(crate)`, so a real
        // `WindowEvent::ScaleFactorChanged` can't be constructed outside the
        // winit crate. `on_scale_factor_changed` is called directly instead:
        // it's the same code the `WindowEvent::ScaleFactorChanged` arm in
        // `handle_window_event` dispatches to.
        let mut app = test_window_app();
        app.on_scale_factor_changed(2.0);
        assert_eq!(
            app.terminal
                .as_ref()
                .unwrap()
                .backend()
                .presenter()
                .last_scale_factor
                .get(),
            Some(2.0)
        );
    }

    #[test]
    fn scale_factor_changed_without_a_window_is_a_no_op_resize() {
        // `test_window_app` has no real winit window (`window: None`), so
        // there is no physical size to re-align the surface to: this must
        // not panic, and must not push a spurious `Event::Resize`.
        let mut app = test_window_app();
        app.on_scale_factor_changed(2.0);
        assert_eq!(poll(&mut app), None);
    }

    #[test]
    fn resize_to_clamps_to_whole_cells_and_pushes_resize_event() {
        // Shared helper behind both `on_resized` and
        // `on_scale_factor_changed`: 8x16 cells, 90x81 px clamps down to
        // 11 cols x 5 rows (88x80 px), not a fractional cell.
        let mut app = test_window_app();
        app.resize_to(winit::dpi::PhysicalSize::new(90, 81));
        assert_eq!(poll(&mut app), Some(Event::Resize(11, 5)));
    }

    #[test]
    fn resize_to_updates_backend_size_immediately() {
        // Regression test for #508: previously `backend.size()` (via `Output::size`) kept
        // reporting the pre-resize dimensions until the app called `Terminal::resize` in
        // response to `Event::Resize`, so polling the backend directly for drift was useless.
        // `resize_to` must now also call `Output::resize` so `size()` agrees with the surface
        // right away, independent of whether/when the app resizes the terminal's own grid.
        let mut app = test_window_app();
        assert_eq!(
            app.terminal.as_ref().unwrap().backend().size(),
            Size::new(10, 5)
        );
        app.resize_to(winit::dpi::PhysicalSize::new(90, 81));
        assert_eq!(
            app.terminal.as_ref().unwrap().backend().size(),
            Size::new(11, 5)
        );
        // `Terminal::size` (the grid itself) is untouched: that stays the app's job, done by
        // calling `Terminal::resize` in response to the `Event::Resize` this same call pushed.
        assert_eq!(app.terminal.as_ref().unwrap().size(), Size::new(10, 5));
    }

    #[test]
    fn resized_below_one_cell_clamps_surface_and_event_to_1x1() {
        // Regression test for #140: an 8x16-cell presenter resized to a
        // window smaller than one cell (4x4 px) must not compute 0 cols/0
        // rows: that would ask `resize_surface` for a zero-size surface,
        // which crashes softbuffer.
        type RecordingApp = WindowApp<
            RecordingPresenter,
            fn(&mut Terminal<WindowBackend<RecordingPresenter>>),
            u64,
            fn(u64, &mut Terminal<WindowBackend<RecordingPresenter>>),
        >;
        let resize_calls = Rc::new(RefCell::new(Vec::new()));
        let presenter = RecordingPresenter {
            resize_calls: resize_calls.clone(),
        };
        let terminal = Terminal::new(WindowBackend::new(presenter));
        let mut app: RecordingApp = WindowApp {
            terminal: Some(terminal),
            app_loop: |_| {},
            on_custom_event: push_custom_event,
            _user_event: PhantomData,
            window: None,
            title: String::new(),
            init_size: InitWindowSize {
                width: 80,
                height: 80,
            },
            attrs: WindowAttrs::default(),
            current_modifiers: KeyModifiers::NONE,
            cursor_px: (0.0, 0.0),
            active_touch: None,
            held_buttons: 0,
            frame_interval: None,
            event_driven: true,
            #[cfg(not(target_arch = "wasm32"))]
            next_frame: std::time::Instant::now(),
            exit_requested: Rc::new(Cell::new(false)),
            skip_present: Rc::new(Cell::new(false)),
            needs_redraw: false,
            consecutive_present_errors: 0,
        };

        app.handle_window_event(WindowEvent::Resized(winit::dpi::PhysicalSize::new(4, 4)));

        // Surface must be resized to at least one full cell (8x16), not
        // 0x0.
        assert_eq!(resize_calls.borrow().as_slice(), &[(8, 16)]);
        // Event::Resize must report the same clamped 1x1 grid, not 0x0.
        assert_eq!(
            app.terminal
                .as_mut()
                .unwrap()
                .backend_mut()
                .poll_event(Duration::ZERO),
            Some(Event::Resize(1, 1))
        );
    }

    // ── needs_redraw (idle/redraw-on-demand, issue #155) ─────────────────────

    #[test]
    fn fresh_app_does_not_need_a_redraw() {
        // `test_window_app` starts with `needs_redraw: false`, unlike the real
        // `resumed()` path, which sets it `true` once the window/surface exists (a real winit
        // `ActiveEventLoop` can't be constructed in a unit test, so `resumed` itself isn't
        // exercised here; see `handle_window_event`/`handle_user_event` below for the parts of
        // the redraw-on-demand logic that are testable without one).
        let app = test_window_app();
        assert!(!app.needs_redraw);
    }

    #[test]
    fn window_event_sets_needs_redraw() {
        // Any real window event (a mouse move here, but any arm other than `RedrawRequested`
        // behaves the same; see `handle_window_event`'s doc comment) should mark that the app
        // loop has something new to react to, so the next `about_to_wait` requests a redraw
        // instead of leaving the loop idle.
        let mut app = test_window_app();
        assert!(!app.needs_redraw);
        app.handle_window_event(WindowEvent::CursorMoved {
            device_id: winit::event::DeviceId::dummy(),
            position: winit::dpi::PhysicalPosition::new(1.0_f64, 1.0_f64),
        });
        assert!(app.needs_redraw);
    }

    #[test]
    fn redraw_requested_does_not_itself_set_needs_redraw() {
        // `RedrawRequested` is the render this flag exists to gate, not a new event to redraw
        // again for: an idle app that gets exactly one `RedrawRequested` (e.g. right after
        // `resumed`) must not perpetually re-arm itself into another one forever.
        let mut app = test_window_app();
        app.handle_window_event(WindowEvent::RedrawRequested);
        assert!(!app.needs_redraw);
    }

    #[test]
    fn user_event_sets_needs_redraw() {
        // A cross-thread `Event::Custom` injection (network, audio, timer, ...) must wake an
        // idle loop into rendering the next frame just like a real window event does.
        let mut app = test_window_app();
        assert!(!app.needs_redraw);
        app.handle_user_event(1);
        assert!(app.needs_redraw);
    }

    #[test]
    fn unhandled_window_events_still_set_needs_redraw() {
        // Even a `WindowEvent` variant with no dedicated handling below (falls through to the
        // `_ => {}` arm in `handle_window_event`'s `match`) should still be treated as "something
        // happened": the flag is set once, up front, before the match runs.
        let mut app = test_window_app();
        app.handle_window_event(WindowEvent::Occluded(true));
        assert!(app.needs_redraw);
    }

    // ── frame-rate cap (target_fps) ───────────────────────────────────────────

    #[test]
    fn target_fps_none_is_redraw_on_demand() {
        // `target_fps: None` leaves `frame_interval` unset, i.e. uncapped whenever a redraw
        // happens; `event_driven: true` is what sends `about_to_wait` down the
        // `needs_redraw`-gated branch.
        let presenter = MockPresenter::default();
        assert_eq!(
            WindowConfig::fit(&presenter, "test", None, true).target_fps(),
            None
        );
    }

    #[test]
    fn target_fps_some_survives_to_the_config() {
        // Regression guard for the wasm32 half of the freeze this mode fixes: `target_fps` used
        // to be dropped on the floor for wasm builds (`frame_interval` was `#[cfg(not(target_arch
        // = "wasm32"))]`), so a browser app asking for continuous rendering silently got
        // redraw-on-demand and rendered one frame for the life of the page. The field is
        // unconditional now; this pins the config end of that, and the `compile-wasm` CI job pins
        // the driver end.
        let presenter = MockPresenter::default();
        assert_eq!(
            WindowConfig::fit(&presenter, "test", Some(60), false).target_fps(),
            Some(60)
        );
    }

    #[test]
    fn event_driven_accessor_reflects_the_config() {
        let presenter = MockPresenter::default();
        assert!(WindowConfig::fit(&presenter, "test", None, true).event_driven());
        assert!(!WindowConfig::fit(&presenter, "test", None, false).event_driven());
    }

    #[test]
    fn target_fps_and_event_driven_combine_independently() {
        // The combination `fit` alone couldn't express before: always redraw (not event-driven)
        // but uncapped (no `target_fps`).
        let presenter = MockPresenter::default();
        let config = WindowConfig::fit(&presenter, "test", None, false);
        assert_eq!(config.target_fps(), None);
        assert!(!config.event_driven());
    }

    #[test]
    fn animated_is_sugar_for_continuous_capped_fit() {
        let presenter = MockPresenter::default();
        let config = WindowConfig::animated(&presenter, "test", 60);
        assert_eq!(config.target_fps(), Some(60));
        assert!(!config.event_driven());
    }

    #[test]
    fn with_run_options_overwrites_target_fps_and_event_driven() {
        let presenter = MockPresenter::default();
        let options = retroglyph_core::app::RunOptions::default()
            .with_target_fps(30)
            .event_driven(false);
        let config = WindowConfig::fit(&presenter, "test", None, true).with_run_options(options);
        assert_eq!(config.target_fps(), Some(30));
        assert!(!config.event_driven());
    }

    #[test]
    fn with_run_options_wins_when_applied_after_fit() {
        // Builder call order decides precedence: `with_run_options` applied last overrides
        // whatever `fit`'s positional `target_fps`/`event_driven` set, giving `RunOptions` the
        // final say the same way it does for `run_on_with`.
        let presenter = MockPresenter::default();
        let config = WindowConfig::fit(&presenter, "test", Some(60), false)
            .with_run_options(retroglyph_core::app::RunOptions::default());
        assert_eq!(config.target_fps(), None);
        assert!(config.event_driven());
    }

    #[cfg(not(target_arch = "wasm32"))]
    #[test]
    fn frame_deadline_in_the_future_parks_the_loop() {
        let now = std::time::Instant::now();
        let next = now + Duration::from_millis(10);
        assert_eq!(
            next_frame_deadline(now, next, Duration::from_millis(16)),
            None
        );
    }

    #[cfg(not(target_arch = "wasm32"))]
    #[test]
    fn frame_deadline_reached_advances_by_exactly_one_interval() {
        // On time (deadline just passed): the next deadline is one interval on from the *deadline*,
        // not from `now`, so a steady loop doesn't drift later and later.
        let interval = Duration::from_millis(16);
        let next = std::time::Instant::now();
        let now = next + Duration::from_micros(200);
        assert_eq!(
            next_frame_deadline(now, next, interval),
            Some(next + interval)
        );
    }

    #[cfg(not(target_arch = "wasm32"))]
    #[test]
    fn overrun_frame_deadline_clamps_to_now_instead_of_bursting() {
        // A frame that blew well past its budget must not leave a backlog of deadlines already in
        // the past, which would render several catch-up frames back to back at full speed.
        let interval = Duration::from_millis(16);
        let next = std::time::Instant::now();
        let now = next + Duration::from_millis(500);
        assert_eq!(next_frame_deadline(now, next, interval), Some(now));
    }

    // ── handle_redraw_requested / present() failure recovery ─────────────────

    type FailingApp = WindowApp<
        FailingPresenter,
        fn(&mut Terminal<WindowBackend<FailingPresenter>>),
        u64,
        fn(u64, &mut Terminal<WindowBackend<FailingPresenter>>),
    >;

    fn failing_app() -> (FailingApp, Rc<Cell<bool>>, Rc<Cell<u32>>) {
        let failing = Rc::new(Cell::new(false));
        let init_surface_calls = Rc::new(Cell::new(0));
        let presenter = FailingPresenter {
            failing: failing.clone(),
            init_surface_calls: init_surface_calls.clone(),
        };
        let terminal = Terminal::new(WindowBackend::new(presenter));
        let app: FailingApp = WindowApp {
            terminal: Some(terminal),
            app_loop: (|_| {}) as fn(&mut Terminal<WindowBackend<FailingPresenter>>),
            on_custom_event: push_custom_event,
            _user_event: PhantomData,
            window: None,
            title: String::new(),
            init_size: InitWindowSize {
                width: 80,
                height: 80,
            },
            attrs: WindowAttrs::default(),
            current_modifiers: KeyModifiers::NONE,
            cursor_px: (0.0, 0.0),
            active_touch: None,
            held_buttons: 0,
            frame_interval: None,
            event_driven: true,
            #[cfg(not(target_arch = "wasm32"))]
            next_frame: std::time::Instant::now(),
            exit_requested: Rc::new(Cell::new(false)),
            skip_present: Rc::new(Cell::new(false)),
            needs_redraw: false,
            consecutive_present_errors: 0,
        };
        (app, failing, init_surface_calls)
    }

    #[test]
    fn successful_presents_never_increment_the_failure_counter() {
        let (mut app, _failing, _init_calls) = failing_app();
        for _ in 0..5 {
            app.handle_redraw_requested();
        }
        assert_eq!(app.consecutive_present_errors, 0);
    }

    #[test]
    fn failing_presents_increment_the_counter_and_stop_short_of_recovery() {
        let (mut app, failing, init_calls) = failing_app();
        failing.set(true);
        for _ in 0..PRESENT_FAILURE_RECOVERY_THRESHOLD - 1 {
            app.handle_redraw_requested();
        }
        assert_eq!(
            app.consecutive_present_errors,
            PRESENT_FAILURE_RECOVERY_THRESHOLD - 1
        );
        // No window to recover from in this test app (`window: None`), but recovery should not
        // even have been attempted yet regardless: confirmed by `try_recover_surface`'s own
        // no-window guard never being reached, i.e. `init_surface` was never called past the
        // initial 0.
        assert_eq!(init_calls.get(), 0);
    }

    #[test]
    fn counter_resets_after_recovering_from_a_failure_streak() {
        let (mut app, failing, _init_calls) = failing_app();
        failing.set(true);
        for _ in 0..5 {
            app.handle_redraw_requested();
        }
        assert_eq!(app.consecutive_present_errors, 5);

        failing.set(false);
        app.handle_redraw_requested();
        assert_eq!(app.consecutive_present_errors, 0);
    }

    #[test]
    fn crossing_the_recovery_threshold_attempts_recovery_without_panicking() {
        // `test_window_app`/`failing_app` have no real winit `Window` (constructing one needs a
        // live event loop, unavailable in a unit test, the same limitation documented on
        // `scale_factor_changed_without_a_window_is_a_no_op_resize` above), so this can't assert
        // `init_surface` actually re-runs; `try_recover_surface`'s own no-window guard is exercised
        // directly below instead. What this does verify: the threshold-crossing call does not
        // panic, and the counter keeps incrementing through and past the threshold rather than
        // resetting or overflowing.
        let (mut app, failing, init_calls) = failing_app();
        failing.set(true);
        for _ in 0..PRESENT_FAILURE_RECOVERY_THRESHOLD {
            app.handle_redraw_requested();
        }
        assert_eq!(
            app.consecutive_present_errors,
            PRESENT_FAILURE_RECOVERY_THRESHOLD
        );
        assert_eq!(
            init_calls.get(),
            0,
            "no window means try_recover_surface's guard skips init_surface"
        );
    }

    #[test]
    fn try_recover_surface_without_a_window_is_a_no_op() {
        let (mut app, _failing, init_calls) = failing_app();
        app.try_recover_surface();
        assert_eq!(init_calls.get(), 0);
    }

    // ── automatic `Terminal::present` on redraw ───────────────────────────────

    /// A [`Presenter`] that mirrors every drawn diff into an in-memory grid (like
    /// [`retroglyph_core::backend::Headless`], but implementing [`Presenter`] instead), so tests
    /// can assert on what was actually presented rather than just on whether `present()` returned
    /// `Ok`.
    #[derive(Default)]
    struct GridRecordingPresenter {
        /// `(x, y) -> glyph` for every cell ever written by `draw_layers`. A real display only
        /// keeps the latest write per cell, which is exactly what repeated `HashMap` inserts give
        /// us here.
        cells: RefCell<std::collections::HashMap<(u16, u16), char>>,
        /// Number of `draw_layers` calls observed, so tests can assert whether a second (and, per
        /// this module's `present`-erases-if-nothing-new-was-drawn finding, harmful) diff was ever
        /// sent.
        draw_calls: Cell<u32>,
    }

    impl Output for GridRecordingPresenter {
        type Error = core::convert::Infallible;

        fn draw<'a, I>(&mut self, _content: I) -> Result<(), Self::Error>
        where
            I: Iterator<Item = DrawCell<'a>>,
        {
            Ok(())
        }

        fn draw_layers<'a, I>(&mut self, content: I) -> Result<(), Self::Error>
        where
            I: Iterator<Item = DrawCell<'a>>,
        {
            self.draw_calls.set(self.draw_calls.get() + 1);
            let mut cells = self.cells.borrow_mut();
            for cell in content {
                cells.insert((cell.pos.x, cell.pos.y), cell.tile.glyph());
            }
            Ok(())
        }

        fn flush(&mut self) -> Result<(), Self::Error> {
            Ok(())
        }

        fn size(&self) -> Size {
            Size::new(10, 5)
        }

        fn clear(&mut self) -> Result<(), Self::Error> {
            Ok(())
        }

        fn resize(&mut self, _size: Size) {}
    }

    impl Presenter for GridRecordingPresenter {
        type SurfaceError = core::convert::Infallible;

        fn init_surface(
            &mut self,
            _window: Arc<dyn crate::presenter::WindowHandle>,
        ) -> Result<(), Self::SurfaceError> {
            Ok(())
        }

        fn resize_surface(&mut self, _width: u32, _height: u32) {}

        fn present(&mut self) -> Result<(), Self::SurfaceError> {
            Ok(())
        }

        fn cell_size(&self) -> (u32, u32) {
            (8, 16)
        }
    }

    type GridRecordingApp = WindowApp<
        GridRecordingPresenter,
        fn(&mut Terminal<WindowBackend<GridRecordingPresenter>>),
        u64,
        fn(u64, &mut Terminal<WindowBackend<GridRecordingPresenter>>),
    >;

    /// Boxed-closure counterparts of [`GridRecordingApp`]'s type parameters, for tests (like
    /// [`skip_present_set_inside_app_loop_suppresses_the_automatic_present`]) whose `app_loop`
    /// needs to capture and mutate a shared flag, which a bare `fn` pointer cannot do.
    type BoxedGridRecordingAppLoop =
        Box<dyn FnMut(&mut Terminal<WindowBackend<GridRecordingPresenter>>)>;
    type BoxedGridRecordingApp = WindowApp<
        GridRecordingPresenter,
        BoxedGridRecordingAppLoop,
        u64,
        fn(u64, &mut Terminal<WindowBackend<GridRecordingPresenter>>),
    >;

    fn recording_app(
        app_loop: fn(&mut Terminal<WindowBackend<GridRecordingPresenter>>),
    ) -> GridRecordingApp {
        let terminal = Terminal::new(WindowBackend::new(GridRecordingPresenter::default()));
        WindowApp {
            terminal: Some(terminal),
            app_loop,
            on_custom_event: push_custom_event,
            _user_event: PhantomData,
            window: None,
            title: String::new(),
            init_size: InitWindowSize {
                width: 80,
                height: 80,
            },
            attrs: WindowAttrs::default(),
            current_modifiers: KeyModifiers::NONE,
            cursor_px: (0.0, 0.0),
            active_touch: None,
            held_buttons: 0,
            frame_interval: None,
            event_driven: true,
            #[cfg(not(target_arch = "wasm32"))]
            next_frame: std::time::Instant::now(),
            exit_requested: Rc::new(Cell::new(false)),
            skip_present: Rc::new(Cell::new(false)),
            needs_redraw: false,
            consecutive_present_errors: 0,
        }
    }

    #[test]
    fn app_loop_that_never_presents_is_still_drawn_by_the_automatic_present() {
        // Case (a): an `app_loop` that draws but never calls `term.present()` itself must still
        // reach the backend: that's the whole point of this driver-side automatic present.
        let mut app = recording_app(|term| {
            term.surface()
                .put((0, 0), '@', retroglyph_core::color::Style::default());
        });
        app.handle_redraw_requested();
        let term = app.terminal.as_ref().unwrap();
        let presenter = term.backend().presenter();
        assert_eq!(presenter.cells.borrow().get(&(0, 0)), Some(&'@'));
        assert_eq!(
            presenter.draw_calls.get(),
            1,
            "exactly one present this frame"
        );
    }

    #[test]
    fn app_loop_that_already_presents_itself_is_not_double_drawn() {
        // Case (b): an `app_loop` that still calls `term.present()` itself (the pre-fix pattern)
        // must keep working, and, crucially, must not have its frame blanked by a second,
        // driver-side `present()` call diffing an now-empty `current` against the just-drawn
        // `previous` (see `Terminal::present`'s doc comment for why that second call would
        // otherwise erase the frame).
        let mut app = recording_app(|term| {
            term.surface()
                .put((0, 0), '@', retroglyph_core::color::Style::default());
            term.present().expect("app_loop's own present");
        });
        app.handle_redraw_requested();
        let term = app.terminal.as_ref().unwrap();
        let presenter = term.backend().presenter();
        assert_eq!(presenter.cells.borrow().get(&(0, 0)), Some(&'@'));
        assert_eq!(
            presenter.draw_calls.get(),
            1,
            "the driver must detect app_loop's own present and skip its automatic one"
        );
    }

    #[test]
    fn skip_present_set_inside_app_loop_suppresses_the_automatic_present() {
        // Simulates an `App::update` returning `Flow::Idle`: `run_app_with_proxy`'s closure draws
        // nothing and sets `skip_present` from inside `app_loop`, the same point in the frame
        // `run_app_with_proxy`'s real closure sets it from. `handle_redraw_requested` must honor
        // it: `Terminal::present` always presents unconditionally (even on an untouched frame),
        // so without this explicit skip it would still run and erase whatever the previous frame
        // left on screen.
        let terminal = Terminal::new(WindowBackend::new(GridRecordingPresenter::default()));
        let skip_present = Rc::new(Cell::new(false));
        let skip_present_in_loop = skip_present.clone();
        let app_loop: BoxedGridRecordingAppLoop =
            Box::new(move |_term| skip_present_in_loop.set(true));
        let mut app: BoxedGridRecordingApp = WindowApp {
            terminal: Some(terminal),
            app_loop,
            on_custom_event: push_custom_event,
            _user_event: PhantomData,
            window: None,
            title: String::new(),
            init_size: InitWindowSize {
                width: 80,
                height: 80,
            },
            attrs: WindowAttrs::default(),
            current_modifiers: KeyModifiers::NONE,
            cursor_px: (0.0, 0.0),
            active_touch: None,
            held_buttons: 0,
            frame_interval: None,
            event_driven: true,
            #[cfg(not(target_arch = "wasm32"))]
            next_frame: std::time::Instant::now(),
            exit_requested: Rc::new(Cell::new(false)),
            skip_present,
            needs_redraw: false,
            consecutive_present_errors: 0,
        };
        app.handle_redraw_requested();
        let term = app.terminal.as_ref().unwrap();
        let presenter = term.backend().presenter();
        assert_eq!(
            presenter.draw_calls.get(),
            0,
            "no present reaches the backend when app_loop sets skip_present"
        );
    }

    #[test]
    fn skip_present_does_not_carry_over_to_the_next_redraw() {
        // `handle_redraw_requested` must reset `skip_present` before running `app_loop`, so a
        // stale `true` from a previous `Idle` frame can't suppress the next frame's present.
        let mut app = recording_app(|term| {
            term.surface()
                .put((0, 0), '@', retroglyph_core::color::Style::default());
        });
        app.skip_present.set(true); // Stale value, as if left over from a prior Idle frame.
        app.handle_redraw_requested();
        let term = app.terminal.as_ref().unwrap();
        let presenter = term.backend().presenter();
        assert_eq!(presenter.cells.borrow().get(&(0, 0)), Some(&'@'));
        assert_eq!(presenter.draw_calls.get(), 1);
    }

    #[test]
    fn present_count_advances_once_per_present_call() {
        let mut term = Terminal::new(WindowBackend::new(GridRecordingPresenter::default()));
        assert_eq!(term.present_count(), 0);
        term.present().expect("present");
        assert_eq!(term.present_count(), 1);
        term.present().expect("present");
        assert_eq!(term.present_count(), 2);
    }

    // ── handle_redraw_requested / unrecoverable (`is_recoverable() == false`) errors ─────────

    type FatalApp = WindowApp<
        FatalPresenter,
        fn(&mut Terminal<WindowBackend<FatalPresenter>>),
        u64,
        fn(u64, &mut Terminal<WindowBackend<FatalPresenter>>),
    >;

    fn fatal_app() -> (FatalApp, Rc<Cell<bool>>, Rc<Cell<u32>>) {
        let failing = Rc::new(Cell::new(false));
        let init_surface_calls = Rc::new(Cell::new(0));
        let presenter = FatalPresenter {
            failing: failing.clone(),
            init_surface_calls: init_surface_calls.clone(),
        };
        let terminal = Terminal::new(WindowBackend::new(presenter));
        let app: FatalApp = WindowApp {
            terminal: Some(terminal),
            app_loop: (|_| {}) as fn(&mut Terminal<WindowBackend<FatalPresenter>>),
            on_custom_event: push_custom_event,
            _user_event: PhantomData,
            window: None,
            title: String::new(),
            init_size: InitWindowSize {
                width: 80,
                height: 80,
            },
            attrs: WindowAttrs::default(),
            current_modifiers: KeyModifiers::NONE,
            cursor_px: (0.0, 0.0),
            active_touch: None,
            held_buttons: 0,
            frame_interval: None,
            event_driven: true,
            #[cfg(not(target_arch = "wasm32"))]
            next_frame: std::time::Instant::now(),
            exit_requested: Rc::new(Cell::new(false)),
            skip_present: Rc::new(Cell::new(false)),
            needs_redraw: false,
            consecutive_present_errors: 0,
        };
        (app, failing, init_surface_calls)
    }

    #[test]
    fn unrecoverable_present_failure_never_attempts_recovery_even_past_the_threshold() {
        // Unlike `FailingPresenter` (recoverable errors, generic threshold-based recovery), a
        // `FatalPresenter` failure is fatal on every single call: `present_failure_action`
        // returns `Fatal` immediately (see the pure-function tests above), so
        // `handle_redraw_requested` must never route it through `try_recover_surface`, no matter
        // how many consecutive failures accumulate past `PRESENT_FAILURE_RECOVERY_THRESHOLD`.
        let (mut app, failing, init_calls) = fatal_app();
        failing.set(true);
        for _ in 0..2 * PRESENT_FAILURE_RECOVERY_THRESHOLD {
            app.handle_redraw_requested();
        }
        assert_eq!(init_calls.get(), 0);
    }

    #[test]
    fn unrecoverable_present_failure_does_not_panic_and_keeps_counting() {
        let (mut app, failing, _init_calls) = fatal_app();
        failing.set(true);
        for _ in 0..5 {
            app.handle_redraw_requested();
        }
        assert_eq!(app.consecutive_present_errors, 5);
    }

    #[test]
    fn recovering_from_an_unrecoverable_failure_streak_still_resets_the_counter() {
        let (mut app, failing, _init_calls) = fatal_app();
        failing.set(true);
        for _ in 0..3 {
            app.handle_redraw_requested();
        }
        assert_eq!(app.consecutive_present_errors, 3);

        failing.set(false);
        app.handle_redraw_requested();
        assert_eq!(app.consecutive_present_errors, 0);
    }
}