velox-renderer 0.1.2

//! Renderer crate with optional backends.
//! No features enabled => stub, compiles fast.

use velox_dom::VNode;
use velox_style::Stylesheet;

/// Apply the renderer-wide UA < author < inline cascade before layout or paint.
///
/// The renderer always calls this once on a freshly-built VNode. Keeping the
/// composition here prevents a backend from accidentally dropping the UA layer.
pub fn style_vnode_with_hover<F>(vnode: &VNode, author: &Stylesheet, is_hovered: &F) -> VNode
where
    F: Fn(&str, &velox_dom::Props) -> bool,
{
    velox_style::apply_with_cascade_with_hover(vnode, author, is_hovered)
}

/// Lifecycle wiring helper — ensures `on_unmounted` / `before_destroy` hooks
/// fire even if the event loop exits via `Drop` rather than `CloseRequested`.
#[allow(dead_code)]
pub struct LifecycleCleanupGuard;

impl Drop for LifecycleCleanupGuard {
    fn drop(&mut self) {
        let _ = std::panic::catch_unwind(velox_core::lifecycle::run_all_destroy_hooks);
    }
}

#[allow(dead_code)]
pub fn ensure_mounted(flag: &mut bool) {
    if !*flag {
        *flag = true;
        velox_core::lifecycle::run_all_mounted_hooks();
    }
}

/// Return the VNode at a child source-index path, if it exists.
pub fn find_node_at_path<'a>(node: &'a VNode, path: &[usize]) -> Option<&'a VNode> {
    let mut cur = node;
    for &idx in path {
        match cur {
            VNode::Element { children, .. } => cur = children.get(idx)?,
            _ => return None,
        }
    }
    Some(cur)
}

/// Unified logical size helper — single rounding point for all frame paths.
/// Delegates to Viewport::from_i32 for consistent clamping; kept as free function for compat.
#[cfg(any(feature = "skia-native", test))]
fn viewport_logical_dimensions(width: i32, height: i32, scale_factor: f32) -> (u32, u32) {
    Viewport::from_i32(width, height, scale_factor).logical_size()
}

#[cfg(feature = "skia-native")]
pub fn logical_size(width: i32, height: i32, scale_factor: f32) -> (u32, u32) {
    viewport_logical_dimensions(width, height, scale_factor)
}

/// Notify resize hooks only after a coalesced physical resize has committed and
/// its logical viewport differs from the last committed size.
#[cfg(any(feature = "skia-native", test))]
fn dispatch_resize_if_changed(
    committed: bool,
    logical: (u32, u32),
    last_resize_size: &mut Option<(u32, u32)>,
) {
    if committed && *last_resize_size != Some(logical) {
        velox_core::lifecycle::run_resize_hooks(logical.0, logical.1);
        *last_resize_size = Some(logical);
    }
}

#[cfg(any(feature = "skia-native", test))]
fn frame_logical_size(
    width: i32,
    height: i32,
    scale_factor: f32,
    committed: bool,
    last_resize_size: &mut Option<(u32, u32)>,
) -> (u32, u32) {
    let logical = viewport_logical_dimensions(width, height, scale_factor);
    dispatch_resize_if_changed(committed, logical, last_resize_size);
    logical
}

/// Per-renderer resize state. Physical events are coalesced here and the
/// committed logical baseline is kept locally to this window/event loop.
#[cfg(any(feature = "skia-native", test))]
struct ResizeState {
    pending_resize: Option<(u32, u32)>,
    last_resize_size: Option<(u32, u32)>,
}

#[cfg(any(feature = "skia-native", test))]
impl ResizeState {
    fn new() -> Self {
        Self {
            pending_resize: None,
            last_resize_size: None,
        }
    }

    fn queue(&mut self, physical: (u32, u32)) {
        self.pending_resize = Some(physical);
    }

    fn take_pending(&mut self) -> Option<(u32, u32)> {
        self.pending_resize.take()
    }

    fn record_initial(&mut self, width: i32, height: i32, scale_factor: f32) -> (u32, u32) {
        let logical = viewport_logical_dimensions(width, height, scale_factor);
        self.last_resize_size = Some(logical);
        logical
    }

    fn frame_logical_size(
        &mut self,
        width: i32,
        height: i32,
        scale_factor: f32,
        committed: bool,
    ) -> (u32, u32) {
        frame_logical_size(
            width,
            height,
            scale_factor,
            committed,
            &mut self.last_resize_size,
        )
    }
}

/// Build hit-test targets from a precomputed layout. No layout recompute here.
#[cfg(feature = "skia-native")]
fn recompute_targets(
    vnode: &velox_dom::VNode,
    layout: &velox_dom::layout::LayoutNode,
    click_targets: &mut Vec<crate::events::ClickTarget>,
    hover_targets: &mut Vec<crate::events::HoverTarget>,
    input_targets: &mut Vec<crate::events::InputTarget>,
) {
    click_targets.clear();
    let mut order = 0;
    crate::events::collect_click_targets(
        vnode,
        layout,
        None,
        crate::events::StackCtx::ROOT,
        &mut order,
        click_targets,
    );
    hover_targets.clear();
    let mut order = 0;
    crate::events::collect_hover_targets(
        vnode,
        layout,
        None,
        crate::events::StackCtx::ROOT,
        &mut order,
        hover_targets,
    );
    // Edit state (focus / caret / selection / blink phase) lives on the targets,
    // and a target rebuild throws it all away. Move the old vector aside and
    // re-apply it onto the freshly collected targets by tree path, so a
    // re-render does not silently drop the caret. `std::mem::take` is only a
    // move — the fresh collection below reuses the same allocation.
    let previous = std::mem::take(input_targets);
    let mut order = 0;
    let mut path = Vec::new();
    crate::events::collect_input_targets(
        vnode,
        layout,
        None,
        crate::events::StackCtx::ROOT,
        &mut path,
        &mut order,
        input_targets,
    );
    crate::events::preserve_input_state(input_targets, &previous, &|p| {
        input_value_char_len(vnode, p)
    });
}

#[cfg(feature = "skia-native")]
fn with_hover_ids(vnode: &velox_dom::VNode, next_id: &mut u32) -> velox_dom::VNode {
    match vnode {
        velox_dom::VNode::Text(_) => vnode.clone(),
        velox_dom::VNode::Element {
            tag,
            props,
            children,
        } => {
            let mut new_props = props.clone();
            if crate::events::is_hoverable(tag, props) {
                let id = *next_id;
                *next_id += 1;
                new_props = new_props.set("data-hover-id", id.to_string());
            }
            let new_children = children
                .iter()
                .map(|c| with_hover_ids(c, next_id))
                .collect();
            velox_dom::VNode::Element {
                tag: tag.clone(),
                props: new_props,
                children: new_children,
            }
        }
    }
}

/// Caret blink half-period, in milliseconds.
///
/// 530 ms is the conventional caret cadence. It is a named const rather than a
/// literal at the call site because the tick interval, the "solid while
/// typing" grace window, and the post-key re-arm deadline must all agree.
#[cfg(feature = "skia-native")]
const CARET_BLINK_MS: u64 = 530;

/// Posts a recurring `UserEvent` to the winit event loop so the caret blink has
/// a clock.
///
/// Why there is a thread at all: the loops run with `ControlFlow::Wait`, so they
/// only wake for real OS events. A bare `Instant::now()` deadline check inside
/// the event closure would therefore never fire while the user is idle, and the
/// caret would freeze solid. winit 0.28 has no timer API, so the tick has to
/// come in as a user event.
///
/// Why this is not a shared-state race: the thread owns nothing but an
/// `EventLoopProxy`, a cloned copy of the tick payload, and an `AtomicBool`
/// stop flag. It has no reference to `input_targets`, the VNode, the renderer,
/// or the presenter — it cannot read or write renderer state at all. Every
/// `blink_on` mutation happens on the event-loop thread, inside the event
/// closure, where the renderer is already single-threaded. This is exactly the
/// property a `Mutex<InputState>` design would have had to give up.
///
/// The thread is joined in `Drop`, so closing the window does not leave a
/// detached ticker posting events at a dead loop.
#[cfg(feature = "skia-native")]
struct CaretBlinkTicker<T: Clone + Send + 'static> {
    stop: std::sync::Arc<std::sync::atomic::AtomicBool>,
    handle: Option<std::thread::JoinHandle<()>>,
    /// The tick payload is moved into the thread, so the struct itself does
    /// not otherwise mention `T`. This marker is what ties the type parameter
    /// to the type; it is zero-sized and owns nothing.
    _tick: std::marker::PhantomData<fn() -> T>,
}

#[cfg(feature = "skia-native")]
impl<T: Clone + Send + 'static> CaretBlinkTicker<T> {
    /// Start ticking. `tick` is the payload posted on every interval — `()` for
    /// the plain loop, [`HmrMessage::KeepWindow`] for the HMR loop, whose
    /// channel is typed.
    fn start(proxy: winit::event_loop::EventLoopProxy<T>, tick: T) -> Self {
        let stop = std::sync::Arc::new(std::sync::atomic::AtomicBool::new(false));
        let stop_thread = std::sync::Arc::clone(&stop);
        let period = std::time::Duration::from_millis(CARET_BLINK_MS);
        let handle = std::thread::Builder::new()
            .name("velox-caret-blink".into())
            .spawn(move || {
                while !stop_thread.load(std::sync::atomic::Ordering::Relaxed) {
                    std::thread::sleep(period);
                    if stop_thread.load(std::sync::atomic::Ordering::Relaxed) {
                        break;
                    }
                    // winit 0.28's `EventLoopProxy<T>::send_event` takes the
                    // user-event payload `T` directly — the platform layer wraps
                    // it into `Event::UserEvent(..)` on delivery. Sending fails
                    // once the loop is gone, which is the exit condition.
                    if proxy.send_event(tick.clone()).is_err() {
                        break;
                    }
                }
            })
            .ok();
        Self {
            stop,
            handle,
            _tick: std::marker::PhantomData,
        }
    }
}

#[cfg(feature = "skia-native")]
impl<T: Clone + Send + 'static> Drop for CaretBlinkTicker<T> {
    fn drop(&mut self) {
        self.stop.store(true, std::sync::atomic::Ordering::Relaxed);
        if let Some(h) = self.handle.take() {
            let _ = h.join();
        }
    }
}

/// Advance the caret blink phase on a ticker event.
///
/// Returns `true` when the phase actually flipped and the frame must be
/// repainted. A tick that arrives inside the "solid while typing" window set by
/// a keypress or a focus change does *not* flip: it only re-arms the deadline,
/// which is what stops the caret from blinking off milliseconds after a
/// keystroke.
#[cfg(feature = "skia-native")]
fn on_caret_blink_tick(
    input_targets: &mut [crate::events::InputTarget],
    blink_deadline: &mut Option<std::time::Instant>,
) -> bool {
    let Some(idx) = crate::events::focused_input_index(input_targets) else {
        return false;
    };
    let now = std::time::Instant::now();
    if let Some(d) = *blink_deadline
        && now < d
    {
        return false;
    }
    input_targets[idx].blink_on = !input_targets[idx].blink_on;
    *blink_deadline = Some(now + std::time::Duration::from_millis(CARET_BLINK_MS));
    true
}

/// Re-arm the blink grace window after a keypress or focus change.
///
/// This is the "caret goes solid while you type" rule: any editing action
/// defers the next phase flip by one cadence period. Without it a keystroke
/// landing just before a tick would be blinked off a few milliseconds later,
/// which reads as flicker rather than as a caret.
#[cfg(feature = "skia-native")]
fn arm_blink_deadline(blink_deadline: &mut Option<std::time::Instant>) {
    *blink_deadline =
        Some(std::time::Instant::now() + std::time::Duration::from_millis(CARET_BLINK_MS));
}

/// Read a declaration out of a cascaded `style` attribute (`"a: b; c: d"`).
#[cfg(feature = "skia-native")]
fn style_decl<'s>(style: &'s str, key: &str) -> Option<&'s str> {
    style
        .split(';')
        .filter_map(|d| d.split_once(':'))
        .find(|(k, _)| k.trim() == key)
        .map(|(_, v)| v.trim())
}

/// The painter's own geometry for a text input's value text: content edges
/// **and** the font size its advances are measured at.
///
/// The click-to-caret mapping is relative to `text_left`, so padding shifts
/// the caret to match the painted glyphs.
///
/// This delegates to [`crate::input_metrics::input_text_metrics`] rather than
/// reading the authored string. It used to look up the literal key
/// `padding-left`, which is never present: the cascade does not expand
/// shorthands, so `padding: 6px 10px` survives under the key `padding` and the
/// lookup missed it, returning `rect.x + 0` while the paint lane drew glyphs at
/// `text_left`. Click-to-caret therefore disagreed with the painted glyphs by
/// the whole left padding, and only an inline `padding-left` happened to work.
/// Reading the *computed* side is the fix; the paint lane already calls the
/// same function, so the two lanes can no longer drift.
///
/// Returning the whole struct — rather than just `.text_left` — is what makes
/// the second half of the bug unrepresentable. The origin used to be shared
/// while the font size was re-derived by a lane-local `parse_px` that accepted
/// only a literal `px` suffix, so `em`/`%`/`rem` and bare inheritance all
/// collapsed to a fallback and every advance came out `16/14` too wide: a
/// click at the far end of the value landed the caret about one character
/// short for every seven clicked. One call, both fields.
///
/// `viewport` is the **logical** `(w, h)` the paint lane resolves `vw`/`vh`
/// against, so `padding-left: 2vw` lands the caret on the glyph in both lanes.
#[cfg(feature = "skia-native")]
fn resolve_text_metrics(
    props: &velox_dom::Props,
    rect: velox_dom::layout::Rect,
    viewport: (f32, f32),
) -> crate::input_metrics::InputTextMetrics {
    crate::input_metrics::input_text_metrics(
        props.attrs.get("style").map(|s| s.as_str()),
        rect,
        viewport,
        // This lane has no inheritance context, so it cannot read the parent's
        // size. The painter's root `TextStyle` IS that context — the size an
        // element that declares no `font-size` of its own paints at — so its
        // constant is the stand-in, and it must be the same number in both
        // lanes. Passing `DEFAULT_ROOT_FONT_SIZE` (16) here instead meant a field
        // with no `font-size` was measured 16/14 too wide and its `em` padding
        // resolved against 16 while paint used 14.
        crate::skia_render::PAINT_ROOT_FONT_SIZE,
    )
}

/// The `value` attribute of the element at `path`, if it is an element.
#[cfg(feature = "skia-native")]
fn input_value_at(vnode: &VNode, path: &[usize]) -> Option<String> {
    match find_node_at_path(vnode, path) {
        Some(VNode::Element { props, .. }) => {
            Some(props.attrs.get("value").cloned().unwrap_or_default())
        }
        _ => None,
    }
}

/// Char length of the value at `path`, 0 when there is no such input.
#[cfg(feature = "skia-native")]
fn input_value_char_len(vnode: &VNode, path: &[usize]) -> usize {
    input_value_at(vnode, path)
        .map(|s| s.chars().count())
        .unwrap_or(0)
}

/// Dispatch an explicit new value to the input at `path` via its `on:input`
/// handler, so the app's state and the next `make_view` see the edit.
#[cfg(feature = "skia-native")]
fn dispatch_input_value(
    last_vnode: &Option<VNode>,
    path: &[usize],
    new_value: &str,
    on_event: &mut impl FnMut(&str, Option<&str>),
) {
    let Some(vnode) = last_vnode else { return };
    let Some(node) = find_node_at_path(vnode, path) else {
        return;
    };
    let VNode::Element { props, .. } = node else {
        return;
    };
    let Some(handler) = props.attrs.get("on:input").cloned() else {
        return;
    };
    on_event(&handler, Some(new_value));
}

/// Apply one editing action to the focused input and dispatch any text change.
///
/// Shared by both event loops — that shared call is what keeps the HMR loop
/// from drifting away from the plain one again. Returns `true` when the caller
/// must repaint.
///
/// Public because a host that drives its own key handling needs the same
/// sequence the loops run, and because "is a keypress additive?" is only
/// testable if both halves of a keypress can be driven in one test.
#[cfg(feature = "skia-native")]
pub fn apply_edit_to_focused(
    input_targets: &mut [crate::events::InputTarget],
    action: crate::events::EditAction,
    last_vnode: &Option<VNode>,
    focused_input: &mut Option<Vec<usize>>,
    on_event: &mut impl FnMut(&str, Option<&str>),
) -> bool {
    let Some(idx) = crate::events::focused_input_index(input_targets) else {
        return false;
    };
    let path = input_targets[idx].path.clone();
    let value = last_vnode
        .as_ref()
        .and_then(|v| input_value_at(v, &path))
        .unwrap_or_default();
    let res = crate::events::apply_edit(&mut input_targets[idx], &value, action);
    if res.submit {
        // Submit always dispatches: with no selection the text is unchanged, and
        // the handler still has to see the submission.
        let v = res.value.clone().unwrap_or_else(|| value.clone());
        dispatch_input_value(last_vnode, &path, &v, on_event);
    } else if let Some(new_value) = &res.value {
        dispatch_input_value(last_vnode, &path, new_value, on_event);
    }
    // `InputTarget::focused` is the single source of truth for focus; the
    // loop's `focused_input` is its index-free mirror, so it has to follow
    // whatever the action did to the flag rather than drift one keypress
    // behind it. Every action except `Blur` leaves focus alone.
    if matches!(action, crate::events::EditAction::Blur) {
        *focused_input = None;
    }
    res.needs_repaint()
}

/// Resolve the VirtualKeyCode of a key event into an editing action, honouring
/// the Shift modifier. Returns `None` for keys that are not editing commands.
///
/// The match is closed on purpose: it is the *editor's* vocabulary, and a key
/// that is not an editing command has no business acquiring one just because an
/// app also listens for it. The author-facing listening vocabulary is
/// [`key_name`], and the two are peers rather than a chain — see
/// [`dispatch_keydown`], which runs both on the same press.
#[cfg(feature = "skia-native")]
pub fn edit_action_for_key(
    keycode: winit::event::VirtualKeyCode,
    shift: bool,
) -> Option<crate::events::EditAction> {
    use crate::events::EditAction;
    use winit::event::VirtualKeyCode as K;
    Some(match keycode {
        K::Left => EditAction::MoveLeft { shift },
        K::Right => EditAction::MoveRight { shift },
        K::Home => EditAction::Home { shift },
        K::End => EditAction::End { shift },
        K::Back => EditAction::Backspace,
        K::Delete => EditAction::Delete,
        K::Return => EditAction::Submit,
        // Esc ends the editing session: focus, selection and caret go, the
        // value stays. This match is closed — there is no fallthrough to user
        // code — so Esc belongs here or the user has no way out of a field.
        K::Escape => EditAction::Blur,
        _ => return None,
    })
}

/// Stable, author-facing name for a key press.
///
/// This is the whole payload contract of `@keydown`, and it is a *string*, not
/// the winit enum, for three reasons:
///
/// - the enum is a dependency's type, so putting it in a `.vx` handler's
///   signature would leak `winit` (and an optional feature) into every app;
/// - winit spells its variants `Key1`, `Numpad0`, `LWin`; an author writes
///   `"1"`, `"0"`, `"MetaLeft"`-or-`"Super"`, never the crate's spelling;
/// - a name is a *value*, so it can be compared, matched, printed and stored,
///   which is what an `if key == "F2"` branch needs.
///
/// The vocabulary is the DOM `KeyboardEvent.key` convention where the two
/// disagree (`"Enter"` not `"Return"`, `" "`-as-`"Space"`, `"Escape"`), and
/// upper-case bare letters so a shifted key is the *same* name as an unshifted
/// one — `shift` is not reported, and folding it into the name would mean an
/// author cannot tell `a` from `A` when they cannot tell them apart either.
/// Anything outside the table becomes `"Unidentified"`.
#[cfg(feature = "skia-native")]
pub fn key_name(keycode: winit::event::VirtualKeyCode) -> &'static str {
    use winit::event::VirtualKeyCode as K;
    match keycode {
        K::A => "A",
        K::B => "B",
        K::C => "C",
        K::D => "D",
        K::E => "E",
        K::F => "F",
        K::G => "G",
        K::H => "H",
        K::I => "I",
        K::J => "J",
        K::K => "K",
        K::L => "L",
        K::M => "M",
        K::N => "N",
        K::O => "O",
        K::P => "P",
        K::Q => "Q",
        K::R => "R",
        K::S => "S",
        K::T => "T",
        K::U => "U",
        K::V => "V",
        K::W => "W",
        K::X => "X",
        K::Y => "Y",
        K::Z => "Z",
        K::Key0 => "0",
        K::Key1 => "1",
        K::Key2 => "2",
        K::Key3 => "3",
        K::Key4 => "4",
        K::Key5 => "5",
        K::Key6 => "6",
        K::Key7 => "7",
        K::Key8 => "8",
        K::Key9 => "9",
        K::F1 => "F1",
        K::F2 => "F2",
        K::F3 => "F3",
        K::F4 => "F4",
        K::F5 => "F5",
        K::F6 => "F6",
        K::F7 => "F7",
        K::F8 => "F8",
        K::F9 => "F9",
        K::F10 => "F10",
        K::F11 => "F11",
        K::F12 => "F12",
        K::Up => "ArrowUp",
        K::Down => "ArrowDown",
        K::Left => "ArrowLeft",
        K::Right => "ArrowRight",
        K::Back => "Backspace",
        K::Delete => "Delete",
        K::Insert => "Insert",
        K::Home => "Home",
        K::End => "End",
        K::PageUp => "PageUp",
        K::PageDown => "PageDown",
        K::Return | K::NumpadEnter => "Enter",
        K::Escape => "Escape",
        K::Tab => "Tab",
        K::Space => "Space",
        K::LShift | K::RShift => "Shift",
        K::LControl | K::RControl => "Control",
        K::LAlt | K::RAlt => "Alt",
        K::LWin | K::RWin => "Meta",
        _ => "Unidentified",
    }
}

/// Focus the input the author named `data-focus-id="id"`.
///
/// The stable-name entry point, for a caller that already knows *which* field it
/// means — a host embedding the renderer that drives focus itself, or a test.
/// Resolution runs through the same [`crate::events::find_focus_id_path`] the
/// `@keydown` grant uses, so "focus the composer" means one thing in this
/// codebase no matter which door it came through.
///
/// Returns `true` when focus actually moved, so an already-focused field is not
/// mistaken for a change and does not have its caret reset.
#[cfg(feature = "skia-native")]
pub fn focus_input_by_id(
    input_targets: &mut [crate::events::InputTarget],
    last_vnode: &Option<VNode>,
    focused_input: &mut Option<Vec<usize>>,
    id: &str,
) -> bool {
    let Some(vnode) = last_vnode.as_ref() else {
        return false;
    };
    let Some(path) = crate::events::find_focus_id_path(vnode, &[], id) else {
        return false;
    };
    // Expressed as a one-focus `KeydownPlan` so this and the key path run the
    // same grant code — the flag, the caret, the anchor, the blink and the
    // index-free mirror are all written in exactly one place.
    let plan = crate::events::KeydownPlan {
        focus_paths: vec![path],
        ..Default::default()
    };
    crate::events::apply_keydown(
        &plan,
        "",
        input_targets,
        focused_input,
        &|p: &[usize]| input_value_char_len(vnode, p),
        &mut (|_: &str, _: Option<&str>| {}),
    )
}

/// Route one key press to the app: declarative focus grants plus every
/// `on:keydown` handler, with the key's [`key_name`] as the payload.
///
/// Shared by both event loops, for the same reason `apply_edit_to_focused` is:
/// the HMR loop drifting from the plain one is not a theoretical risk, it is the
/// specific bug that indirection exists to prevent. Returns `true` when the
/// caller must repaint.
///
/// Both loops call this from the key arm *before* the reload/quit guard and
/// outside the `match` that routes editing keys, and it returns no instruction
/// to consume the key. That placement is the additive contract made structural
/// instead of documented: `dispatch_keydown` and the editing branch are
/// siblings, so swallowing a keystroke would take deleting the adjacency to do.
#[cfg(feature = "skia-native")]
pub fn dispatch_keydown(
    keycode: winit::event::VirtualKeyCode,
    input_targets: &mut [crate::events::InputTarget],
    last_vnode: &Option<VNode>,
    focused_input: &mut Option<Vec<usize>>,
    on_event: &mut impl FnMut(&str, Option<&str>),
) -> bool {
    let name = crate::key_name(keycode);
    let Some(vnode) = last_vnode.as_ref() else {
        return false;
    };
    let plan = crate::events::plan_keydown(vnode, name);
    crate::events::apply_keydown(
        &plan,
        name,
        input_targets,
        focused_input,
        &|p: &[usize]| input_value_char_len(vnode, p),
        on_event,
    )
}

/// Inject the caret/selection/focus contract onto every text input in the tree.
///
/// Paint never reads live event state. It reads these five attrs off the styled
/// VNode, which is what makes the caret headless-testable — a golden can be
/// produced from a VNode alone, with no event loop running:
///
/// - `caret`       — caret position as a char index into the value
/// - `caret_blink` — `"true"` when the caret bar is visible this frame
/// - `sel_start`   — selection start char index (== `caret` when collapsed)
/// - `sel_end`     — selection end char index
/// - `focused`     — `"true"` when the input holds keyboard focus
///
/// Runs *after* `style_vnode_with_hover` (the single style-cascade application
/// site) and before layout, so it adds no second cascade and no second
/// rounding. Every text input always carries all five attrs, so paint never has
/// to invent a default for a missing one.
#[cfg(feature = "skia-native")]
fn inject_input_caret_attrs(vnode: &VNode, focused: Option<&crate::events::InputTarget>) -> VNode {
    /// Descend one level. `on_path` means "this node is an ancestor of (or is)
    /// the focused input"; `focused_path` is the remainder to match.
    fn walk(
        node: &VNode,
        focused_path: Option<&[usize]>,
        on_path: bool,
        focused: Option<&crate::events::InputTarget>,
    ) -> VNode {
        match node {
            VNode::Text(_) => node.clone(),
            VNode::Element {
                tag,
                props,
                children,
            } => {
                let mut new_props = props.clone();
                if crate::events::is_text_input(tag, props) {
                    let (is_focused, cursor, blink, sel_start, sel_end) = match focused {
                        Some(t) if on_path => {
                            let (a, b) = t.selection().unwrap_or((t.cursor, t.cursor));
                            (true, t.cursor, t.blink_on, a, b)
                        }
                        // Unfocused inputs still get the attrs, so paint reads
                        // a total contract rather than guessing at absence.
                        _ => (false, 0, true, 0, 0),
                    };
                    new_props = new_props
                        .set("focused", if is_focused { "true" } else { "false" })
                        .set("caret", cursor.to_string())
                        .set("caret_blink", if blink { "true" } else { "false" })
                        .set("sel_start", sel_start.to_string())
                        .set("sel_end", sel_end.to_string());
                }
                let new_children = children
                    .iter()
                    .enumerate()
                    .map(|(i, child)| {
                        let child_on_path = on_path
                            && focused_path
                                .and_then(|p| p.first())
                                .map(|first| *first == i)
                                .unwrap_or(false);
                        let rest = if child_on_path {
                            focused_path.map(|p| &p[1..])
                        } else {
                            None
                        };
                        walk(child, rest, child_on_path, focused)
                    })
                    .collect();
                VNode::Element {
                    tag: tag.clone(),
                    props: new_props,
                    children: new_children,
                }
            }
        }
    }

    let focused_path = focused.map(|t| t.path.as_slice());
    walk(vnode, focused_path, true, focused)
}

/// Effective font family of a styled element, for caret hit testing.
#[cfg(feature = "skia-native")]
fn resolve_font_family(props: &velox_dom::Props) -> String {
    props
        .attrs
        .get("style")
        .and_then(|s| style_decl(s, "font-family"))
        .filter(|f| !f.is_empty())
        .unwrap_or("system-ui, sans-serif")
        .to_string()
}

/// The **logical** `(w, h)` the paint lane will resolve `vw`/`vh` against this
/// frame, read from the live surface.
///
/// Read from the surface, not a constant, precisely so it cannot disagree with
/// paint: `render_frame` resolves viewport units against the same surface
/// dimensions through the same [`logical_size`] rounding. A surface-less
/// (headless) renderer yields `(0.0, 0.0)`, which only matters for
/// viewport-unit padding — and there is nothing painted to disagree with.
#[cfg(feature = "skia-native")]
fn surface_logical_viewport(
    renderer: &skia_backend::SkiaRenderer,
    scale_factor: f32,
) -> (f32, f32) {
    match &renderer.surface {
        Some(s) => {
            let (w, h) = logical_size(s.width, s.height, scale_factor);
            (w as f32, h as f32)
        }
        None => (0.0, 0.0),
    }
}

/// Move keyboard focus to the text input under the cursor (or drop focus),
/// and place the caret.
///
/// Shared by both event loops for the same reason `apply_edit_to_focused` is:
/// the HMR loop used to have no focus handling at all, and a second copy of
/// this is how the drift happened.
///
/// Caret placement differs by case, and the difference is deliberate:
/// - clicking an input that did **not** have focus is a focus *gain*, so the
///   caret goes to the end of the value (the conventional behaviour, and what
///   makes "click a field and start typing" replace rather than prepend);
/// - clicking an input that already had focus is a reposition, so the caret
///   goes to the clicked glyph via [`crate::events::click_to_char_index`].
///
/// Returns `true` when the caller must repaint — i.e. focus actually changed or
/// the caret moved.
///
/// `viewport` is the **logical** `(w, h)` the paint lane uses this frame (see
/// [`surface_logical_viewport`]). It is threaded in rather than reconstructed
/// here because the caret's origin is computed from the same lengths paint
/// resolves: pass a different viewport and a `vw`/`vh` field puts its caret in
/// one place and its glyphs in another.
#[cfg(feature = "skia-native")]
fn apply_click_focus(
    input_targets: &mut [crate::events::InputTarget],
    last_vnode: &Option<VNode>,
    focused_input: &mut Option<Vec<usize>>,
    x: f32,
    y: f32,
    scale_factor: f32,
    viewport: (f32, f32),
) -> bool {
    let Some(idx) = crate::events::hit_test_input_index(input_targets, x, y) else {
        // Clicked empty space: blur. Clearing the selection is what stops a
        // stale highlight from surviving on a field the user has left. The
        // caret position is deliberately kept, so re-entering the field
        // restores where the user was.
        let changed = crate::events::blur_focused_input(input_targets);
        return changed || focused_input.take().is_some();
    };

    let was_focused = input_targets[idx].focused;
    let rect = input_targets[idx].rect;
    let path = input_targets[idx].path.clone();
    let mut caret: Option<usize> = None;
    let mut value_len: Option<usize> = None;
    if let Some(vnode) = last_vnode.as_ref()
        && let Some(VNode::Element { props, .. }) = find_node_at_path(vnode, &path)
    {
        let value = props.attrs.get("value").cloned().unwrap_or_default();
        value_len = Some(value.chars().count());
        // ONE call for both the origin and the font size. Taking the origin
        // from here and the size from anywhere else is the defect this shape
        // removes: two lanes that must agree, each holding its own constant.
        let metrics = resolve_text_metrics(props, rect, viewport);
        let font_size = metrics.font_size;
        let config = crate::text::TextRenderConfig::new(&resolve_font_family(props), font_size);
        // Measure through the renderer's own text stack (single measure path,
        // skia-aware under `skia-native`) rather than a local heuristic, so the
        // caret lands on the glyph the paint lane will actually draw.
        let measure =
            |s: &str| crate::text::TextMeasurer::measure_with_scale(s, &config, scale_factor).0;
        caret = Some(crate::events::click_to_char_index(
            &value,
            rect,
            x,
            metrics.text_left,
            font_size,
            &measure,
        ));
    }

    // The one place focus is granted. `focus_input` also blurs any other field
    // that held focus, so a click straight from one field to another cannot
    // leave two of them flagged.
    crate::events::focus_input(input_targets, idx);
    input_targets[idx].blink_on = true;
    // A plain click is never a selection-extension; it collapses one.
    input_targets[idx].anchor = None;
    // `cursor` is a char index, so "the end" is the char count, not `len()`.
    input_targets[idx].cursor = if was_focused {
        caret.unwrap_or(input_targets[idx].cursor)
    } else {
        // Focus gain: caret to the end of the value.
        value_len.unwrap_or(input_targets[idx].cursor)
    };
    *focused_input = Some(path);
    true
}

pub mod event_binding;
pub mod events;
pub mod hmr;
// The single authority on an `<input>`'s content-box geometry. Public so both
// `skia_render`'s paint lane and `lib.rs`'s hit-test lane reach the SAME
// function instead of each growing an inset of its own, and so a test can
// assert against it without re-deriving the painter's arithmetic.
pub mod input_metrics;
pub mod text;
pub mod viewport;

pub use hmr::{DEFAULT_HMR_PORT, HmrMessage, hmr_config, run_hmr_client};
pub use viewport::{LogicalSize, PhysicalSize, Viewport};

// Native Skia GL helper module (feature-gated)
#[cfg(feature = "skia-native")]
mod skia_gl;
// Skia surface and renderer helpers (feature-gated)
#[cfg(feature = "skia-native")]
mod skia_render;
#[cfg(feature = "skia-native")]
pub mod skia_surface;
// Softbuffer presenter for window rendering (feature-gated)
#[cfg(feature = "skia-native")]
mod presenter;
#[cfg(feature = "skia-native")]
pub use skia_render::skia_impl::render_vnode_to_rgba;
// Test/diagnostic only, and public only because `tests/` is a separate crate
// that cannot see a private module. The decode counter is the honest way to
// assert the persistent `<img src>` cache works; a timing assertion would
// measure the machine instead of the cache. See `IMAGE_DECODE_COUNT`.
#[cfg(feature = "skia-native")]
pub use skia_render::skia_impl::{image_decode_count, reset_image_decode_count};
#[cfg(feature = "skia-native")]
pub use skia_render::{render_vnode_to_raster_png, render_vnode_to_raster_png_with_scale};

/// In-memory representation of a mounted tree (stubbed for now).
pub struct RenderTree {
    pub root: VNode,
    pub node_count: usize,
    pub text_count: usize,
}

#[derive(Debug, Clone, PartialEq)]
pub struct A11yNode {
    pub id: usize,
    pub role: String,
    pub name: String,
    pub rect: velox_dom::layout::Rect,
    pub children: Vec<A11yNode>,
}

#[derive(Debug, Clone, PartialEq)]
pub struct A11yTree {
    pub root: A11yNode,
}

pub trait HmrRenderer {
    /// Initialize the rendering backend (e.g. create DirectContext, verify GPU).
    fn init() -> Result<(), String>
    where
        Self: Sized;

    /// Mount a VNode into the renderer for display.
    fn mount(&mut self, vnode: VNode) -> Result<(), String>;

    /// Hot-update the rendered VNode.
    fn hot_update(&mut self, new_vnode: VNode) -> Result<(), String>;

    /// Get the raw window handle for platform integration.
    fn get_window_handle(&self) -> *mut std::ffi::c_void;
}

/// High-level renderer lifecycle trait. Backends implement this to expose
/// a consistent `new -> mount -> hot_update` workflow that returns `Result`
/// instead of panicking on failure.
pub trait VeloxRenderer {
    /// Construct a new renderer instance.
    fn new() -> Result<Self, String>
    where
        Self: Sized;

    /// Mount a VNode tree for rendering, returning an error on failure.
    fn mount(&mut self, vnode: VNode) -> Result<(), String>;

    /// Hot-replace the rendered VNode tree, returning an error on failure.
    fn hot_update(&mut self, new_vnode: VNode) -> Result<(), String>;
}

fn summarize(v: &VNode, counts: &mut (usize, usize)) {
    match v {
        VNode::Text(_) => {
            counts.0 += 1;
            counts.1 += 1;
        }
        VNode::Element { children, .. } => {
            counts.0 += 1;
            for c in children {
                summarize(c, counts);
            }
        }
    }
}

fn build_render_tree(v: &VNode) -> RenderTree {
    let mut counts = (0, 0);
    summarize(v, &mut counts);
    RenderTree {
        root: v.clone(),
        node_count: counts.0,
        text_count: counts.1,
    }
}

fn vnode_text_content(node: &VNode) -> String {
    match node {
        VNode::Text(t) => t.clone(),
        VNode::Element { children, .. } => {
            let mut out = String::new();
            for ch in children {
                let s = vnode_text_content(ch);
                if !s.is_empty() {
                    if !out.is_empty() {
                        out.push(' ');
                    }
                    out.push_str(&s);
                }
            }
            out
        }
    }
}

fn a11y_role_for(tag: &str, props: &velox_dom::Props) -> String {
    if let Some(role) = props.attrs.get("role") {
        return role.clone();
    }
    match tag {
        "button" => "button",
        "img" => "image",
        "input" => "textbox",
        "label" => "label",
        "a" => "link",
        _ => "group",
    }
    .to_string()
}

fn a11y_name_for(tag: &str, props: &velox_dom::Props, node: &VNode) -> String {
    if let Some(label) = props.attrs.get("aria-label") {
        return label.clone();
    }
    if tag == "img"
        && let Some(alt) = props.attrs.get("alt")
    {
        return alt.clone();
    }
    vnode_text_content(node)
}

fn build_a11y_tree_with_layout(
    vnode: &VNode,
    layout: &velox_dom::layout::LayoutNode,
    next_id: &mut usize,
) -> A11yNode {
    let id = *next_id;
    *next_id += 1;
    match vnode {
        VNode::Text(t) => A11yNode {
            id,
            role: "text".to_string(),
            name: t.clone(),
            rect: layout.rect,
            children: Vec::new(),
        },
        VNode::Element {
            tag,
            props,
            children,
            ..
        } => {
            let mut child_nodes = Vec::new();
            for ch_layout in &layout.children {
                if ch_layout.display_none {
                    continue;
                }
                if let Some(src_idx) = ch_layout.source_index
                    && let Some(ch) = children.get(src_idx)
                {
                    child_nodes.push(build_a11y_tree_with_layout(ch, ch_layout, next_id));
                }
            }
            A11yNode {
                id,
                role: a11y_role_for(tag, props),
                name: a11y_name_for(tag, props, vnode),
                rect: layout.rect,
                children: child_nodes,
            }
        }
    }
}

/// Build the accessibility tree for `vnode` laid out at `width` x `height`.
///
/// DELIBERATE EXCEPTION to R-8's single-funnel invariant: this function runs its
/// own `compute_layout` and does not go through the renderer's `prepare_frame`.
///
/// The reason is that this is not a render path. It takes no `Stylesheet`, paints
/// nothing, and is given no `SkiaSurface`; its only output is a tree of roles,
/// names and rects. Funnelling it would mean inventing a stylesheet and a surface
/// for a caller that has neither, and then applying a style cascade to a tree
/// whose geometry — not its appearance — is what is being reported. It is named
/// here so the exception is on the record rather than discovered later as a
/// "missing" funnel site.
pub fn build_a11y_tree(vnode: &VNode, width: i32, height: i32) -> A11yTree {
    let layout = velox_dom::layout::compute_layout(vnode, width, height);
    let mut next_id = 1;
    let root = build_a11y_tree_with_layout(vnode, &layout, &mut next_id);
    A11yTree { root }
}

/// Minimal renderer trait. Backends implement this to expose a consistent API.
pub trait Renderer {
    fn backend_name(&self) -> &'static str;
    fn mount(&self, vnode: &VNode) -> Result<RenderTree, String>;
}

// Real Skia backend only when `skia-native` is enabled.
#[cfg(feature = "skia-native")]
pub mod skia_backend {
    use crate::HmrRenderer;
    use crate::VeloxRenderer;
    #[cfg(feature = "skia-native")]
    use crate::skia_gl;
    #[cfg(feature = "skia-native")]
    use crate::skia_surface;
    #[cfg(feature = "skia-native")]
    use raw_window_handle::HasRawWindowHandle;
    use velox_dom::VNode;

    pub fn init() -> Result<(), String> {
        match skia_gl::create_context() {
            Ok(gl_ctx) => match gl_ctx.into_direct_context() {
                Some(_dctx) => {
                    log::info!("skia backend: init OK (DirectContext created)");
                    Ok(())
                }
                None => Err("skia backend: init failed: couldn't create DirectContext".to_string()),
            },
            Err(e) => Err(format!("skia backend: init failed: {}", e)),
        }
    }

    pub struct SkiaRenderer {
        pub surface: Option<skia_surface::SkiaSurface>,
        pub vnode: Option<VNode>,
    }

    impl SkiaRenderer {
        pub fn with_window(
            window: &impl HasRawWindowHandle,
            width: i32,
            height: i32,
        ) -> Result<Self, String> {
            match skia_surface::create_window_surface_from_handle(window, width, height) {
                Ok(s) => Ok(SkiaRenderer {
                    surface: Some(s),
                    vnode: None,
                }),
                Err(e) => Err(e),
            }
        }

        pub fn present(&mut self) -> Result<(), String> {
            if let Some(s) = &mut self.surface {
                s.present()
            } else {
                Ok(())
            }
        }

        pub fn resize(&mut self, width: i32, height: i32) -> Result<(), String> {
            if let Some(s) = &mut self.surface {
                s.resize(width, height)
            } else {
                Ok(())
            }
        }
    }

    impl crate::Renderer for SkiaRenderer {
        fn backend_name(&self) -> &'static str {
            "skia"
        }
        fn mount(&self, vnode: &velox_dom::VNode) -> Result<crate::RenderTree, String> {
            Ok(crate::build_render_tree(vnode))
        }
    }

    impl HmrRenderer for SkiaRenderer {
        fn init() -> Result<(), String> {
            init()
        }

        fn mount(&mut self, vnode: VNode) -> Result<(), String> {
            self.vnode = Some(vnode);
            Ok(())
        }

        fn hot_update(&mut self, new_vnode: VNode) -> Result<(), String> {
            self.vnode = Some(new_vnode);
            Ok(())
        }

        fn get_window_handle(&self) -> *mut std::ffi::c_void {
            std::ptr::null_mut()
        }
    }

    impl VeloxRenderer for SkiaRenderer {
        fn new() -> Result<Self, String> {
            Ok(SkiaRenderer {
                surface: None,
                vnode: None,
            })
        }

        fn mount(&mut self, vnode: VNode) -> Result<(), String> {
            self.vnode = Some(vnode);
            Ok(())
        }

        fn hot_update(&mut self, new_vnode: VNode) -> Result<(), String> {
            self.vnode = Some(new_vnode);
            Ok(())
        }
    }
}

// Skia stub backend to allow compiling with `--features skia` without native deps.
#[cfg(all(feature = "skia", not(feature = "skia-native")))]
pub mod skia_backend {
    pub fn init() -> Result<(), String> {
        Ok(())
    }

    pub struct SkiaRenderer;
    impl crate::Renderer for SkiaRenderer {
        fn backend_name(&self) -> &'static str {
            "skia"
        }
        fn mount(&self, vnode: &velox_dom::VNode) -> Result<crate::RenderTree, String> {
            Ok(crate::build_render_tree(vnode))
        }
    }
    impl crate::HmrRenderer for SkiaRenderer {
        fn init() -> Result<(), String> {
            Ok(())
        }
        fn mount(&mut self, _vnode: velox_dom::VNode) -> Result<(), String> {
            Ok(())
        }
        fn hot_update(&mut self, _new_vnode: velox_dom::VNode) -> Result<(), String> {
            Ok(())
        }
        fn get_window_handle(&self) -> *mut std::ffi::c_void {
            std::ptr::null_mut()
        }
    }
    impl crate::VeloxRenderer for SkiaRenderer {
        fn new() -> Result<Self, String> {
            Ok(SkiaRenderer)
        }
        fn mount(&mut self, _vnode: velox_dom::VNode) -> Result<(), String> {
            Ok(())
        }
        fn hot_update(&mut self, _new_vnode: velox_dom::VNode) -> Result<(), String> {
            Ok(())
        }
    }
}

/// Stub init used when no backend features are enabled.
#[cfg(not(feature = "skia"))]
pub fn init() -> Result<(), String> {
    // Intentionally empty — no backend enabled
    Ok(())
}

// Simple identifier of the selected backend, useful for tests.
#[cfg(feature = "skia")]
pub const BACKEND: &str = "skia";
#[cfg(not(feature = "skia"))]
pub const BACKEND: &str = "stub";

pub fn backend_name() -> &'static str {
    BACKEND
}

/// Feature-selected renderer type and constructor for tests and examples.
#[cfg(any(feature = "skia", feature = "skia-native"))]
pub type SelectedRenderer = skia_backend::SkiaRenderer;
#[cfg(not(feature = "skia"))]
pub struct StubRenderer;
#[cfg(not(feature = "skia"))]
pub type SelectedRenderer = StubRenderer;
#[cfg(not(feature = "skia"))]
impl Renderer for StubRenderer {
    fn backend_name(&self) -> &'static str {
        "stub"
    }
    fn mount(&self, vnode: &VNode) -> Result<RenderTree, String> {
        Ok(build_render_tree(vnode))
    }
}
#[cfg(not(feature = "skia"))]
impl HmrRenderer for StubRenderer {
    fn init() -> Result<(), String> {
        Ok(())
    }
    fn mount(&mut self, _vnode: VNode) -> Result<(), String> {
        Ok(())
    }
    fn hot_update(&mut self, _new_vnode: VNode) -> Result<(), String> {
        Ok(())
    }
    fn get_window_handle(&self) -> *mut std::ffi::c_void {
        std::ptr::null_mut()
    }
}
#[cfg(not(feature = "skia"))]
impl VeloxRenderer for StubRenderer {
    fn new() -> Result<Self, String> {
        Ok(StubRenderer)
    }
    fn mount(&mut self, _vnode: VNode) -> Result<(), String> {
        Ok(())
    }
    fn hot_update(&mut self, _new_vnode: VNode) -> Result<(), String> {
        Ok(())
    }
}

/// Construct the feature-selected renderer.
pub fn new_selected_renderer() -> SelectedRenderer {
    #[cfg(feature = "skia-native")]
    {
        // Construct SkiaRenderer with no surface for the default selected renderer.
        skia_backend::SkiaRenderer {
            surface: None,
            vnode: None,
        }
    }
    #[cfg(all(feature = "skia", not(feature = "skia-native")))]
    {
        skia_backend::SkiaRenderer
    }
    #[cfg(not(feature = "skia"))]
    {
        StubRenderer
    }
}

pub use events::Runtime as EventRuntime;

/// Test helper: exercise a small Skia draw path (native-only).
#[cfg(all(feature = "skia-native", unix))]
pub fn skia_draw_test_frame() -> Result<(), String> {
    crate::skia_gl::draw_gpu_test_frame(256, 256)
}

/// Convenience wrapper to create a Skia `DirectContext` from the crate root.
///
/// Returns the owning `GlDirectContext` pair, which holds the headless EGL
/// context alongside the `DirectContext`: handing back a bare `DirectContext`
/// would return a handle whose GL context had already been destroyed.
#[cfg(all(feature = "skia-native", unix))]
pub fn create_direct_context() -> Result<crate::skia_gl::GlDirectContext, String> {
    crate::skia_gl::create_direct_context()
}

/// Extracts the human-readable message from a caught panic payload
/// (`panic!("literal")` stores `&'static str`, `format!`-based panics store
/// `String`). Used so compositor failures caught by `catch_unwind` are
/// surfaced instead of silently discarded (CX-13 / F-23).
#[cfg(feature = "skia-native")]
fn panic_detail(payload: Box<dyn std::any::Any + Send>) -> String {
    if let Some(s) = payload.downcast_ref::<&'static str>() {
        (*s).to_string()
    } else if let Some(s) = payload.downcast_ref::<String>() {
        s.clone()
    } else {
        "unknown panic payload".to_string()
    }
}

// ---------------------------------------------------------------------------
// Shared window-event arms (Task 5.6b)
// ---------------------------------------------------------------------------
//
// `run_window_vnode_skia` and `run_window_vnode_skia_with_hmr` are the same loop
// written twice, and six `WindowEvent` arms were copy-pasted between them. Two
// copies means every fix has to be made twice, and this repo has already paid
// for that: the HMR keyboard arm shipped as a bare `_ => {}` stub, and the HMR
// loop had no `ReceivedCharacter` arm at all, so typing silently did nothing
// under HMR. Each function below is the single implementation of one arm, and
// both loops call it.
//
// Two rules, both load-bearing:
//
//   * These functions never touch `ControlFlow`. The loops keep every
//     `*control_flow = ...` assignment, because parking on `Wait` (not `Poll`,
//     not `WaitUntil`) is right for a non-game application: `Poll` would burn a
//     core while idle and `WaitUntil` would swallow the events these arms
//     handle. `tests/event_loop_arms.rs` pins that, and it is easier to keep
//     pinned when the control flow never leaves the loop.
//   * Shared state is passed as arguments instead of moving into a struct, so
//     the three arms that are *not* here keep reading the loop's own locals
//     under their own names — see the note on why those three stayed behind.
//
// Why six of the nine, and not nine:
//
//   * Extracted: `CloseRequested`, `Resized`, `ScaleFactorChanged`,
//     `CursorMoved`, `ModifiersChanged`, `MouseWheel`.
//   * Left in place: the left-click, `KeyboardInput` and `ReceivedCharacter`
//     arms. `tests/event_loop_arms.rs` pins their bodies as *literal source
//     text* inside each loop — `apply_click_focus(`, `&mut focused_input`,
//     `hit_test_click`, `edit_action_for_key(keycode, shift_held)`,
//     `arm_blink_deadline(&mut blink_deadline)`, `EditAction::Insert(c)` — and
//     `the_duplicated_arms_of_both_loops_make_the_same_calls` compares their
//     callee sequences so a second divergence cannot appear beside the one
//     known HMR-only `last_vnode` republish. Moving the bodies out of the
//     loops deletes that guard along with the duplicate: the drift test would
//     compare two identical one-line call sites and never see the arms again.
//     Those three are the arms the caret and typing regressions both lived in,
//     so they keep a live drift test instead of a second copy. Task 5.6 step 3
//     is what actually finishes the job — see the report for why it needs
//     production-code change before it can exist.

/// `WindowEvent::CloseRequested` — shared by both loops.
///
/// Returns `true` so the *caller* performs the exit. `ControlFlow` belongs to
/// the loop, and keeping the assignment at the call site keeps it visible to
/// the structural test.
#[cfg(feature = "skia-native")]
fn window_close_requested() -> bool {
    velox_core::lifecycle::run_all_destroy_hooks();
    true
}

/// `WindowEvent::Resized` — shared by both loops.
///
/// R-L3: coalesce. A drag produces many `Resized` events and only the last one
/// per frame may materialise, so this records the pending size and asks for a
/// redraw. It must not touch the raster surface; surface recreation belongs to
/// `RedrawRequested`.
#[cfg(feature = "skia-native")]
fn window_resized(
    resize_state: &mut ResizeState,
    window_opt: &Option<winit::window::Window>,
    size: (u32, u32),
) {
    // `ResizeState::queue` is the production `pending_resize = Some(..)` path.
    resize_state.queue(size);
    if let Some(w) = window_opt.as_ref() {
        w.request_redraw();
    }
}

/// `WindowEvent::ScaleFactorChanged` — shared by both loops.
///
/// R-M1: the physical cursor must not move when the scale factor changes, so
/// the logical `mouse_pos` is rescaled by `old / new` in the same step that
/// adopts the new factor. R-L3: the pending resize and the surface scale are
/// updated here, but nothing is recreated — layout and hit-test refresh wait
/// for `RedrawRequested` (single layout/frame, R-H5).
#[cfg(feature = "skia-native")]
fn window_scale_factor_changed(
    scale_factor: &mut f32,
    mouse_pos: &mut (f32, f32),
    resize_state: &mut ResizeState,
    surface: &mut Option<crate::skia_surface::SkiaSurface>,
    window_opt: &Option<winit::window::Window>,
    new_scale: f64,
    new_inner_size: (u32, u32),
) {
    let old_scale = *scale_factor;
    *scale_factor = new_scale as f32;
    if old_scale.is_finite() && old_scale > 0.0 && scale_factor.is_finite() && *scale_factor > 0.0 {
        mouse_pos.0 = mouse_pos.0 * old_scale / *scale_factor;
        mouse_pos.1 = mouse_pos.1 * old_scale / *scale_factor;
    }
    resize_state.queue(new_inner_size);
    if let Some(s) = surface.as_mut() {
        s.set_scale_factor(*scale_factor);
    }
    if let Some(w) = window_opt.as_ref() {
        w.request_redraw();
    }
}

/// `WindowEvent::CursorMoved` — shared by both loops.
///
/// winit reports physical pixels while hit-testing and layout are logical, so
/// the position is divided by the scale factor. Only a *change* of hover asks
/// for a redraw: a cursor move within the same target paints identically.
#[cfg(feature = "skia-native")]
fn window_cursor_moved(
    scale_factor: f32,
    mouse_pos: &mut (f32, f32),
    hover_targets: &[crate::events::HoverTarget],
    hovered_id: &mut Option<u32>,
    window_opt: &Option<winit::window::Window>,
    position: (f32, f32),
) {
    *mouse_pos = (position.0 / scale_factor, position.1 / scale_factor);
    let now_hovered = crate::events::hit_test_hover(hover_targets, mouse_pos.0, mouse_pos.1);
    if now_hovered != *hovered_id {
        *hovered_id = now_hovered;
        if let Some(w) = window_opt.as_ref() {
            w.request_redraw();
        }
    }
}

/// `WindowEvent::ModifiersChanged` — shared by both loops.
///
/// Latches Shift for Shift-extends-selection. winit 0.28 deprecates
/// `KeyboardInput::modifiers` in favour of this event, so the caret reads the
/// latched value instead of the deprecated per-key field.
#[cfg(feature = "skia-native")]
fn window_modifiers_changed(shift_held: &mut bool, shift: bool) {
    *shift_held = shift;
}

/// Convert a winit scroll delta into logical pixels, negated so that wheel-down
/// increases the scroll offset and reveals content below.
///
/// winit reports positive `y` for wheel-up; one line is 40 logical px, and
/// `PixelDelta` is physical so it needs the scale factor.
#[cfg(feature = "skia-native")]
fn window_wheel_delta_y(delta: winit::event::MouseScrollDelta, scale_factor: f32) -> f32 {
    match delta {
        winit::event::MouseScrollDelta::LineDelta(_, y) => -y * 40.0,
        winit::event::MouseScrollDelta::PixelDelta(pos) => -(pos.y as f32) / scale_factor,
    }
}

/// `WindowEvent::MouseWheel` — shared by both loops.
///
/// Scrollable overflow: the deepest scrollable under the cursor, clamped by
/// `apply_wheel_scroll`, which returns whether the offset actually moved. A
/// wheel event that arrives before the first frame has no layout to scroll and
/// must be a no-op rather than a panic.
#[cfg(feature = "skia-native")]
fn window_mouse_wheel(
    scale_factor: f32,
    mouse_pos: (f32, f32),
    last_layout: &Option<velox_dom::layout::LayoutNode>,
    scroll_offsets: &mut std::collections::HashMap<Vec<usize>, f32>,
    window_opt: &Option<winit::window::Window>,
    delta: winit::event::MouseScrollDelta,
) {
    let delta_y = window_wheel_delta_y(delta, scale_factor);
    if let Some(layout) = last_layout.as_ref()
        && crate::events::apply_wheel_scroll(
            layout,
            mouse_pos.0,
            mouse_pos.1,
            scroll_offsets,
            delta_y,
        )
        && let Some(w) = window_opt.as_ref()
    {
        w.request_redraw();
    }
}

// Behavioural tests for the shared arms.
//
// The loops themselves can only run inside a live winit window, and winit is an
// optional dependency rather than a dev-dependency, so nothing under `tests/`
// can name a `WindowEvent` and reach these functions. `tests/event_loop_arms.rs`
// pins the *source text* of what is left in the loops, which catches the two
// copies drifting apart from each other but says nothing about whether either
// one is correct. These tests are that missing check: each one states an
// observable property of a shared arm, so deleting or breaking the shared
// implementation turns them red.
//
// They run under `cargo test --features velox-renderer/skia-native`; a plain
// `cargo test -p velox-renderer` does not enable `skia-native` and does not
// compile the loops or these tests at all.
#[cfg(all(test, feature = "skia-native"))]
mod window_event_shared_tests {
    use super::*;
    use std::collections::HashMap;
    use winit::dpi::PhysicalPosition;
    use winit::event::MouseScrollDelta;

    /// Stand-in for the loop's `window_opt`. A `None` window is the case that
    /// matters for these assertions: every shared arm must still do its state
    /// work, and must simply skip the redraw request.
    fn no_window() -> Option<winit::window::Window> {
        None
    }

    fn hover_target(x: i32, y: i32, w: i32, h: i32, id: u32) -> crate::events::HoverTarget {
        crate::events::HoverTarget {
            rect: velox_dom::layout::Rect { x, y, w, h },
            id,
            z_index: 0,
            order: 0,
            clip: None,
            sc: crate::events::StackCtx::ROOT,
        }
    }

    #[test]
    fn close_requested_tells_the_caller_to_exit() {
        // `ControlFlow` is the loop's to own, so the shared arm signals instead
        // of setting it. A false here would leave the window running.
        assert!(window_close_requested());
    }

    #[test]
    fn resized_keeps_only_the_last_size_of_a_drag() {
        // R-L3 coalescing: a drag emits many `Resized` events and only the
        // final one may be committed, so a resize storm cannot queue a backlog.
        let mut state = ResizeState::new();
        let window = no_window();
        window_resized(&mut state, &window, (800, 600));
        window_resized(&mut state, &window, (1024, 768));
        assert_eq!(state.take_pending(), Some((1024, 768)));
        assert_eq!(state.take_pending(), None);
    }

    #[test]
    fn scale_factor_change_rescales_the_cursor_so_the_physical_point_is_stable() {
        // R-M1: the physical cursor does not move when the scale factor does,
        // so the logical position has to be divided by the same ratio.
        let mut state = ResizeState::new();
        let window = no_window();
        let mut surface: Option<crate::skia_surface::SkiaSurface> = None;
        let mut scale_factor = 1.0f32;
        let mut mouse_pos = (100.0f32, 200.0f32);

        window_scale_factor_changed(
            &mut scale_factor,
            &mut mouse_pos,
            &mut state,
            &mut surface,
            &window,
            2.0,
            (1920, 1080),
        );

        assert_eq!(scale_factor, 2.0);
        assert_eq!(mouse_pos, (50.0, 100.0));
        // The pending resize is queued by the same arm (R-L3).
        assert_eq!(state.take_pending(), Some((1920, 1080)));
    }

    #[test]
    fn scale_factor_change_never_divides_by_a_degenerate_old_scale() {
        // A 0, negative or NaN old scale cannot produce a meaningful ratio.
        // Rescaling by it anyway would poison `mouse_pos` with NaN or infinity,
        // after which every hover hit-test silently misses forever.
        for degenerate in [0.0f32, -1.0, f32::NAN, f32::INFINITY] {
            let mut state = ResizeState::new();
            let window = no_window();
            let mut surface: Option<crate::skia_surface::SkiaSurface> = None;
            let mut scale_factor = degenerate;
            let mut mouse_pos = (10.0f32, 20.0f32);

            window_scale_factor_changed(
                &mut scale_factor,
                &mut mouse_pos,
                &mut state,
                &mut surface,
                &window,
                2.0,
                (800, 600),
            );

            assert_eq!(
                scale_factor, 2.0,
                "old_scale={degenerate}: new scale adopted"
            );
            assert_eq!(
                mouse_pos,
                (10.0, 20.0),
                "old_scale={degenerate}: cursor left alone"
            );
            // The pending resize is not conditional on the cursor arithmetic.
            assert_eq!(state.take_pending(), Some((800, 600)));
        }
    }

    #[test]
    fn cursor_moved_converts_physical_pixels_to_logical_units() {
        // winit reports physical pixels; layout and hit-testing are logical.
        // Skipping this division puts the cursor half a screen away at 2x.
        let window = no_window();
        let mut mouse_pos = (0.0f32, 0.0f32);
        let mut hovered_id: Option<u32> = None;

        window_cursor_moved(
            2.0,
            &mut mouse_pos,
            &[],
            &mut hovered_id,
            &window,
            (300.0, 200.0),
        );

        assert_eq!(mouse_pos, (150.0, 100.0));
    }

    #[test]
    fn cursor_moved_latches_and_clears_the_hover_target_under_the_logical_cursor() {
        let window = no_window();
        let targets = [hover_target(0, 0, 200, 200, 7)];
        let mut mouse_pos = (0.0f32, 0.0f32);
        let mut hovered_id: Option<u32> = None;

        // 300 physical px at 2.0 is 150 logical, inside the 0..200 target.
        window_cursor_moved(
            2.0,
            &mut mouse_pos,
            &targets,
            &mut hovered_id,
            &window,
            (300.0, 200.0),
        );
        assert_eq!(hovered_id, Some(7));

        // Leaving the target has to clear the latch, or the hover style sticks.
        window_cursor_moved(
            2.0,
            &mut mouse_pos,
            &targets,
            &mut hovered_id,
            &window,
            (900.0, 800.0),
        );
        assert_eq!(hovered_id, None);
    }

    #[test]
    fn modifiers_changed_latches_the_shift_state_the_caret_reads() {
        // winit 0.28 deprecated the per-key `KeyboardInput::modifiers` field, so
        // Shift-extends-selection depends on this latch being the only writer.
        let mut shift_held = false;
        window_modifiers_changed(&mut shift_held, true);
        assert!(shift_held);
        window_modifiers_changed(&mut shift_held, false);
        assert!(!shift_held);
    }

    #[test]
    fn wheel_deltas_become_logical_pixels_in_the_natural_direction() {
        // winit reports positive y for wheel-up; the offset must grow when the
        // wheel is pushed down, and one line is 40 logical px.
        assert_eq!(
            window_wheel_delta_y(MouseScrollDelta::LineDelta(0.0, 1.0), 1.0),
            -40.0
        );
        assert_eq!(
            window_wheel_delta_y(MouseScrollDelta::LineDelta(0.0, -1.0), 1.0),
            40.0
        );
        // The horizontal component is not a vertical scroll.
        assert_eq!(
            window_wheel_delta_y(MouseScrollDelta::LineDelta(7.0, 1.0), 1.0),
            -40.0
        );
        // `PixelDelta` is physical, so it needs the scale factor.
        assert_eq!(
            window_wheel_delta_y(
                MouseScrollDelta::PixelDelta(PhysicalPosition::new(0.0, -100.0)),
                2.0,
            ),
            50.0
        );
    }

    #[test]
    fn a_wheel_event_before_the_first_frame_is_a_no_op() {
        // A wheel can arrive before any `RedrawRequested`, when there is no
        // layout to scroll. That must be inert, not a panic.
        let window = no_window();
        let mut scroll_offsets: HashMap<Vec<usize>, f32> = HashMap::new();
        window_mouse_wheel(
            1.0,
            (10.0, 10.0),
            &None,
            &mut scroll_offsets,
            &window,
            MouseScrollDelta::LineDelta(0.0, 3.0),
        );
        assert!(scroll_offsets.is_empty());
    }
}

/// Run a Skia window whose contents are produced by `make_view`.
///
/// # Viewport contract (1A / X-H1) — Viewport Root Normalization (CX-04)
///
/// `make_view` is `FnMut(w: u32, h: u32) -> (VNode, Stylesheet)` where
/// `(w, h)` are **logical viewport dimensions** — `Viewport::from_i32(physical, scale).logical_size()`.
/// The renderer calls it:
/// * once on startup,
/// * on every `RedrawRequested` / resize / DPI change with the *current* logical `w,h`.
///   Templates must not ignore `(w,h)` (no `|_w, _h|`). The canonical
///   responsive pattern is a viewport-filling root:
///   `width: 100%; min-height: 100vh` on `.app` (see `veloxc/templates/project/src/App.vx`).
///   With `width:100%` and `min-height:100vh`, `compute_layout(vnode, w as i32, h as i32)`
///   reflows visibly on every window resize — no element hidden when it should be visible
///   (Flutter invariant). Callers may also thread `(w,h)` into style/layout decisions if needed.
///
/// ## Root normalization
///
/// `velox_dom::layout::root_is_viewport_filling` ensures the *first* VNode (index 0 / `None`)
/// always fills the logical viewport — even without explicit `width:100%`. The expanded
/// predicate `is_viewport_filling(style, is_root_index)` additionally treats `100% | 100vw | 100dvw | 100vh | 100dvh | min-height:100%|vh|dvh`
/// as viewport-filling, validated via `compute_layout` percent chains (`height:100%` fills
/// when parent is definite) and `vw/vh/dvh` units that resolve against `Viewport::logical_size`.
#[cfg(feature = "skia-native")]
pub fn run_window_vnode_skia<F, G, H>(
    title: &str,
    mut make_view: F,
    mut on_event: G,
    mut get_title: H,
) -> Result<(), String>
where
    F: FnMut(u32, u32) -> (velox_dom::VNode, Stylesheet) + 'static,
    G: FnMut(&str, Option<&str>) + 'static,
    H: FnMut() -> String + 'static,
{
    use winit::dpi::PhysicalSize;
    use winit::event::{ElementState, Event, MouseButton, StartCause, WindowEvent};
    use winit::event_loop::{ControlFlow, EventLoop};
    use winit::window::WindowBuilder;

    // Headless mode: when no compositor is available (CI, containers, SSH)
    // we still create the window + run the event loop but skip softbuffer
    // presentation, rendering offscreen only. Enable with VELOX_HEADLESS=1
    // or it is auto-detected via presenter::is_compositor_available().
    let headless_env = std::env::var("VELOX_HEADLESS").as_deref() == Ok("1")
        || !crate::presenter::is_compositor_available();

    let mut last_vnode: Option<velox_dom::VNode> = None;
    let mut _hmr_pending = false;

    // Prepare the winit backend: force X11 if Wayland socket is stale to
    // avoid winit's Wayland backend calling process::exit() on EPIPE.
    let _headless_check = crate::presenter::prepare_backend();

    // Try to create a winit event loop and window. In headless mode or when
    // no compositor is available, both can fail (winit may panic instead of
    // returning Err) — we catch this and proceed with headless rendering.
    //
    // A NON-display failure is not headless-eligible: it is returned as a
    // typed `VeloxError::Window`, never panicked. The old `panic!` here was
    // caught by the `catch_unwind` below and silently converted into the same
    // headless fallback as a broken pipe, which masked real configuration
    // errors (bad size, closed display fd) as "running headless".
    // Factored out of the `let` below: clippy::type_complexity fires on the
    // inline annotation. Behaviour-identical alias.
    type WindowOutcome = Result<
        (
            Option<winit::event_loop::EventLoop<()>>,
            Option<winit::window::Window>,
            PhysicalSize<u32>,
            f32,
        ),
        String,
    >;
    let window_outcome: WindowOutcome =
        std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
            let event_loop = EventLoop::new();
            let window_result = WindowBuilder::new()
                .with_title(title)
                .with_inner_size(PhysicalSize::new(800, 600))
                .build(&event_loop);
            match window_result {
                Ok(w) => {
                    let size = w.inner_size();
                    let sf = w.scale_factor() as f32;
                    Ok((Some(event_loop), Some(w), size, sf))
                }
                Err(e) => {
                    let msg = e.to_string();
                    let lower = msg.to_ascii_lowercase();
                    let is_display_err = lower.contains("broken pipe")
                        || lower.contains("os error 32")
                        || lower.contains("no compositor")
                        || lower.contains("no display server")
                        || lower.contains("failed to connect");
                    if headless_env || is_display_err {
                        log::warn!("window creation failed — continuing in headless mode: {msg}");
                        Ok((Some(event_loop), None, PhysicalSize::new(800, 600), 1.0))
                    } else {
                        Err(
                            velox_dom::VeloxError::Window(format!("failed to create window: {e}"))
                                .to_string(),
                        )
                    }
                }
            }
        }))
        .unwrap_or_else(|payload| {
            // Surface the panic payload (usually the compositor error, e.g.
            // broken pipe) instead of silently masking it — log::warn is
            // invisible without an initialized logger (CX-13 / F-23).
            eprintln!(
                "[velox] window/event loop creation panicked — continuing in headless mode: {}",
                panic_detail(payload)
            );
            Ok((None, None, PhysicalSize::new(800, 600), 1.0))
        });
    let (event_loop_opt, window, window_size, scale_factor) = window_outcome?;

    let window_opt: Option<winit::window::Window> = window;
    let mut renderer = match crate::skia_surface::SkiaSurface::new_raster(
        window_size.width as i32,
        window_size.height as i32,
    ) {
        Ok(surface) => skia_backend::SkiaRenderer {
            surface: Some(surface),
            vnode: None,
        },
        Err(e) => {
            return Err(format!("failed to create SkiaSurface: {e}"));
        }
    };
    let mut presenter: Option<crate::presenter::SoftbufferPresenter> = None;
    if let Some(w) = window_opt.as_ref() {
        // softbuffer::Context::new() can panic when the display server is
        // unreachable even though DISPLAY/WAYLAND_DISPLAY are set (broken pipe).
        // Catch such panics and degrade to headless rendering.
        match std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
            crate::presenter::SoftbufferPresenter::new(w, window_size.width, window_size.height)
        })) {
            Ok(Ok(p)) => presenter = Some(p),
            Ok(Err(e)) => {
                if headless_env {
                    log::warn!(
                        "softbuffer presenter unavailable — continuing in headless mode: {e}"
                    );
                } else {
                    return Err(format!("failed to create softbuffer presenter: {e}"));
                }
            }
            Err(payload) => {
                // Surface the panic payload (softbuffer panics with a
                // broken pipe when the display is unreachable) instead of
                // silently masking it (CX-13 / F-23).
                eprintln!(
                    "[velox] softbuffer presenter creation panicked — continuing in headless mode: {}",
                    panic_detail(payload)
                );
            }
        }
    }
    let mut scale_factor = scale_factor;
    let mut mouse_pos = (0.0f32, 0.0f32);
    let mut hovered_id: Option<u32> = None;
    let mut click_targets: Vec<crate::events::ClickTarget> = Vec::new();
    let mut hover_targets: Vec<crate::events::HoverTarget> = Vec::new();
    let mut input_targets: Vec<crate::events::InputTarget> = Vec::new();
    // Path (child source indices) to the focused text input, if any.
    let mut focused_input: Option<Vec<usize>> = None;
    // Instant before which the caret must stay solid regardless of tick
    // timing. Armed by every editing key and by focus changes.
    let mut blink_deadline: Option<std::time::Instant> = None;
    // Shift state, latched by the shared `window_modifiers_changed` below.
    // winit 0.28 deprecates `KeyboardInput::modifiers` in favour of the
    // `ModifiersChanged` event, so the caret reads this latched value instead
    // of the deprecated per-key field.
    let mut shift_held = false;
    // Scrollable overflow model: wheel clamping + deepest hit_test
    let mut scroll_offsets: std::collections::HashMap<Vec<usize>, f32> =
        std::collections::HashMap::new();
    let mut last_layout: Option<velox_dom::layout::LayoutNode> = None;
    // Lifecycle: ensure on_mounted fires once on first RedrawRequested and
    // on_unmounted/before_destroy fire on CloseRequested or drop.
    let _lifecycle_guard = LifecycleCleanupGuard;
    let mut did_mount = false;
    // R-L3: coalesce rapid resize drags — only last size per frame materializes.
    // Surface recreation (raster_n32_premul) is deferred to RedrawRequested.
    let mut resize_state = ResizeState::new();

    // Render first frame immediately before entering the event loop.
    // This ensures the window has content even on platforms where
    // request_redraw() from NewEvents(StartCause::Init) may not trigger
    // a RedrawRequested event (e.g. certain Wayland/X11 compositors).

    if let Some(s) = &mut renderer.surface {
        s.set_scale_factor(scale_factor);
        let (vw, vh) = resize_state.record_initial(s.width, s.height, scale_factor);
        let (vnode_raw, sheet) = make_view(vw, vh);
        let mut next_id = 1u32;
        let vnode_tagged = with_hover_ids(&vnode_raw, &mut next_id);
        let vnode = crate::style_vnode_with_hover(&vnode_tagged, &sheet, &|_tag, props| {
            props
                .attrs
                .get("data-hover-id")
                .and_then(|v| v.parse::<u32>().ok())
                .map(|id| Some(id) == hovered_id)
                .unwrap_or(false)
        });
        // First frame: no input can be focused yet, so every text input gets
        // the unfocused defaults. Injected anyway so the attr contract holds on
        // frame one and paint never has to special-case absence.
        let vnode = crate::inject_input_caret_attrs(&vnode, None);
        // Publish the pre-loop frame so a click or keystroke that arrives before
        // the first RedrawRequested is handled against real layout/value data
        // rather than an empty `last_vnode`. Without this, `apply_click_focus`
        // and `apply_edit_to_focused` see `None` and either drop the edit or put
        // the caret at 0 — the regression the HMR loop already guards against
        // by assigning `last_vnode` here.
        last_vnode = Some(vnode.clone());
        let mut layout = velox_dom::layout::compute_layout(&vnode, vw as i32, vh as i32);
        {
            let mut path = Vec::new();
            crate::events::apply_scroll_offsets(&mut layout, &scroll_offsets, &mut path);
        }
        last_layout = Some(layout.clone());
        recompute_targets(
            &vnode,
            &layout,
            &mut click_targets,
            &mut hover_targets,
            &mut input_targets,
        );
        // Render and present the initial frame so the window has immediate content.
        if let Err(e) = crate::skia_render::skia_impl::render_frame(s, &vnode, &layout, &sheet) {
            log::error!("skia initial render error: {}", e);
        }
        if let Some(presenter) = presenter.as_mut()
            && let Err(e) = presenter.present(s)
        {
            log::error!("skia initial present error: {}", e);
        }
    }

    if let Some(event_loop) = event_loop_opt {
        // The event loop can panic if the display server becomes unreachable
        // (e.g. "Io error: Broken pipe") — catch that and degrade gracefully.
        let _ = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
            // Caret blink clock. `EventLoop::new()` is an alias for
            // `EventLoopBuilder::new().build()`, and that constructor calls
            // `with_user_event()`, so the user-event channel is already enabled
            // here and `create_proxy()` needs no builder change. Held in a local
            // inside the unwind closure so its `Drop` joins the thread when the
            // loop ends.
            let _blink_ticker = crate::CaretBlinkTicker::start(event_loop.create_proxy(), ());
            event_loop.run(move |event, _, control_flow| {
                *control_flow = ControlFlow::Wait;
                match event {
                    Event::NewEvents(StartCause::Init) => {
                        if let Some(w) = window_opt.as_ref() {
                            w.request_redraw();
                        }
                    }
                    Event::WindowEvent {
                        event: WindowEvent::CloseRequested,
                        ..
                    } => {
                        // Destroy hooks run inside the shared arm; the exit stays
                        // here so `ControlFlow` never leaves the loop.
                        if window_close_requested() {
                            *control_flow = ControlFlow::Exit;
                        }
                    }
                    Event::WindowEvent {
                        event: WindowEvent::Resized(new_size),
                        ..
                    } => {
                        window_resized(
                            &mut resize_state,
                            &window_opt,
                            (new_size.width, new_size.height),
                        );
                    }
                    Event::WindowEvent {
                        event:
                            WindowEvent::ScaleFactorChanged {
                                scale_factor: new_scale,
                                new_inner_size,
                                ..
                            },
                        ..
                    } => {
                        window_scale_factor_changed(
                            &mut scale_factor,
                            &mut mouse_pos,
                            &mut resize_state,
                            &mut renderer.surface,
                            &window_opt,
                            new_scale,
                            (new_inner_size.width, new_inner_size.height),
                        );
                    }
                    Event::WindowEvent {
                        event: WindowEvent::CursorMoved { position, .. },
                        ..
                    } => {
                        window_cursor_moved(
                            scale_factor,
                            &mut mouse_pos,
                            &hover_targets,
                            &mut hovered_id,
                            &window_opt,
                            (position.x as f32, position.y as f32),
                        );
                    }
                    Event::WindowEvent {
                        event:
                            WindowEvent::MouseInput {
                                state: ElementState::Pressed,
                                button: MouseButton::Left,
                                ..
                            },
                        ..
                    } => {
                        // Text-input focus: clicking a text field focuses it and
                        // places the caret; clicking anywhere else drops focus and
                        // clears the selection. This runs BEFORE the click-handler
                        // test and is deliberately independent of it — see the
                        // redraw hoist at the bottom of this arm.
                        crate::apply_click_focus(
                            &mut input_targets,
                            &last_vnode,
                            &mut focused_input,
                            mouse_pos.0,
                            mouse_pos.1,
                            scale_factor,
                            surface_logical_viewport(&renderer, scale_factor),
                        );
                        let mut handled_click = false;
                        if let Some((handler, payload_opt)) =
                            crate::events::hit_test_click(&click_targets, mouse_pos.0, mouse_pos.1)
                        {
                            let payload_owned =
                                payload_opt.map(|p| p.to_string()).unwrap_or_else(|| {
                                    format!("{{\"x\":{},\"y\":{}}}", mouse_pos.0, mouse_pos.1)
                                });
                            on_event(handler, Some(&payload_owned));
                            velox_core::lifecycle::run_all_updated_hooks();
                            if let Some(s) = &mut renderer.surface {
                                let (vw, vh) = logical_size(s.width, s.height, scale_factor);
                                let (vnode_raw, sheet) = make_view(vw, vh);
                                let mut next_id = 1u32;
                                let vnode_tagged = with_hover_ids(&vnode_raw, &mut next_id);
                                let vnode = crate::style_vnode_with_hover(
                                    &vnode_tagged,
                                    &sheet,
                                    &|_tag, props| {
                                        props
                                            .attrs
                                            .get("data-hover-id")
                                            .and_then(|v| v.parse::<u32>().ok())
                                            .map(|id| Some(id) == hovered_id)
                                            .unwrap_or(false)
                                    },
                                );
                                let vnode = crate::inject_input_caret_attrs(
                                    &vnode,
                                    crate::events::focused_input_index(&input_targets)
                                        .and_then(|i| input_targets.get(i)),
                                );
                                let mut layout =
                                    velox_dom::layout::compute_layout(&vnode, vw as i32, vh as i32);
                                {
                                    let mut path = Vec::new();
                                    crate::events::apply_scroll_offsets(
                                        &mut layout,
                                        &scroll_offsets,
                                        &mut path,
                                    );
                                }
                                last_layout = Some(layout.clone());
                                recompute_targets(
                                    &vnode,
                                    &layout,
                                    &mut click_targets,
                                    &mut hover_targets,
                                    &mut input_targets,
                                );
                            }
                            handled_click = true;
                        }
                        if let Some(w) = window_opt.as_ref() {
                            if handled_click {
                                w.set_title(&get_title());
                            }
                            // HOISTED OUT of the `hit_test_click` arm on purpose.
                            // This used to live inside it, so a click that hit
                            // empty space changed focus and then never repainted:
                            // the focus ring and caret were stuck at their old
                            // values. Focus is now state the frame depends on, so
                            // any change to it must schedule a redraw.
                            w.request_redraw();
                        }
                    }
                    Event::WindowEvent {
                        event: WindowEvent::ModifiersChanged(mods),
                        ..
                    } => {
                        window_modifiers_changed(&mut shift_held, mods.shift());
                    }
                    Event::WindowEvent {
                        event: WindowEvent::KeyboardInput { input, .. },
                        ..
                    } => {
                        // Handle keyboard shortcuts and text-input editing keys.
                        use winit::event::VirtualKeyCode;
                        if let Some(keycode) = input.virtual_keycode
                            && input.state == ElementState::Pressed
                        {
                            // `@keydown` bindings first, and additively: this
                            // grants focus and calls handlers, it never
                            // consumes the key, so every branch below still
                            // runs exactly as it did before any keydown
                            // binding existed.
                            if crate::dispatch_keydown(
                                keycode,
                                &mut input_targets,
                                &last_vnode,
                                &mut focused_input,
                                &mut on_event,
                            ) {
                                arm_blink_deadline(&mut blink_deadline);
                                velox_core::lifecycle::run_all_updated_hooks();
                                if let Some(w) = window_opt.as_ref() {
                                    w.request_redraw();
                                }
                            }
                            match keycode {
                                // Reload (app exits, dev server restarts it) and quit.
                                // Both are withheld while a text field has focus: with a
                                // field focused these keys are *text*, so an unguarded
                                // shortcut means typing "r" into a search box kills the
                                // application. With nothing focused they are the only way
                                // out of the window. When the guard fails the key falls
                                // through to the editor below, which is what lets the
                                // character still reach the field.
                                VirtualKeyCode::R | VirtualKeyCode::Q
                                    if !crate::events::any_input_focused(&input_targets) =>
                                {
                                    velox_core::lifecycle::run_all_destroy_hooks();
                                    *control_flow = ControlFlow::Exit;
                                }
                                _ => {
                                    // Every remaining editing key routes through the
                                    // one shared call, so the HMR loop cannot drift
                                    // from this one again.
                                    if let Some(action) =
                                        crate::edit_action_for_key(keycode, shift_held)
                                    {
                                        let changed = crate::apply_edit_to_focused(
                                            &mut input_targets,
                                            action,
                                            &last_vnode,
                                            &mut focused_input,
                                            &mut on_event,
                                        );
                                        if changed {
                                            // Caret goes solid while you type.
                                            arm_blink_deadline(&mut blink_deadline);
                                            velox_core::lifecycle::run_all_updated_hooks();
                                            if let Some(w) = window_opt.as_ref() {
                                                w.request_redraw();
                                            }
                                        }
                                    }
                                }
                            }
                        }
                    }
                    Event::UserEvent(()) => {
                        // Caret blink tick from `CaretBlinkTicker`. See that
                        // type for why a thread is involved and why it is not a
                        // shared-state race.
                        if crate::on_caret_blink_tick(&mut input_targets, &mut blink_deadline)
                            && let Some(w) = window_opt.as_ref()
                        {
                            w.request_redraw();
                        }
                    }
                    Event::WindowEvent {
                        event: WindowEvent::ReceivedCharacter(c),
                        ..
                    } => {
                        // Printable characters go to the focused text input.
                        if !c.is_control()
                            && c != '\u{7f}'
                            && crate::apply_edit_to_focused(
                                &mut input_targets,
                                crate::events::EditAction::Insert(c),
                                &last_vnode,
                                &mut focused_input,
                                &mut on_event,
                            )
                        {
                            // Caret goes solid while you type.
                            arm_blink_deadline(&mut blink_deadline);
                            velox_core::lifecycle::run_all_updated_hooks();
                            if let Some(w) = window_opt.as_ref() {
                                w.request_redraw();
                            }
                        }
                    }
                    Event::WindowEvent {
                        event: WindowEvent::MouseWheel { delta, .. },
                        ..
                    } => {
                        window_mouse_wheel(
                            scale_factor,
                            mouse_pos,
                            &last_layout,
                            &mut scroll_offsets,
                            &window_opt,
                            delta,
                        );
                    }
                    Event::RedrawRequested(_) => {
                        // R-L3: materialize any coalesced resize exactly once per frame.
                        let mut committed_resize = false;
                        if let Some((pw, ph)) = resize_state.take_pending() {
                            // `ResizeState::take_pending` is the production `pending_resize.take()` path.
                            match renderer.resize(pw as i32, ph as i32) {
                                Ok(()) => committed_resize = true,
                                Err(e) => {
                                    log::warn!("renderer resize failed ({}x{}): {}", pw, ph, e);
                                }
                            }
                            if let Some(presenter) = presenter.as_mut()
                                && let Err(e) = presenter.resize(pw, ph)
                            {
                                log::warn!("presenter resize failed: {}", e);
                            }
                        }
                        // First mount: fire on_mounted once.
                        ensure_mounted(&mut did_mount);
                        // Render VNode -> Skia frame and present.
                        if let Some(s) = &mut renderer.surface {
                            s.set_scale_factor(scale_factor);
                            let (vw, vh) = resize_state.frame_logical_size(
                                s.width,
                                s.height,
                                scale_factor,
                                committed_resize,
                            );
                            let (vnode_raw, sheet) = make_view(vw, vh);
                            let mut next_id = 1u32;
                            let vnode_tagged = with_hover_ids(&vnode_raw, &mut next_id);
                            let vnode = crate::style_vnode_with_hover(
                                &vnode_tagged,
                                &sheet,
                                &|_tag, props| {
                                    props
                                        .attrs
                                        .get("data-hover-id")
                                        .and_then(|v| v.parse::<u32>().ok())
                                        .map(|id| Some(id) == hovered_id)
                                        .unwrap_or(false)
                                },
                            );
                            // Caret/selection/focus contract for the paint lane.
                            // After the style cascade, before layout: it adds no
                            // second cascade and no second rounding site.
                            let vnode = crate::inject_input_caret_attrs(
                                &vnode,
                                crate::events::focused_input_index(&input_targets)
                                    .and_then(|i| input_targets.get(i)),
                            );
                            last_vnode = Some(vnode.clone());
                            let mut layout =
                                velox_dom::layout::compute_layout(&vnode, vw as i32, vh as i32);
                            {
                                let mut path = Vec::new();
                                crate::events::apply_scroll_offsets(
                                    &mut layout,
                                    &scroll_offsets,
                                    &mut path,
                                );
                            }
                            last_layout = Some(layout.clone());
                            recompute_targets(
                                &vnode,
                                &layout,
                                &mut click_targets,
                                &mut hover_targets,
                                &mut input_targets,
                            );
                            if let Err(e) = crate::skia_render::skia_impl::render_frame(
                                s, &vnode, &layout, &sheet,
                            ) {
                                log::error!("skia render error: {}", e);
                            }
                            if let Some(presenter) = presenter.as_mut()
                                && let Err(e) = presenter.present(s)
                            {
                                log::error!("skia present error: {}", e);
                            }
                        }
                    }
                    _ => {}
                }
            });
        }));
    } else {
        // Headless mode — no event loop, just run the initial render and return.
        log::info!("running in headless mode (no event loop)");
        velox_core::lifecycle::run_all_destroy_hooks();
    }
    Ok(())
}

/// What the caught window/loop construction yields: the event loop and its HMR
/// proxy (both survive even when the window does not), the window itself, its
/// physical size, and the DPI scale factor.
#[cfg(feature = "skia-native")]
type WindowBootstrap = (
    Option<winit::event_loop::EventLoop<HmrMessage>>,
    Option<winit::event_loop::EventLoopProxy<HmrMessage>>,
    Option<winit::window::Window>,
    winit::dpi::PhysicalSize<u32>,
    f32,
);

/// HMR variant — same viewport contract as [`run_window_vnode_skia`] (see its docs).
/// `make_view` is called with logical `(w, h)` on every frame/resize; templates use
/// `width:100%` / `min-height:100vh` so the viewport change is visibly reflected.
#[cfg(feature = "skia-native")]
pub fn run_window_vnode_skia_with_hmr<F, G, H>(
    title: &str,
    mut make_view: F,
    mut on_event: G,
    mut get_title: H,
    hmr_rx: std::sync::Arc<std::sync::Mutex<std::sync::mpsc::Receiver<HmrMessage>>>,
) -> Result<(), String>
where
    F: FnMut(u32, u32) -> (velox_dom::VNode, Stylesheet) + 'static,
    G: FnMut(&str, Option<&str>) + 'static,
    H: FnMut() -> String + 'static,
{
    use winit::dpi::PhysicalSize;
    use winit::event::{ElementState, Event, MouseButton, StartCause, WindowEvent};
    use winit::event_loop::{ControlFlow, EventLoopBuilder};
    use winit::window::WindowBuilder;

    let headless_env_hmr = std::env::var("VELOX_HEADLESS").as_deref() == Ok("1")
        || !crate::presenter::is_compositor_available();

    // Prepare the winit backend: force X11 if Wayland socket is stale to
    // avoid winit's Wayland backend calling process::exit() on EPIPE.
    let _headless_check_hmr = crate::presenter::prepare_backend();

    // Try to create a winit event loop and window. In headless mode or when
    // no compositor is available, both can fail (winit may panic instead of
    // returning Err) — we catch this and proceed with headless rendering.
    // Use EventLoop<HmrMessage> so the HMR thread can forward messages
    // directly via EventLoopProxy::send_event(HmrMessage) without any
    // shared Mutex<Receiver>.
    // Same contract as the plain loop above: a non-display window failure is
    // a typed `VeloxError::Window` returned as `Err`, never a `panic!` the
    // `catch_unwind` would launder into headless mode.
    let window_outcome_hmr: Result<WindowBootstrap, String> =
        std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
            let event_loop = EventLoopBuilder::<HmrMessage>::with_user_event().build();
            let proxy = event_loop.create_proxy();
            let window_result = WindowBuilder::new()
                .with_title(title)
                .with_inner_size(PhysicalSize::new(800, 600))
                .build(&event_loop);
            match window_result {
                Ok(w) => {
                    let size = w.inner_size();
                    let sf = w.scale_factor() as f32;
                    Ok((Some(event_loop), Some(proxy), Some(w), size, sf))
                }
                Err(e) => {
                    let msg = e.to_string();
                    let lower = msg.to_ascii_lowercase();
                    let is_display_err = lower.contains("broken pipe")
                        || lower.contains("os error 32")
                        || lower.contains("no compositor")
                        || lower.contains("no display server")
                        || lower.contains("failed to connect");
                    if headless_env_hmr || is_display_err {
                        log::warn!("window creation failed — continuing in headless mode: {msg}");
                        Ok((
                            Some(event_loop),
                            Some(proxy),
                            None,
                            PhysicalSize::new(800, 600),
                            1.0,
                        ))
                    } else {
                        Err(
                            velox_dom::VeloxError::Window(format!("failed to create window: {e}"))
                                .to_string(),
                        )
                    }
                }
            }
        }))
        .unwrap_or_else(|payload| {
            // Surface the panic payload (usually the compositor error, e.g.
            // broken pipe) instead of silently masking it — log::warn is
            // invisible without an initialized logger (CX-13 / F-23).
            eprintln!(
                "[velox] window/event loop creation panicked — continuing in headless mode: {}",
                panic_detail(payload)
            );
            Ok((None, None, None, PhysicalSize::new(800, 600), 1.0))
        });
    let (event_loop_opt, proxy_opt, window, window_size, scale_factor) = window_outcome_hmr?;

    // Spawn the HMR forwarding thread. It takes sole ownership of the
    // Receiver (moved out of the Arc<Mutex>) and forwards each HmrMessage
    // directly via EventLoopProxy::send_event(HmrMessage). No Mutex is
    // held across blocking recv(). Wrap in catch_unwind so HMR panics
    // never bring down the app (acceptance: catch_unwind around HMR loop).
    if let Some(proxy) = proxy_opt {
        match std::sync::Arc::try_unwrap(hmr_rx) {
            Ok(mutex) => {
                // `into_inner`, never `expect`: a poisoned mutex still owns a
                // valid `Receiver`, and panicking here would kill the app over
                // a previous panic's side effect. The receiver is taken as-is.
                let rx = mutex
                    .into_inner()
                    .unwrap_or_else(|poisoned| poisoned.into_inner());
                std::thread::spawn(move || {
                    let _ = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
                        while let Ok(msg) = rx.recv() {
                            if proxy.send_event(msg).is_err() {
                                break;
                            }
                        }
                    }));
                });
            }
            Err(shared) => {
                // Fallback: another Arc clone exists (unusual). Poll without
                // holding the lock across blocking recv.
                std::thread::spawn(move || {
                    let _ = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
                        loop {
                            let res = {
                                // `into_inner`, never `expect`: poisoning must
                                // not panic the HMR thread (which would kill
                                // forwarding silently). The guard is dropped
                                // before `recv` either way — no lock is held
                                // across the blocking call.
                                let guard = shared
                                    .lock()
                                    .unwrap_or_else(|poisoned| poisoned.into_inner());
                                guard.try_recv()
                            };
                            match res {
                                Ok(msg) => {
                                    if proxy.send_event(msg).is_err() {
                                        break;
                                    }
                                }
                                Err(std::sync::mpsc::TryRecvError::Empty) => {
                                    std::thread::sleep(std::time::Duration::from_millis(16));
                                }
                                Err(std::sync::mpsc::TryRecvError::Disconnected) => break,
                            }
                        }
                    }));
                });
            }
        }
    }
    // NOTE: hmr_rx has been moved into the forwarding thread. The event loop
    // now receives HmrMessage via Event::UserEvent(msg) without any Mutex.

    let window_opt: Option<winit::window::Window> = window;
    let mut scale_factor = scale_factor;
    let mut renderer = match crate::skia_surface::SkiaSurface::new_raster(
        window_size.width as i32,
        window_size.height as i32,
    ) {
        Ok(surface) => skia_backend::SkiaRenderer {
            surface: Some(surface),
            vnode: None,
        },
        Err(e) => {
            return Err(format!("failed to create SkiaSurface: {e}"));
        }
    };
    let mut presenter: Option<crate::presenter::SoftbufferPresenter> = None;
    if let Some(w) = window_opt.as_ref() {
        // softbuffer::Context::new() can panic when the display server is
        // unreachable even though DISPLAY/WAYLAND_DISPLAY are set (broken pipe).
        // Catch such panics and degrade to headless rendering.
        match std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
            crate::presenter::SoftbufferPresenter::new(w, window_size.width, window_size.height)
        })) {
            Ok(Ok(p)) => presenter = Some(p),
            Ok(Err(e)) => {
                if headless_env_hmr || !crate::presenter::is_compositor_available() {
                    log::warn!(
                        "softbuffer presenter unavailable — continuing in headless mode: {e}"
                    );
                } else {
                    return Err(format!("failed to create softbuffer presenter: {e}"));
                }
            }
            Err(payload) => {
                // Surface the panic payload (softbuffer panics with a
                // broken pipe when the display is unreachable) instead of
                // silently masking it (CX-13 / F-23).
                eprintln!(
                    "[velox] softbuffer presenter creation panicked — continuing in headless mode: {}",
                    panic_detail(payload)
                );
            }
        }
    }
    let mut mouse_pos = (0.0f32, 0.0f32);
    let mut hovered_id: Option<u32> = None;
    let mut click_targets: Vec<crate::events::ClickTarget> = Vec::new();
    let mut hover_targets: Vec<crate::events::HoverTarget> = Vec::new();
    let mut input_targets: Vec<crate::events::InputTarget> = Vec::new();
    // Was `_last_vnode` until 5cc4a4f, which renamed it and made it live. The
    // earlier HMR-deadlock fix (7585674) introduced this binding *unused*, so
    // typing, Backspace and Return stayed dead under HMR; 5cc4a4f is what
    // actually fixed them, by giving the value a real name and wiring it into
    // the ported input handling.
    let mut last_vnode: Option<velox_dom::VNode> = None;
    // Path (child source indices) to the focused text input, if any.
    // Mirrors the plain loop's state so the two loops stay behaviourally equal.
    let mut focused_input: Option<Vec<usize>> = None;
    // Instant before which the caret must stay solid regardless of tick timing.
    let mut blink_deadline: Option<std::time::Instant> = None;
    // Shift state, latched by the shared `window_modifiers_changed` below —
    // see the plain loop for why the per-key `modifiers` field is not used.
    let mut shift_held = false;
    let mut scroll_offsets_hmr: std::collections::HashMap<Vec<usize>, f32> =
        std::collections::HashMap::new();
    let mut last_layout_hmr: Option<velox_dom::layout::LayoutNode> = None;
    let _lifecycle_guard_hmr = LifecycleCleanupGuard;
    let mut did_mount_hmr = false;
    // R-L3: coalesce rapid resize drags in HMR loop too (only last size per frame).
    let mut resize_state = ResizeState::new();

    // Render first frame immediately before entering the event loop.
    // This ensures the window has content even on platforms where
    // request_redraw() from NewEvents(StartCause::Init) may not trigger
    // a RedrawRequested event (e.g. certain Wayland/X11 compositors).
    if let Some(s) = &mut renderer.surface {
        s.set_scale_factor(scale_factor);
        let (vw, vh) = resize_state.record_initial(s.width, s.height, scale_factor);
        let (vnode_raw, sheet) = make_view(vw, vh);
        let mut next_id = 1u32;
        let vnode_tagged = with_hover_ids(&vnode_raw, &mut next_id);
        let vnode = crate::style_vnode_with_hover(&vnode_tagged, &sheet, &|_tag, props| {
            props
                .attrs
                .get("data-hover-id")
                .and_then(|v| v.parse::<u32>().ok())
                .map(|id| Some(id) == hovered_id)
                .unwrap_or(false)
        });
        // First frame: no input can be focused yet, so every text input gets the
        // unfocused defaults.
        let vnode = crate::inject_input_caret_attrs(&vnode, None);
        last_vnode = Some(vnode.clone());
        let mut layout = velox_dom::layout::compute_layout(&vnode, vw as i32, vh as i32);
        {
            let mut path = Vec::new();
            crate::events::apply_scroll_offsets(&mut layout, &scroll_offsets_hmr, &mut path);
        }
        last_layout_hmr = Some(layout.clone());
        recompute_targets(
            &vnode,
            &layout,
            &mut click_targets,
            &mut hover_targets,
            &mut input_targets,
        );
        // Render and present the initial frame so the window has immediate content.
        if let Err(e) = crate::skia_render::skia_impl::render_frame(s, &vnode, &layout, &sheet) {
            log::error!("skia initial render error: {}", e);
        }
        if let Some(presenter) = presenter.as_mut()
            && let Err(e) = presenter.present(s)
        {
            log::error!("skia initial present error: {}", e);
        }
    }

    if let Some(event_loop) = event_loop_opt {
        // The event loop can panic if the display server becomes unreachable
        // (e.g. "Io error: Broken pipe") — catch that and degrade gracefully.
        let _ = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
            // Caret blink clock for the HMR loop. `EventLoop::run` consumes
            // `self`, so the proxy has to be made before the call. This loop's
            // channel is typed `HmrMessage`, so the tick rides
            // `HmrMessage::KeepWindow` — see that arm for why.
            let _blink_ticker =
                crate::CaretBlinkTicker::start(event_loop.create_proxy(), HmrMessage::KeepWindow);
            event_loop.run(move |event, _, control_flow| {
                *control_flow = ControlFlow::Wait;
                match event {
                    Event::UserEvent(msg) => match msg {
                        HmrMessage::FullReload => {
                            velox_core::lifecycle::run_all_destroy_hooks();
                            *control_flow = ControlFlow::Exit;
                        }
                        HmrMessage::HotReload { module_path: _ } => {
                            // Rebuild view and refresh all hit-test targets including
                            // input_targets, then request a redraw so the new VNode
                            // is painted on the next frame.
                            if let Some(s) = renderer.surface.as_ref() {
                                let (vw, vh) = logical_size(s.width, s.height, scale_factor);
                                let (vnode_raw, _) = make_view(vw, vh);
                                if let Err(e) = HmrRenderer::hot_update(&mut renderer, vnode_raw) {
                                    log::error!("hot_update failed: {}", e);
                                }
                                // Recompute all targets from the updated view so
                                // input focus / hit-test stays in sync after HMR.
                                if let Some(s2) = renderer.surface.as_ref() {
                                    let (vw2, vh2) =
                                        logical_size(s2.width, s2.height, scale_factor);
                                    let (vnode2_raw, sheet2) = make_view(vw2, vh2);
                                    let mut nid = 1u32;
                                    let tagged = with_hover_ids(&vnode2_raw, &mut nid);
                                    let vnode2 = crate::style_vnode_with_hover(
                                        &tagged,
                                        &sheet2,
                                        &|_tag, props| {
                                            props
                                                .attrs
                                                .get("data-hover-id")
                                                .and_then(|v| v.parse::<u32>().ok())
                                                .map(|id| Some(id) == hovered_id)
                                                .unwrap_or(false)
                                        },
                                    );
                                    // Caret attrs survive a hot reload: the
                                    // focused path is matched by tree path, and
                                    // `recompute_targets` below re-applies the
                                    // edit state onto the new targets.
                                    let vnode2 = crate::inject_input_caret_attrs(
                                        &vnode2,
                                        crate::events::focused_input_index(&input_targets)
                                            .and_then(|i| input_targets.get(i)),
                                    );
                                    last_vnode = Some(vnode2.clone());
                                    let mut layout = velox_dom::layout::compute_layout(
                                        &vnode2, vw2 as i32, vh2 as i32,
                                    );
                                    {
                                        let mut path = Vec::new();
                                        crate::events::apply_scroll_offsets(
                                            &mut layout,
                                            &scroll_offsets_hmr,
                                            &mut path,
                                        );
                                    }
                                    last_layout_hmr = Some(layout.clone());
                                    recompute_targets(
                                        &vnode2,
                                        &layout,
                                        &mut click_targets,
                                        &mut hover_targets,
                                        &mut input_targets,
                                    );
                                }
                            }
                            if let Some(w) = window_opt.as_ref() {
                                w.request_redraw();
                            }
                        }
                        // `KeepWindow` is overloaded as the HMR loop's caret
                        // blink tick. The HMR channel is typed `HmrMessage`, and
                        // winit 0.28 has no timer API, so the ticker posts a
                        // no-op HMR message on the same channel rather than
                        // forcing the loop to be untyped. The dev server's real
                        // keep-alives land here too, and both are no-ops, so
                        // overloading costs nothing.
                        HmrMessage::KeepWindow => {
                            if crate::on_caret_blink_tick(&mut input_targets, &mut blink_deadline)
                                && let Some(w) = window_opt.as_ref()
                            {
                                w.request_redraw();
                            }
                        }
                    },
                    Event::NewEvents(StartCause::Init) => {
                        if let Some(w) = window_opt.as_ref() {
                            w.request_redraw();
                        }
                    }
                    Event::WindowEvent {
                        event: WindowEvent::CloseRequested,
                        ..
                    } => {
                        // Destroy hooks run inside the shared arm; the exit stays
                        // here so `ControlFlow` never leaves the loop.
                        if window_close_requested() {
                            *control_flow = ControlFlow::Exit;
                        }
                    }
                    Event::WindowEvent {
                        event: WindowEvent::Resized(new_size),
                        ..
                    } => {
                        window_resized(
                            &mut resize_state,
                            &window_opt,
                            (new_size.width, new_size.height),
                        );
                    }
                    Event::WindowEvent {
                        event:
                            WindowEvent::ScaleFactorChanged {
                                scale_factor: new_scale,
                                new_inner_size,
                                ..
                            },
                        ..
                    } => {
                        window_scale_factor_changed(
                            &mut scale_factor,
                            &mut mouse_pos,
                            &mut resize_state,
                            &mut renderer.surface,
                            &window_opt,
                            new_scale,
                            (new_inner_size.width, new_inner_size.height),
                        );
                    }
                    Event::WindowEvent {
                        event: WindowEvent::CursorMoved { position, .. },
                        ..
                    } => {
                        window_cursor_moved(
                            scale_factor,
                            &mut mouse_pos,
                            &hover_targets,
                            &mut hovered_id,
                            &window_opt,
                            (position.x as f32, position.y as f32),
                        );
                    }
                    Event::WindowEvent {
                        event:
                            WindowEvent::MouseInput {
                                state: ElementState::Pressed,
                                button: MouseButton::Left,
                                ..
                            },
                        ..
                    } => {
                        // Text-input focus: identical to the plain loop, and now
                        // actually present. This arm previously had NO focus
                        // handling at all, which is why the whole caret was
                        // unreachable under HMR.
                        crate::apply_click_focus(
                            &mut input_targets,
                            &last_vnode,
                            &mut focused_input,
                            mouse_pos.0,
                            mouse_pos.1,
                            scale_factor,
                            surface_logical_viewport(&renderer, scale_factor),
                        );
                        let mut handled_click = false;
                        if let Some((handler, payload_opt)) =
                            crate::events::hit_test_click(&click_targets, mouse_pos.0, mouse_pos.1)
                        {
                            let payload_owned =
                                payload_opt.map(|p| p.to_string()).unwrap_or_else(|| {
                                    format!("{{\"x\":{},\"y\":{}}}", mouse_pos.0, mouse_pos.1)
                                });
                            on_event(handler, Some(&payload_owned));
                            velox_core::lifecycle::run_all_updated_hooks();
                            if let Some(s) = &mut renderer.surface {
                                let (vw, vh) = logical_size(s.width, s.height, scale_factor);
                                let (vnode_raw, sheet) = make_view(vw, vh);
                                let mut next_id = 1u32;
                                let vnode_tagged = with_hover_ids(&vnode_raw, &mut next_id);
                                let vnode = crate::style_vnode_with_hover(
                                    &vnode_tagged,
                                    &sheet,
                                    &|_tag, props| {
                                        props
                                            .attrs
                                            .get("data-hover-id")
                                            .and_then(|v| v.parse::<u32>().ok())
                                            .map(|id| Some(id) == hovered_id)
                                            .unwrap_or(false)
                                    },
                                );
                                let vnode = crate::inject_input_caret_attrs(
                                    &vnode,
                                    crate::events::focused_input_index(&input_targets)
                                        .and_then(|i| input_targets.get(i)),
                                );
                                last_vnode = Some(vnode.clone());
                                let mut layout =
                                    velox_dom::layout::compute_layout(&vnode, vw as i32, vh as i32);
                                {
                                    let mut path = Vec::new();
                                    crate::events::apply_scroll_offsets(
                                        &mut layout,
                                        &scroll_offsets_hmr,
                                        &mut path,
                                    );
                                }
                                last_layout_hmr = Some(layout.clone());
                                recompute_targets(
                                    &vnode,
                                    &layout,
                                    &mut click_targets,
                                    &mut hover_targets,
                                    &mut input_targets,
                                );
                            }
                            handled_click = true;
                        }
                        if let Some(w) = window_opt.as_ref() {
                            if handled_click {
                                w.set_title(&get_title());
                            }
                            // HOISTED OUT of the `hit_test_click` arm, for the
                            // same reason as the plain loop: this arm had the
                            // identical nesting bug, so a click on empty space
                            // changed focus and never repainted.
                            w.request_redraw();
                        }
                    }
                    Event::WindowEvent {
                        event: WindowEvent::ModifiersChanged(mods),
                        ..
                    } => {
                        window_modifiers_changed(&mut shift_held, mods.shift());
                    }
                    Event::WindowEvent {
                        event: WindowEvent::KeyboardInput { input, .. },
                        ..
                    } => {
                        // Keyboard shortcuts and text-input editing keys.
                        use winit::event::VirtualKeyCode;
                        if let Some(keycode) = input.virtual_keycode
                            && input.state == ElementState::Pressed
                        {
                            // `@keydown` bindings, identical to the plain
                            // loop's — including the additive contract, which
                            // is what stops an HMR session from losing F2-focus
                            // the moment a rebuild is watched.
                            if crate::dispatch_keydown(
                                keycode,
                                &mut input_targets,
                                &last_vnode,
                                &mut focused_input,
                                &mut on_event,
                            ) {
                                crate::arm_blink_deadline(&mut blink_deadline);
                                velox_core::lifecycle::run_all_updated_hooks();
                                if let Some(w) = window_opt.as_ref() {
                                    w.request_redraw();
                                }
                            }
                            match keycode {
                                // Reload / quit. Identical to the plain loop's arm
                                // including the focus guard — an unguarded shortcut
                                // here would mean typing "r" into a field exits the
                                // process under HMR only, which is the kind of drift
                                // that survives review because it is "the other loop".
                                VirtualKeyCode::R | VirtualKeyCode::Q
                                    if !crate::events::any_input_focused(&input_targets) =>
                                {
                                    velox_core::lifecycle::run_all_destroy_hooks();
                                    *control_flow = ControlFlow::Exit;
                                }
                                // Ported from the plain loop. This arm used to be
                                // a bare `_ => {}` stub with no `focused_input` in
                                // scope at all, so typing did nothing under HMR.
                                _ => {
                                    if let Some(action) =
                                        crate::edit_action_for_key(keycode, shift_held)
                                    {
                                        let changed = crate::apply_edit_to_focused(
                                            &mut input_targets,
                                            action,
                                            &last_vnode,
                                            &mut focused_input,
                                            &mut on_event,
                                        );
                                        if changed {
                                            // Caret goes solid while you type.
                                            crate::arm_blink_deadline(&mut blink_deadline);
                                            velox_core::lifecycle::run_all_updated_hooks();
                                            if let Some(w) = window_opt.as_ref() {
                                                w.request_redraw();
                                            }
                                        }
                                    }
                                }
                            }
                        }
                    }
                    Event::WindowEvent {
                        event: WindowEvent::ReceivedCharacter(c),
                        ..
                    } => {
                        // Printable characters go to the focused text input.
                        // Ported from the plain loop; there was no
                        // `ReceivedCharacter` arm anywhere in this loop, so
                        // typing inserted nothing under HMR.
                        if !c.is_control()
                            && c != '\u{7f}'
                            && crate::apply_edit_to_focused(
                                &mut input_targets,
                                crate::events::EditAction::Insert(c),
                                &last_vnode,
                                &mut focused_input,
                                &mut on_event,
                            )
                        {
                            // Caret goes solid while you type.
                            crate::arm_blink_deadline(&mut blink_deadline);
                            velox_core::lifecycle::run_all_updated_hooks();
                            if let Some(w) = window_opt.as_ref() {
                                w.request_redraw();
                            }
                        }
                    }
                    Event::WindowEvent {
                        event: WindowEvent::MouseWheel { delta, .. },
                        ..
                    } => {
                        window_mouse_wheel(
                            scale_factor,
                            mouse_pos,
                            &last_layout_hmr,
                            &mut scroll_offsets_hmr,
                            &window_opt,
                            delta,
                        );
                    }
                    Event::RedrawRequested(_) => {
                        // R-L3: materialize any coalesced resize exactly once per frame.
                        let mut committed_resize = false;
                        if let Some((pw, ph)) = resize_state.take_pending() {
                            // `ResizeState::take_pending` is the production `pending_resize.take()` path.
                            match renderer.resize(pw as i32, ph as i32) {
                                Ok(()) => committed_resize = true,
                                Err(e) => {
                                    log::warn!("renderer resize failed ({}x{}): {}", pw, ph, e);
                                }
                            }
                            if let Some(presenter) = presenter.as_mut()
                                && let Err(e) = presenter.resize(pw, ph)
                            {
                                log::warn!("presenter resize failed: {}", e);
                            }
                        }
                        ensure_mounted(&mut did_mount_hmr);
                        // Render VNode -> Skia frame and present.
                        if let Some(s) = &mut renderer.surface {
                            s.set_scale_factor(scale_factor);
                            let (vw, vh) = resize_state.frame_logical_size(
                                s.width,
                                s.height,
                                scale_factor,
                                committed_resize,
                            );
                            let (vnode_raw, sheet) = make_view(vw, vh);
                            let mut next_id = 1u32;
                            let vnode_tagged = with_hover_ids(&vnode_raw, &mut next_id);
                            let vnode = crate::style_vnode_with_hover(
                                &vnode_tagged,
                                &sheet,
                                &|_tag, props| {
                                    props
                                        .attrs
                                        .get("data-hover-id")
                                        .and_then(|v| v.parse::<u32>().ok())
                                        .map(|id| Some(id) == hovered_id)
                                        .unwrap_or(false)
                                },
                            );
                            // Caret/selection/focus contract for the paint lane.
                            let vnode = crate::inject_input_caret_attrs(
                                &vnode,
                                crate::events::focused_input_index(&input_targets)
                                    .and_then(|i| input_targets.get(i)),
                            );
                            last_vnode = Some(vnode.clone());
                            let mut layout =
                                velox_dom::layout::compute_layout(&vnode, vw as i32, vh as i32);
                            {
                                let mut path = Vec::new();
                                crate::events::apply_scroll_offsets(
                                    &mut layout,
                                    &scroll_offsets_hmr,
                                    &mut path,
                                );
                            }
                            last_layout_hmr = Some(layout.clone());
                            recompute_targets(
                                &vnode,
                                &layout,
                                &mut click_targets,
                                &mut hover_targets,
                                &mut input_targets,
                            );
                            if let Err(e) = crate::skia_render::skia_impl::render_frame(
                                s, &vnode, &layout, &sheet,
                            ) {
                                log::error!("skia render error: {}", e);
                            }
                            if let Some(presenter) = presenter.as_mut()
                                && let Err(e) = presenter.present(s)
                            {
                                log::error!("softbuffer present error: {}", e);
                            }
                        }
                    }
                    _ => {}
                }
            });
        }));
    } else {
        // Headless mode — no event loop, just run the initial render and return.
        log::info!("running in headless mode (no event loop)");
        velox_core::lifecycle::run_all_destroy_hooks();
    }
    Ok(())
}

#[cfg(test)]
mod resize_state_tests {
    use super::ResizeState;
    use std::cell::RefCell;
    use std::rc::Rc;
    use std::sync::{Mutex, MutexGuard, OnceLock};
    use velox_core::lifecycle::{
        cleanup_component, clear_current_component, generate_component_id, on_resize,
        set_current_component,
    };
    use velox_core::signal::Signal;

    fn resize_test_guard() -> MutexGuard<'static, ()> {
        static LOCK: OnceLock<Mutex<()>> = OnceLock::new();
        LOCK.get_or_init(|| Mutex::new(()))
            .lock()
            .unwrap_or_else(|poisoned| poisoned.into_inner())
    }

    fn register_recorder(calls: Rc<RefCell<Vec<(u32, u32)>>>) -> usize {
        let id = generate_component_id();
        set_current_component(id);
        on_resize(move |width, height| calls.borrow_mut().push((width, height)));
        clear_current_component();
        id
    }

    #[test]
    fn renderer_resize_state_coalesces_raw_events_and_dispatches_once() {
        let _guard = resize_test_guard();
        let calls = Rc::new(RefCell::new(Vec::new()));
        let id = register_recorder(calls.clone());
        let mut state = ResizeState::new();

        assert_eq!(state.record_initial(640, 480, 1.0), (640, 480));
        state.queue((800, 600));
        state.queue((900, 700));
        // Raw events only update the pending size; they do not dispatch hooks.
        assert!(calls.borrow().is_empty());
        assert_eq!(state.take_pending(), Some((900, 700)));
        assert_eq!(state.frame_logical_size(900, 700, 1.0, true), (900, 700));
        assert_eq!(state.take_pending(), None);
        assert_eq!(state.frame_logical_size(900, 700, 1.0, true), (900, 700));

        assert_eq!(&*calls.borrow(), &[(900, 700)]);
        cleanup_component(id);
    }

    #[test]
    fn renderer_resize_state_initial_logical_size_suppresses_notification() {
        let _guard = resize_test_guard();
        let calls = Rc::new(RefCell::new(Vec::new()));
        let id = register_recorder(calls.clone());
        let mut state = ResizeState::new();

        assert_eq!(state.record_initial(800, 600, 1.0), (800, 600));
        assert_eq!(state.frame_logical_size(800, 600, 1.0, true), (800, 600));
        assert!(calls.borrow().is_empty());
        cleanup_component(id);
    }

    #[test]
    fn renderer_resize_state_uses_viewport_logical_pixels_for_dpi() {
        let _guard = resize_test_guard();
        let calls = Rc::new(RefCell::new(Vec::new()));
        let id = register_recorder(calls.clone());
        let mut state = ResizeState::new();

        assert_eq!(state.record_initial(1600, 1200, 2.0), (800, 600));
        state.queue((2000, 1000));
        assert_eq!(state.take_pending(), Some((2000, 1000)));
        assert_eq!(state.frame_logical_size(2000, 1000, 2.0, true), (1000, 500));
        assert_eq!(&*calls.borrow(), &[(1000, 500)]);
        cleanup_component(id);
    }

    #[test]
    fn two_windows_with_equal_committed_sizes_both_dispatch() {
        let _guard = resize_test_guard();
        let calls = Rc::new(RefCell::new(Vec::new()));
        let id = register_recorder(calls.clone());
        let mut first_window = ResizeState::new();
        let mut second_window = ResizeState::new();

        // Each window owns its baseline; both legitimately commit 900x600.
        first_window.record_initial(800, 600, 1.0);
        second_window.record_initial(700, 600, 1.0);
        first_window.queue((900, 600));
        second_window.queue((900, 600));

        first_window.frame_logical_size(900, 600, 1.0, true);
        second_window.frame_logical_size(900, 600, 1.0, true);

        assert_eq!(&*calls.borrow(), &[(900, 600), (900, 600)]);
        cleanup_component(id);
    }

    #[test]
    fn renderer_resize_state_runs_all_hooks_and_allows_signal_mutation() {
        let _guard = resize_test_guard();
        let first_calls = Rc::new(RefCell::new(Vec::new()));
        let second_calls = Rc::new(RefCell::new(Vec::new()));
        let signal = Rc::new(Signal::new(0u32));
        let id = generate_component_id();

        set_current_component(id);
        {
            let calls = first_calls.clone();
            on_resize(move |width, height| calls.borrow_mut().push((width, height)));
        }
        {
            let calls = second_calls.clone();
            let signal = signal.clone();
            on_resize(move |width, height| {
                calls.borrow_mut().push((width, height));
                signal.set(width + height);
            });
        }
        clear_current_component();

        let mut state = ResizeState::new();
        state.record_initial(100, 100, 1.0);
        state.queue((200, 150));
        assert_eq!(state.take_pending(), Some((200, 150)));
        assert_eq!(state.frame_logical_size(200, 150, 1.0, true), (200, 150));

        assert_eq!(&*first_calls.borrow(), &[(200, 150)]);
        assert_eq!(&*second_calls.borrow(), &[(200, 150)]);
        assert_eq!(signal.get(), 350);
        cleanup_component(id);
    }

    #[test]
    fn cleanup_component_removes_only_its_resize_hook() {
        let _guard = resize_test_guard();
        let removed_calls = Rc::new(RefCell::new(Vec::new()));
        let retained_calls = Rc::new(RefCell::new(Vec::new()));
        let removed_id = register_recorder(removed_calls.clone());
        let retained_id = register_recorder(retained_calls.clone());

        cleanup_component(removed_id);
        let mut state = ResizeState::new();
        state.record_initial(100, 100, 1.0);
        state.frame_logical_size(200, 150, 1.0, true);

        assert!(removed_calls.borrow().is_empty());
        assert_eq!(&*retained_calls.borrow(), &[(200, 150)]);
        cleanup_component(retained_id);
    }
}

// ---------------------------------------------------------------------------
// Task 5.6a — characterisation tests for the text-input edit and focus path.
//
// `apply_click_focus`, `apply_edit_to_focused` and `edit_action_for_key` are the
// three functions both copies of the event loop call into, and until this
// module landed none of them had any test coverage at all. A dedup that dropped
// or re-pointed an arm would have shipped silently, which is the failure mode
// these tests exist to prevent.
//
// They live here rather than in `tests/` because all three are *private* `fn`s
// behind `#[cfg(feature = "skia-native")]`; an integration test can only reach
// the crate's public API.
#[cfg(all(test, feature = "skia-native"))]
mod edit_focus_tests {
    use super::{
        apply_click_focus, apply_edit_to_focused, edit_action_for_key, inject_input_caret_attrs,
        recompute_targets, style_vnode_with_hover,
    };
    use crate::events::{EditAction, InputTarget};
    use velox_dom::{Props, VNode};
    use winit::event::VirtualKeyCode as K;

    const W: i32 = 400;
    const H: i32 = 300;
    /// Absolutely positioned so the field's rect is known exactly: the click
    /// tests below use literal coordinates and must not depend on layout order.
    const FIELD_STYLE: &str = "position:absolute;left:20px;top:20px;width:200px;height:40px;font-size:15px;padding-left:2px";
    const VALUE: &str = "hello";
    const HANDLER: &str = "set_value";
    /// The field spans x 20..220, y 20..60; its text starts at x 20 + 2px padding.
    const TEXT_X: f32 = 22.0;
    const TEXT_Y: f32 = 40.0;
    const EMPTY_X: f32 = 350.0;
    const EMPTY_Y: f32 = 250.0;

    /// `(handler, value)` pairs handed to the app, in dispatch order.
    type Dispatches = Vec<(String, Option<String>)>;

    /// One text input on a page, built through the frame order the loops use:
    /// style cascade, caret-attr injection, layout, then target collection.
    /// Going through the real pipeline is the point — a hand-assembled target
    /// vector would not exercise the `path`/`rect` plumbing these functions read.
    struct Scene {
        /// `None` models the window before the first `RedrawRequested`, when the
        /// loop has painted a frame but has not published it as `last_vnode`.
        vnode: Option<VNode>,
        inputs: Vec<InputTarget>,
        /// The loops' index-free mirror of "which input is focused". It is
        /// asserted on below, because after Esc it has to follow the blur
        /// rather than keep naming a field that is no longer focused.
        focused: Option<Vec<usize>>,
    }

    fn scene(attrs: &[(&str, &str)]) -> Scene {
        let mut p = Props::new()
            .set("type", "text")
            .set("style", FIELD_STYLE)
            .set("value", VALUE)
            .set("on:input", HANDLER);
        for (k, v) in attrs.iter().copied() {
            p = p.set(k, v);
        }
        let raw = VNode::Element {
            tag: "div".into(),
            props: Props::new().set("style", "background:#000000"),
            children: vec![VNode::Element {
                tag: "input".into(),
                props: p,
                children: vec![],
            }],
        };
        let sheet = velox_style::Stylesheet::default();
        let styled = style_vnode_with_hover(&raw, &sheet, &|_tag: &str, _p: &Props| false);
        let vnode = inject_input_caret_attrs(&styled, None);
        let layout = velox_dom::layout::compute_layout(&vnode, W, H);
        let mut clicks: Vec<crate::events::ClickTarget> = Vec::new();
        let mut hovers: Vec<crate::events::HoverTarget> = Vec::new();
        let mut inputs: Vec<InputTarget> = Vec::new();
        recompute_targets(&vnode, &layout, &mut clicks, &mut hovers, &mut inputs);
        assert_eq!(inputs.len(), 1, "fixture must yield exactly one text input");
        Scene {
            vnode: Some(vnode),
            inputs,
            focused: None,
        }
    }

    impl Scene {
        /// Focus the field with the caret `cursor` chars in.
        fn focused_at(mut self, cursor: usize) -> Self {
            self.inputs[0].focused = true;
            self.inputs[0].cursor = cursor;
            self
        }
    }

    /// Run one edit, returning whether the caller must repaint and everything
    /// dispatched to the app. Asserting on the dispatch log, not just the
    /// caret, is the point: a caret that moves while the handler is never
    /// called is precisely the silent-typing failure this suite must not miss.
    fn run_edit(s: &mut Scene, action: EditAction) -> (bool, Dispatches) {
        let mut log: Dispatches = Vec::new();
        let repaint = {
            let mut sink = |name: &str, data: Option<&str>| {
                log.push((name.to_string(), data.map(str::to_string)))
            };
            apply_edit_to_focused(&mut s.inputs, action, &s.vnode, &mut s.focused, &mut sink)
        };
        (repaint, log)
    }

    /// Click at (x, y), returning whether the caller must repaint. `focused` is
    /// the loop's own mirror of the focus flag, kept in `Scene` so the blur tests
    /// can assert it alongside the flag itself.
    fn run_click(s: &mut Scene, x: f32, y: f32) -> bool {
        // The viewport the loops pass is the live surface's logical size; this
        // module has no surface, so it passes the page it laid out against
        // (`W` x `H`) at scale 1. Only viewport-unit lengths (`vw`/`vh`) can
        // tell the two apart, and none of the fixtures below declare one.
        apply_click_focus(
            &mut s.inputs,
            &s.vnode,
            &mut s.focused,
            x,
            y,
            1.0,
            (W as f32, H as f32),
        )
    }

    // -- apply_edit_to_focused ------------------------------------------------

    #[test]
    fn insert_at_caret_dispatches_new_value_and_advances_caret() {
        let mut s = scene(&[]).focused_at(5);
        let (repaint, log) = run_edit(&mut s, EditAction::Insert('X'));
        assert!(repaint, "an inserted char must repaint");
        assert_eq!(log, vec![(HANDLER.to_string(), Some("helloX".into()))]);
        assert_eq!(s.inputs[0].cursor, 6);
    }

    #[test]
    fn insert_lands_at_the_caret_not_at_the_end() {
        let mut s = scene(&[]).focused_at(0);
        let (repaint, log) = run_edit(&mut s, EditAction::Insert('X'));
        assert!(repaint);
        assert_eq!(log, vec![(HANDLER.to_string(), Some("Xhello".into()))]);
        assert_eq!(s.inputs[0].cursor, 1);
    }

    #[test]
    fn backspace_removes_the_char_before_the_caret() {
        let mut s = scene(&[]).focused_at(5);
        let (repaint, log) = run_edit(&mut s, EditAction::Backspace);
        assert!(repaint);
        assert_eq!(log, vec![(HANDLER.to_string(), Some("hell".into()))]);
        assert_eq!(s.inputs[0].cursor, 4);
    }

    #[test]
    fn submit_dispatches_the_unchanged_value() {
        let mut s = scene(&[]).focused_at(5);
        let (repaint, log) = run_edit(&mut s, EditAction::Submit);
        assert!(
            repaint,
            "submit must repaint even though the text is unchanged"
        );
        assert_eq!(
            log,
            vec![(HANDLER.to_string(), Some("hello".into()))],
            "a submit with no selection must still reach the handler"
        );
        assert_eq!(s.inputs[0].cursor, 5);
    }

    #[test]
    fn caret_move_repaints_without_announcing_a_text_change() {
        let mut s = scene(&[]).focused_at(3);
        let (repaint, log) = run_edit(&mut s, EditAction::MoveLeft { shift: false });
        assert!(repaint, "a caret move is a visual change and must repaint");
        assert!(
            log.is_empty(),
            "a caret move must not dispatch a value change"
        );
        assert_eq!(s.inputs[0].cursor, 2);
    }

    #[test]
    fn shift_caret_move_anchors_a_selection() {
        let mut s = scene(&[]).focused_at(3);
        let (repaint, log) = run_edit(&mut s, EditAction::MoveLeft { shift: true });
        assert!(repaint);
        assert!(log.is_empty());
        assert_eq!(s.inputs[0].cursor, 2);
        assert_eq!(
            s.inputs[0].anchor,
            Some(3),
            "a shift-move must anchor at the caret's old position"
        );
    }

    #[test]
    fn no_focused_input_is_a_no_op() {
        let mut s = scene(&[]);
        assert!(!s.inputs[0].focused);
        let (repaint, log) = run_edit(&mut s, EditAction::Insert('X'));
        assert!(!repaint, "an edit with nothing focused must not repaint");
        assert!(log.is_empty());
    }

    #[test]
    fn backspace_at_caret_zero_changes_nothing() {
        let mut s = scene(&[]).focused_at(0);
        let (repaint, log) = run_edit(&mut s, EditAction::Backspace);
        assert!(!repaint, "backspace at index 0 is a no-op, not a repaint");
        assert!(log.is_empty(), "backspace at index 0 must not dispatch");
        assert_eq!(s.inputs[0].cursor, 0);
    }

    /// The target holds no value of its own — the tree does. This is the coupling
    /// that makes an unpublished frame (below) a correctness problem rather than
    /// a cosmetic one.
    #[test]
    fn dispatch_uses_the_value_in_the_tree_not_the_target() {
        let mut s = scene(&[("value", "world")]).focused_at(5);
        let (repaint, log) = run_edit(&mut s, EditAction::Backspace);
        assert!(repaint);
        assert_eq!(log, vec![(HANDLER.to_string(), Some("worl".into()))]);
    }

    // -- edit_action_for_key --------------------------------------------------

    /// Every key the two loops route to the editor. This is the mapping a dedup
    /// could silently transpose, and `Back`/`Delete`/`Return` are exactly the
    /// keys whose mis-wiring stays invisible until someone types.
    #[test]
    fn editing_keys_map_to_their_actions() {
        use EditAction::*;
        let cases: &[(K, bool, EditAction)] = &[
            (K::Left, false, MoveLeft { shift: false }),
            (K::Left, true, MoveLeft { shift: true }),
            (K::Right, false, MoveRight { shift: false }),
            (K::Right, true, MoveRight { shift: true }),
            (K::Home, false, Home { shift: false }),
            (K::Home, true, Home { shift: true }),
            (K::End, false, End { shift: false }),
            (K::End, true, End { shift: true }),
            (K::Back, false, Backspace),
            (K::Delete, false, Delete),
            (K::Return, false, Submit),
            // Esc is an editing key even though it changes no text: it is the
            // only way out of a field, and the match has no fallthrough to user
            // code, so anything absent here simply cannot happen.
            (K::Escape, false, Blur),
        ];
        for (key, shift, want) in cases {
            assert_eq!(
                edit_action_for_key(*key, *shift),
                Some(*want),
                "{key:?} with shift={shift}"
            );
        }
    }

    /// A printable key must map to `None`: characters reach the editor through
    /// `ReceivedCharacter`, so claiming one here would insert it twice.
    ///
    /// `Escape` is deliberately *not* in this list. It used to be, and that
    /// assertion was what pinned the bug: with Esc mapping to `None` and no
    /// other code handling it, the key was inert and a focused field could not
    /// be left. Esc maps to [`EditAction::Blur`] and the blur behaviour is
    /// asserted below.
    #[test]
    fn non_editing_keys_produce_no_action() {
        for key in [K::A, K::R, K::Key1, K::Space, K::Tab, K::Up, K::Down] {
            assert_eq!(
                edit_action_for_key(key, false),
                None,
                "{key:?} is not an editing key"
            );
        }
    }

    // -- Esc unfocuses --------------------------------------------------------

    /// Esc on a focused field drops focus, the selection and the caret, and
    /// leaves the text alone.
    ///
    /// The caret goes because Esc is an explicit "I am done here" rather than a
    /// click that wandered off — that is the difference from the empty-space
    /// blur above, which deliberately keeps the caret. The value must not
    /// change: Esc is not an edit, so the app must not be woken with an
    /// `on:input` it did not cause.
    #[test]
    fn escape_on_a_focused_field_blurs_it_and_drops_its_caret_and_selection() {
        let mut s = scene(&[]).focused_at(4);
        s.inputs[0].anchor = Some(1);
        assert_eq!(s.inputs[0].selection(), Some((1, 4)));

        let (repaint, log) = run_edit(&mut s, EditAction::Blur);

        assert!(
            repaint,
            "the focus ring and highlight disappear, so it repaints"
        );
        assert!(!s.inputs[0].focused, "Esc must leave the field");
        assert_eq!(
            s.inputs[0].anchor, None,
            "a selection cannot outlive the focus"
        );
        assert_eq!(s.inputs[0].cursor, 0, "Esc clears the caret");
        assert!(
            s.inputs[0].blink_on,
            "the caret comes back solid, not blinking"
        );
        assert!(
            log.is_empty(),
            "Esc changes no text, so nothing is dispatched"
        );
        assert_eq!(
            s.focused, None,
            "the loop's mirror of the focus must follow the blur, or it names a \
             field that is no longer focused"
        );
    }

    /// Esc with nothing focused is free — no repaint, no dispatch.
    ///
    /// Same contract as every other no-op keypress: the loops branch on
    /// `needs_repaint()`, and a wake-up here would repaint for nothing.
    #[test]
    fn escape_with_nothing_focused_is_a_no_op() {
        let mut s = scene(&[]);
        assert!(!s.inputs[0].focused);
        let (repaint, log) = run_edit(&mut s, EditAction::Blur);
        assert!(
            !repaint,
            "there was no focus to drop, so there is nothing to redraw"
        );
        assert!(log.is_empty());
        assert_eq!(s.focused, None);
    }

    /// Esc reaches the editor at all only because the key map claims it, so pin
    /// the mapping through the same call the loops make.
    #[test]
    fn escape_is_routed_to_the_blur_action() {
        assert_eq!(
            edit_action_for_key(K::Escape, false),
            Some(EditAction::Blur)
        );
        assert_eq!(
            edit_action_for_key(K::Escape, true),
            Some(EditAction::Blur),
            "Shift does not change what Esc means"
        );
    }

    // -- apply_click_focus ----------------------------------------------------

    #[test]
    fn focus_gain_puts_the_caret_at_the_end_of_the_value() {
        let mut s = scene(&[]);
        assert_eq!(s.inputs[0].cursor, 0);
        assert!(run_click(&mut s, TEXT_X + 1.0, TEXT_Y));
        assert!(s.inputs[0].focused);
        assert_eq!(
            s.inputs[0].cursor, 5,
            "focus gain must put the caret at the end, so typing appends"
        );
    }

    #[test]
    fn click_into_a_focused_field_repositions_the_caret() {
        let mut s = scene(&[]).focused_at(5);
        assert!(run_click(&mut s, TEXT_X + 1.0, TEXT_Y));
        assert!(s.inputs[0].focused);
        assert_eq!(
            s.inputs[0].cursor, 0,
            "clicking the first glyph must move the caret there, not leave it at the end"
        );
    }

    /// The regression this lane had: the origin was computed from a lookup of
    /// the literal key `padding-left`, which the cascade never writes for an
    /// authored shorthand. `padding: 6px 30px` therefore resolved to
    /// `rect.x + 0` while the painter drew the first glyph at `rect.x + 30`, so
    /// every caret sat 30px left of the glyph it claimed to be on. With the
    /// field focused, a click on the *first* glyph was measured 30px further
    /// right than it is and landed past the end of the value.
    #[test]
    fn shorthand_padding_places_the_caret_on_the_painted_glyph() {
        const SHORTHAND_STYLE: &str = "position:absolute;left:20px;top:20px;width:200px;height:40px;font-size:15px;padding:6px 30px";
        let mut s = scene(&[("style", SHORTHAND_STYLE)]).focused_at(5);
        let rect = s.inputs[0].rect;
        assert_eq!(rect.x, 20, "fixture must stay absolutely positioned");
        let text_left = rect.x as f32 + 30.0;

        assert!(run_click(&mut s, text_left + 1.0, TEXT_Y));
        assert_eq!(
            s.inputs[0].cursor, 0,
            "a click on the first glyph must put the caret before it, whatever \
             syntax the author used for the padding"
        );
    }

    /// The origin is not merely "shifted": it must equal the painter's
    /// `text_left` for every spelling of the same declaration, because both
    /// lanes now read the same computed value out of
    /// `input_metrics::input_text_metrics`.
    #[test]
    fn text_origin_agrees_with_the_paint_authority_for_every_padding_spelling() {
        use velox_dom::Props;
        let rect = velox_dom::layout::Rect {
            x: 20,
            y: 20,
            w: 200,
            h: 40,
        };
        let viewport = (W as f32, H as f32);
        let authored = [
            "padding: 6px 30px",
            "padding-left: 30px",
            "padding: 6px 30px; padding-left: 30px",
            "padding-left: 30px; padding: 6px 30px",
            "padding: 30px 6px 6px 30px",
        ];
        for decl in authored {
            let props = Props::new().set("style", decl);
            assert_eq!(
                super::resolve_text_metrics(&props, rect, viewport).text_left,
                50.0,
                "`{decl}` must resolve the same first-glyph x as the paint lane"
            );
            // …and the authority itself, not a local copy of it.
            let authority = crate::input_metrics::input_text_metrics(
                props.attrs.get("style").map(|s| s.as_str()),
                rect,
                viewport,
                velox_dom::layout::DEFAULT_ROOT_FONT_SIZE,
            );
            assert_eq!(
                super::resolve_text_metrics(&props, rect, viewport).text_left,
                authority.text_left,
                "`{decl}`: the hit-test lane must not re-derive what paint computed"
            );
        }
        // A viewport-unit left padding must resolve against the viewport the
        // caller threaded in — the same one paint resolves `vw` against.
        let props = Props::new().set("style", "padding-left: 5vw");
        assert_eq!(
            super::resolve_text_metrics(&props, rect, viewport).text_left,
            rect.x as f32 + 0.05 * W as f32
        );
    }

    /// The caret index a click at `click_x` leaves on a field styled `style`.
    ///
    /// The field is pre-focused on purpose: `apply_click_focus` sends the caret
    /// to the END of the value on focus gain, so a never-focused fixture would
    /// report "end of value" for every click x and any comparison built on it
    /// would pass without ever running the code under test.
    ///
    /// Overriding `style` and `value` on top of `FIELD_STYLE` matters for the
    /// same reason: `FIELD_STYLE` pins `font-size:15px`, and every fixture that
    /// inherits it silently opts out of the undeclared-font-size case these
    /// tests exist to cover.
    fn caret_from_click(style: &str, value: &str, click_x: f32) -> usize {
        let mut s = scene(&[("style", style), ("value", value)]).focused_at(0);
        assert!(
            run_click(&mut s, click_x, TEXT_Y),
            "the click at x={click_x} must land on the field, not on empty space"
        );
        s.inputs[0].cursor
    }

    /// Assert that two styles describing visually identical fields answer
    /// identically at every click x across the field, and that the answers are
    /// non-degenerate.
    ///
    /// `equal_windows` sweeps the whole field rather than probing one x on
    /// purpose. A single probe can land on a char boundary where two disagreeing
    /// advances happen to round to the same index, and the test would pass with
    /// the defect still in place; sweeping makes that unfalsifiable at one lucky
    /// coordinate.
    fn assert_carets_agree_across_field(equal: (&str, &str), value: &str) {
        let (a, b) = equal;
        // Collect the WHOLE profile before asserting, so a failure reports how
        // far apart the two lanes end up rather than only the first coordinate
        // where they parted.
        let mut profile: Vec<(f32, usize, usize)> = Vec::new();
        let mut indices: Vec<usize> = Vec::new();
        let mut x = 20.0;
        while x <= 219.0 {
            let ca = caret_from_click(a, value, x);
            let cb = caret_from_click(b, value, x);
            if ca != cb {
                profile.push((x, ca, cb));
            }
            indices.push(ca);
            x += 1.0;
        }
        let worst = profile
            .iter()
            .map(|&(_, u, v)| (u as i64 - v as i64).abs())
            .max()
            .unwrap_or(0);
        assert!(
            profile.is_empty(),
            "`{a}` and `{b}` paint identically, so the caret must land on the same \
             glyph at every click x. They disagree at {} of 200 coordinates, by up \
             to {worst} character(s); first divergence: {:?}",
            profile.len(),
            profile.first()
        );
        // Anti-vacuity: a lane that ignored the click entirely, or that always
        // answered "end of value", would satisfy the equality above trivially.
        let len = value.chars().count();
        assert!(
            indices.iter().any(|&i| i > 0 && i < len),
            "every click resolved to an end-of-value caret, so this compared \
             nothing: {indices:?}"
        );
    }

    /// The click-to-caret lane must measure advances at the size the PAINT lane
    /// draws them at, and must not hold a second answer to "how big is this
    /// field".
    ///
    /// The proof is differential and needs no font metric, no geometry constant
    /// and no knowledge of which face is bundled: an input that declares no
    /// `font-size` paints at the painter's root size — that is what "declares
    /// none" means — which is by definition the same size an input declaring
    /// that size *explicitly* paints at. The two fixtures are therefore
    /// identical on screen, so a click at the same x must leave the caret on
    /// the same glyph, whatever each lane measured with.
    ///
    /// This is exactly the shipped-boilerplate-shaped hole: the UA sheet
    /// declares no `font-size` for `input`, so a field that never says it
    /// inherits 14px from the painter's root `TextStyle` while the hit-test lane
    /// fell back to its own 16px. Every advance came out `16/14` wide, so a
    /// click at the far end of the value landed the caret roughly one character
    /// short for every seven clicked.
    #[test]
    fn the_caret_lane_measures_at_the_size_an_undeclared_font_size_paints_at() {
        // Wide glyphs, and enough of them that a 14%-wide advance error cannot
        // hide inside one character's slack over the 200px field.
        assert_carets_agree_across_field(
            (
                "position:absolute;left:20px;top:20px;width:200px;height:40px",
                "position:absolute;left:20px;top:20px;width:200px;height:40px;font-size:14px",
            ),
            &"M".repeat(20),
        );
    }

    /// The origin half of the same defect, which survived after the origin was
    /// first routed through the shared authority: an `em` padding resolves
    /// against the ELEMENT's font size, so an element that inherits its size has
    /// to resolve `4em` against the inherited size in *both* lanes. The hit-test
    /// lane passed its own 16px where paint inherits 14px, so the caret sat a
    /// whole `4em` — 16px — to the right of the glyph it claimed to be on.
    #[test]
    fn em_padding_resolves_against_the_same_inherited_size_in_both_lanes() {
        // Vertical padding stays 0 so the only thing under test is the
        // horizontal origin, and the field's box cannot move out from under the
        // literal click coordinates the sweep uses.
        assert_carets_agree_across_field(
            (
                "position:absolute;left:20px;top:20px;width:200px;height:40px;padding:0 4em",
                "position:absolute;left:20px;top:20px;width:200px;height:40px;padding:0 4em;font-size:14px",
            ),
            &"M".repeat(20),
        );
    }

    /// A relative `font-size` is resolved against the inherited size by the
    /// cascade, so `2em` on a field that inherits 14px paints at 28px — and
    /// nothing about that declaration contains a `px` suffix for a lane-local
    /// literal parser to find.
    #[test]
    fn a_relative_font_size_reaches_the_caret_lane_at_its_resolved_size() {
        assert_carets_agree_across_field(
            (
                "position:absolute;left:20px;top:20px;width:200px;height:40px;font-size:2em",
                "position:absolute;left:20px;top:20px;width:200px;height:40px;font-size:28px",
            ),
            &"M".repeat(20),
        );
    }

    #[test]
    fn clicking_empty_space_blurs_clears_the_selection_and_keeps_the_caret() {
        let mut s = scene(&[]).focused_at(5);
        s.inputs[0].anchor = Some(1);
        assert!(
            run_click(&mut s, EMPTY_X, EMPTY_Y),
            "a blur is a visual change"
        );
        assert!(!s.inputs[0].focused);
        assert_eq!(
            s.inputs[0].anchor, None,
            "a stale highlight must not survive a blur"
        );
        assert_eq!(
            s.inputs[0].cursor, 5,
            "blur keeps the caret, so re-entering the field restores position"
        );
    }

    #[test]
    fn clicking_empty_space_with_nothing_focused_changes_nothing() {
        let mut s = scene(&[]);
        assert!(
            !run_click(&mut s, EMPTY_X, EMPTY_Y),
            "a click on empty space with nothing focused is not a change"
        );
    }

    // -- the `last_vnode` window ----------------------------------------------
    //
    // Both loops declare `last_vnode` as `None` and only assign it on
    // `RedrawRequested`, so before the loop's first redraw it is empty. The HMR
    // loop additionally publishes its pre-loop frame; these tests pin what the
    // plain loop must do to match, and what happens if it does not.

    /// With no published frame there is nothing to measure or to read a value
    /// from, so focus gain leaves the caret at 0 — which turns "click a field
    /// and type" into a *prepend*.
    #[test]
    fn focus_gain_with_no_published_frame_puts_the_caret_at_zero() {
        let mut s = scene(&[]);
        s.vnode = None;
        assert!(run_click(&mut s, TEXT_X + 1.0, TEXT_Y));
        assert!(s.inputs[0].focused);
        assert_eq!(
            s.inputs[0].cursor, 0,
            "with no frame to measure, focus gain leaves the caret at 0 and typing prepends"
        );
    }

    /// The other half of the same window: the edit mutates the caret, reports
    /// "repaint", and reaches the app not at all. The keystroke is lost and the
    /// caret is left corrupted — the value reads as `""`, so the caret is
    /// clamped to 0 before the insert and lands at 1 rather than 6.
    #[test]
    fn insert_with_no_published_frame_is_silently_dropped() {
        let mut s = scene(&[]).focused_at(5);
        s.vnode = None;
        let (repaint, log) = run_edit(&mut s, EditAction::Insert('X'));
        assert!(
            repaint,
            "the edit still reports a repaint, so the loop schedules a frame for nothing"
        );
        assert!(
            log.is_empty(),
            "no dispatch at all: the app never sees the keystroke, yet the caret moved"
        );
        assert_eq!(
            s.inputs[0].cursor, 1,
            "the caret is clamped to an empty value and corrupted, not advanced to 6"
        );
    }

    /// `last_vnode` is what the two functions above read, so both loops must
    /// publish the frame they render *before* they start dispatching input, not
    /// only once the first `RedrawRequested` arrives. Structural — the loops need
    /// a live window — but it is the only thing that binds the requirement to the
    /// loops themselves rather than to one call site.
    #[test]
    fn both_loops_publish_their_pre_loop_frame_before_they_start() {
        let src = include_str!("lib.rs");
        // Assembled from two halves on purpose: `include_str!` reads this test
        // module too, and a needle written out in full would match its own
        // source and look like a third loop.
        let decl = format!("last_vnode: Option<{}::VNode> = None;", "velox_dom");
        let mut loops = 0usize;
        let mut rest = src;
        while let Some(at) = rest.find(decl.as_str()) {
            loops += 1;
            let body = &rest[at..];
            let run_at = body
                .find("event_loop.run(")
                .unwrap_or_else(|| panic!("loop {loops} has no `event_loop.run(`"));
            assert!(
                body[..run_at].contains("last_vnode = Some("),
                "loop {loops} must assign last_vnode in its pre-loop frame; until it does, every \
                 click and keystroke before the first redraw is handled against an empty value"
            );
            rest = &body[run_at..];
        }
        assert_eq!(loops, 2, "expected the plain loop and the HMR loop");
    }
}