quadraui 0.6.0

//! In-process headless test driver for [`AppLogic`] TUI apps.
//!
//! [`TuiDriver`] drives a *whole* [`AppLogic`] impl — the same type the
//! shipping `tui_*` examples instantiate — through the real
//! event → `handle` → `render` path, but against ratatui's in-memory
//! [`TestBackend`] instead of a live terminal. No TTY, no pty: it runs
//! under `cargo test` and is deterministic.
//!
//! ## What it tests that the primitive round-trips don't
//!
//! The per-primitive paint/click round-trips (e.g. `tui/tree.rs::tests`)
//! call a single `draw_*` fn on a hand-built struct and assert on the
//! buffer + the `hit_test` return value. They never construct an app,
//! never dispatch a [`UiEvent`] through [`AppLogic::handle`], and never
//! touch the run loop. [`TuiDriver`] tests the *example's wiring*: feed
//! a keystroke or a click, observe the re-rendered screen. It catches a
//! handler routing a click to the wrong target, a missing
//! [`Reaction::Redraw`], or stale state — none of which the primitive
//! tests can see.
//!
//! ## No drift from production
//!
//! [`Self::render`] and [`Self::dispatch`] call the same
//! [`super::run::render_frame`] / [`super::run::dispatch_event`] the live
//! runner uses, so the test path renders and pre-processes events
//! (text-selection, Ctrl-C copy) identically to `tui::run`. Mouse input
//! is a direct [`UiEvent::MouseDown`] in backend coordinates — no SGR
//! escape-sequence math, which is the ergonomic win over a pty runner
//! for click-heavy primitives.
//!
//! ## Limitations
//!
//! It renders into a `TestBackend` buffer, so it does **not** exercise
//! real ANSI/escape emission — terminal-protocol bugs (raw-mode setup,
//! escape parsing, SGR mouse decoding) are out of scope and need a
//! pty-based smoke test instead.
//!
//! ```no_run
//! # use quadraui::tui::testing::TuiDriver;
//! # use quadraui::{AppLogic, Backend, Reaction, UiEvent};
//! # fn demo<A: AppLogic + Default>() {
//! let mut driver = TuiDriver::new(A::default(), 100, 30);
//! driver.type_char('x');
//! assert!(driver.screen_contains("…"));
//! # }
//! ```

use std::cell::RefCell;
use std::rc::Rc;

use ratatui::backend::TestBackend;
use ratatui::Terminal;

/// Re-exported so a downstream test crate can name `Color`/`Modifier` (e.g.
/// `style.modifiers.contains(Modifier::ITALIC)`) without adding its own
/// `ratatui` dependency — it already depends on `quadraui` to get
/// [`TuiDriver`]. Scoped to `tui::testing` rather than the crate root to
/// avoid colliding with [`crate::types::Color`], which is re-exported there
/// already.
pub use ratatui::style::{Color, Modifier};

use crate::backend::Backend;
use crate::runner::{AppLogic, Reaction};
use crate::shell::{ShellApp, ShellConfig};
use crate::testing::driver_core::DriverCore;
use crate::testing::{
    Anchor, ConformanceDriver, DriverInput, FrameInventory, LogicalViewport, TextRun,
};
use crate::tui::backend::TuiBackend;
use crate::tui::run::{dispatch_event, render_frame, EventOutcome};
use crate::tui::text::char_cell_width;
use crate::{
    ButtonMask, Key, Modifiers, MouseButton, NamedKey, Point, Rect, ScrollDelta, UiEvent, WidgetId,
};

/// Build a [`TuiDriver`] that wraps `app` in the full
/// [`crate::shell_adapter::ShellAdapter`] stack, mirroring exactly what
/// [`crate::tui::shell_runner::run_with_shell`] does at runtime — but
/// returning a testable driver instead of entering the live event loop.
///
/// Use this constructor in tests that need to verify the full
/// `ShellApp → ShellAdapter → dispatch_event` integration path, e.g.
/// confirming that `register_text_region()` is reached via
/// `ShellAdapter::render()` and that the selection pipeline (drag → Ctrl-C)
/// works end-to-end for `run_with_shell` callers.
///
/// # Example
///
/// ```no_run
/// # use quadraui::tui::testing::driver_with_shell;
/// # use quadraui::{ShellApp, ShellConfig, Backend, ShellContext, Reaction, UiEvent};
/// # struct MyApp;
/// # impl ShellApp for MyApp {
/// #     fn render_content(&self, _: &mut dyn Backend, _: &quadraui::compose::app_shell::AppShellLayout) {}
/// #     fn handle(&mut self, _: UiEvent, _: &mut dyn Backend, _: &ShellContext) -> Reaction { Reaction::Continue }
/// # }
/// let config = ShellConfig::new("Demo", vec![]);
/// let mut driver = driver_with_shell(MyApp, config, 80, 24);
/// assert!(driver.screen_contains("Demo"));
/// ```
pub fn driver_with_shell<A: ShellApp + 'static>(
    app: A,
    config: ShellConfig,
    width: u16,
    height: u16,
) -> TuiDriver<impl AppLogic> {
    let adapter = crate::shell_adapter::build_shell_adapter(app, config);
    TuiDriver::new(adapter, width, height)
}

/// The style painted onto one rendered cell: foreground, background, and
/// text modifiers (bold/italic/underline/…).
///
/// Mirrors `ratatui::buffer::Cell::style()`'s three fields but drops the
/// `sub_modifier`/`underline_color` ratatui carries for *diffing* two
/// styles against each other — a rendered cell has no "subtracted"
/// modifiers, only the resolved `add_modifier` bits ratatui reports back as
/// [`Modifier`], which is what a driver test wants to assert against.
/// `fg`/`bg` are ratatui's own [`Color`] (re-exported as
/// [`self::Color`], see [`TuiDriver::style_at`]) rather than
/// `quadraui::types::Color` because that's what the rasterisers actually
/// paint into the buffer — comparing against a theme token means wrapping
/// the token with [`crate::tui::ratatui_color`] first.
///
/// `#[non_exhaustive]`: fields may grow (e.g. `underline_color`, dropped
/// from this first cut — see above) without that being a breaking change
/// for callers. `CellStyle` is only ever returned, never constructed, by
/// consumers, so this only forbids exhaustive destructuring, not field
/// access.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[non_exhaustive]
pub struct CellStyle {
    pub fg: Color,
    pub bg: Color,
    pub modifiers: Modifier,
}

/// Drives an [`AppLogic`] impl headlessly for tests.
///
/// Construct with [`Self::new`] (which runs `setup` + paints the first
/// frame), poke it with [`Self::press`] / [`Self::type_char`] /
/// [`Self::press_named`] / [`Self::click`], and read the rendered grid
/// back with [`Self::screen`] / [`Self::screen_contains`] / [`Self::find`],
/// or its style with [`Self::style_at`] / [`Self::styled_row`].
pub struct TuiDriver<A: AppLogic> {
    core: DriverCore<TuiBackend, A>,
    terminal: Rc<RefCell<Terminal<TestBackend>>>,
}

impl<A: AppLogic> TuiDriver<A> {
    /// Build a driver for `app` on a `width`×`height` cell grid, run the
    /// app's `setup` hook, and paint the first frame.
    pub fn new(app: A, width: u16, height: u16) -> Self {
        let terminal =
            Terminal::new(TestBackend::new(width, height)).expect("TestBackend terminal");
        let terminal = Rc::new(RefCell::new(terminal));
        let mut backend = TuiBackend::new();
        // Wire the same `Terminal` this driver renders through as the
        // nested dialog loop's paint target — see
        // `tui::services`'s module doc, "Dialogs" section. Lets a test
        // drive `PlatformServices::show_file_open_dialog` et al. through
        // a real `AppLogic` via [`Self::queue_dialog_events`], the same
        // shape `super::run::run_with` wires for the live runner.
        let dialog_surface: Rc<RefCell<dyn crate::tui::services::DialogSurface>> = terminal.clone();
        backend.tui_services().set_dialog_surface(dialog_surface);
        // Default to **scripted** dialog mode with an empty queue, not
        // "read real crossterm input": an empty
        // `scripted_dialog_events` still synthesizes a single Escape on
        // the nested loop's first iteration (see
        // `TuiPlatformServices::next_dialog_events`), so a dialog opened
        // without a prior `Self::queue_dialog_events` call resolves
        // immediately via Escape instead of blocking on
        // `crossterm::event::poll` against this *process's* real stdin —
        // which under `cargo test` has no controlling terminal and would
        // otherwise spin the nested loop forever (`poll` erroring
        // immediately, over and over, with nothing to wait on). A test
        // that wants real scripted input calls `queue_dialog_events`
        // before triggering the dialog, same as always.
        backend
            .tui_services()
            .queue_dialog_events(std::iter::empty());
        // Seed the viewport from the driver's terminal size BEFORE setup,
        // exactly as the live runner does. Without this,
        // `app.setup()` would read the default 80×24 viewport instead of
        // the requested `width`×`height`, so any setup-time sizing (e.g.
        // `TerminalApp` spawning its PTY) would use the wrong dimensions.
        backend.begin_frame(crate::Viewport::new(width as f32, height as f32, 1.0));
        let mut app = app;
        app.setup(&mut backend);
        let mut driver = Self {
            core: DriverCore::new(app, backend),
            terminal,
        };
        driver.render();
        driver
    }

    /// Seed the nested dialog loop's scripted input — see
    /// [`crate::tui::services::TuiPlatformServices::queue_dialog_events`].
    /// Must be called *before* the event that triggers a
    /// `show_file_open_dialog`/`show_file_save_dialog`/
    /// `show_message_dialog` call — the loop resolves synchronously
    /// inside that one [`Self::dispatch`]/[`Self::press`]-family call,
    /// with no way to be fed more input mid-call.
    pub fn queue_dialog_events(&self, events: impl IntoIterator<Item = UiEvent>) {
        self.core
            .backend()
            .tui_services()
            .queue_dialog_events(events);
    }

    /// Repaint one frame through the shared production render path.
    ///
    /// Consumes any pending [`crate::Backend::request_full_repaint`]
    /// request the same way the live runner's
    /// `tui::run::TuiRunner::step`/`pump` frame loop does — `Terminal::clear()` before
    /// this frame's `render_frame` call — so a driver test can exercise
    /// the same "next frame paints as if blank" behaviour a real terminal
    /// session gets, with no real terminal to observe the clear on
    /// directly.
    pub fn render(&mut self) {
        let full_repaint = self.core.backend_mut().take_full_repaint_requested();
        let (backend, app) = self.core.parts_mut();
        let mut terminal = self.terminal.borrow_mut();
        if full_repaint {
            terminal.clear().expect("TestBackend clear is infallible");
        }
        render_frame(&mut terminal, backend, app).expect("TestBackend render is infallible");
    }

    /// Resize the underlying `TestBackend`'s physical cell buffer to
    /// `width`×`height` — **without** touching [`TuiBackend`]'s cached
    /// [`crate::Viewport`] or any layout state a previous [`Self::render`]
    /// cached (e.g. [`Self::tab_center`]'s `cached_tab_bar_layout`). Those
    /// stay exactly as they were after the last repaint, at whatever size
    /// the driver last actually painted.
    ///
    /// This deliberately does **not** go through [`Self::render`] /
    /// `Terminal::draw`'s `autoresize`: that path re-derives `TuiBackend`'s
    /// per-frame `Viewport` from the buffer's own post-resize `frame.area()`
    /// on *every* call, so a full `Self::render` right after a resize always
    /// self-heals — a test that only ever resized-then-rendered would never
    /// observe a mismatch. To construct the exact stale-layout-vs-shrunk-buffer
    /// race a test calls this to shrink *only* the physical buffer first, then
    /// reads [`Self::backend`]'s still-stale, larger viewport (or a cached
    /// layout) to drive a click or a direct paint against it, asserting
    /// the result degrades gracefully — truncates, ignores an
    /// out-of-bounds hit — instead of panicking. Call [`Self::render`]
    /// afterward, once the test is done observing the stale state, to
    /// restore a consistent frame at the new size (it will resync
    /// `TuiBackend`'s viewport to match, same as the live runner does on a
    /// real terminal resize).
    ///
    /// Uses `ratatui::backend::TestBackend::resize` directly on the raw
    /// cell buffer; no `Terminal::resize`/`autoresize` bookkeeping
    /// (viewport area, cursor position, scrollback wiring) runs here
    /// either — this is a strictly lower-level knob than either.
    pub fn resize(&mut self, width: u16, height: u16) {
        self.terminal
            .borrow_mut()
            .backend_mut()
            .resize(width, height);
    }

    /// Feed one synthetic event through the **full production pipeline**:
    /// backend translation ([`TuiBackend::translate_injected`] — drag
    /// state, accelerator matching, double-click folding) followed by the
    /// shared [`dispatch_event`] for each resulting event. Repaints on
    /// redraw and latches `exited`. Returns the strongest [`Reaction`]
    /// (Exit > Redraw > Continue) for convenient assertions.
    ///
    /// Routing through `translate_injected` (not straight to
    /// `dispatch_event`) is what makes drag sequences work: a `MouseDown`
    /// in a registered text region begins a `TextSelection` drag, and a
    /// following `MouseMoved` is translated into `TextSelectionChanged` —
    /// exactly as `wait_events` does for real crossterm input.
    pub fn dispatch(&mut self, event: UiEvent) -> Reaction {
        if self.core.exited() {
            return Reaction::Exit;
        }
        let mut result = Reaction::Continue;
        let translated = self.core.backend_mut().translate_injected(vec![event]);
        for ev in translated {
            let outcome = {
                let (backend, app) = self.core.parts_mut();
                dispatch_event(ev, backend, app)
            };
            match &outcome {
                EventOutcome::Continue => {}
                EventOutcome::Redraw => self.render(),
                // Arm the same bookkeeping the live runner
                // would via `Backend::request_frame_in` — harmless on a
                // driver with no real timer loop, and lets a test assert
                // on `backend().frame_requests()` /
                // `backend().pending_frame_delay()` afterward.
                EventOutcome::RedrawAfter(d) => {
                    self.core.backend_mut().request_frame_in(*d);
                }
                EventOutcome::Exit => {
                    self.core.mark_exited();
                    return Reaction::Exit;
                }
            }
            // `Redraw` always wins over `RedrawAfter`, and
            // two `RedrawAfter`s in the same batch keep the *earlier*
            // deadline rather than whichever arrived first — see
            // `Reaction::merge`'s doc.
            result = result.merge(outcome.into());
        }
        result
    }

    /// Deliver every [`UiEvent::User`] payload queued via
    /// [`crate::Backend::waker`] since the last call, through the same
    /// production dispatch pipeline [`Self::dispatch`] uses.
    ///
    /// Unlike `dispatch`, this doesn't take an event — it drains
    /// [`TuiBackend::drain_user_events`] (the same `user_events` queue the
    /// live runner's `poll_events`/`wait_events` fold in) and dispatches
    /// whatever a background thread queued via `backend().waker()` in the
    /// meantime. This is the driver-tier proof that a background thread
    /// can wake the UI at all: call `backend().waker()` to get the
    /// `Send + Sync` closure, invoke it from a spawned thread, join, then
    /// call this to observe the delivery — see `tests::` in this module
    /// for the full round trip.
    pub fn pump_user_events(&mut self) -> Reaction {
        if self.core.exited() {
            return Reaction::Exit;
        }
        let events = self.core.backend_mut().drain_user_events();
        let mut result = Reaction::Continue;
        for ev in events {
            let outcome = {
                let (backend, app) = self.core.parts_mut();
                dispatch_event(ev, backend, app)
            };
            match &outcome {
                EventOutcome::Continue => {}
                EventOutcome::Redraw => self.render(),
                // See `Self::dispatch`'s identical arm.
                EventOutcome::RedrawAfter(d) => {
                    self.core.backend_mut().request_frame_in(*d);
                }
                EventOutcome::Exit => {
                    self.core.mark_exited();
                    return Reaction::Exit;
                }
            }
            // See `Self::dispatch`'s identical merge.
            result = result.merge(outcome.into());
        }
        result
    }

    /// Run one [`AppLogic::tick`], applying its [`Reaction`] exactly as
    /// the live runner's loop does.
    ///
    /// The live TUI loop calls `app.tick(backend)` once after every event
    /// batch — including the empty batch a bare poll timeout produces —
    /// and matches the returned `Reaction` the same four ways
    /// [`Self::dispatch`] matches an `EventOutcome`: nothing on
    /// `Continue`, repaint on `Redraw`, `Backend::request_frame_in` on
    /// `RedrawAfter`, latch `exited` on `Exit`. See `tui::run::TuiRunner::step`/`pump`.
    ///
    /// Time-driven app state — a spinner frame, a caret blink, a
    /// countdown, a background-job poll — lives entirely in `tick`, so
    /// without this a driver test can only ever observe an app's
    /// event-driven half. Nothing here *waits*: the driver has no event
    /// loop and no real timer, so a test drives the animation by calling
    /// this once per frame it wants to advance rather than sleeping. Pair
    /// it with [`TuiBackend::frame_requests`] /
    /// [`TuiBackend::pending_frame_delay`] (via [`Self::backend`]) to
    /// assert on the *scheduling* the app asked for, which is otherwise
    /// invisible from the painted screen.
    ///
    /// Short-circuits to [`Reaction::Exit`] once the app has exited,
    /// matching [`Self::dispatch`].
    pub fn tick(&mut self) -> Reaction {
        if self.core.exited() {
            return Reaction::Exit;
        }
        let reaction = {
            let (backend, app) = self.core.parts_mut();
            app.tick(backend)
        };
        match reaction {
            Reaction::Continue => Reaction::Continue,
            Reaction::Redraw => {
                self.render();
                Reaction::Redraw
            }
            Reaction::RedrawAfter(d) => {
                self.core.backend_mut().request_frame_in(d);
                Reaction::RedrawAfter(d)
            }
            Reaction::Exit => {
                self.core.mark_exited();
                Reaction::Exit
            }
        }
    }

    /// Press a key (no modifiers).
    pub fn press(&mut self, key: Key) -> Reaction {
        DriverInput::press(self, key)
    }

    /// Type a single character key (no modifiers).
    pub fn type_char(&mut self, c: char) -> Reaction {
        DriverInput::type_char(self, c)
    }

    /// Press a named (non-printable) key, e.g. [`NamedKey::Enter`].
    pub fn press_named(&mut self, key: NamedKey) -> Reaction {
        DriverInput::press_named(self, key)
    }

    /// Press a character key with Ctrl held (e.g. `ctrl_char('c')` to
    /// trigger the runner's copy-on-selection path).
    pub fn ctrl_char(&mut self, c: char) -> Reaction {
        DriverInput::ctrl_char(self, c)
    }

    /// Left-click at backend coordinates `(x, y)` (cell units for TUI),
    /// delivered as a [`UiEvent::MouseDown`] — the event primitives'
    /// hit-test paths consume.
    pub fn click(&mut self, x: f32, y: f32) -> Reaction {
        DriverInput::click(self, x, y)
    }

    /// Press the left mouse button down at `(x, y)`. Begins a drag if it
    /// lands on a draggable target (text region, scrollbar thumb).
    pub fn mouse_down(&mut self, x: f32, y: f32) -> Reaction {
        self.dispatch(UiEvent::MouseDown {
            widget: None,
            button: MouseButton::Left,
            position: Point::new(x, y),
            modifiers: Modifiers::default(),
        })
    }

    /// Move the cursor to `(x, y)` with the left button held. During an
    /// active drag this is translated to the drag's high-level event
    /// (e.g. [`UiEvent::TextSelectionChanged`] /
    /// [`UiEvent::ScrollOffsetChanged`]).
    pub fn mouse_move(&mut self, x: f32, y: f32) -> Reaction {
        self.dispatch(UiEvent::MouseMoved {
            position: Point::new(x, y),
            buttons: ButtonMask {
                left: true,
                ..ButtonMask::default()
            },
        })
    }

    /// Release the left mouse button at `(x, y)`, ending any active drag.
    pub fn mouse_up(&mut self, x: f32, y: f32) -> Reaction {
        self.dispatch(UiEvent::MouseUp {
            widget: None,
            button: MouseButton::Left,
            position: Point::new(x, y),
        })
    }

    /// Left-button drag from `(x0, y0)` to `(x1, y1)`: down → move → up.
    pub fn drag(&mut self, x0: f32, y0: f32, x1: f32, y1: f32) -> Reaction {
        DriverInput::drag(self, x0, y0, x1, y1)
    }

    /// Deliver a [`UiEvent::DoubleClick`] at `(x, y)` directly — the same
    /// event `TuiBackend`'s `DoubleClickDetector` would eventually
    /// synthesise from two close-together `MouseDown`s, but produced here
    /// with no dependence on wall-clock timing. A test that
    /// fired two `click()`s and relied on the 400ms/1.5-cell detector
    /// window was a race against real time — flaky under load, identical
    /// on the 1st and 1000th call here since no clock is consulted.
    ///
    /// Still routed through [`Self::dispatch`] (backend translation +
    /// `dispatch_event`), not straight to `app.handle`, so it sees the
    /// same pre-processing (e.g. `clear_selection_display`) a real
    /// detector-folded double click would — see the module doc's "No
    /// drift from production" note. `apply_dispatch`/`DoubleClickDetector`
    /// both pass a `DoubleClick` event through unchanged, so this doesn't
    /// risk a second fold.
    pub fn double_click(&mut self, x: f32, y: f32) -> Reaction {
        self.dispatch(UiEvent::DoubleClick {
            widget: None,
            position: Point::new(x, y),
        })
    }

    /// Press-and-release the middle mouse button at `(x, y)`, delivered as
    /// a [`UiEvent::MouseDown`] with [`MouseButton::Middle`]. Shorthand for
    /// [`Self::click_with`] with the default modifiers.
    pub fn middle_click(&mut self, x: f32, y: f32) -> Reaction {
        self.click_with(MouseButton::Middle, Modifiers::default(), x, y)
    }

    /// Press-and-release the right mouse button at `(x, y)`, delivered as
    /// a [`UiEvent::MouseDown`] with [`MouseButton::Right`]. Shorthand for
    /// [`Self::click_with`] with the default modifiers.
    pub fn right_click(&mut self, x: f32, y: f32) -> Reaction {
        self.click_with(MouseButton::Right, Modifiers::default(), x, y)
    }

    /// Click at `(x, y)` with an arbitrary [`MouseButton`] and
    /// [`Modifiers`], delivered as a [`UiEvent::MouseDown`] — the general
    /// form [`Self::click`]/[`Self::middle_click`]/[`Self::right_click`]
    /// build on. Lets a test assert modifier-gated behaviour (e.g.
    /// Ctrl-click) without hand-assembling the event.
    pub fn click_with(
        &mut self,
        button: MouseButton,
        modifiers: Modifiers,
        x: f32,
        y: f32,
    ) -> Reaction {
        self.dispatch(UiEvent::MouseDown {
            widget: None,
            button,
            position: Point::new(x, y),
            modifiers,
        })
    }

    /// Enable or disable double-click folding for subsequent
    /// [`Self::click`]/[`Self::mouse_down`] calls. Defaults
    /// to `true` — today's behaviour, where two `click()`s landing inside
    /// `TuiBackend`'s `DoubleClickDetector` window (400ms, 1.5 cells) fold
    /// into a single `DoubleClick`. Pass `false` when a test means two
    /// *separate* single clicks — e.g. exercising click-outside-to-dismiss
    /// twice in a row — so it deterministically gets two `MouseDown`s
    /// instead of a result that depends on how much wall-clock time
    /// elapsed between the two `dispatch` calls.
    pub fn set_double_click_folding(&mut self, enabled: bool) {
        self.core.backend_mut().set_double_click_folding(enabled);
    }

    /// Whether the app has returned [`Reaction::Exit`].
    pub fn exited(&self) -> bool {
        self.core.exited()
    }

    /// The current rendered screen as newline-joined rows.
    ///
    /// Wide-char aware: strides by each glyph's
    /// [`char_cell_width`], the same way [`Self::row_cells`] does, rather
    /// than one buffer column per iteration. A double-width glyph's
    /// reserved continuation cell is always blank by construction and never
    /// reaches `next` in `Buffer::diff`'s output, so `TestBackend` never
    /// actually writes it — it just retains whatever was in that column
    /// *before* this frame, which for a freshly-painted glyph is
    /// `Cell::default()`'s `symbol()`, `" "`. Reading column-by-column
    /// therefore inserted a phantom space after every wide glyph that was
    /// never really there (a real terminal shows the glyph occupying both
    /// columns with nothing "under" it), which made `screen_contains`/`screen_has`
    /// — the direct backer of the conformance suite's `assert_screen_has` step —
    /// report a needle spanning a wide glyph as absent even though
    /// [`Self::find_bounds`] (already wide-char aware) found it painted as one
    /// contiguous run. Striding here brings `screen()` in line with what
    /// `row_cells`/`find_bounds`/`inventory` already saw, and with what
    /// [`crate::tui::vt_testing::TuiVtDriver::screen`]'s vt100-observed
    /// twin reports for the identical content.
    pub fn screen(&self) -> String {
        let terminal = self.terminal.borrow();
        let buf = terminal.backend().buffer();
        let area = buf.area;
        let mut out = String::new();
        for y in area.top()..area.bottom() {
            let mut x = area.left();
            while x < area.right() {
                let symbol = buf[(x, y)].symbol();
                let ch = symbol.chars().next().unwrap_or(' ');
                out.push_str(symbol);
                x += char_cell_width(ch).max(1);
            }
            out.push('\n');
        }
        out
    }

    /// True if any rendered row contains `needle`.
    pub fn screen_contains(&self, needle: &str) -> bool {
        self.screen().contains(needle)
    }

    /// The style a rasteriser painted onto one cell: foreground, background,
    /// and text modifiers (bold/italic/underline/…).
    ///
    /// [`Self::screen`] only surfaces glyphs, so a driver test can prove a
    /// cell exists but not, e.g., that it's the *preview*-tab italic rather
    /// than the permanent-tab plain style — both stringify
    /// identically. `style_at` reads the same buffer cell `screen()` does,
    /// just its [`ratatui::buffer::Cell::style`] instead of its symbol.
    ///
    /// `fg`/`bg`/`modifiers` are `ratatui`'s own [`Color`]/[`Modifier`],
    /// re-exported from this module (`quadraui::tui::testing::Modifier`) so
    /// a downstream test crate can assert on them (`style.modifiers
    /// .contains(Modifier::ITALIC)`) without adding its own `ratatui`
    /// dependency — it already depends on `quadraui` to get `TuiDriver`.
    pub fn style_at(&self, x: u16, y: u16) -> Option<CellStyle> {
        let terminal = self.terminal.borrow();
        let buf = terminal.backend().buffer();
        let area = buf.area;
        if x < area.left() || x >= area.right() || y < area.top() || y >= area.bottom() {
            return None;
        }
        let cell = &buf[(x, y)];
        Some(CellStyle {
            fg: cell.fg,
            bg: cell.bg,
            modifiers: cell.modifier,
        })
    }

    /// [`Self::style_at`] for every cell of row `y`, left to right — one
    /// entry per cell, so a test can assert "every cell of the tab label is
    /// italic" without indexing arithmetic. Empty if `y` is out of range.
    ///
    /// `styled_row(y).len()` equals the buffer width (`area.width`) for any
    /// in-range `y` — always one entry per cell, unlike [`Self::screen`],
    /// whose row length before the trailing `\n` is only the same count for
    /// buffers with no wide characters: `screen()` concatenates each cell's
    /// `symbol()`, so a double-width glyph's continuation cell contributes
    /// zero chars there, while `styled_row` still emits an entry for it
    /// (its char defaults to `' '`, since a continuation cell's `symbol()`
    /// is empty).
    pub fn styled_row(&self, y: u16) -> Vec<(char, CellStyle)> {
        let terminal = self.terminal.borrow();
        let buf = terminal.backend().buffer();
        let area = buf.area;
        if y < area.top() || y >= area.bottom() {
            return Vec::new();
        }
        (area.left()..area.right())
            .map(|x| {
                let cell = &buf[(x, y)];
                let ch = cell.symbol().chars().next().unwrap_or(' ');
                (
                    ch,
                    CellStyle {
                        fg: cell.fg,
                        bg: cell.bg,
                        modifiers: cell.modifier,
                    },
                )
            })
            .collect()
    }

    /// Access the app state for test assertions.
    ///
    /// Useful when the test app records side-effects (e.g. last copied text,
    /// selection changes) that would otherwise require screen-scraping.
    pub fn app(&self) -> &A {
        self.core.app()
    }

    /// Mutable access to the app state for tests that need to poke state
    /// directly rather than through a scripted [`UiEvent`] — e.g. asserting
    /// a render-time invariant (like editor cursor placement) across a
    /// state change with no dedicated event/handler of its own.
    pub fn app_mut(&mut self) -> &mut A {
        self.core.app_mut()
    }

    /// Access the backend for test assertions (e.g. active selection state,
    /// drag state).
    pub fn backend(&self) -> &TuiBackend {
        self.core.backend()
    }

    /// The `(x, y)` position `Terminal::draw` last applied to the
    /// underlying `TestBackend` via `Frame::set_cursor_position` —
    /// reflecting [`super::run::render_frame`]'s `take_last_cursor_position`
    /// handoff from `TuiBackend::draw_editor`.
    ///
    /// Note `TestBackend` tracks only the last-set position, not whether
    /// the cursor is currently hidden — so this always returns the most
    /// recent position ever applied, even on a later frame that painted no
    /// editor and would hide the real terminal cursor. Fine for asserting
    /// "the editor's cursor position reached the Frame", not for asserting
    /// hide/show transitions.
    pub fn terminal_cursor_position(&mut self) -> Option<(u16, u16)> {
        self.terminal
            .borrow_mut()
            .get_cursor_position()
            .ok()
            .map(|p| (p.x, p.y))
    }

    /// This row's cells as `(char, cell_x, cell_width)` triples, in
    /// left-to-right order — the shared scan [`Self::find_bounds`] and
    /// [`ConformanceDriver::inventory`] both walk.
    ///
    /// Skips the reserved continuation cell(s) a double-width glyph's
    /// rendering leaves behind: ratatui resets them to a blank cell whose
    /// `symbol()` reads back as `" "`, indistinguishable from a real space
    /// by content alone, so the *width* of the character just placed (via
    /// [`char_cell_width`]), not the following cell's symbol, drives how
    /// far `x` advances. Without this, two adjacent double-width
    /// characters (e.g. CJK) get a spurious blank cell wedged between them
    /// in the reconstructed row, and a needle spanning them would never
    /// match.
    fn row_cells(&self, y: u16) -> Vec<(char, u16, u16)> {
        let terminal = self.terminal.borrow();
        let buf = terminal.backend().buffer();
        let area = buf.area;
        let mut cells = Vec::new();
        let mut x = area.left();
        while x < area.right() {
            let symbol = buf[(x, y)].symbol();
            let ch = symbol.chars().next().unwrap_or(' ');
            let w = char_cell_width(ch).max(1);
            cells.push((ch, x, w));
            x += w;
        }
        cells
    }

    /// Cell bounds of the first row containing `needle`, wide-char aware
    /// (see [`Self::row_cells`]): a needle spanning two double-width
    /// glyphs (CJK, most emoji) matches and reports the full cell span,
    /// not just its first character's cell.
    pub fn find_bounds(&self, needle: &str) -> Option<Rect> {
        let needle: Vec<char> = needle.chars().collect();
        if needle.is_empty() {
            return None;
        }
        let area = self.terminal.borrow().backend().buffer().area;
        for y in area.top()..area.bottom() {
            let cells = self.row_cells(y);
            if cells.len() < needle.len() {
                continue;
            }
            if let Some(start) = cells
                .windows(needle.len())
                .position(|w| w.iter().map(|(c, _, _)| *c).eq(needle.iter().copied()))
            {
                let (_, start_x, _) = cells[start];
                let (_, last_x, last_w) = cells[start + needle.len() - 1];
                let width = (last_x + last_w).saturating_sub(start_x);
                return Some(Rect::new(start_x as f32, y as f32, width as f32, 1.0));
            }
        }
        None
    }

    /// Coordinates of the first row containing `needle`, at the center of
    /// the matched span's *first* cell — preserved from earlier behaviour
    /// (a click there always lands inside the match regardless of span width)
    /// even though [`Self::find_bounds`] now reports the full wide-char-aware
    /// span. Callers that want the full span's center use `find_bounds`
    /// directly (e.g. [`ConformanceDriver::click_text_at`]'s `Anchor::Center`).
    pub fn find(&self, needle: &str) -> Option<(f32, f32)> {
        self.find_bounds(needle)
            .map(|b| (b.x + 0.5, b.y + b.height / 2.0))
    }

    /// Cell coordinates of tab `tab_idx`'s center in the tab bar `bar`, for
    /// [`Self::click`] to land on that specific tab.
    ///
    /// `find` can't do this job: every tab paints the same chrome (and a
    /// closed/active tab's label may repeat elsewhere on screen), so there
    /// is no needle that disambiguates "tab 3" from "tab 0". This instead
    /// resolves against the [`crate::TabBarLayout`] `TuiBackend::draw_tab_bar`
    /// cached the last time `bar` painted — `None` if `bar` didn't paint
    /// this frame, or if `tab_idx` is scrolled out of view behind the
    /// bar's `scroll_offset`.
    pub fn tab_center(&self, bar: &WidgetId, tab_idx: usize) -> Option<(f32, f32)> {
        let (rect, layout) = self.core.backend().cached_tab_bar_layout(bar)?;
        let (cx, cy) = layout.tab_center(tab_idx)?;
        Some((rect.x + cx, rect.y + cy))
    }

    /// Cell coordinates of tab `tab_idx`'s close-button center in the tab
    /// bar `bar` — the `×`/`●` glyph every tab shares, so [`Self::find`]
    /// can't target one tab's close button over another's.
    ///
    /// `None` for the same reasons as [`Self::tab_center`], plus: the tab
    /// drew no close button this frame (`is_closable: false` on the tab,
    /// or `show_tab_close: false` on the bar).
    pub fn tab_close_center(&self, bar: &WidgetId, tab_idx: usize) -> Option<(f32, f32)> {
        let (rect, layout) = self.core.backend().cached_tab_bar_layout(bar)?;
        let (cx, cy) = layout.tab_close_center(tab_idx)?;
        Some((rect.x + cx, rect.y + cy))
    }
}

/// The four raw primitives [`DriverInput`]'s default `press`/`type_char`/
/// `press_named`/`ctrl_char`/`click`/`drag` methods build on — see that
/// trait's doc for why `dispatch`/`mouse_down`/`mouse_move`/`mouse_up`
/// stay required (genuinely per-backend) rather than shared.
/// `TuiDriver` doesn't also adopt [`crate::testing::PixelClickConformance`]
/// — its `ConformanceDriver::click_text_at`/`drag_text`/`scroll_at` stay
/// bespoke below, since TUI's cell-unit `+0.5`/`-0.5` edge offsets (vs the
/// pixel backends' `+1.0`/`-1.0`) and `self.screen()`-dump panic messages
/// are real, load-bearing divergences, not accidental duplication.
impl<A: AppLogic> DriverInput for TuiDriver<A> {
    fn dispatch(&mut self, event: UiEvent) -> Reaction {
        self.dispatch(event)
    }

    fn mouse_down(&mut self, x: f32, y: f32) -> Reaction {
        self.mouse_down(x, y)
    }

    fn mouse_move(&mut self, x: f32, y: f32) -> Reaction {
        self.mouse_move(x, y)
    }

    fn mouse_up(&mut self, x: f32, y: f32) -> Reaction {
        self.mouse_up(x, y)
    }
}

impl<A: AppLogic> ConformanceDriver for TuiDriver<A> {
    type App = A;

    fn new_fixture(app: Self::App, viewport: LogicalViewport) -> Self {
        // TUI's native unit *is* the cell, so a `LogicalViewport` maps
        // straight through — no char_width/line_height scaling needed
        // (contrast `GtkDriver::new_fixture`, a pixel backend).
        TuiDriver::new(app, viewport.cols as u16, viewport.rows as u16)
    }

    fn backend_caps(&self) -> crate::BackendCaps {
        // Straight off the real `TuiBackend` this driver wraps — never a
        // re-statement.
        crate::Backend::backend_caps(self.core.backend())
    }

    fn press_named(&mut self, key: NamedKey) {
        TuiDriver::press_named(self, key);
    }

    fn type_char(&mut self, c: char) {
        TuiDriver::type_char(self, c);
    }

    fn ctrl_char(&mut self, c: char) {
        TuiDriver::ctrl_char(self, c);
    }

    fn click_text_at(&mut self, needle: &str, at: Anchor) {
        let bounds = self
            .find_bounds(needle)
            .unwrap_or_else(|| panic!("TuiDriver: {needle:?} not painted:\n{}", self.screen()));
        let y = bounds.y + bounds.height / 2.0;
        let x = match at {
            Anchor::Center => bounds.x + bounds.width / 2.0,
            // Half a cell in from each edge — the outermost cell's
            // center — so the click reliably lands inside the run even
            // for a single-cell-wide match.
            Anchor::LeftEdge => bounds.x + 0.5,
            Anchor::RightEdge => bounds.x + bounds.width - 0.5,
        };
        self.click(x, y);
    }

    fn drag_text(&mut self, from: &str, to: &str) {
        let (x0, y0) = self
            .find(from)
            .unwrap_or_else(|| panic!("TuiDriver: {from:?} not painted:\n{}", self.screen()));
        let (x1, y1) = self
            .find(to)
            .unwrap_or_else(|| panic!("TuiDriver: {to:?} not painted:\n{}", self.screen()));
        self.drag(x0, y0, x1, y1);
    }

    fn scroll_at(&mut self, needle: &str, lines: i32) {
        let (x, y) = self
            .find(needle)
            .unwrap_or_else(|| panic!("TuiDriver: {needle:?} not painted:\n{}", self.screen()));
        let line_height = self.core.backend().line_height();
        self.dispatch(UiEvent::Scroll {
            widget: None,
            delta: ScrollDelta::new(0.0, lines as f32 * line_height),
            position: Point::new(x, y),
        });
    }

    fn inventory(&self) -> FrameInventory {
        let area = self.terminal.borrow().backend().buffer().area;
        let mut text_runs = Vec::new();
        for y in area.top()..area.bottom() {
            let cells = self.row_cells(y);
            let mut run: Option<(String, u16, u16)> = None; // (text, start_x, end_x)
            for (ch, x, w) in cells {
                if ch == ' ' {
                    if let Some((text, start_x, end_x)) = run.take() {
                        text_runs.push(TextRun {
                            text,
                            bounds: Rect::new(
                                start_x as f32,
                                y as f32,
                                (end_x - start_x) as f32,
                                1.0,
                            ),
                        });
                    }
                    continue;
                }
                match &mut run {
                    Some((text, _start_x, end_x)) => {
                        text.push(ch);
                        *end_x = x + w;
                    }
                    None => run = Some((ch.to_string(), x, x + w)),
                }
            }
            if let Some((text, start_x, end_x)) = run {
                text_runs.push(TextRun {
                    text,
                    bounds: Rect::new(start_x as f32, y as f32, (end_x - start_x) as f32, 1.0),
                });
            }
        }
        FrameInventory {
            text_runs,
            // Zones registered this frame via `Backend::register_zone` —
            // currently the shell-chrome zones `AppShell::render` records
            // (activity-bar items, sidebar header/content, status bar,
            // ...). Primitives that don't yet call `register_zone`
            // contribute no zone.
            zones: self.core.backend().zones().to_vec(),
        }
    }

    fn screen_has(&self, needle: &str) -> bool {
        self.screen_contains(needle)
    }

    fn exited(&self) -> bool {
        TuiDriver::exited(self)
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::Viewport;
    use std::cell::{Cell, RefCell};
    use std::rc::Rc;

    /// Minimal app that records the viewport it observes in `setup()`.
    ///
    /// `setup()` must see the driver's real terminal size, not the
    /// `Viewport::default()` (80×24) that `TuiBackend::new()` seeds. An app
    /// that sizes a side effect from the setup-time viewport (e.g. `TerminalApp`
    /// spawning its PTY) would otherwise under-fill the pane until the first
    /// resize event.
    struct SetupViewportRecorder {
        seen: Rc<Cell<Option<Viewport>>>,
    }

    impl AppLogic for SetupViewportRecorder {
        type AreaId = ();

        fn setup(&mut self, backend: &mut dyn Backend) {
            self.seen.set(Some(backend.viewport()));
        }

        fn render(&self, _backend: &mut dyn Backend, _area: ()) {}

        fn handle(&mut self, _event: UiEvent, _backend: &mut dyn Backend) -> Reaction {
            Reaction::Continue
        }
    }

    /// App that requests a full repaint whenever it handles
    /// a key press, and otherwise does nothing.
    struct FullRepaintRequester;

    impl AppLogic for FullRepaintRequester {
        type AreaId = ();

        fn render(&self, _backend: &mut dyn Backend, _area: ()) {}

        fn handle(&mut self, event: UiEvent, backend: &mut dyn Backend) -> Reaction {
            if matches!(event, UiEvent::KeyPressed { .. }) {
                backend.request_full_repaint();
                return Reaction::Redraw;
            }
            Reaction::Continue
        }
    }

    /// `TuiDriver::dispatch` routes through [`TuiDriver::render`] on a
    /// `Redraw` reaction, exactly like the live runner's frame loop — so a
    /// `Backend::request_full_repaint` call made from `handle` must be
    /// consumed by that same `render` call, not left pending. Verified
    /// indirectly: if `render` had consumed it, a direct
    /// `take_full_repaint_requested` call made *after* the dispatch returns
    /// `false` (already cleared); if `render` never consulted it at all, that
    /// same call would still see the original `true` and this assertion would
    /// fail.
    #[test]
    fn dispatch_consumes_a_pending_full_repaint_request_via_render() {
        let mut driver = TuiDriver::new(FullRepaintRequester, 20, 5);

        let reaction = driver.press(Key::Named(NamedKey::Enter));
        assert_eq!(reaction, Reaction::Redraw);

        assert!(
            !driver.core.backend_mut().take_full_repaint_requested(),
            "TuiDriver::render must consume the pending full-repaint \
             request during the redraw `dispatch` already triggered"
        );
    }

    /// Prove `render` doesn't just *consume* the pending flag (see the test
    /// above) but actually forces `ratatui::Terminal::clear()`'s effect —
    /// the next frame repaints every cell unconditionally, even one the
    /// app's own (unchanged) content never touches.
    ///
    /// `FullRepaintRequester::render` never paints anything, so once the
    /// first frame lands the widget-side content stays blank forever —
    /// ratatui's diff cache always agrees "nothing changed" and would
    /// normally skip re-sending every cell. We simulate the exact kind of
    /// external drift `request_full_repaint`'s doc names (a PTY writing
    /// straight into the shared terminal, bypassing ratatui's own
    /// `Buffer` tracking) by calling `ratatui::backend::Backend::draw`
    /// directly on the `TestBackend`, underneath `Terminal`'s diff — a
    /// stale glyph ratatui's cache has no idea appeared.
    ///
    /// Without a pending full-repaint request, a plain redraw leaves that
    /// stale glyph in place (diff sees no change at that cell, so it's
    /// never told to repaint it) — matching the symptom. With
    /// `request_full_repaint()` set first, `render` must clear it, proving
    /// `Terminal::clear()` actually ran rather than the flag merely being
    /// read and discarded.
    #[test]
    fn render_actually_clears_stale_content_outside_the_diff_cache() {
        use ratatui::backend::Backend as RatatuiBackend;
        use ratatui::buffer::Cell;

        let mut driver = TuiDriver::new(FullRepaintRequester, 10, 3);

        // Inject a "stale" glyph directly into the physical backend,
        // underneath `Terminal`'s own previous-buffer diff tracking —
        // ratatui has no record that this cell ever changed.
        let mut stale = Cell::default();
        stale.set_symbol("X");
        RatatuiBackend::draw(
            driver.terminal.borrow_mut().backend_mut(),
            std::iter::once((0u16, 0u16, &stale)),
        )
        .expect("TestBackend draw is infallible");
        assert!(
            driver.screen_contains("X"),
            "sanity: the injected stale glyph must be visible before either render"
        );

        // A plain redraw with nothing pending: ratatui's diff still
        // believes this cell is unchanged (blank), so the stale glyph
        // survives.
        driver.render();
        assert!(
            driver.screen_contains("X"),
            "a redraw with no full-repaint request pending must not touch \
             cells the diff cache believes are unchanged"
        );

        // Now request a full repaint and redraw again: `render` must
        // consume the request by calling `Terminal::clear()` before
        // painting, which resets the diff cache so every cell —
        // including the stale one the app's own content never wrote to
        // this frame — repaints unconditionally back to blank.
        driver.core.backend_mut().request_full_repaint();
        driver.render();
        assert!(
            !driver.screen_contains("X"),
            "render() must actually call Terminal::clear() when a full \
             repaint was requested, not just consume the flag — the stale \
             glyph should be gone once every cell repaints unconditionally"
        );
    }

    /// `setup()` must observe the driver's real terminal dimensions, not
    /// the default 80×24 viewport. An app that sizes a side effect from the
    /// setup-time viewport (e.g. PTY initialization) would under-fill the
    /// pane until first interaction.
    #[test]
    fn setup_sees_real_viewport_not_default() {
        let seen = Rc::new(Cell::new(None));
        let app = SetupViewportRecorder { seen: seen.clone() };
        // Deliberately not 80×24 so a regression to the default is visible.
        let _driver = TuiDriver::new(app, 120, 40);

        let vp = seen.get().expect("setup() should have run");
        assert_eq!(
            (vp.width, vp.height),
            (120.0, 40.0),
            "setup() must see the driver's real size, not Viewport::default() (80×24)"
        );
    }

    use crate::primitives::status_bar::{StatusBar, StatusBarSegment};
    use crate::types::{Color, WidgetId};

    /// Paints one status-bar line whose text is given verbatim (no
    /// automatic padding), so a test can control exactly which glyphs are
    /// adjacent — needed to exercise the double-width continuation-cell
    /// case `find`/`find_bounds` must handle correctly.
    struct OneLineApp {
        text: &'static str,
    }

    impl AppLogic for OneLineApp {
        type AreaId = ();

        fn render(&self, backend: &mut dyn Backend, _area: ()) {
            backend.draw_status_bar_interactive(
                Rect::new(0.0, 0.0, 30.0, 1.0),
                &StatusBar {
                    id: WidgetId::new("status"),
                    left_segments: vec![StatusBarSegment {
                        text: self.text.to_string(),
                        fg: Color::rgb(255, 255, 255),
                        bg: Color::rgb(0, 0, 0),
                        bold: false,
                        action_id: None,
                    }],
                    right_segments: vec![],
                },
                &crate::InteractionState::new(),
            );
        }

        fn handle(&mut self, _event: UiEvent, _backend: &mut dyn Backend) -> Reaction {
            Reaction::Continue
        }
    }

    /// Regression guard: two adjacent double-width (CJK) glyphs must be
    /// found as one 4-cell-wide span, not silently unmatched because
    /// ratatui wedges a blank continuation cell between them that isn't a
    /// real space.
    #[test]
    fn find_bounds_is_wide_char_aware_for_adjacent_cjk_glyphs() {
        let driver = TuiDriver::new(
            OneLineApp {
                text: "你好 world"
            },
            30,
            3,
        );

        let cjk = driver.find_bounds("你好").expect(
            "adjacent double-width glyphs should match as one span, not be split by a \
             spurious blank continuation cell",
        );
        assert_eq!(
            (cjk.x, cjk.width),
            (0.0, 4.0),
            "two double-width glyphs occupy 4 cells total"
        );

        // `find` returns the first cell's center, not the span's midpoint,
        // which preserves earlier behaviour where a click there always lands
        // inside the match regardless of span width.
        let (x, y) = driver.find("你好").expect("find should also locate it");
        assert_eq!((x, y), (0.5, 0.5));

        // "world" is offset past the 4-cell CJK run and the separating
        // space — proves the continuation-cell fix didn't just get the
        // *first* match right by accident.
        let world = driver
            .find_bounds("world")
            .expect("world should be found after the CJK run");
        assert_eq!(world.x, cjk.x + cjk.width + 1.0);
    }

    /// Regression guard: adjacent double-width glyphs must read back as a
    /// contiguous substring, with no phantom space inserted for the wide
    /// glyph's (always-blank) continuation cell — the same needle
    /// [`find_bounds`] above already located as one 4-cell span. Before
    /// this fix `screen()` walked one buffer column per iteration, so
    /// `screen_contains`/`screen_has` (the direct backer of the
    /// conformance suite's `assert_screen_has` step) disagreed with
    /// `find_bounds`/`inventory` on the exact same frame.
    #[test]
    fn screen_is_wide_char_aware_for_adjacent_cjk_glyphs() {
        let driver = TuiDriver::new(
            OneLineApp {
                text: "你好 world"
            },
            30,
            3,
        );

        assert!(
            driver.screen_contains("你好"),
            "screen() must not insert a phantom space inside a contiguous \
             double-width run:\n{}",
            driver.screen()
        );
        assert!(
            driver.screen_contains("你好 world"),
            "the whole line must read back exactly as painted:\n{}",
            driver.screen()
        );
    }

    /// `ConformanceDriver` smoke test for the TUI impl: `new_fixture` (the
    /// `LogicalViewport`-aligned constructor), `scroll_at`, and
    /// `inventory` all round-trip through the promoted trait.
    #[test]
    fn conformance_driver_new_fixture_scroll_at_and_inventory_round_trip() {
        let driver: TuiDriver<OneLineApp> = ConformanceDriver::new_fixture(
            OneLineApp {
                text: "你好 world"
            },
            LogicalViewport::new(30, 3),
        );
        assert_eq!(
            (
                driver.backend().viewport().width,
                driver.backend().viewport().height
            ),
            (30.0, 3.0),
            "new_fixture's LogicalViewport should map straight through to TUI cells"
        );

        let inv = ConformanceDriver::inventory(&driver);
        let texts: Vec<&str> = inv.text_runs().iter().map(|t| t.text.as_str()).collect();
        assert!(
            texts.contains(&"你好") && texts.contains(&"world"),
            "inventory() should synthesize a TextRun per whitespace-separated \
             run on the cell grid, wide-char aware: {texts:?}"
        );

        // scroll_at must not panic when the target text exists — the
        // dispatched Scroll event's downstream handling is exercised by
        // primitive-level tests; this just confirms the driver-level
        // find-then-dispatch plumbing works.
        let mut driver = driver;
        driver.scroll_at("world", 1);
    }

    /// Records every `UiEvent` the app's `handle()` receives, verbatim —
    /// lets a test assert exactly what reached the app (button, modifiers,
    /// `DoubleClick` vs `MouseDown`) without screen-scraping.
    #[derive(Default)]
    struct EventRecorder {
        events: Rc<RefCell<Vec<UiEvent>>>,
    }

    impl AppLogic for EventRecorder {
        type AreaId = ();

        fn render(&self, _backend: &mut dyn Backend, _area: ()) {}

        fn handle(&mut self, event: UiEvent, _backend: &mut dyn Backend) -> Reaction {
            self.events.borrow_mut().push(event);
            Reaction::Continue
        }
    }

    /// `double_click` must deliver exactly one `UiEvent::DoubleClick` at
    /// the given position, with no prior `MouseDown`, and do so
    /// identically whether it's the 1st or 1000th call — the deterministic
    /// alternative to two `click()`s racing `DoubleClickDetector`'s 400ms
    /// wall-clock window.
    #[test]
    fn double_click_is_deterministic_across_many_calls() {
        let events = Rc::new(RefCell::new(Vec::new()));
        let app = EventRecorder {
            events: events.clone(),
        };
        let mut driver = TuiDriver::new(app, 20, 5);

        for i in 0..1000 {
            events.borrow_mut().clear();
            driver.double_click(3.0, 2.0);
            let recorded = events.borrow();
            assert_eq!(
                recorded.len(),
                1,
                "call {i}: double_click should deliver exactly one event, got {recorded:?}"
            );
            assert_eq!(
                recorded[0],
                UiEvent::DoubleClick {
                    widget: None,
                    position: Point::new(3.0, 2.0),
                },
                "call {i}: expected a single DoubleClick with no prior MouseDown"
            );
        }
    }

    /// `middle_click` delivers `MouseDown { button: MouseButton::Middle, .. }`.
    #[test]
    fn middle_click_delivers_middle_mouse_down() {
        let events = Rc::new(RefCell::new(Vec::new()));
        let app = EventRecorder {
            events: events.clone(),
        };
        let mut driver = TuiDriver::new(app, 20, 5);

        driver.middle_click(4.0, 1.0);

        let recorded = events.borrow();
        assert_eq!(recorded.len(), 1);
        assert_eq!(
            recorded[0],
            UiEvent::MouseDown {
                widget: None,
                button: MouseButton::Middle,
                position: Point::new(4.0, 1.0),
                modifiers: Modifiers::default(),
            }
        );
    }

    /// `right_click` delivers `MouseDown { button: MouseButton::Right, .. }`.
    #[test]
    fn right_click_delivers_right_mouse_down() {
        let events = Rc::new(RefCell::new(Vec::new()));
        let app = EventRecorder {
            events: events.clone(),
        };
        let mut driver = TuiDriver::new(app, 20, 5);

        driver.right_click(4.0, 1.0);

        let recorded = events.borrow();
        assert_eq!(recorded.len(), 1);
        assert_eq!(
            recorded[0],
            UiEvent::MouseDown {
                widget: None,
                button: MouseButton::Right,
                position: Point::new(4.0, 1.0),
                modifiers: Modifiers::default(),
            }
        );
    }

    /// `click_with` threads an arbitrary button + modifiers through to the
    /// app, e.g. a Ctrl-held left click.
    #[test]
    fn click_with_delivers_button_and_modifiers() {
        let events = Rc::new(RefCell::new(Vec::new()));
        let app = EventRecorder {
            events: events.clone(),
        };
        let mut driver = TuiDriver::new(app, 20, 5);

        let ctrl = Modifiers {
            ctrl: true,
            ..Modifiers::default()
        };
        driver.click_with(MouseButton::Left, ctrl, 2.0, 3.0);

        let recorded = events.borrow();
        assert_eq!(recorded.len(), 1);
        assert_eq!(
            recorded[0],
            UiEvent::MouseDown {
                widget: None,
                button: MouseButton::Left,
                position: Point::new(2.0, 3.0),
                modifiers: ctrl,
            }
        );
    }

    /// With folding disabled, two `click()` calls in immediate succession
    /// (well inside `DoubleClickDetector`'s 400ms/1.5-cell window) must
    /// deliver two `MouseDown` events and zero `DoubleClick` — proving the
    /// toggle actually suppresses folding rather than just being ignored.
    #[test]
    fn folding_disabled_keeps_two_clicks_separate() {
        let events = Rc::new(RefCell::new(Vec::new()));
        let app = EventRecorder {
            events: events.clone(),
        };
        let mut driver = TuiDriver::new(app, 20, 5);
        driver.set_double_click_folding(false);

        driver.click(5.0, 5.0);
        driver.click(5.0, 5.0);

        let recorded = events.borrow();
        let mouse_downs = recorded
            .iter()
            .filter(|e| matches!(e, UiEvent::MouseDown { .. }))
            .count();
        let double_clicks = recorded
            .iter()
            .filter(|e| matches!(e, UiEvent::DoubleClick { .. }))
            .count();
        assert_eq!(
            (mouse_downs, double_clicks),
            (2, 0),
            "folding disabled should yield two MouseDowns and no DoubleClick, got {recorded:?}"
        );
    }

    /// Sanity check for the *default* (folding enabled) behaviour this
    /// issue must not regress: two `click()`s at the same position can
    /// still fold into a `DoubleClick` via the real detector when a test
    /// wants that (contrast the deterministic `double_click` helper above,
    /// which needs no timing at all).
    #[test]
    fn folding_enabled_by_default_still_folds_two_clicks() {
        let events = Rc::new(RefCell::new(Vec::new()));
        let app = EventRecorder {
            events: events.clone(),
        };
        let mut driver = TuiDriver::new(app, 20, 5);

        driver.click(5.0, 5.0);
        driver.click(5.0, 5.0);

        let recorded = events.borrow();
        let double_clicks = recorded
            .iter()
            .filter(|e| matches!(e, UiEvent::DoubleClick { .. }))
            .count();
        assert_eq!(
            double_clicks, 1,
            "default folding should still fold two same-position clicks: {recorded:?}"
        );
    }

    use crate::primitives::tab_bar::{TabBar, TabItem};
    use crate::theme::Theme;
    use crate::types::WidgetId as QWidgetId;

    /// Paints a two-tab `TabBar` — one permanent, one preview — so a test
    /// can assert `style_at`/`styled_row` distinguish them. Mirrors the
    /// `OneLineApp` pattern above but drives `draw_tab_bar` instead of
    /// `draw_status_bar`.
    struct TabBarApp {
        theme: Theme,
    }

    impl AppLogic for TabBarApp {
        type AreaId = ();

        fn render(&self, backend: &mut dyn Backend, _area: ()) {
            backend.set_theme(self.theme);
            backend.draw_tab_bar(
                Rect::new(0.0, 0.0, 20.0, 1.0),
                &TabBar {
                    id: QWidgetId::new("tabs"),
                    tabs: vec![
                        TabItem {
                            label: "perm".to_string(),
                            is_active: false,
                            is_dirty: false,
                            is_preview: false,
                            is_closable: false,
                        },
                        TabItem {
                            label: "prev".to_string(),
                            is_active: false,
                            is_dirty: false,
                            is_preview: true,
                            is_closable: false,
                        },
                    ],
                    scroll_offset: 0,
                    right_segments: vec![],
                    active_accent: None,
                    show_tab_close: false,
                    compact: false,
                },
                None,
            );
        }

        fn handle(&mut self, _event: UiEvent, _backend: &mut dyn Backend) -> Reaction {
            Reaction::Continue
        }
    }

    /// Acceptance criterion: `style_at` distinguishes a preview tab from a
    /// permanent one by `Modifier::ITALIC` and by the theme's
    /// `tab_preview_inactive_fg` vs `tab_inactive_fg` foreground — neither
    /// of which `screen()` can see, since both tabs stringify identically
    #[test]
    fn style_at_distinguishes_preview_tab_from_permanent_tab() {
        let theme = Theme::default();
        let driver = TuiDriver::new(TabBarApp { theme }, 20, 1);

        // Neither tab reserves a close button (`is_closable: false` on
        // both), so each gets a trailing `TAB_SEPARATOR_COLS` column
        // instead: "perm" occupies cells 0..5 ("perm" + separator at 4),
        // "prev" occupies cells 5..10 ("prev" + separator at 9); the rest
        // of the 20-cell bar is blank fill.
        assert!(driver.screen_contains("perm│prev│"));

        let perm_style = driver
            .style_at(0, 0)
            .expect("(0, 0) is inside the 20x1 buffer");
        let preview_style = driver
            .style_at(5, 0)
            .expect("(5, 0) is inside the 20x1 buffer");

        assert!(
            !perm_style.modifiers.contains(Modifier::ITALIC),
            "permanent tab must not be italic: {perm_style:?}"
        );
        assert!(
            preview_style.modifiers.contains(Modifier::ITALIC),
            "preview tab must be italic: {preview_style:?}"
        );

        assert_eq!(
            perm_style.fg,
            crate::tui::ratatui_color(theme.tab_inactive_fg),
            "permanent tab's fg should be the theme's tab_inactive_fg token"
        );
        assert_eq!(
            preview_style.fg,
            crate::tui::ratatui_color(theme.tab_preview_inactive_fg),
            "preview tab's fg should be the theme's tab_preview_inactive_fg token"
        );
    }

    /// `style_at` outside the buffer returns `None` rather than panicking
    /// (acceptance criterion) — both past the right/bottom edge and on the
    /// exact boundary row/column, which is the first out-of-range value.
    #[test]
    fn style_at_out_of_bounds_returns_none() {
        let driver = TuiDriver::new(
            TabBarApp {
                theme: Theme::default(),
            },
            20,
            1,
        );

        assert!(
            driver.style_at(20, 0).is_none(),
            "x == width is out of range"
        );
        assert!(
            driver.style_at(0, 1).is_none(),
            "y == height is out of range"
        );
        assert!(
            driver.style_at(100, 100).is_none(),
            "far outside is out of range"
        );
        assert!(
            driver.style_at(19, 0).is_some(),
            "x == width - 1 is the last valid column"
        );
    }

    /// `styled_row(y).len()` equals the buffer width for any in-range `y`
    /// (acceptance criterion), and is empty for an out-of-range row rather
    /// than panicking.
    #[test]
    fn styled_row_length_matches_buffer_width() {
        let driver = TuiDriver::new(
            TabBarApp {
                theme: Theme::default(),
            },
            20,
            1,
        );

        let row = driver.styled_row(0);
        assert_eq!(row.len(), 20, "row length should equal buffer width");
        assert_eq!(
            row.iter().map(|(ch, _)| *ch).collect::<String>(),
            // 10 chars of content ("perm" + "│" + "prev" + "│") + 10
            // spaces of blank fill = 20 chars. Not byte-sliced like the
            // old all-ASCII literal: "│" is multi-byte in UTF-8, so a
            // byte-index slice would land mid-codepoint.
            "perm│prev│          ",
            "styled_row's chars should match screen()'s glyphs left to right"
        );
        // Same per-cell distinction as style_at, reachable via styled_row.
        // Column 4 is the separator after "perm", not part of either tab's
        // label — the preview tab's own first cell is column 5.
        assert!(!row[0].1.modifiers.contains(Modifier::ITALIC));
        assert!(row[5].1.modifiers.contains(Modifier::ITALIC));

        assert!(
            driver.styled_row(1).is_empty(),
            "out-of-range row should be empty, not panic"
        );
    }

    /// A compact tab bar with no close buttons (`show_tab_close: false`).
    /// Drives the real `TuiBackend::draw_tab_bar`/`tab_bar_layout`
    /// measurer end to end — not a hand-rolled `TabMeasure` — so a
    /// regression in any of the backend's own measurer call sites breaks
    /// here, not only in `tab_bar.rs`'s rasteriser-level unit test.
    struct NoCloseTabBarApp {
        active: usize,
    }

    impl NoCloseTabBarApp {
        const RECT: Rect = Rect::new(0.0, 0.0, 20.0, 1.0);

        fn bar(&self) -> TabBar {
            TabBar {
                id: QWidgetId::new("tabs"),
                tabs: vec![
                    TabItem {
                        label: "Problems".into(),
                        is_active: self.active == 0,
                        is_dirty: false,
                        is_preview: false,
                        is_closable: false,
                    },
                    TabItem {
                        label: "Output".into(),
                        is_active: self.active == 1,
                        is_dirty: false,
                        is_preview: false,
                        is_closable: false,
                    },
                ],
                scroll_offset: 0,
                right_segments: vec![],
                active_accent: None,
                show_tab_close: false,
                compact: true,
            }
        }
    }

    impl AppLogic for NoCloseTabBarApp {
        type AreaId = ();

        #[allow(deprecated)] // `Backend::draw_tab_bar` returns `TabBarHits` — issue #823
        fn render(&self, backend: &mut dyn Backend, _area: ()) {
            backend.draw_tab_bar(Self::RECT, &self.bar(), None);
        }

        #[allow(deprecated)] // `Backend::tab_bar_layout` returns `TabBarHits` — issue #823
        fn handle(&mut self, event: UiEvent, backend: &mut dyn Backend) -> Reaction {
            let UiEvent::MouseDown { position, .. } = event else {
                return Reaction::Continue;
            };
            let hits = backend.tab_bar_layout(Self::RECT, &self.bar());
            let x = position.x as f64;
            for (i, (sx, ex)) in hits.slot_positions.iter().enumerate() {
                if x >= *sx && x < *ex {
                    self.active = i;
                    return Reaction::Redraw;
                }
            }
            Reaction::Continue
        }
    }

    /// Acceptance criterion for quadraui's compact-tab-bar separator: the
    /// backend's own measurer (not a test's hand-rolled `TabMeasure`)
    /// reserves a `TAB_SEPARATOR_CHAR` column after each no-close tab, and
    /// that column stays part of the tab it follows rather than becoming a
    /// dead click zone.
    #[test]
    fn no_close_tab_bar_separator_click_through_backend_measurer_activates_its_tab() {
        let app = NoCloseTabBarApp { active: 1 };
        let mut driver = TuiDriver::new(app, 20, 1);

        assert!(
            driver.screen_contains("Problems│Output│"),
            "the real backend measurer should reserve a separator column after each no-close tab"
        );

        let (sep_x, sep_y) = driver
            .find("│")
            .expect("separator glyph should be visible on the 20-cell bar");
        driver.click(sep_x, sep_y);
        assert_eq!(
            driver.app().active,
            0,
            "clicking the separator painted right after tab 0's label should activate tab 0, \
             not fall through to a dead zone"
        );
    }

    // ─── quadraui#594 ────────────────────────────────────────────────────

    /// Three closable tabs on a 20-cell bar. `handle()` hit-tests raw
    /// `MouseDown` clicks against the *live*, non-painting
    /// [`crate::Backend::tab_bar_layout`] query (mirrors the "cached
    /// layout hit-test pattern" `gtk/testing.rs`'s `ToggleStatusBarApp`
    /// uses) — close regions win over tab bodies, matching
    /// `TabBarLayout::hit_regions`' close-before-body ordering. Exercises
    /// `TuiDriver::tab_center`/`tab_close_center` end-to-end: the driver
    /// locates a specific tab's click target with no hardcoded
    /// coordinates, and the resulting click reaches the app through the
    /// ordinary event path.
    struct InteractiveTabBarApp {
        active: usize,
        closed_idx: Option<usize>,
    }

    impl InteractiveTabBarApp {
        const RECT: Rect = Rect::new(0.0, 0.0, 20.0, 1.0);

        fn bar(&self) -> TabBar {
            TabBar {
                id: QWidgetId::new("tabs"),
                tabs: (0..3)
                    .map(|i| TabItem {
                        label: format!("tab{i}"),
                        is_active: i == self.active,
                        is_dirty: false,
                        is_preview: false,
                        is_closable: true,
                    })
                    .collect(),
                scroll_offset: 0,
                right_segments: vec![],
                active_accent: None,
                show_tab_close: true,
                compact: false,
            }
        }
    }

    impl AppLogic for InteractiveTabBarApp {
        type AreaId = ();

        #[allow(deprecated)] // `Backend::draw_tab_bar` returns `TabBarHits` — issue #823
        fn render(&self, backend: &mut dyn Backend, _area: ()) {
            backend.draw_tab_bar(Self::RECT, &self.bar(), None);
        }

        #[allow(deprecated)] // `Backend::tab_bar_layout` returns `TabBarHits` — issue #823
        fn handle(&mut self, event: UiEvent, backend: &mut dyn Backend) -> Reaction {
            let UiEvent::MouseDown { position, .. } = event else {
                return Reaction::Continue;
            };
            let hits = backend.tab_bar_layout(Self::RECT, &self.bar());
            let x = position.x as f64;
            // Close regions before tab bodies — mirrors
            // `TabBarLayout::hit_regions`' ordering (close-before-body).
            for (i, close) in hits.close_bounds.iter().enumerate() {
                if let Some((sx, ex)) = close {
                    if x >= *sx && x < *ex {
                        self.closed_idx = Some(i);
                        return Reaction::Redraw;
                    }
                }
            }
            for (i, (sx, ex)) in hits.slot_positions.iter().enumerate() {
                if x >= *sx && x < *ex {
                    self.active = i;
                    return Reaction::Redraw;
                }
            }
            Reaction::Continue
        }
    }

    /// Acceptance criterion: with a 3-tab bar rendered,
    /// `driver.click(tab_center(&bar, 1))` activates tab 1.
    #[test]
    fn tab_center_click_activates_target_tab() {
        let app = InteractiveTabBarApp {
            active: 0,
            closed_idx: None,
        };
        let mut driver = TuiDriver::new(app, 20, 1);
        let bar_id = QWidgetId::new("tabs");

        assert_eq!(driver.app().active, 0);
        let (x, y) = driver
            .tab_center(&bar_id, 1)
            .expect("tab 1 should be visible on a 20-cell bar with 3 short tabs");
        driver.click(x, y);

        assert_eq!(
            driver.app().active,
            1,
            "clicking tab 1's center should activate it"
        );
    }

    /// Acceptance criterion: `driver.click(tab_close_center(&bar, 1))`
    /// closes tab 1 and does **not** merely activate it.
    #[test]
    fn tab_close_center_click_closes_target_tab_not_just_activates() {
        let app = InteractiveTabBarApp {
            active: 0,
            closed_idx: None,
        };
        let mut driver = TuiDriver::new(app, 20, 1);
        let bar_id = QWidgetId::new("tabs");

        let (x, y) = driver
            .tab_close_center(&bar_id, 1)
            .expect("tab 1 is closable and should have a close button");
        driver.click(x, y);

        assert_eq!(
            driver.app().closed_idx,
            Some(1),
            "clicking tab 1's close button should close it"
        );
        assert_ne!(
            driver.app().active,
            1,
            "closing tab 1 must not merely activate it — active should stay at its prior value"
        );
    }

    /// The rect a consumer hit-tests against must be the *same* rect
    /// `draw_tab_bar` painted. `tab_close_center` resolves against the
    /// [`crate::TabBarLayout`] `TuiBackend` cached at paint time; this test
    /// takes that exact point and runs it back through the authoritative
    /// no-paint accessor, [`crate::Backend::resolve_tab_bar_layout`] +
    /// [`crate::TabBarLayout::hit_test`] — not the deprecated
    /// `TabBarHits`/`tab_bar_layout` pair — and asserts it resolves to
    /// `TabBarHit::TabClose(1)`, not a plain tab-select. If this is green,
    /// the TUI backend's paint and hit-test rects agree.
    #[test]
    fn tab_close_center_resolves_to_authoritative_tab_close_hit() {
        use crate::primitives::tab_bar::TabBarHit;

        let app = InteractiveTabBarApp {
            active: 0,
            closed_idx: None,
        };
        let mut driver = TuiDriver::new(app, 20, 1);
        let bar_id = QWidgetId::new("tabs");
        let bar = driver.app().bar();

        let (x, y) = driver
            .tab_close_center(&bar_id, 1)
            .expect("tab 1 is closable and should have a close button");

        let resolved = crate::Backend::resolve_tab_bar_layout(
            driver.core.backend_mut(),
            InteractiveTabBarApp::RECT,
            &bar,
        );
        // `resolve_tab_bar_layout` is bar-relative (see its doc), while
        // `tab_close_center` returns absolute driver/screen coordinates —
        // undo the `rect.x`/`rect.y` shift `TuiDriver::tab_close_center`
        // applied before handing the point to `hit_test`.
        let local_x = x - InteractiveTabBarApp::RECT.x;
        let local_y = y - InteractiveTabBarApp::RECT.y;

        assert_eq!(
            resolved.hit_test(local_x, local_y),
            TabBarHit::TabClose(1),
            "the close-glyph rect `draw_tab_bar` painted must be the same \
             rect a consumer hit-tests against"
        );
    }

    /// Acceptance criterion: `tab_close_center` returns `None` for a tab
    /// rendered with `is_closable: false`.
    #[test]
    fn tab_close_center_none_when_tab_not_closable() {
        struct NonClosableTabBarApp;

        impl AppLogic for NonClosableTabBarApp {
            type AreaId = ();

            fn render(&self, backend: &mut dyn Backend, _area: ()) {
                backend.draw_tab_bar(
                    Rect::new(0.0, 0.0, 20.0, 1.0),
                    &TabBar {
                        id: QWidgetId::new("tabs"),
                        tabs: vec![TabItem {
                            label: "a".to_string(),
                            is_active: true,
                            is_dirty: false,
                            is_preview: false,
                            is_closable: false,
                        }],
                        scroll_offset: 0,
                        right_segments: vec![],
                        active_accent: None,
                        show_tab_close: true,
                        compact: false,
                    },
                    None,
                );
            }

            fn handle(&mut self, _event: UiEvent, _backend: &mut dyn Backend) -> Reaction {
                Reaction::Continue
            }
        }

        let driver = TuiDriver::new(NonClosableTabBarApp, 20, 1);
        let bar_id = QWidgetId::new("tabs");

        assert!(
            driver.tab_center(&bar_id, 0).is_some(),
            "the tab itself is visible"
        );
        assert!(
            driver.tab_close_center(&bar_id, 0).is_none(),
            "a non-closable tab drew no close button, so its center must be None"
        );
    }

    /// Acceptance criterion: `tab_center` returns `None` for a tab hidden
    /// behind the bar's `scroll_offset`.
    #[test]
    fn tab_center_none_when_scrolled_out_of_view() {
        struct ScrolledTabBarApp;

        impl AppLogic for ScrolledTabBarApp {
            type AreaId = ();

            fn render(&self, backend: &mut dyn Backend, _area: ()) {
                backend.draw_tab_bar(
                    Rect::new(0.0, 0.0, 5.0, 1.0),
                    &TabBar {
                        id: QWidgetId::new("tabs"),
                        tabs: vec![
                            TabItem {
                                label: "a".to_string(),
                                is_active: false,
                                is_dirty: false,
                                is_preview: false,
                                is_closable: true,
                            },
                            TabItem {
                                label: "b".to_string(),
                                is_active: true,
                                is_dirty: false,
                                is_preview: false,
                                is_closable: true,
                            },
                            TabItem {
                                label: "c".to_string(),
                                is_active: false,
                                is_dirty: false,
                                is_preview: false,
                                is_closable: true,
                            },
                        ],
                        // Bar is far too narrow (5 cells) for all 3 tabs
                        // (each ~3 cells with a close button); TUI honours
                        // this caller-supplied offset directly rather than
                        // computing its own (no scroll arrows in TUI).
                        scroll_offset: 1,
                        right_segments: vec![],
                        active_accent: None,
                        show_tab_close: true,
                        compact: false,
                    },
                    None,
                );
            }

            fn handle(&mut self, _event: UiEvent, _backend: &mut dyn Backend) -> Reaction {
                Reaction::Continue
            }
        }

        let driver = TuiDriver::new(ScrolledTabBarApp, 5, 1);
        let bar_id = QWidgetId::new("tabs");

        assert!(
            driver.tab_center(&bar_id, 0).is_none(),
            "tab 0 is scrolled out of view behind scroll_offset=1"
        );
        assert!(driver.tab_close_center(&bar_id, 0).is_none());
        assert!(
            driver.tab_center(&bar_id, 1).is_some(),
            "tab 1 (the scroll target) should still be visible"
        );
    }

    // ── Background-thread wake ──────────────────────────────────────────
    //
    // Before `Backend::waker` existed, there was no `Send`-safe way to get
    // a value from a background thread into the app at all — every
    // runner was poll-only (grep any `run.rs` for `mpsc`/`Waker`/
    // `channel(`: zero hits). These tests are the driver-tier proof this
    // is fixed for TUI: a real `std::thread::spawn`'d thread calls the
    // closure `Backend::waker` hands out, and the payload it carries
    // reaches `AppLogic::handle` and changes what's rendered — the same
    // production `dispatch_event` pipeline every other driver test in this
    // module exercises, not a bypass.

    /// Renders whatever string the most recent `UiEvent::User` carried, or
    /// "waiting" if none has arrived yet — so the test can assert on a
    /// *rendered* screen change, not just that `handle` was called.
    struct BackgroundResultApp {
        last: String,
    }

    impl BackgroundResultApp {
        fn new() -> Self {
            Self {
                last: "waiting".to_string(),
            }
        }
    }

    impl AppLogic for BackgroundResultApp {
        type AreaId = ();

        fn render(&self, backend: &mut dyn Backend, _area: ()) {
            backend.draw_text_display(
                Rect::new(0.0, 0.0, 40.0, 1.0),
                &crate::TextDisplay {
                    id: QWidgetId::new("result"),
                    lines: vec![crate::TextDisplayLine {
                        spans: vec![crate::types::StyledSpan::plain(self.last.clone())],
                        decoration: Default::default(),
                        timestamp: None,
                    }],
                    scroll_offset: 0,
                    auto_scroll: true,
                    max_lines: 0,
                    has_focus: false,
                    title: None,
                    show_scrollbar: false,
                },
            );
        }

        fn handle(&mut self, event: UiEvent, _backend: &mut dyn Backend) -> Reaction {
            match event {
                UiEvent::User(payload) => {
                    if let Some(text) = payload.downcast_ref::<String>() {
                        self.last = text.clone();
                        Reaction::Redraw
                    } else {
                        Reaction::Continue
                    }
                }
                _ => Reaction::Continue,
            }
        }
    }

    /// Calling the closure `Backend::waker()` returns from another thread
    /// delivers a `UiEvent::User` that reaches `AppLogic::handle` and
    /// produces a rendered change.
    #[test]
    fn waker_delivers_background_thread_result_to_rendered_output() {
        let mut driver = TuiDriver::new(BackgroundResultApp::new(), 40, 3);
        assert!(
            driver.screen_contains("waiting"),
            "initial render should show the placeholder"
        );

        let waker = driver.backend().waker();
        let handle = std::thread::spawn(move || {
            waker(crate::UserPayload::new("done from background".to_string()));
        });
        handle.join().expect("background thread must not panic");

        let reaction = driver.pump_user_events();
        assert_eq!(
            reaction,
            Reaction::Redraw,
            "delivering the queued UiEvent::User must trigger a redraw"
        );
        assert!(
            driver.screen_contains("done from background"),
            "the background thread's payload must reach rendered output"
        );
        assert!(
            !driver.screen_contains("waiting"),
            "the placeholder must be gone once the real result renders"
        );
    }

    /// The closure `waker()` returns is reusable and thread-agnostic: two
    /// separate background threads can each call it once, and both
    /// payloads are delivered in the order they were pushed.
    #[test]
    fn waker_delivers_multiple_background_payloads_in_order() {
        let mut driver = TuiDriver::new(BackgroundResultApp::new(), 40, 3);

        let waker1 = driver.backend().waker();
        let waker2 = driver.backend().waker();
        let t1 = std::thread::spawn(move || {
            waker1(crate::UserPayload::new("first".to_string()));
        });
        t1.join().unwrap();
        let t2 = std::thread::spawn(move || {
            waker2(crate::UserPayload::new("second".to_string()));
        });
        t2.join().unwrap();

        driver.pump_user_events();
        assert!(
            driver.screen_contains("second"),
            "the later payload must win — both were delivered in push order"
        );
    }

    use crate::compose::app_shell::{AppShellLayout, PanelDefinition};
    use crate::shell::ShellContext;
    use crate::types::Icon;

    /// Minimal `ShellApp` that flips `nerd_fonts_enabled` once in `setup()`
    /// — the pattern `Backend::set_nerd_fonts`'s own doc recommends for a
    /// flag that's static for the process lifetime.
    struct NerdFontsToggleApp {
        nerd_fonts: bool,
    }

    impl ShellApp for NerdFontsToggleApp {
        fn render_content(&self, _backend: &mut dyn Backend, _layout: &AppShellLayout) {}

        fn handle(
            &mut self,
            _event: UiEvent,
            _backend: &mut dyn Backend,
            _ctx: &ShellContext,
        ) -> Reaction {
            Reaction::Continue
        }

        fn setup(&mut self, backend: &mut dyn Backend) {
            backend.set_nerd_fonts(self.nerd_fonts);
        }
    }

    /// The `ShellConfig` used by acceptance tests: one top panel and one
    /// bottom item, each carrying a `with_panel_icon` override whose
    /// glyph/fallback halves are unmistakable non-ASCII symbols — chosen so
    /// they can't collide with any chrome text (titles, tooltips) the shell
    /// itself paints.
    fn nerd_fonts_toggle_config() -> ShellConfig {
        let panel_id = WidgetId::new("panel:explorer");
        let bottom_id = WidgetId::new("panel:settings");
        ShellConfig::new(
            "Demo",
            vec![PanelDefinition {
                id: panel_id.clone(),
                icon: "X".to_string(),
                tooltip: "Explorer".to_string(),
                title: "EXPLORER".to_string(),
            }],
        )
        .with_bottom_items(vec![PanelDefinition {
            id: bottom_id.clone(),
            icon: "Y".to_string(),
            tooltip: "Settings".to_string(),
            title: "SETTINGS".to_string(),
        }])
        .with_panel_icon(panel_id, Icon::new("\u{f07c}", "\u{25c6}"))
        .with_panel_icon(bottom_id, Icon::new("\u{f013}", "\u{25a0}"))
    }

    /// A `ShellConfig` panel-icon override, rendered through
    /// `driver_with_shell` — the consumer path a real `ShellApp` takes —
    /// with `nerd_fonts_enabled` off, paints the fallback.
    #[test]
    fn shell_config_panel_icon_paints_fallback_when_nerd_fonts_off() {
        let driver = driver_with_shell(
            NerdFontsToggleApp { nerd_fonts: false },
            nerd_fonts_toggle_config(),
            40,
            10,
        );
        assert!(
            driver.screen_contains("\u{25c6}"),
            "nerd_fonts off must paint the top panel's fallback glyph"
        );
        assert!(
            !driver.screen_contains("\u{f07c}"),
            "nerd_fonts off must not paint the top panel's Nerd Font glyph"
        );
    }

    /// The same config with `nerd_fonts_enabled` on paints the glyph.
    #[test]
    fn shell_config_panel_icon_paints_glyph_when_nerd_fonts_on() {
        let driver = driver_with_shell(
            NerdFontsToggleApp { nerd_fonts: true },
            nerd_fonts_toggle_config(),
            40,
            10,
        );
        assert!(
            driver.screen_contains("\u{f07c}"),
            "nerd_fonts on must paint the top panel's Nerd Font glyph"
        );
        assert!(
            !driver.screen_contains("\u{25c6}"),
            "nerd_fonts on must not paint the top panel's fallback glyph"
        );
    }

    /// Bottom-item half: fallback when nerd_fonts is off — pins that the
    /// override reaches `bottom_items`, not just `panels`.
    #[test]
    fn shell_config_panel_icon_paints_fallback_for_bottom_item_when_nerd_fonts_off() {
        let driver = driver_with_shell(
            NerdFontsToggleApp { nerd_fonts: false },
            nerd_fonts_toggle_config(),
            40,
            10,
        );
        assert!(
            driver.screen_contains("\u{25a0}"),
            "nerd_fonts off must paint the bottom item's fallback glyph"
        );
        assert!(
            !driver.screen_contains("\u{f013}"),
            "nerd_fonts off must not paint the bottom item's Nerd Font glyph"
        );
    }

    /// Bottom-item half: glyph when nerd_fonts is on.
    #[test]
    fn shell_config_panel_icon_paints_glyph_for_bottom_item_when_nerd_fonts_on() {
        let driver = driver_with_shell(
            NerdFontsToggleApp { nerd_fonts: true },
            nerd_fonts_toggle_config(),
            40,
            10,
        );
        assert!(
            driver.screen_contains("\u{f013}"),
            "nerd_fonts on must paint the bottom item's Nerd Font glyph"
        );
        assert!(
            !driver.screen_contains("\u{25a0}"),
            "nerd_fonts on must not paint the bottom item's fallback glyph"
        );
    }

    /// `resize` must shrink the physical `TestBackend` buffer immediately,
    /// but leave `TuiBackend`'s cached viewport exactly as it was after the
    /// last `render()` — the stale-layout-vs-shrunk-buffer condition,
    /// reproducible from a driver test instead of only a hand-built
    /// `Buffer`/`Rect` pair.
    #[test]
    fn resize_shrinks_buffer_without_touching_cached_viewport() {
        let mut driver = TuiDriver::new(
            OneLineApp {
                text: "hello resize world",
            },
            30,
            3,
        );
        assert_eq!(
            (
                driver.backend().viewport().width,
                driver.backend().viewport().height
            ),
            (30.0, 3.0),
            "sanity: TuiBackend's viewport should start matching the driver's initial size"
        );

        driver.resize(5, 1);

        // The physical buffer shrank immediately...
        let area = driver.terminal.borrow().backend().buffer().area;
        assert_eq!(
            (area.width, area.height),
            (5, 1),
            "resize() must shrink the TestBackend's real cell buffer right away"
        );

        // ...but TuiBackend's cached viewport must NOT have moved: `resize`
        // never reaches into `TuiBackend` at all, only `render()` does.
        assert_eq!(
            (
                driver.backend().viewport().width,
                driver.backend().viewport().height
            ),
            (30.0, 3.0),
            "resize() must not touch TuiBackend's cached viewport — it should stay stale \
             at the pre-resize size until the next render()"
        );

        // Driving a repaint against a buffer now far smaller than the
        // app's content assumes must not panic — the stale-layout-vs-shrunk-
        // buffer condition, reproduced here at driver tier rather than by
        // hand-building a `Buffer`/`Rect` pair.
        driver.render();

        // render()'s autoresize rebases TuiBackend's viewport to
        // the new, smaller size before `app.render()` runs, so it's back
        // in sync afterward.
        assert_eq!(
            (
                driver.backend().viewport().width,
                driver.backend().viewport().height
            ),
            (5.0, 1.0),
            "render() after resize() must resync TuiBackend's viewport to the new size"
        );
    }
}