input_pipeline/

pointer_sensor_scale_handler.rs

// Copyright 2022 The Fuchsia Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

//!
use crate::input_handler::{InputHandlerStatus, UnhandledInputHandler};
use crate::utils::Position;
use crate::{input_device, metrics, mouse_binding};
use async_trait::async_trait;
use fuchsia_inspect::health::Reporter;

use metrics_registry::*;
use std::cell::RefCell;
use std::num::FpCategory;
use std::rc::Rc;

pub struct PointerSensorScaleHandler {
    mutable_state: RefCell<MutableState>,

    /// The inventory of this handler's Inspect status.
    pub inspect_status: InputHandlerStatus,

    /// The metrics logger.
    metrics_logger: metrics::MetricsLogger,
}

struct MutableState {
    /// The time of the last processed mouse move event.
    last_move_timestamp: Option<zx::MonotonicInstant>,
    /// The time of the last processed mouse scroll event.
    last_scroll_timestamp: Option<zx::MonotonicInstant>,
}

/// For tick based scrolling, PointerSensorScaleHandler scales tick * 120 to logical
/// pixel.
const PIXELS_PER_TICK: f32 = 120.0;

/// TODO(https://fxbug.dev/42059911): Temporary apply a linear scale factor to scroll to make it feel
/// faster.
const SCALE_SCROLL: f32 = 2.0;

#[async_trait(?Send)]
impl UnhandledInputHandler for PointerSensorScaleHandler {
    async fn handle_unhandled_input_event(
        self: Rc<Self>,
        unhandled_input_event: input_device::UnhandledInputEvent,
    ) -> Vec<input_device::InputEvent> {
        fuchsia_trace::duration!(c"input", c"pointer_sensor_scale_handler");
        match unhandled_input_event.clone() {
            input_device::UnhandledInputEvent {
                device_event:
                    input_device::InputDeviceEvent::Mouse(mouse_binding::MouseEvent {
                        location:
                            mouse_binding::MouseLocation::Relative(mouse_binding::RelativeLocation {
                                millimeters: raw_motion,
                            }),
                        wheel_delta_v,
                        wheel_delta_h,
                        // Only the `Move` phase carries non-zero motion.
                        phase: phase @ mouse_binding::MousePhase::Move,
                        affected_buttons,
                        pressed_buttons,
                        is_precision_scroll,
                        wake_lease,
                    }),
                device_descriptor:
                    input_device::InputDeviceDescriptor::Mouse(mouse_binding::MouseDeviceDescriptor {
                        absolute_x_range,
                        absolute_y_range,
                        buttons,
                        counts_per_mm,
                        device_id,
                        wheel_h_range,
                        wheel_v_range,
                    }),
                event_time,
                trace_id,
            } => {
                fuchsia_trace::duration!(c"input", c"pointer_sensor_scale_handler[processing]");
                let tracing_id = trace_id.unwrap_or_else(|| 0.into());
                fuchsia_trace::flow_step!(c"input", c"event_in_input_pipeline", tracing_id);

                self.inspect_status.count_received_event(&event_time);
                let scaled_motion = self.scale_motion(raw_motion, event_time);
                let input_event = input_device::InputEvent {
                    device_event: input_device::InputDeviceEvent::Mouse(
                        mouse_binding::MouseEvent {
                            location: mouse_binding::MouseLocation::Relative(
                                mouse_binding::RelativeLocation { millimeters: scaled_motion },
                            ),
                            wheel_delta_v,
                            wheel_delta_h,
                            phase,
                            affected_buttons,
                            pressed_buttons,
                            is_precision_scroll,
                            wake_lease,
                        },
                    ),
                    device_descriptor: input_device::InputDeviceDescriptor::Mouse(
                        mouse_binding::MouseDeviceDescriptor {
                            absolute_x_range,
                            absolute_y_range,
                            buttons,
                            counts_per_mm,
                            device_id,
                            wheel_h_range,
                            wheel_v_range,
                        },
                    ),
                    event_time,
                    handled: input_device::Handled::No,
                    trace_id,
                };
                vec![input_event]
            }
            input_device::UnhandledInputEvent {
                device_event:
                    input_device::InputDeviceEvent::Mouse(mouse_binding::MouseEvent {
                        location,
                        wheel_delta_v,
                        wheel_delta_h,
                        phase: phase @ mouse_binding::MousePhase::Wheel,
                        affected_buttons,
                        pressed_buttons,
                        is_precision_scroll,
                        wake_lease,
                    }),
                device_descriptor:
                    input_device::InputDeviceDescriptor::Mouse(mouse_binding::MouseDeviceDescriptor {
                        absolute_x_range,
                        absolute_y_range,
                        buttons,
                        counts_per_mm,
                        device_id,
                        wheel_h_range,
                        wheel_v_range,
                    }),
                event_time,
                trace_id,
            } => {
                fuchsia_trace::duration!(c"input", c"pointer_sensor_scale_handler[processing]");
                if let Some(trace_id) = trace_id {
                    fuchsia_trace::flow_step!(
                        c"input",
                        c"event_in_input_pipeline",
                        trace_id.into()
                    );
                }

                self.inspect_status.count_received_event(&event_time);
                let scaled_wheel_delta_v = self.scale_scroll(wheel_delta_v, event_time);
                let scaled_wheel_delta_h = self.scale_scroll(wheel_delta_h, event_time);
                let input_event = input_device::InputEvent {
                    device_event: input_device::InputDeviceEvent::Mouse(
                        mouse_binding::MouseEvent {
                            location,
                            wheel_delta_v: scaled_wheel_delta_v,
                            wheel_delta_h: scaled_wheel_delta_h,
                            phase,
                            affected_buttons,
                            pressed_buttons,
                            is_precision_scroll,
                            wake_lease,
                        },
                    ),
                    device_descriptor: input_device::InputDeviceDescriptor::Mouse(
                        mouse_binding::MouseDeviceDescriptor {
                            absolute_x_range,
                            absolute_y_range,
                            buttons,
                            counts_per_mm,
                            device_id,
                            wheel_h_range,
                            wheel_v_range,
                        },
                    ),
                    event_time,
                    handled: input_device::Handled::No,
                    trace_id,
                };
                vec![input_event]
            }
            _ => vec![input_device::InputEvent::from(unhandled_input_event)],
        }
    }

    fn set_handler_healthy(self: std::rc::Rc<Self>) {
        self.inspect_status.health_node.borrow_mut().set_ok();
    }

    fn set_handler_unhealthy(self: std::rc::Rc<Self>, msg: &str) {
        self.inspect_status.health_node.borrow_mut().set_unhealthy(msg);
    }
}

// The minimum reasonable delay between intentional mouse movements.
// This value
// * Is used to compensate for time compression if the driver gets
//   backlogged.
// * Is set to accommodate up to 10 kHZ event reporting.
//
// TODO(https://fxbug.dev/42181307): Use the polling rate instead of event timestamps.
const MIN_PLAUSIBLE_EVENT_DELAY: zx::MonotonicDuration = zx::MonotonicDuration::from_micros(100);

// The maximum reasonable delay between intentional mouse movements.
// This value is used to compute speed for the first mouse motion after
// a long idle period.
//
// Alternatively:
// 1. The code could use the uncapped delay. However, this would lead to
//    very slow initial motion after a long idle period.
// 2. Wait until a second report comes in. However, older mice generate
//    reports at 125 HZ, which would mean an 8 msec delay.
//
// TODO(https://fxbug.dev/42181307): Use the polling rate instead of event timestamps.
const MAX_PLAUSIBLE_EVENT_DELAY: zx::MonotonicDuration = zx::MonotonicDuration::from_millis(50);

const MAX_SENSOR_COUNTS_PER_INCH: f32 = 20_000.0; // From https://sensor.fyi/sensors
const MAX_SENSOR_COUNTS_PER_MM: f32 = MAX_SENSOR_COUNTS_PER_INCH / 12.7;
const MIN_MEASURABLE_DISTANCE_MM: f32 = 1.0 / MAX_SENSOR_COUNTS_PER_MM;
const MAX_PLAUSIBLE_EVENT_DELAY_SECS: f32 = MAX_PLAUSIBLE_EVENT_DELAY.into_nanos() as f32 / 1E9;
const MIN_MEASURABLE_VELOCITY_MM_PER_SEC: f32 =
    MIN_MEASURABLE_DISTANCE_MM / MAX_PLAUSIBLE_EVENT_DELAY_SECS;

// Define the buckets which determine which mapping to use.
// * Speeds below the beginning of the medium range use the low-speed mapping.
// * Speeds within the medium range use the medium-speed mapping.
// * Speeds above the end of the medium range use the high-speed mapping.
const MEDIUM_SPEED_RANGE_BEGIN_MM_PER_SEC: f32 = 32.0;
const MEDIUM_SPEED_RANGE_END_MM_PER_SEC: f32 = 150.0;

// A linear factor affecting the responsiveness of the pointer to motion.
// A higher numbness indicates lower responsiveness.
const NUMBNESS: f32 = 37.5;

impl PointerSensorScaleHandler {
    /// Creates a new [`PointerSensorScaleHandler`].
    ///
    /// Returns `Rc<Self>`.
    pub fn new(
        input_handlers_node: &fuchsia_inspect::Node,
        metrics_logger: metrics::MetricsLogger,
    ) -> Rc<Self> {
        let inspect_status = InputHandlerStatus::new(
            input_handlers_node,
            "pointer_sensor_scale_handler",
            /* generates_events */ false,
        );
        Rc::new(Self {
            mutable_state: RefCell::new(MutableState {
                last_move_timestamp: None,
                last_scroll_timestamp: None,
            }),
            inspect_status,
            metrics_logger,
        })
    }

    // Linearly scales `movement_mm_per_sec`.
    //
    // Given the values of `MEDIUM_SPEED_RANGE_BEGIN_MM_PER_SEC` and
    // `NUMBNESS` above, this results in downscaling the motion.
    fn scale_low_speed(movement_mm_per_sec: f32) -> f32 {
        const LINEAR_SCALE_FACTOR: f32 = MEDIUM_SPEED_RANGE_BEGIN_MM_PER_SEC / NUMBNESS;
        LINEAR_SCALE_FACTOR * movement_mm_per_sec
    }

    // Quadratically scales `movement_mm_per_sec`.
    //
    // The scale factor is chosen so that the composite curve is
    // continuous as the speed transitions from the low-speed
    // bucket to the medium-speed bucket.
    //
    // Note that the composite curve is _not_ differentiable at the
    // transition from low-speed to medium-speed, since the
    // slope on the left side of the point
    // (MEDIUM_SPEED_RANGE_BEGIN_MM_PER_SEC / NUMBNESS)
    // is different from the slope on the right side of the point
    // (2 * MEDIUM_SPEED_RANGE_BEGIN_MM_PER_SEC / NUMBNESS).
    //
    // However, the transition works well enough in practice.
    fn scale_medium_speed(movement_mm_per_sec: f32) -> f32 {
        const QUARDRATIC_SCALE_FACTOR: f32 = 1.0 / NUMBNESS;
        QUARDRATIC_SCALE_FACTOR * movement_mm_per_sec * movement_mm_per_sec
    }

    // Linearly scales `movement_mm_per_sec`.
    //
    // The parameters are chosen so that
    // 1. The composite curve is continuous as the speed transitions
    //    from the medium-speed bucket to the high-speed bucket.
    // 2. The composite curve is differentiable.
    fn scale_high_speed(movement_mm_per_sec: f32) -> f32 {
        // Use linear scaling equal to the slope of `scale_medium_speed()`
        // at the transition point.
        const LINEAR_SCALE_FACTOR: f32 = 2.0 * (MEDIUM_SPEED_RANGE_END_MM_PER_SEC / NUMBNESS);

        // Compute offset so the composite curve is continuous.
        const Y_AT_MEDIUM_SPEED_RANGE_END_MM_PER_SEC: f32 =
            MEDIUM_SPEED_RANGE_END_MM_PER_SEC * MEDIUM_SPEED_RANGE_END_MM_PER_SEC / NUMBNESS;
        const OFFSET: f32 = Y_AT_MEDIUM_SPEED_RANGE_END_MM_PER_SEC
            - LINEAR_SCALE_FACTOR * MEDIUM_SPEED_RANGE_END_MM_PER_SEC;

        // Apply the computed transformation.
        LINEAR_SCALE_FACTOR * movement_mm_per_sec + OFFSET
    }

    // Scales Euclidean velocity by one of the scale_*_speed_motion() functions above,
    // choosing the function based on `MEDIUM_SPEED_RANGE_BEGIN_MM_PER_SEC` and
    // `MEDIUM_SPEED_RANGE_END_MM_PER_SEC`.
    fn scale_euclidean_velocity(raw_velocity: f32) -> f32 {
        if (0.0..MEDIUM_SPEED_RANGE_BEGIN_MM_PER_SEC).contains(&raw_velocity) {
            Self::scale_low_speed(raw_velocity)
        } else if (MEDIUM_SPEED_RANGE_BEGIN_MM_PER_SEC..MEDIUM_SPEED_RANGE_END_MM_PER_SEC)
            .contains(&raw_velocity)
        {
            Self::scale_medium_speed(raw_velocity)
        } else {
            Self::scale_high_speed(raw_velocity)
        }
    }

    /// Scales `movement_mm`.
    fn scale_motion(&self, movement_mm: Position, event_time: zx::MonotonicInstant) -> Position {
        // Determine the duration of this `movement`.
        let elapsed_time_secs =
            match self.mutable_state.borrow_mut().last_move_timestamp.replace(event_time) {
                Some(last_event_time) => (event_time - last_event_time)
                    .clamp(MIN_PLAUSIBLE_EVENT_DELAY, MAX_PLAUSIBLE_EVENT_DELAY),
                None => MAX_PLAUSIBLE_EVENT_DELAY,
            }
            .into_nanos() as f32
                / 1E9;

        // Compute the velocity in each dimension.
        let x_mm_per_sec = movement_mm.x / elapsed_time_secs;
        let y_mm_per_sec = movement_mm.y / elapsed_time_secs;

        let euclidean_velocity =
            f32::sqrt(x_mm_per_sec * x_mm_per_sec + y_mm_per_sec * y_mm_per_sec);
        if euclidean_velocity < MIN_MEASURABLE_VELOCITY_MM_PER_SEC {
            // Avoid division by zero that would come from computing `scale_factor` below.
            return movement_mm;
        }

        // Compute the scaling factor to be applied to each dimension.
        //
        // Geometrically, this is a bit dodgy when there's movement along both
        // dimensions. Specifically: the `OFFSET` for high-speed motion should be
        // constant, but the way its used here scales the offset based on velocity.
        //
        // Nonetheless, this works well enough in practice.
        let scale_factor = Self::scale_euclidean_velocity(euclidean_velocity) / euclidean_velocity;

        // Apply the scale factor and return the result.
        let scaled_movement_mm = scale_factor * movement_mm;

        match (scaled_movement_mm.x.classify(), scaled_movement_mm.y.classify()) {
            (FpCategory::Infinite | FpCategory::Nan, _)
            | (_, FpCategory::Infinite | FpCategory::Nan) => {
                // Backstop, in case the code above missed some cases of bad arithmetic.
                // Avoid sending `Infinite` or `Nan` values, since such values will
                // poison the `current_position` in `MouseInjectorHandlerInner`.
                // That manifests as the pointer becoming invisible, and never
                // moving again.
                //
                // TODO(https://fxbug.dev/42181389) Add a triage rule to highlight the
                // implications of this message.
                self.metrics_logger.log_error(
                    InputPipelineErrorMetricDimensionEvent::PointerSensorScaleHandlerScaledMotionInvalid,
                    std::format!(
                        "skipped motion; scaled movement of {:?} is infinite or NaN; x is {:?}, and y is {:?}",
                        scaled_movement_mm,
                        scaled_movement_mm.x.classify(),
                        scaled_movement_mm.y.classify(),
                ));
                Position { x: 0.0, y: 0.0 }
            }
            _ => scaled_movement_mm,
        }
    }

    /// `scroll_mm` scale with the curve algorithm.
    /// `scroll_tick` scale with 120.
    fn scale_scroll(
        &self,
        wheel_delta: Option<mouse_binding::WheelDelta>,
        event_time: zx::MonotonicInstant,
    ) -> Option<mouse_binding::WheelDelta> {
        match wheel_delta {
            None => None,
            Some(mouse_binding::WheelDelta {
                raw_data: mouse_binding::RawWheelDelta::Ticks(tick),
                ..
            }) => Some(mouse_binding::WheelDelta {
                raw_data: mouse_binding::RawWheelDelta::Ticks(tick),
                physical_pixel: Some(tick as f32 * PIXELS_PER_TICK),
            }),
            Some(mouse_binding::WheelDelta {
                raw_data: mouse_binding::RawWheelDelta::Millimeters(mm),
                ..
            }) => {
                // Determine the duration of this `scroll`.
                let elapsed_time_secs =
                    match self.mutable_state.borrow_mut().last_scroll_timestamp.replace(event_time)
                    {
                        Some(last_event_time) => (event_time - last_event_time)
                            .clamp(MIN_PLAUSIBLE_EVENT_DELAY, MAX_PLAUSIBLE_EVENT_DELAY),
                        None => MAX_PLAUSIBLE_EVENT_DELAY,
                    }
                    .into_nanos() as f32
                        / 1E9;

                let velocity = mm.abs() / elapsed_time_secs;

                if velocity < MIN_MEASURABLE_VELOCITY_MM_PER_SEC {
                    // Avoid division by zero that would come from computing
                    // `scale_factor` below.
                    return Some(mouse_binding::WheelDelta {
                        raw_data: mouse_binding::RawWheelDelta::Millimeters(mm),
                        physical_pixel: Some(SCALE_SCROLL * mm),
                    });
                }

                let scale_factor = Self::scale_euclidean_velocity(velocity) / velocity;

                // Apply the scale factor and return the result.
                let scaled_scroll_mm = SCALE_SCROLL * scale_factor * mm;

                if scaled_scroll_mm.is_infinite() || scaled_scroll_mm.is_nan() {
                    self.metrics_logger.log_error(
                        InputPipelineErrorMetricDimensionEvent::PointerSensorScaleHandlerScaledScrollInvalid,
                        std::format!(
                            "skipped scroll; scaled scroll of {:?} is infinite or NaN.",
                            scaled_scroll_mm,
                    ));
                    return Some(mouse_binding::WheelDelta {
                        raw_data: mouse_binding::RawWheelDelta::Millimeters(mm),
                        physical_pixel: Some(SCALE_SCROLL * mm),
                    });
                }

                Some(mouse_binding::WheelDelta {
                    raw_data: mouse_binding::RawWheelDelta::Millimeters(mm),
                    physical_pixel: Some(scaled_scroll_mm),
                })
            }
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::input_handler::InputHandler;
    use crate::testing_utilities;
    use assert_matches::assert_matches;
    use maplit::hashset;
    use std::cell::Cell;
    use std::ops::Add;
    use test_util::{assert_gt, assert_lt, assert_near};
    use {fuchsia_async as fasync, fuchsia_inspect};

    const COUNTS_PER_MM: f32 = 12.0;
    const DEVICE_DESCRIPTOR: input_device::InputDeviceDescriptor =
        input_device::InputDeviceDescriptor::Mouse(mouse_binding::MouseDeviceDescriptor {
            device_id: 0,
            absolute_x_range: None,
            absolute_y_range: None,
            wheel_v_range: None,
            wheel_h_range: None,
            buttons: None,
            counts_per_mm: COUNTS_PER_MM as u32,
        });

    // Maximum tolerable difference between "equal" scale factors. This is
    // likely higher than FP rounding error can explain, but still small
    // enough that there would be no user-perceptible difference.
    //
    // Rationale for not being user-perceptible: this requires the raw
    // movement to have a count of 100,000, before there's a unit change
    // in the scaled motion.
    //
    // On even the highest resolution sensor (per https://sensor.fyi/sensors),
    // that would require 127mm (5 inches) of motion within one sampling
    // interval.
    //
    // In the unlikely case that the high resolution sensor is paired
    // with a low polling rate, that works out to 127mm/8msec, or _at least_
    // 57 km/hr.
    const SCALE_EPSILON: f32 = 1.0 / 100_000.0;

    std::thread_local! {static NEXT_EVENT_TIME: Cell<i64> = Cell::new(0)}

    fn make_unhandled_input_event(
        mouse_event: mouse_binding::MouseEvent,
    ) -> input_device::UnhandledInputEvent {
        let event_time = NEXT_EVENT_TIME.with(|t| {
            let old = t.get();
            t.set(old + 1);
            old
        });
        input_device::UnhandledInputEvent {
            device_event: input_device::InputDeviceEvent::Mouse(mouse_event),
            device_descriptor: DEVICE_DESCRIPTOR.clone(),
            event_time: zx::MonotonicInstant::from_nanos(event_time),
            trace_id: None,
        }
    }

    #[fuchsia::test]
    async fn pointer_sensor_scale_handler_initialized_with_inspect_node() {
        let inspector = fuchsia_inspect::Inspector::default();
        let fake_handlers_node = inspector.root().create_child("input_handlers_node");
        let _handler =
            PointerSensorScaleHandler::new(&fake_handlers_node, metrics::MetricsLogger::default());
        diagnostics_assertions::assert_data_tree!(inspector, root: {
            input_handlers_node: {
                pointer_sensor_scale_handler: {
                    events_received_count: 0u64,
                    events_handled_count: 0u64,
                    last_received_timestamp_ns: 0u64,
                    "fuchsia.inspect.Health": {
                        status: "STARTING_UP",
                        // Timestamp value is unpredictable and not relevant in this context,
                        // so we only assert that the property is present.
                        start_timestamp_nanos: diagnostics_assertions::AnyProperty
                    },
                }
            }
        });
    }

    #[fasync::run_singlethreaded(test)]
    async fn pointer_sensor_scale_handler_inspect_counts_events() {
        let inspector = fuchsia_inspect::Inspector::default();
        let fake_handlers_node = inspector.root().create_child("input_handlers_node");
        let handler =
            PointerSensorScaleHandler::new(&fake_handlers_node, metrics::MetricsLogger::default());

        let event_time1 = zx::MonotonicInstant::get();
        let event_time2 = event_time1.add(zx::MonotonicDuration::from_micros(1));
        let event_time3 = event_time2.add(zx::MonotonicDuration::from_micros(1));

        let input_events = vec![
            testing_utilities::create_mouse_event(
                mouse_binding::MouseLocation::Absolute(Position { x: 0.0, y: 0.0 }),
                None, /* wheel_delta_v */
                None, /* wheel_delta_h */
                None, /* is_precision_scroll */
                mouse_binding::MousePhase::Wheel,
                hashset! {},
                hashset! {},
                event_time1,
                &DEVICE_DESCRIPTOR,
            ),
            testing_utilities::create_mouse_event(
                mouse_binding::MouseLocation::Relative(mouse_binding::RelativeLocation {
                    millimeters: Position { x: 1.5 / COUNTS_PER_MM, y: 4.5 / COUNTS_PER_MM },
                }),
                None, /* wheel_delta_v */
                None, /* wheel_delta_h */
                None, /* is_precision_scroll */
                mouse_binding::MousePhase::Move,
                hashset! {},
                hashset! {},
                event_time2,
                &DEVICE_DESCRIPTOR,
            ),
            // Should not count non-mouse input events.
            testing_utilities::create_fake_input_event(event_time2),
            // Should not count received events that have already been handled.
            testing_utilities::create_mouse_event_with_handled(
                mouse_binding::MouseLocation::Absolute(Position { x: 0.0, y: 0.0 }),
                None, /* wheel_delta_v */
                None, /* wheel_delta_h */
                None, /* is_precision_scroll */
                mouse_binding::MousePhase::Wheel,
                hashset! {},
                hashset! {},
                event_time3,
                &DEVICE_DESCRIPTOR,
                input_device::Handled::Yes,
            ),
        ];

        for input_event in input_events {
            let _ = handler.clone().handle_input_event(input_event).await;
        }

        let last_received_event_time: u64 = event_time2.into_nanos().try_into().unwrap();

        diagnostics_assertions::assert_data_tree!(inspector, root: {
            input_handlers_node: {
                pointer_sensor_scale_handler: {
                    events_received_count: 2u64,
                    events_handled_count: 0u64,
                    last_received_timestamp_ns: last_received_event_time,
                    "fuchsia.inspect.Health": {
                        status: "STARTING_UP",
                        // Timestamp value is unpredictable and not relevant in this context,
                        // so we only assert that the property is present.
                        start_timestamp_nanos: diagnostics_assertions::AnyProperty
                    },
                }
            }
        });
    }

    // While its generally preferred to write tests against the public API of
    // a module, these tests
    // 1. Can't be written against the public API (since that API doesn't
    //    provide a way to control which curve is used for scaling), and
    // 2. Validate important properties of the module.
    mod internal_computations {
        use super::*;

        #[fuchsia::test]
        fn transition_from_low_to_medium_is_continuous() {
            assert_near!(
                PointerSensorScaleHandler::scale_low_speed(MEDIUM_SPEED_RANGE_BEGIN_MM_PER_SEC),
                PointerSensorScaleHandler::scale_medium_speed(MEDIUM_SPEED_RANGE_BEGIN_MM_PER_SEC),
                SCALE_EPSILON
            );
        }

        // As noted in `scale_motion()`, the offset will be applied imperfectly,
        // so the externally visible transition may not be continuous.
        //
        // However, it's still valuable to verify that the internal building block
        // works as intended.
        #[fuchsia::test]
        fn transition_from_medium_to_high_is_continuous() {
            assert_near!(
                PointerSensorScaleHandler::scale_medium_speed(MEDIUM_SPEED_RANGE_END_MM_PER_SEC),
                PointerSensorScaleHandler::scale_high_speed(MEDIUM_SPEED_RANGE_END_MM_PER_SEC),
                SCALE_EPSILON
            );
        }
    }

    mod motion_scaling_mm {
        use super::*;

        #[ignore]
        #[fuchsia::test(allow_stalls = false)]
        async fn plot_example_curve() {
            let duration = zx::MonotonicDuration::from_millis(8);
            for count in 1..1000 {
                let scaled_count = get_scaled_motion_mm(
                    Position { x: count as f32 / COUNTS_PER_MM, y: 0.0 },
                    duration,
                )
                .await;
                log::error!("{}, {}", count, scaled_count.x);
            }
        }

        async fn get_scaled_motion_mm(
            movement_mm: Position,
            duration: zx::MonotonicDuration,
        ) -> Position {
            let inspector = fuchsia_inspect::Inspector::default();
            let test_node = inspector.root().create_child("test_node");
            let handler =
                PointerSensorScaleHandler::new(&test_node, metrics::MetricsLogger::default());

            // Send a don't-care value through to seed the last timestamp.
            let input_event = input_device::UnhandledInputEvent {
                device_event: input_device::InputDeviceEvent::Mouse(mouse_binding::MouseEvent {
                    location: mouse_binding::MouseLocation::Relative(Default::default()),
                    wheel_delta_v: None,
                    wheel_delta_h: None,
                    phase: mouse_binding::MousePhase::Move,
                    affected_buttons: hashset! {},
                    pressed_buttons: hashset! {},
                    is_precision_scroll: None,
                    wake_lease: None.into(),
                }),
                device_descriptor: DEVICE_DESCRIPTOR.clone(),
                event_time: zx::MonotonicInstant::from_nanos(0),
                trace_id: None,
            };
            handler.clone().handle_unhandled_input_event(input_event).await;

            // Send in the requested motion.
            let input_event = input_device::UnhandledInputEvent {
                device_event: input_device::InputDeviceEvent::Mouse(mouse_binding::MouseEvent {
                    location: mouse_binding::MouseLocation::Relative(
                        mouse_binding::RelativeLocation { millimeters: movement_mm },
                    ),
                    wheel_delta_v: None,
                    wheel_delta_h: None,
                    phase: mouse_binding::MousePhase::Move,
                    affected_buttons: hashset! {},
                    pressed_buttons: hashset! {},
                    is_precision_scroll: None,
                    wake_lease: None.into(),
                }),
                device_descriptor: DEVICE_DESCRIPTOR.clone(),
                event_time: zx::MonotonicInstant::from_nanos(duration.into_nanos()),
                trace_id: None,
            };
            let transformed_events =
                handler.clone().handle_unhandled_input_event(input_event).await;

            // Provide a useful debug message if the transformed event doesn't have the expected
            // overall structure.
            assert_matches!(
                transformed_events.as_slice(),
                [input_device::InputEvent {
                    device_event: input_device::InputDeviceEvent::Mouse(
                        mouse_binding::MouseEvent {
                            location: mouse_binding::MouseLocation::Relative(
                                mouse_binding::RelativeLocation { .. }
                            ),
                            ..
                        }
                    ),
                    ..
                }]
            );

            // Return the transformed motion.
            if let input_device::InputEvent {
                device_event:
                    input_device::InputDeviceEvent::Mouse(mouse_binding::MouseEvent {
                        location:
                            mouse_binding::MouseLocation::Relative(mouse_binding::RelativeLocation {
                                millimeters: movement_mm,
                            }),
                        ..
                    }),
                ..
            } = transformed_events[0]
            {
                movement_mm
            } else {
                unreachable!()
            }
        }

        fn velocity_to_mm(velocity_mm_per_sec: f32, duration: zx::MonotonicDuration) -> f32 {
            velocity_mm_per_sec * (duration.into_nanos() as f32 / 1E9)
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn low_speed_horizontal_motion_scales_linearly() {
            const TICK_DURATION: zx::MonotonicDuration = zx::MonotonicDuration::from_millis(8);
            const MOTION_A_MM: f32 = 1.0 / COUNTS_PER_MM;
            const MOTION_B_MM: f32 = 2.0 / COUNTS_PER_MM;
            assert_lt!(
                MOTION_B_MM,
                velocity_to_mm(MEDIUM_SPEED_RANGE_BEGIN_MM_PER_SEC, TICK_DURATION)
            );

            let scaled_a =
                get_scaled_motion_mm(Position { x: MOTION_A_MM, y: 0.0 }, TICK_DURATION).await;
            let scaled_b =
                get_scaled_motion_mm(Position { x: MOTION_B_MM, y: 0.0 }, TICK_DURATION).await;
            assert_near!(scaled_b.x / scaled_a.x, 2.0, SCALE_EPSILON);
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn low_speed_vertical_motion_scales_linearly() {
            const TICK_DURATION: zx::MonotonicDuration = zx::MonotonicDuration::from_millis(8);
            const MOTION_A_MM: f32 = 1.0 / COUNTS_PER_MM;
            const MOTION_B_MM: f32 = 2.0 / COUNTS_PER_MM;
            assert_lt!(
                MOTION_B_MM,
                velocity_to_mm(MEDIUM_SPEED_RANGE_BEGIN_MM_PER_SEC, TICK_DURATION)
            );

            let scaled_a =
                get_scaled_motion_mm(Position { x: 0.0, y: MOTION_A_MM }, TICK_DURATION).await;
            let scaled_b =
                get_scaled_motion_mm(Position { x: 0.0, y: MOTION_B_MM }, TICK_DURATION).await;
            assert_near!(scaled_b.y / scaled_a.y, 2.0, SCALE_EPSILON);
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn low_speed_45degree_motion_scales_dimensions_equally() {
            const TICK_DURATION: zx::MonotonicDuration = zx::MonotonicDuration::from_millis(8);
            const MOTION_MM: f32 = 1.0 / COUNTS_PER_MM;
            assert_lt!(
                MOTION_MM,
                velocity_to_mm(MEDIUM_SPEED_RANGE_BEGIN_MM_PER_SEC, TICK_DURATION)
            );

            let scaled =
                get_scaled_motion_mm(Position { x: MOTION_MM, y: MOTION_MM }, TICK_DURATION).await;
            assert_near!(scaled.x, scaled.y, SCALE_EPSILON);
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn medium_speed_motion_scales_quadratically() {
            const TICK_DURATION: zx::MonotonicDuration = zx::MonotonicDuration::from_millis(8);
            const MOTION_A_MM: f32 = 7.0 / COUNTS_PER_MM;
            const MOTION_B_MM: f32 = 14.0 / COUNTS_PER_MM;
            assert_gt!(
                MOTION_A_MM,
                velocity_to_mm(MEDIUM_SPEED_RANGE_BEGIN_MM_PER_SEC, TICK_DURATION)
            );
            assert_lt!(
                MOTION_B_MM,
                velocity_to_mm(MEDIUM_SPEED_RANGE_END_MM_PER_SEC, TICK_DURATION)
            );

            let scaled_a =
                get_scaled_motion_mm(Position { x: MOTION_A_MM, y: 0.0 }, TICK_DURATION).await;
            let scaled_b =
                get_scaled_motion_mm(Position { x: MOTION_B_MM, y: 0.0 }, TICK_DURATION).await;
            assert_near!(scaled_b.x / scaled_a.x, 4.0, SCALE_EPSILON);
        }

        // Given the handling of `OFFSET` for high-speed motion, (see comment
        // in `scale_motion()`), high speed motion scaling is _not_ linear for
        // the range of values of practical interest.
        //
        // Thus, this tests verifies a weaker property.
        #[fuchsia::test(allow_stalls = false)]
        async fn high_speed_motion_scaling_is_increasing() {
            const TICK_DURATION: zx::MonotonicDuration = zx::MonotonicDuration::from_millis(8);
            const MOTION_A_MM: f32 = 16.0 / COUNTS_PER_MM;
            const MOTION_B_MM: f32 = 20.0 / COUNTS_PER_MM;
            assert_gt!(
                MOTION_A_MM,
                velocity_to_mm(MEDIUM_SPEED_RANGE_END_MM_PER_SEC, TICK_DURATION)
            );

            let scaled_a =
                get_scaled_motion_mm(Position { x: MOTION_A_MM, y: 0.0 }, TICK_DURATION).await;
            let scaled_b =
                get_scaled_motion_mm(Position { x: MOTION_B_MM, y: 0.0 }, TICK_DURATION).await;
            assert_gt!(scaled_b.x, scaled_a.x)
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn zero_motion_maps_to_zero_motion() {
            const TICK_DURATION: zx::MonotonicDuration = zx::MonotonicDuration::from_millis(8);
            let scaled = get_scaled_motion_mm(Position { x: 0.0, y: 0.0 }, TICK_DURATION).await;
            assert_eq!(scaled, Position::zero())
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn zero_duration_does_not_crash() {
            get_scaled_motion_mm(
                Position { x: 1.0 / COUNTS_PER_MM, y: 0.0 },
                zx::MonotonicDuration::from_millis(0),
            )
            .await;
        }
    }

    mod scroll_scaling_tick {
        use super::*;
        use test_case::test_case;

        #[test_case(mouse_binding::MouseEvent {
            location: mouse_binding::MouseLocation::Relative(mouse_binding::RelativeLocation {
                millimeters: Position::zero(),
            }),
            wheel_delta_v: Some(mouse_binding::WheelDelta {
                raw_data: mouse_binding::RawWheelDelta::Ticks(1),
                physical_pixel: None,
            }),
            wheel_delta_h: None,
            phase: mouse_binding::MousePhase::Wheel,
            affected_buttons: hashset! {},
            pressed_buttons: hashset! {},
            is_precision_scroll: None,
            wake_lease: None.into(),
        } => (Some(PIXELS_PER_TICK), None); "v")]
        #[test_case(mouse_binding::MouseEvent {
            location: mouse_binding::MouseLocation::Relative(mouse_binding::RelativeLocation {
                millimeters: Position::zero(),
            }),
            wheel_delta_v: None,
            wheel_delta_h: Some(mouse_binding::WheelDelta {
                raw_data: mouse_binding::RawWheelDelta::Ticks(1),
                physical_pixel: None,
            }),
            phase: mouse_binding::MousePhase::Wheel,
            affected_buttons: hashset! {},
            pressed_buttons: hashset! {},
            is_precision_scroll: None,
            wake_lease: None.into(),
        } => (None, Some(PIXELS_PER_TICK)); "h")]
        #[fuchsia::test(allow_stalls = false)]
        async fn scaled(event: mouse_binding::MouseEvent) -> (Option<f32>, Option<f32>) {
            let inspector = fuchsia_inspect::Inspector::default();
            let test_node = inspector.root().create_child("test_node");
            let handler =
                PointerSensorScaleHandler::new(&test_node, metrics::MetricsLogger::default());
            let unhandled_event = make_unhandled_input_event(event);

            let events = handler.clone().handle_unhandled_input_event(unhandled_event).await;
            assert_matches!(
                events.as_slice(),
                [input_device::InputEvent {
                    device_event: input_device::InputDeviceEvent::Mouse(
                        mouse_binding::MouseEvent { .. }
                    ),
                    ..
                }]
            );
            if let input_device::InputEvent {
                device_event:
                    input_device::InputDeviceEvent::Mouse(mouse_binding::MouseEvent {
                        wheel_delta_v,
                        wheel_delta_h,
                        ..
                    }),
                ..
            } = events[0].clone()
            {
                match (wheel_delta_v, wheel_delta_h) {
                    (None, None) => return (None, None),
                    (None, Some(delta_h)) => return (None, delta_h.physical_pixel),
                    (Some(delta_v), None) => return (delta_v.physical_pixel, None),
                    (Some(delta_v), Some(delta_h)) => {
                        return (delta_v.physical_pixel, delta_h.physical_pixel);
                    }
                }
            } else {
                unreachable!();
            }
        }
    }

    mod scroll_scaling_mm {
        use super::*;
        use pretty_assertions::assert_eq;

        async fn get_scaled_scroll_mm(
            wheel_delta_v_mm: Option<f32>,
            wheel_delta_h_mm: Option<f32>,
            duration: zx::MonotonicDuration,
        ) -> (Option<mouse_binding::WheelDelta>, Option<mouse_binding::WheelDelta>) {
            let inspector = fuchsia_inspect::Inspector::default();
            let test_node = inspector.root().create_child("test_node");
            let handler =
                PointerSensorScaleHandler::new(&test_node, metrics::MetricsLogger::default());

            // Send a don't-care value through to seed the last timestamp.
            let input_event = input_device::UnhandledInputEvent {
                device_event: input_device::InputDeviceEvent::Mouse(mouse_binding::MouseEvent {
                    location: mouse_binding::MouseLocation::Relative(Default::default()),
                    wheel_delta_v: Some(mouse_binding::WheelDelta {
                        raw_data: mouse_binding::RawWheelDelta::Millimeters(1.0),
                        physical_pixel: None,
                    }),
                    wheel_delta_h: None,
                    phase: mouse_binding::MousePhase::Wheel,
                    affected_buttons: hashset! {},
                    pressed_buttons: hashset! {},
                    is_precision_scroll: None,
                    wake_lease: None.into(),
                }),
                device_descriptor: DEVICE_DESCRIPTOR.clone(),
                event_time: zx::MonotonicInstant::from_nanos(0),
                trace_id: None,
            };
            handler.clone().handle_unhandled_input_event(input_event).await;

            // Send in the requested motion.
            let input_event = input_device::UnhandledInputEvent {
                device_event: input_device::InputDeviceEvent::Mouse(mouse_binding::MouseEvent {
                    location: mouse_binding::MouseLocation::Relative(Default::default()),
                    wheel_delta_v: match wheel_delta_v_mm {
                        None => None,
                        Some(delta) => Some(mouse_binding::WheelDelta {
                            raw_data: mouse_binding::RawWheelDelta::Millimeters(delta),
                            physical_pixel: None,
                        }),
                    },
                    wheel_delta_h: match wheel_delta_h_mm {
                        None => None,
                        Some(delta) => Some(mouse_binding::WheelDelta {
                            raw_data: mouse_binding::RawWheelDelta::Millimeters(delta),
                            physical_pixel: None,
                        }),
                    },
                    phase: mouse_binding::MousePhase::Wheel,
                    affected_buttons: hashset! {},
                    pressed_buttons: hashset! {},
                    is_precision_scroll: None,
                    wake_lease: None.into(),
                }),
                device_descriptor: DEVICE_DESCRIPTOR.clone(),
                event_time: zx::MonotonicInstant::from_nanos(duration.into_nanos()),
                trace_id: None,
            };
            let transformed_events =
                handler.clone().handle_unhandled_input_event(input_event).await;

            assert_eq!(transformed_events.len(), 1);

            if let input_device::InputEvent {
                device_event:
                    input_device::InputDeviceEvent::Mouse(mouse_binding::MouseEvent {
                        wheel_delta_v: delta_v,
                        wheel_delta_h: delta_h,
                        ..
                    }),
                ..
            } = transformed_events[0].clone()
            {
                return (delta_v, delta_h);
            } else {
                unreachable!()
            }
        }

        fn velocity_to_mm(velocity_mm_per_sec: f32, duration: zx::MonotonicDuration) -> f32 {
            velocity_mm_per_sec * (duration.into_nanos() as f32 / 1E9)
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn low_speed_horizontal_scroll_scales_linearly() {
            const TICK_DURATION: zx::MonotonicDuration = zx::MonotonicDuration::from_millis(8);
            const MOTION_A_MM: f32 = 1.0 / COUNTS_PER_MM;
            const MOTION_B_MM: f32 = 2.0 / COUNTS_PER_MM;
            assert_lt!(
                MOTION_B_MM,
                velocity_to_mm(MEDIUM_SPEED_RANGE_BEGIN_MM_PER_SEC, TICK_DURATION)
            );

            let (_, scaled_a_h) =
                get_scaled_scroll_mm(None, Some(MOTION_A_MM), TICK_DURATION).await;

            let (_, scaled_b_h) =
                get_scaled_scroll_mm(None, Some(MOTION_B_MM), TICK_DURATION).await;

            match (scaled_a_h, scaled_b_h) {
                (Some(a_h), Some(b_h)) => {
                    assert_ne!(a_h.physical_pixel, None);
                    assert_ne!(b_h.physical_pixel, None);
                    assert_ne!(a_h.physical_pixel.unwrap(), 0.0);
                    assert_ne!(b_h.physical_pixel.unwrap(), 0.0);
                    assert_near!(
                        b_h.physical_pixel.unwrap() / a_h.physical_pixel.unwrap(),
                        2.0,
                        SCALE_EPSILON
                    );
                }
                _ => {
                    panic!("wheel delta is none");
                }
            }
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn low_speed_vertical_scroll_scales_linearly() {
            const TICK_DURATION: zx::MonotonicDuration = zx::MonotonicDuration::from_millis(8);
            const MOTION_A_MM: f32 = 1.0 / COUNTS_PER_MM;
            const MOTION_B_MM: f32 = 2.0 / COUNTS_PER_MM;
            assert_lt!(
                MOTION_B_MM,
                velocity_to_mm(MEDIUM_SPEED_RANGE_BEGIN_MM_PER_SEC, TICK_DURATION)
            );

            let (scaled_a_v, _) =
                get_scaled_scroll_mm(Some(MOTION_A_MM), None, TICK_DURATION).await;

            let (scaled_b_v, _) =
                get_scaled_scroll_mm(Some(MOTION_B_MM), None, TICK_DURATION).await;

            match (scaled_a_v, scaled_b_v) {
                (Some(a_v), Some(b_v)) => {
                    assert_ne!(a_v.physical_pixel, None);
                    assert_ne!(b_v.physical_pixel, None);
                    assert_near!(
                        b_v.physical_pixel.unwrap() / a_v.physical_pixel.unwrap(),
                        2.0,
                        SCALE_EPSILON
                    );
                }
                _ => {
                    panic!("wheel delta is none");
                }
            }
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn medium_speed_horizontal_scroll_scales_quadratically() {
            const TICK_DURATION: zx::MonotonicDuration = zx::MonotonicDuration::from_millis(8);
            const MOTION_A_MM: f32 = 7.0 / COUNTS_PER_MM;
            const MOTION_B_MM: f32 = 14.0 / COUNTS_PER_MM;
            assert_gt!(
                MOTION_A_MM,
                velocity_to_mm(MEDIUM_SPEED_RANGE_BEGIN_MM_PER_SEC, TICK_DURATION)
            );
            assert_lt!(
                MOTION_B_MM,
                velocity_to_mm(MEDIUM_SPEED_RANGE_END_MM_PER_SEC, TICK_DURATION)
            );

            let (_, scaled_a_h) =
                get_scaled_scroll_mm(None, Some(MOTION_A_MM), TICK_DURATION).await;

            let (_, scaled_b_h) =
                get_scaled_scroll_mm(None, Some(MOTION_B_MM), TICK_DURATION).await;

            match (scaled_a_h, scaled_b_h) {
                (Some(a_h), Some(b_h)) => {
                    assert_ne!(a_h.physical_pixel, None);
                    assert_ne!(b_h.physical_pixel, None);
                    assert_ne!(a_h.physical_pixel.unwrap(), 0.0);
                    assert_ne!(b_h.physical_pixel.unwrap(), 0.0);
                    assert_near!(
                        b_h.physical_pixel.unwrap() / a_h.physical_pixel.unwrap(),
                        4.0,
                        SCALE_EPSILON
                    );
                }
                _ => {
                    panic!("wheel delta is none");
                }
            }
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn medium_speed_vertical_scroll_scales_quadratically() {
            const TICK_DURATION: zx::MonotonicDuration = zx::MonotonicDuration::from_millis(8);
            const MOTION_A_MM: f32 = 7.0 / COUNTS_PER_MM;
            const MOTION_B_MM: f32 = 14.0 / COUNTS_PER_MM;
            assert_gt!(
                MOTION_A_MM,
                velocity_to_mm(MEDIUM_SPEED_RANGE_BEGIN_MM_PER_SEC, TICK_DURATION)
            );
            assert_lt!(
                MOTION_B_MM,
                velocity_to_mm(MEDIUM_SPEED_RANGE_END_MM_PER_SEC, TICK_DURATION)
            );

            let (scaled_a_v, _) =
                get_scaled_scroll_mm(Some(MOTION_A_MM), None, TICK_DURATION).await;

            let (scaled_b_v, _) =
                get_scaled_scroll_mm(Some(MOTION_B_MM), None, TICK_DURATION).await;

            match (scaled_a_v, scaled_b_v) {
                (Some(a_v), Some(b_v)) => {
                    assert_ne!(a_v.physical_pixel, None);
                    assert_ne!(b_v.physical_pixel, None);
                    assert_near!(
                        b_v.physical_pixel.unwrap() / a_v.physical_pixel.unwrap(),
                        4.0,
                        SCALE_EPSILON
                    );
                }
                _ => {
                    panic!("wheel delta is none");
                }
            }
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn high_speed_horizontal_scroll_scaling_is_inreasing() {
            const TICK_DURATION: zx::MonotonicDuration = zx::MonotonicDuration::from_millis(8);
            const MOTION_A_MM: f32 = 16.0 / COUNTS_PER_MM;
            const MOTION_B_MM: f32 = 20.0 / COUNTS_PER_MM;
            assert_gt!(
                MOTION_A_MM,
                velocity_to_mm(MEDIUM_SPEED_RANGE_END_MM_PER_SEC, TICK_DURATION)
            );

            let (_, scaled_a_h) =
                get_scaled_scroll_mm(None, Some(MOTION_A_MM), TICK_DURATION).await;

            let (_, scaled_b_h) =
                get_scaled_scroll_mm(None, Some(MOTION_B_MM), TICK_DURATION).await;

            match (scaled_a_h, scaled_b_h) {
                (Some(a_h), Some(b_h)) => {
                    assert_ne!(a_h.physical_pixel, None);
                    assert_ne!(b_h.physical_pixel, None);
                    assert_ne!(a_h.physical_pixel.unwrap(), 0.0);
                    assert_ne!(b_h.physical_pixel.unwrap(), 0.0);
                    assert_gt!(b_h.physical_pixel.unwrap(), a_h.physical_pixel.unwrap());
                }
                _ => {
                    panic!("wheel delta is none");
                }
            }
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn high_speed_vertical_scroll_scaling_is_inreasing() {
            const TICK_DURATION: zx::MonotonicDuration = zx::MonotonicDuration::from_millis(8);
            const MOTION_A_MM: f32 = 16.0 / COUNTS_PER_MM;
            const MOTION_B_MM: f32 = 20.0 / COUNTS_PER_MM;
            assert_gt!(
                MOTION_A_MM,
                velocity_to_mm(MEDIUM_SPEED_RANGE_END_MM_PER_SEC, TICK_DURATION)
            );

            let (scaled_a_v, _) =
                get_scaled_scroll_mm(Some(MOTION_A_MM), None, TICK_DURATION).await;

            let (scaled_b_v, _) =
                get_scaled_scroll_mm(Some(MOTION_B_MM), None, TICK_DURATION).await;

            match (scaled_a_v, scaled_b_v) {
                (Some(a_v), Some(b_v)) => {
                    assert_ne!(a_v.physical_pixel, None);
                    assert_ne!(b_v.physical_pixel, None);
                    assert_gt!(b_v.physical_pixel.unwrap(), a_v.physical_pixel.unwrap());
                }
                _ => {
                    panic!("wheel delta is none");
                }
            }
        }
    }

    mod metadata_preservation {
        use super::*;
        use test_case::test_case;

        #[test_case(mouse_binding::MouseEvent {
            location: mouse_binding::MouseLocation::Relative(mouse_binding::RelativeLocation {
                millimeters: Position { x: 1.5 / COUNTS_PER_MM, y: 4.5 / COUNTS_PER_MM },
            }),
            wheel_delta_v: None,
            wheel_delta_h: None,
            phase: mouse_binding::MousePhase::Move,
            affected_buttons: hashset! {},
            pressed_buttons: hashset! {},
            is_precision_scroll: None,
            wake_lease: None.into(),
        }; "move event")]
        #[test_case(mouse_binding::MouseEvent {
            location: mouse_binding::MouseLocation::Relative(mouse_binding::RelativeLocation {
                millimeters: Position::zero(),
            }),
            wheel_delta_v: Some(mouse_binding::WheelDelta {
                raw_data: mouse_binding::RawWheelDelta::Ticks(1),
                physical_pixel: None,
            }),
            wheel_delta_h: None,
            phase: mouse_binding::MousePhase::Wheel,
            affected_buttons: hashset! {},
            pressed_buttons: hashset! {},
            is_precision_scroll: None,
            wake_lease: None.into(),
        }; "wheel event")]
        #[fuchsia::test(allow_stalls = false)]
        async fn does_not_consume_event(event: mouse_binding::MouseEvent) {
            let inspector = fuchsia_inspect::Inspector::default();
            let test_node = inspector.root().create_child("test_node");
            let handler =
                PointerSensorScaleHandler::new(&test_node, metrics::MetricsLogger::default());
            let input_event = make_unhandled_input_event(event);
            assert_matches!(
                handler.clone().handle_unhandled_input_event(input_event).await.as_slice(),
                [input_device::InputEvent { handled: input_device::Handled::No, .. }]
            );
        }

        // Downstream handlers, and components consuming the `MouseEvent`, may be
        // sensitive to the speed of motion. So it's important to preserve timestamps.
        #[test_case(mouse_binding::MouseEvent {
            location: mouse_binding::MouseLocation::Relative(mouse_binding::RelativeLocation {
                millimeters: Position { x: 1.5 / COUNTS_PER_MM, y: 4.5 / COUNTS_PER_MM },
            }),
            wheel_delta_v: None,
            wheel_delta_h: None,
            phase: mouse_binding::MousePhase::Move,
            affected_buttons: hashset! {},
            pressed_buttons: hashset! {},
            is_precision_scroll: None,
            wake_lease: None.into(),
        }; "move event")]
        #[test_case(mouse_binding::MouseEvent {
            location: mouse_binding::MouseLocation::Relative(mouse_binding::RelativeLocation {
                millimeters: Position::zero(),
            }),
            wheel_delta_v: Some(mouse_binding::WheelDelta {
                raw_data: mouse_binding::RawWheelDelta::Ticks(1),
                physical_pixel: None,
            }),
            wheel_delta_h: None,
            phase: mouse_binding::MousePhase::Wheel,
            affected_buttons: hashset! {},
            pressed_buttons: hashset! {},
            is_precision_scroll: None,
            wake_lease: None.into(),
        }; "wheel event")]
        #[fuchsia::test(allow_stalls = false)]
        async fn preserves_event_time(event: mouse_binding::MouseEvent) {
            let inspector = fuchsia_inspect::Inspector::default();
            let test_node = inspector.root().create_child("test_node");
            let handler =
                PointerSensorScaleHandler::new(&test_node, metrics::MetricsLogger::default());
            let mut input_event = make_unhandled_input_event(event);
            const EVENT_TIME: zx::MonotonicInstant = zx::MonotonicInstant::from_nanos(42);
            input_event.event_time = EVENT_TIME;

            let events = handler.clone().handle_unhandled_input_event(input_event).await;
            assert_eq!(events.len(), 1, "{events:?} should be length 1");
            assert_eq!(events[0].event_time, EVENT_TIME);
        }

        #[test_case(
            mouse_binding::MouseEvent {
                location: mouse_binding::MouseLocation::Relative(mouse_binding::RelativeLocation {
                    millimeters: Position::zero(),
                }),
                wheel_delta_v: Some(mouse_binding::WheelDelta {
                    raw_data: mouse_binding::RawWheelDelta::Ticks(1),
                    physical_pixel: Some(1.0),
                }),
                wheel_delta_h: None,
                phase: mouse_binding::MousePhase::Wheel,
                affected_buttons: hashset! {},
                pressed_buttons: hashset! {},
                is_precision_scroll: Some(mouse_binding::PrecisionScroll::No),
            wake_lease: None.into(),
            } => matches input_device::InputEvent {
                device_event: input_device::InputDeviceEvent::Mouse(mouse_binding::MouseEvent {
                    is_precision_scroll: Some(mouse_binding::PrecisionScroll::No),
                    ..
                }),
                ..
            }; "no")]
        #[test_case(
            mouse_binding::MouseEvent {
                location: mouse_binding::MouseLocation::Relative(mouse_binding::RelativeLocation {
                    millimeters: Position::zero(),
                }),
                wheel_delta_v: Some(mouse_binding::WheelDelta {
                    raw_data: mouse_binding::RawWheelDelta::Ticks(1),
                    physical_pixel: Some(1.0),
                }),
                wheel_delta_h: None,
                phase: mouse_binding::MousePhase::Wheel,
                affected_buttons: hashset! {},
                pressed_buttons: hashset! {},
                is_precision_scroll: Some(mouse_binding::PrecisionScroll::Yes),
                wake_lease: None.into(),
            } => matches input_device::InputEvent {
                device_event: input_device::InputDeviceEvent::Mouse(mouse_binding::MouseEvent {
                    is_precision_scroll: Some(mouse_binding::PrecisionScroll::Yes),
                    ..
                }),
                ..
            }; "yes")]
        #[fuchsia::test(allow_stalls = false)]
        async fn preserves_is_precision_scroll(
            event: mouse_binding::MouseEvent,
        ) -> input_device::InputEvent {
            let inspector = fuchsia_inspect::Inspector::default();
            let test_node = inspector.root().create_child("test_node");
            let handler =
                PointerSensorScaleHandler::new(&test_node, metrics::MetricsLogger::default());
            let input_event = make_unhandled_input_event(event);

            handler.clone().handle_unhandled_input_event(input_event).await[0].clone()
        }
    }
}