diff --git a/core/src/ui/render_state.rs b/core/src/ui/render_state.rs
index 4bd7a90..476dc78 100644
--- a/core/src/ui/render_state.rs
+++ b/core/src/ui/render_state.rs
@@ -34,6 +34,10 @@ impl UiRenderState {
         self.resized = true;
     }
 
+    pub fn output_size(&self) -> Vec2 {
+        self.output_size
+    }
+
     pub fn update<'a>(&mut self, root: impl Into<Option<&'a StrongWidget>>, rsc: &mut dyn UiRsc) {
         // safety mechanism for memory leaks; might wanna return a result instead so user can
         // decide whether to panic or not
diff --git a/src/default/mod.rs b/src/default/mod.rs
index ea00eb7..0577309 100644
--- a/src/default/mod.rs
+++ b/src/default/mod.rs
@@ -64,6 +64,12 @@ impl DefaultUiState {
     }
 }
 
+impl WakeTaskQueue for Window {
+    fn wake(&self) {
+        self.request_redraw();
+    }
+}
+
 pub trait HasDefaultUiState: Sized + 'static {
     fn default_state(&self) -> &DefaultUiState;
     fn default_state_mut(&mut self) -> &mut DefaultUiState;
@@ -105,8 +111,8 @@ pub struct DefaultRsc<State: 'static> {
 }
 
 impl<State> DefaultRsc<State> {
-    fn init(window: Arc<Window>) -> (Self, TaskMsgReceiver<Self>) {
-        let (tasks, recv) = Tasks::init(window);
+    pub fn init(wake: Arc<dyn WakeTaskQueue>) -> (Self, TaskMsgReceiver<Self>) {
+        let (tasks, recv) = Tasks::init(wake);
         (
             Self {
                 ui: Default::default(),
diff --git a/src/default/task.rs b/src/default/task.rs
index 36b55c1..80588b9 100644
--- a/src/default/task.rs
+++ b/src/default/task.rs
@@ -13,7 +13,13 @@ use tokio::{
         unbounded_channel as async_channel,
     },
 };
-use winit::window::Window;
+
+/// Wakes the host so it applies queued task updates. A task reaches the
+/// application only through [`TaskCtx::update`], so this is all the queue
+/// needs of a platform.
+pub trait WakeTaskQueue: Send + Sync + 'static {
+    fn wake(&self);
+}
 
 pub type TaskMsgSender<Rsc> = SyncSender<Box<dyn TaskUpdate<Rsc>>>;
 pub type TaskMsgReceiver<Rsc> = SyncReceiver<Box<dyn TaskUpdate<Rsc>>>;
@@ -23,29 +29,32 @@ impl<F: FnOnce(&mut Rsc::State, &mut Rsc) + Send, Rsc: HasState> TaskUpdate<Rsc>
 
 pub struct Tasks<Rsc: HasState> {
     start: AsyncSender<BoxTask>,
-    window: Arc<Window>,
+    wake: Arc<dyn WakeTaskQueue>,
     msg_send: SyncSender<Box<dyn TaskUpdate<Rsc>>>,
 }
 
 pub struct TaskCtx<Rsc: HasState> {
     send: TaskMsgSender<Rsc>,
+    wake: Arc<dyn WakeTaskQueue>,
 }
 
 impl<Rsc: HasState> TaskCtx<Rsc> {
     pub fn update(&mut self, f: impl TaskUpdate<Rsc> + 'static) {
-        let _ = self.send.send(Box::new(f));
+        if self.send.send(Box::new(f)).is_ok() {
+            self.wake.wake();
+        }
     }
 }
 impl<Rsc: HasState + 'static> TaskCtx<Rsc> {
-    fn new(send: TaskMsgSender<Rsc>) -> Self {
-        Self { send }
+    fn new(send: TaskMsgSender<Rsc>, wake: Arc<dyn WakeTaskQueue>) -> Self {
+        Self { send, wake }
     }
 }
 
 type BoxTask = Pin<Box<dyn Future<Output = ()> + Send>>;
 
 impl<Rsc: HasState> Tasks<Rsc> {
-    pub fn init(window: Arc<Window>) -> (Self, TaskMsgReceiver<Rsc>) {
+    pub fn init(wake: Arc<dyn WakeTaskQueue>) -> (Self, TaskMsgReceiver<Rsc>) {
         let (start, start_recv) = async_channel();
         let (msgs, msgs_recv) = sync_channel();
         std::thread::spawn(|| {
@@ -56,7 +65,7 @@ impl<Rsc: HasState> Tasks<Rsc> {
             Self {
                 start,
                 msg_send: msgs,
-                window,
+                wake,
             },
             msgs_recv,
         )
@@ -67,10 +76,9 @@ impl<Rsc: HasState> Tasks<Rsc> {
         F::CallOnceFuture: Send,
     {
         let send = self.msg_send.clone();
-        let window = self.window.clone();
+        let wake = self.wake.clone();
         let _ = self.start.send(Box::pin(async move {
-            task(TaskCtx::new(send)).await;
-            window.request_redraw();
+            task(TaskCtx::new(send, wake)).await;
         }));
     }
 }
@@ -80,3 +88,48 @@ async fn listen(mut recv: AsyncReceiver<BoxTask>) {
         tokio::spawn(task);
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::{sync::mpsc::sync_channel, time::Duration};
+
+    struct TestRsc;
+
+    impl HasState for TestRsc {
+        type State = usize;
+    }
+
+    /// Signals rather than counts, so the test waits for a wake instead of
+    /// racing the task thread to sample it.
+    struct WakeSignal(std::sync::mpsc::SyncSender<()>);
+
+    impl WakeTaskQueue for WakeSignal {
+        fn wake(&self) {
+            let _ = self.0.send(());
+        }
+    }
+
+    #[test]
+    fn every_update_wakes_the_host() {
+        let (woken, wakes) = sync_channel(8);
+        let (mut tasks, updates) = Tasks::<TestRsc>::init(Arc::new(WakeSignal(woken)));
+
+        tasks.spawn(async move |mut ctx| {
+            ctx.update(|state: &mut usize, _| *state += 1);
+            ctx.update(|state: &mut usize, _| *state += 2);
+        });
+
+        let second = Duration::from_secs(1);
+        let (mut state, mut rsc) = (0, TestRsc);
+        for _ in 0..2 {
+            wakes.recv_timeout(second).expect("no wake for an update");
+            updates.recv_timeout(second).unwrap()(&mut state, &mut rsc);
+        }
+        assert_eq!(state, 3);
+        assert!(
+            wakes.recv_timeout(Duration::from_millis(100)).is_err(),
+            "woken with nothing to apply"
+        );
+    }
+}
diff --git a/src/harness.rs b/src/harness.rs
new file mode 100644
index 0000000..1a268ff
--- /dev/null
+++ b/src/harness.rs
@@ -0,0 +1,154 @@
+//! A ui with no window: build a tree, run frames, move a pointer, and read
+//! back where widgets landed.
+//!
+//! It does not draw. A claim about pixels still needs a real surface.
+
+use crate::prelude::*;
+use std::{sync::Arc, time::Duration};
+
+/// The harness drains the update queue itself, so there is no loop to wake.
+struct NoWake;
+
+impl WakeTaskQueue for NoWake {
+    fn wake(&self) {}
+}
+
+#[derive(Default)]
+pub struct HarnessState {
+    pub root: Option<StrongWidget>,
+}
+
+impl HasRoot for HarnessState {
+    fn set_root(&mut self, root: StrongWidget) {
+        self.root = Some(root);
+    }
+}
+
+pub struct Harness {
+    pub rsc: DefaultRsc<HarnessState>,
+    pub render: UiRenderState,
+    pub state: HarnessState,
+    updates: TaskMsgReceiver<DefaultRsc<HarnessState>>,
+    cursor: CursorState,
+}
+
+impl Harness {
+    /// `size` is the output in physical pixels.
+    pub fn new(size: impl Into<Vec2>) -> Self {
+        let (rsc, updates) = DefaultRsc::init(Arc::new(NoWake));
+        let mut render = UiRenderState::new();
+        render.resize(size);
+        Self {
+            rsc,
+            render,
+            state: HarnessState::default(),
+            updates,
+            cursor: CursorState::default(),
+        }
+    }
+
+    pub fn size(&self) -> Vec2 {
+        self.render.output_size()
+    }
+
+    pub fn resize(&mut self, size: impl Into<Vec2>) {
+        self.render.resize(size);
+    }
+
+    /// Sets the root and lays it out, so a pointer event has something to hit.
+    pub fn set_root<T>(&mut self, widget: impl WidgetLike<DefaultRsc<HarnessState>, T>) {
+        widget.set_root(&mut self.rsc, &mut self.state);
+        self.frame();
+    }
+
+    pub fn needs_redraw(&self) -> bool {
+        self.render
+            .needs_redraw(&self.state.root, self.rsc.widgets())
+    }
+
+    pub fn apply_updates(&mut self) -> usize {
+        let mut applied = 0;
+        while let Ok(update) = self.updates.try_recv() {
+            update(&mut self.state, &mut self.rsc);
+            applied += 1;
+        }
+        applied
+    }
+
+    /// Waits for a task's first update, then applies everything waiting.
+    /// False if none arrived in time.
+    #[must_use]
+    pub fn await_update(&mut self, timeout: Duration) -> bool {
+        let Ok(update) = self.updates.recv_timeout(timeout) else {
+            return false;
+        };
+        update(&mut self.state, &mut self.rsc);
+        self.apply_updates();
+        true
+    }
+
+    /// Lays the tree out and builds its primitives.
+    pub fn frame(&mut self) {
+        self.apply_updates();
+        self.render.update(&self.state.root, &mut self.rsc);
+    }
+
+    /// Where the last frame put a widget, or `None` if it drew nothing.
+    pub fn region(&self, id: &impl IdLike) -> Option<PixelRegion> {
+        self.render.window_region(id)
+    }
+
+    pub fn move_to(&mut self, pos: impl Into<Vec2>) {
+        self.cursor.pos = pos.into();
+        self.cursor.exists = true;
+        self.sense();
+    }
+
+    pub fn leave(&mut self) {
+        self.cursor.exists = false;
+        self.sense();
+    }
+
+    pub fn press(&mut self, button: CursorButton) {
+        self.button(button).update(true);
+        self.sense();
+    }
+
+    pub fn release(&mut self, button: CursorButton) {
+        self.button(button).update(false);
+        self.sense();
+    }
+
+    /// A wheel carries no position, so this goes wherever
+    /// [`move_to`](Self::move_to) last put the cursor -- nowhere, until it has
+    /// been called.
+    pub fn scroll(&mut self, delta: impl Into<Vec2>) {
+        self.cursor.scroll_delta = delta.into();
+        self.sense();
+    }
+
+    pub fn click(&mut self, pos: impl Into<Vec2>) {
+        self.move_to(pos);
+        self.press(CursorButton::Left);
+        self.release(CursorButton::Left);
+    }
+
+    fn button(&mut self, button: CursorButton) -> &mut ActivationState {
+        let buttons = &mut self.cursor.buttons;
+        match button {
+            CursorButton::Left => &mut buttons.left,
+            CursorButton::Middle => &mut buttons.middle,
+            CursorButton::Right => &mut buttons.right,
+        }
+    }
+
+    /// Dispatches against the last [`frame`](Self::frame)'s layout, which is
+    /// what a window delivers input against too.
+    fn sense(&mut self) {
+        let cursor = self.cursor.clone();
+        let size = self.render.output_size();
+        self.render
+            .run_sensors(&mut self.rsc, &mut self.state, cursor, size);
+        self.cursor.end_frame();
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
index 05ef101..1e50b70 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -7,6 +7,7 @@
 
 pub mod default;
 pub mod event;
+pub mod harness;
 pub mod widget;
 
 pub use iris_core as core;
diff --git a/tests/harness.rs b/tests/harness.rs
new file mode 100644
index 0000000..52262c8
--- /dev/null
+++ b/tests/harness.rs
@@ -0,0 +1,140 @@
+//! Layout, hit testing and task updates, driven without a window.
+
+use std::{cell::RefCell, rc::Rc, time::Duration};
+
+use iris::harness::Harness;
+use iris::prelude::*;
+
+/// `PixelRegion` neither compares nor prints.
+fn corners(h: &Harness, id: &impl IdLike) -> (f32, f32, f32, f32) {
+    let region = h.region(id).expect("widget drew nothing");
+    (
+        region.top_left.x,
+        region.top_left.y,
+        region.bot_right.x,
+        region.bot_right.y,
+    )
+}
+
+/// A fixed 100 wide, and the rest of the 400 to its neighbour.
+fn two_rects(h: &mut Harness) -> (WidgetId, WidgetId) {
+    let left = rect(Color::RED).width(100).add(&mut h.rsc);
+    let right = rect(Color::BLUE).add(&mut h.rsc);
+    h.set_root((left, right).span(Dir::RIGHT));
+    (left.id(), right.id())
+}
+
+#[test]
+fn a_span_gives_each_child_the_width_it_asked_for() {
+    let mut h = Harness::new((400, 200));
+    let (left, right) = two_rects(&mut h);
+
+    assert_eq!(corners(&h, &left), (0.0, 0.0, 100.0, 200.0));
+    assert_eq!(corners(&h, &right), (100.0, 0.0, 400.0, 200.0));
+}
+
+#[test]
+fn resizing_relays_out_against_the_new_output() {
+    let mut h = Harness::new((400, 200));
+    let (left, right) = two_rects(&mut h);
+
+    h.resize((800, 100));
+    assert!(h.needs_redraw());
+    h.frame();
+
+    assert_eq!(corners(&h, &left), (0.0, 0.0, 100.0, 100.0));
+    assert_eq!(corners(&h, &right), (100.0, 0.0, 800.0, 100.0));
+}
+
+#[test]
+fn a_press_reaches_only_the_widget_under_the_cursor() {
+    let mut h = Harness::new((400, 200));
+    let clicks = Rc::new(RefCell::new(Vec::new()));
+
+    let (on_left, on_right) = (clicks.clone(), clicks.clone());
+    let left = rect(Color::RED)
+        .width(100)
+        .on(CursorSense::click(), move |_, _| {
+            on_left.borrow_mut().push("left")
+        })
+        .add(&mut h.rsc);
+    let right = rect(Color::BLUE)
+        .on(CursorSense::click(), move |_, _| {
+            on_right.borrow_mut().push("right")
+        })
+        .add(&mut h.rsc);
+    h.set_root((left, right).span(Dir::RIGHT));
+
+    h.click((50, 100));
+    assert_eq!(*clicks.borrow(), ["left"]);
+
+    h.click((300, 100));
+    assert_eq!(*clicks.borrow(), ["left", "right"]);
+}
+
+#[test]
+fn hover_ends_when_the_cursor_leaves_the_window() {
+    let mut h = Harness::new((400, 200));
+    let hovered = Rc::new(RefCell::new(0));
+    let ended = Rc::new(RefCell::new(0));
+
+    let (h_count, e_count) = (hovered.clone(), ended.clone());
+    let widget = rect(Color::RED)
+        .on(CursorSense::HoverStart, move |_, _| {
+            *h_count.borrow_mut() += 1
+        })
+        .on(CursorSense::HoverEnd, move |_, _| {
+            *e_count.borrow_mut() += 1
+        })
+        .add(&mut h.rsc);
+    h.set_root(widget);
+
+    h.move_to((200, 100));
+    assert_eq!((*hovered.borrow(), *ended.borrow()), (1, 0));
+
+    // A second sample inside the same widget is not a second hover.
+    h.move_to((210, 100));
+    assert_eq!((*hovered.borrow(), *ended.borrow()), (1, 0));
+
+    h.leave();
+    assert_eq!((*hovered.borrow(), *ended.borrow()), (1, 1));
+}
+
+#[test]
+fn a_task_update_reaches_the_tree() {
+    let mut h = Harness::new((400, 200));
+    let widget = rect(Color::RED).add(&mut h.rsc);
+    h.set_root(widget.task_on(CursorSense::click(), async move |mut ctx| {
+        ctx.update(move |_, rsc| widget(rsc).color = Color::BLUE);
+    }));
+
+    h.click((200, 100));
+
+    assert!(
+        h.await_update(Duration::from_secs(5)),
+        "the task sent no update"
+    );
+    assert_eq!(h.rsc[widget].color, Color::BLUE);
+}
+
+#[test]
+fn a_wheel_scrolls_the_content_and_stops_at_its_end() {
+    let mut h = Harness::new((400, 200));
+    // Twice the window's height, so there is 200 to scroll.
+    let top = rect(Color::RED).height(200).add(&mut h.rsc);
+    let bottom = rect(Color::BLUE).height(200).add(&mut h.rsc);
+    h.set_root((top, bottom).span(Dir::DOWN).scrollable());
+    h.move_to((200, 100));
+
+    // `Scroll` starts snapped to the end.
+    assert_eq!(corners(&h, &top).1, -200.0);
+
+    // The handler scales a wheel line by 50.
+    h.scroll((0, 1));
+    h.frame();
+    assert_eq!(corners(&h, &top).1, -150.0);
+
+    h.scroll((0, 10));
+    h.frame();
+    assert_eq!(corners(&h, &top).1, 0.0);
+}