denise-drm 0.26.0

//! The scanout surface: dumb buffers, modeset, and page flips.

use denise::{Frame, MAX_DAMAGE_RECTS, PixelFormat, Rect, Size, Surface, SurfaceError};
use drm::Device as _;
use drm::DriverCapability;
use drm::buffer::Buffer as _;
use drm::control::{
    Device as ControlDevice, Event, Mode, PageFlipFlags, connector, crtc, dumbbuffer::DumbBuffer,
    framebuffer,
};
use drm_fourcc::DrmFourcc;

use crate::device::Card;
use crate::error::DrmError;
use crate::mode::{self, ModePreference, OutputPreference};
use crate::swapchain::Swapchain;

/// Bits per pixel of the scanout format.
const BPP: u32 = 32;
/// Colour depth, excluding the ignored high byte.
const DEPTH: u32 = 24;

/// When a queued flip actually reaches the panel.
///
/// A real trade, not a quality setting. Which way it should go depends on what is
/// on the screen, and the default here is chosen for the kind of thing Denise is
/// built for rather than for the kind of thing a compositor is built for.
#[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
pub enum PresentMode {
    /// Flip immediately, part-way through a scan-out if necessary. Tears.
    ///
    /// The default, and measured on a Pi 3 A+ driving 1920x1080 the reason is
    /// plain: waiting for vblank costs about 17 ms of latency and was described by
    /// the person operating it as lagging several milliseconds behind the whole
    /// time. Flipping immediately removes that wait entirely.
    ///
    /// The cost is a horizontal seam where the panel switched buffers mid-frame.
    /// With damage tracking most updates are a few thousand pixels, so the seam is
    /// small, brief and in practice invisible — a control panel redrawing a button
    /// is nothing like a compositor scrolling a window.
    ///
    /// This used to say "reconsider for signage or anything with large fast-moving
    /// content, where a tear crosses something worth looking at". A scrolling
    /// viewport turned out to be exactly that, and the flicker was reported from a
    /// Pi within a day of the gallery gaining one. So the mode is no longer a
    /// promise about every frame: **a frame whose damage covers a quarter of the
    /// screen's rows or more flips at vblank anyway**, and this asks for async
    /// flips on the frames where the seam is short and the latency is felt. See
    /// `flip_flags_for`, including why it counts rows rather than pixels, and
    /// why it counts the rows covered rather than the rows spanned.
    ///
    /// Requires `DRM_CAP_ASYNC_PAGE_FLIP`; drivers without it fall back to
    /// [`PresentMode::Vsync`], and [`DrmSurface::present_mode`] reports what was
    /// actually obtained.
    ///
    /// **This mode paces the caller only on its large frames.** See
    /// [`DrmSurface`].
    #[default]
    Immediate,

    /// Flip at the next vblank. Never tears.
    ///
    /// A flip queued just after a vblank cannot land before the next one, so this
    /// costs on the order of one refresh period — about 17 ms at 60 Hz. Every
    /// tear-free system pays it.
    ///
    /// In exchange, [`Surface::acquire`] blocks until the flip retires, so the
    /// display paces the render loop for free and an application needs no frame
    /// timing of its own.
    Vsync,
}

/// How to bring the display up.
#[derive(Clone, Copy, Debug)]
pub struct SurfaceConfig {
    /// Which output to drive.
    pub output: OutputPreference,
    /// Which mode to set on it.
    pub mode: ModePreference,
    /// How many scanout buffers to rotate through.
    ///
    /// Two by default. Three trades latency for smoothness, which is the wrong
    /// trade for a panel someone is touching.
    pub buffers: usize,

    /// Whether to wait for vblank before showing a frame.
    pub present_mode: PresentMode,
}

impl Default for SurfaceConfig {
    fn default() -> Self {
        Self {
            output: OutputPreference::Auto,
            mode: ModePreference::Preferred,
            buffers: 2,
            present_mode: PresentMode::Vsync,
        }
    }
}

/// One scanout buffer: the allocation, its framebuffer id, and its CPU mapping.
#[derive(Debug)]
struct Scanout {
    dumb: DumbBuffer,
    fb: framebuffer::Handle,
    /// Start of the mapping, as `u32` words.
    ptr: *mut u32,
    /// Length of the mapping in words.
    words: usize,
    /// Length of the mapping in bytes, for `munmap`.
    bytes: usize,
}

impl Scanout {
    fn new(card: &Card, size: Size) -> Result<Self, DrmError> {
        let mut dumb = card
            .create_dumb_buffer((size.width, size.height), DrmFourcc::Xrgb8888, BPP)
            .map_err(|source| DrmError::Allocate {
                width: size.width,
                height: size.height,
                source,
            })?;

        let fb = card
            .add_framebuffer(&dumb, DEPTH, BPP)
            .map_err(DrmError::AddFramebuffer)?;

        // The `drm` crate's mapping unmaps itself on drop, which cannot work here:
        // the mapping has to outlive the call that made it, and a `Frame` handed to
        // the renderer borrows from it. So take the pointer and forget the guard,
        // making this code responsible for the `munmap` in `DrmSurface::drop`.
        // Mapping once at start-up also saves an mmap/munmap pair every frame.
        let (ptr, bytes) = {
            let mut mapping = card.map_dumb_buffer(&mut dumb).map_err(DrmError::Map)?;
            let slice: &mut [u8] = &mut mapping;
            let ptr = slice.as_mut_ptr();
            let bytes = slice.len();
            core::mem::forget(mapping);
            (ptr, bytes)
        };

        let mut scanout = Self {
            dumb,
            fb,
            // SAFETY: `mmap` returns page-aligned memory, which satisfies `u32`
            // alignment. The cast does not change the region's extent; `words`
            // below accounts for the narrower element type.
            ptr: ptr.cast::<u32>(),
            words: bytes / 4,
            bytes,
        };

        // A freshly allocated dumb buffer holds whatever was in that memory.
        // Without this, the modeset shows one frame of garbage before the first
        // repaint lands.
        scanout.pixels_mut().fill(0);

        Ok(scanout)
    }

    fn pixels_mut(&mut self) -> &mut [u32] {
        // SAFETY: `ptr` and `words` come from a single successful mapping of this
        // buffer, which stays mapped until `DrmSurface::drop` unmaps it. `&mut
        // self` rules out any other live reference to the same region.
        unsafe { core::slice::from_raw_parts_mut(self.ptr, self.words) }
    }
}

/// Above this share of the surface's **rows**, a frame flips at vblank even
/// under [`PresentMode::Immediate`].
///
/// A quarter, as a numerator over [`TEAR_FREE_DENOMINATOR`] so the comparison
/// stays in integers. The number is not delicate: real damage either spans a
/// control, which is a few dozen rows, or something that moved a whole column,
/// which is nearly all of them. There is very little in between.
const TEAR_FREE_NUMERATOR: u32 = 1;
const TEAR_FREE_DENOMINATOR: u32 = 4;

/// Which page flip this frame gets: async, or paced by vblank.
///
/// **The tear is not the whole cost of tearing.** An async flip lands wherever
/// the beam happens to be, which for a button redrawing itself puts a seam a few
/// pixels tall somewhere nobody is looking. For a frame that moved everything —
/// a scrolling viewport — the seam crosses the thing being read, which is
/// exactly the case [`PresentMode::Immediate`]'s own documentation says to
/// reconsider. It reads as flicker, and it was reported as flicker.
///
/// The second half is pacing. An async flip never blocks, so a loop that redraws
/// while input keeps arriving runs as fast as the CPU allows: a Pi 3 A+ paints a
/// scrolled 1920x1080 viewport in about 14.5 ms, so it will spend a whole core
/// producing frames that tear, one after another. A vblank-paced flip makes
/// [`Surface::acquire`] wait for the retire, which caps the loop at the refresh
/// rate for free. The frame that most needs not to tear is the same frame that
/// most needs the brakes.
///
/// So the mode follows the damage rather than being set once for everything: the
/// low latency [`PresentMode::Immediate`] exists for is kept where it is felt —
/// a press lighting a button — and given up on the frames where it is neither
/// felt nor affordable.
///
/// # Rows, not area
///
/// The first version of this compared the damaged *area* against the surface,
/// and a Pi still flashed occasionally. The gallery's sidebar is 300 by 1016 on
/// a 1920x1080 panel: **14.7% of the pixels, and 94% of the scanlines.** It went
/// out async and tore across almost the whole height of the screen.
///
/// A tear is a horizontal seam, and it appears when the buffer changes under the
/// beam part-way down. What decides whether it is visible is therefore how many
/// **rows** the damage spans, not how much of the surface it covers. A full-width
/// toolbar forty rows tall can tear freely — the seam is a thin band that is gone
/// next frame. A narrow column down the whole screen cannot.
///
/// # Covered, not spanned
///
/// Which leaves how to count the rows when the damage is in several pieces. The
/// first answer here was the bounding box, on the reasoning that one flip
/// changes the buffer for every rectangle at once, so the beam can seam anywhere
/// between them. True, and it measures the wrong thing: the beam can seam there,
/// but nobody can *see* it there. Outside the damage both buffers hold the same
/// pixels — that is what repainting to the buffer's age guarantees — and a seam
/// between two identical images is not a seam.
///
/// So it counts the rows the damage actually covers, which is what the earlier
/// reasoning was reaching for anyway: the sidebar covers 1016 rows whichever way
/// it is counted, and still waits. What changes is the frame this backend was
/// never meant to catch. The gallery keeps a spinner turning at the top of the
/// screen, so every frame while a pointer is somewhere in the lower two thirds
/// carried a 48-row spinner, a 24-row cursor, and a bounding box spanning the
/// eight hundred untouched rows between them — vblank-paced, all of it, from a
/// rule written for scrolling. Counting coverage puts that frame back at 72 rows
/// and back on the async path it was on in 0.13.0.
///
/// An empty damage list means the caller presented without saying what changed,
/// which cannot be assumed to be small.
fn flip_flags_for(mode: PresentMode, damage: &[Rect], surface: Size) -> PageFlipFlags {
    let synced = PageFlipFlags::EVENT;
    let immediate = PageFlipFlags::EVENT | PageFlipFlags::ASYNC;

    if mode == PresentMode::Vsync {
        return synced;
    }
    if damage.is_empty() {
        return synced;
    }

    let rows = damaged_rows(damage, surface);

    if rows * TEAR_FREE_DENOMINATOR >= surface.height * TEAR_FREE_NUMERATOR {
        synced
    } else {
        immediate
    }
}

/// How many of the surface's scanlines the damage covers, counting an overlap
/// once.
///
/// The vertical extents, merged. Rectangles arrive in no particular order and
/// may overlap, so this sorts them by top edge — an insertion sort over at most
/// [`MAX_DAMAGE_RECTS`] items, in a fixed array, because this runs once per
/// frame on a Pi and must not allocate — and then sweeps, extending the run
/// while the next span starts before the current one ends.
///
/// Rows outside the surface cannot tear, so each span is clipped first. A list
/// longer than the tracker's own capacity is something this backend has no
/// business guessing about: it reports the full height, and the caller gets a
/// vblank.
fn damaged_rows(damage: &[Rect], surface: Size) -> u32 {
    if damage.len() > MAX_DAMAGE_RECTS {
        return surface.height;
    }

    let bottom_edge = surface.height as i32;
    let mut spans = [(0i32, 0i32); MAX_DAMAGE_RECTS];
    let mut len = 0;

    for rect in damage {
        let top = rect.y.clamp(0, bottom_edge);
        let bottom = rect.bottom().clamp(0, bottom_edge);
        if bottom <= top {
            continue;
        }
        let mut i = len;
        while i > 0 && spans[i - 1].0 > top {
            spans[i] = spans[i - 1];
            i -= 1;
        }
        spans[i] = (top, bottom);
        len += 1;
    }

    let mut rows: u32 = 0;
    let mut i = 0;
    while i < len {
        let (start, mut end) = spans[i];
        i += 1;
        // Sorted by top edge, so anything that starts at or before this run's
        // current end belongs to the same run — and may extend it.
        while i < len && spans[i].0 <= end {
            end = end.max(spans[i].1);
            i += 1;
        }
        rows += (end - start) as u32;
    }

    rows
}

/// A display brought up under our control, scanning out CPU-rendered buffers.
///
/// Takes DRM master on construction and gives it back on drop, restoring whatever
/// the CRTC was showing before. A clean exit and a panic both hand the console
/// back, rather than leaving a black screen that needs a power cycle.
///
/// # Pacing is the caller's job under [`PresentMode::Immediate`]
///
/// Under [`PresentMode::Vsync`], [`acquire`](Surface::acquire) blocks until the
/// previous flip retires, so a bare `loop { acquire; draw; present }` runs at
/// exactly the refresh rate and costs nothing extra.
///
/// Under [`PresentMode::Immediate`] — the default — a small frame does not wait.
/// The same loop runs as fast as the CPU allows and will happily use a whole core
/// drawing frames no one will ever see. An application must either draw only when
/// something changed, which damage tracking makes natural, or keep a frame
/// deadline of its own. `examples/kiosk` does both.
///
/// This is not a flaw in async flips; it is what removing the wait means. It is
/// also why a *large* frame gives the wait back: see `flip_flags_for`, where
/// the same decision that keeps a seam off a scrolling viewport is what stops the
/// loop repainting it a hundred times a second.
#[derive(Debug)]
pub struct DrmSurface {
    // `pub(crate)` for the cursor plane, which lives in its own module and needs
    // the card and the CRTC to talk to.
    pub(crate) card: Card,
    pub(crate) crtc: crtc::Handle,
    connector: connector::Handle,
    buffers: Vec<Scanout>,
    swapchain: Swapchain,
    size: Size,
    /// Row stride in pixels, from the driver's pitch. Rarely equals the width.
    stride: u32,
    /// A flip has been queued and its completion event not yet read.
    flip_pending: bool,
    /// The mode actually in force, after checking what the driver supports.
    present_mode: PresentMode,
    saved_crtc: Option<crtc::Info>,
    mode_name: String,
    /// The hardware cursor plane's buffer, allocated on first use. `None` until
    /// an application asks for a sprite, because a panel driven by touch never
    /// wants one and should not pay for the allocation.
    pub(crate) cursor: Option<crate::cursor::CursorBuffer>,
}

impl DrmSurface {
    /// Brings up the display.
    pub fn new(card: Card, config: SurfaceConfig) -> Result<Self, DrmError> {
        card.become_master()?;

        let (handles, infos) = card.connectors()?;
        let selection = mode::select(&infos, config.output, config.mode)?;
        let connector = handles[selection.connector];
        let crtc = card.crtc_for(connector)?;

        // Re-read the connector for the driver's own `Mode`, since the selection
        // policy works on a copy that deliberately drops the timing details.
        let info = card
            .get_connector(connector, false)
            .map_err(DrmError::Resources)?;
        let mode: Mode = info.modes()[selection.mode];
        let (width, height) = mode.size();
        let size = Size::new(u32::from(width), u32::from(height));

        let saved_crtc = card.get_crtc(crtc).ok();

        let mut buffers = Vec::with_capacity(config.buffers);
        for _ in 0..Swapchain::new(config.buffers).count() {
            buffers.push(Scanout::new(&card, size)?);
        }

        // The pitch is the driver's, not ours: it is padded for alignment and is
        // routinely wider than the visible row. Everything downstream addresses
        // rows through this, never through the width.
        let pitch = buffers[0].dumb.pitch();
        if !pitch.is_multiple_of(4) {
            return Err(DrmError::UnalignedPitch { pitch });
        }

        // Ask the driver rather than assume. Requesting an async flip on hardware
        // that cannot do one fails the ioctl every frame, which would turn a
        // latency preference into a display that never updates.
        let async_capable = card
            .get_driver_capability(DriverCapability::ASyncPageFlip)
            .is_ok_and(|supported| supported != 0);

        let present_mode = match config.present_mode {
            PresentMode::Immediate if async_capable => PresentMode::Immediate,
            _ => PresentMode::Vsync,
        };

        card.set_crtc(crtc, Some(buffers[0].fb), (0, 0), &[connector], Some(mode))
            .map_err(|source| DrmError::SetMode {
                mode: format!("{width}x{height}"),
                crtc: u32::from(crtc),
                source,
            })?;

        // Buffer 0 is now being scanned out, so the next frame must not draw into
        // it. Recording the modeset as a presentation advances past it.
        let mut swapchain = Swapchain::new(config.buffers);
        swapchain.presented();

        Ok(Self {
            card,
            crtc,
            connector,
            buffers,
            swapchain,
            size,
            stride: pitch / 4,
            flip_pending: false,
            present_mode,
            saved_crtc,
            mode_name: format!("{width}x{height}@{}", mode.vrefresh()),
            cursor: None,
        })
    }

    /// Opens the first display-capable device and brings it up.
    pub fn open(config: SurfaceConfig) -> Result<Self, DrmError> {
        Self::new(Card::open_first()?, config)
    }

    /// The open card, for driving DRM objects the surface does not own —
    /// video planes above all. One process is DRM master, so anything else
    /// touching the display **must** go through this card rather than a
    /// second open, which would either fail or fight. `denise-video` is the
    /// consumer this seam exists for.
    pub fn card(&self) -> &Card {
        &self.card
    }

    /// The CRTC being driven, for placing planes on it.
    pub fn crtc(&self) -> drm::control::crtc::Handle {
        self.crtc
    }

    /// The mode in force, for logging.
    pub fn mode_name(&self) -> &str {
        &self.mode_name
    }

    /// Row stride in pixels.
    pub fn stride(&self) -> u32 {
        self.stride
    }

    /// Number of buffers in rotation.
    pub fn buffer_count(&self) -> usize {
        self.buffers.len()
    }

    /// The presentation mode actually in force.
    ///
    /// May be [`PresentMode::Vsync`] even when [`PresentMode::Immediate`] was
    /// asked for, if the driver does not advertise `DRM_CAP_ASYNC_PAGE_FLIP`.
    pub fn present_mode(&self) -> PresentMode {
        self.present_mode
    }

    /// Blocks until any queued flip has actually happened.
    ///
    /// This is the vsync wait, and it is where the frame loop should spend its
    /// idle time: the process sleeps in the kernel until the scanout engine is
    /// done, instead of spinning to guess when that was.
    ///
    /// How long that sleep lasts is the driver's business, not ours, and not every
    /// driver makes it last. `virtio-gpu` under a hypervisor completes the flip as
    /// soon as the host acknowledges it, so this returns immediately and the loop
    /// runs at thousands of frames a second on a 75 Hz mode. Real scanout hardware
    /// — vc4 on a Pi, for one — retires the flip at vblank and this blocks for the
    /// rest of the frame.
    ///
    /// A caller that must not spin when the driver declines to pace it needs its
    /// own frame deadline on top. That belongs in the event loop, with input, and
    /// arrives with it.
    fn wait_for_flip(&mut self) -> Result<(), DrmError> {
        while self.flip_pending {
            let events = self.card.receive_events().map_err(DrmError::WaitVblank)?;
            for event in events {
                if matches!(event, Event::PageFlip(_)) {
                    self.flip_pending = false;
                }
            }
        }
        Ok(())
    }
}

impl Surface for DrmSurface {
    fn size(&self) -> Size {
        self.size
    }

    fn scale_factor(&self) -> f32 {
        // DRM has no notion of a scale factor. A panel's physical size is known,
        // but turning that into a UI scale is policy, and policy does not belong
        // in the backend.
        1.0
    }

    fn format(&self) -> PixelFormat {
        PixelFormat::Xrgb8888
    }

    fn acquire(&mut self) -> Result<Frame<'_>, SurfaceError> {
        // The buffer we are about to hand out may still be on screen until the
        // previous flip retires. Drawing into it before then is what tearing is.
        self.wait_for_flip()?;

        let index = self.swapchain.current();
        let age = self.swapchain.age();
        let size = self.size;
        let stride = self.stride;

        Frame::new(
            self.buffers[index].pixels_mut(),
            size,
            stride,
            PixelFormat::Xrgb8888,
            age,
        )
    }

    fn present(&mut self, damage: &[Rect]) -> Result<(), SurfaceError> {
        // Damage cannot restrict the *upload* — a page flip swaps whole buffers,
        // and wiring partial updates in would need atomic modesetting and
        // `FB_DAMAGE_CLIPS`, which most drivers ignore. It can decide something
        // else, though: whether this particular frame is one a tear would show
        // on. See `flip_flags_for`.
        let index = self.swapchain.current();
        let fb = self.buffers[index].fb;
        let flags = flip_flags_for(self.present_mode, damage, self.size);

        self.card
            .page_flip(self.crtc, fb, flags, None)
            .map_err(DrmError::PageFlip)?;

        self.flip_pending = true;
        self.swapchain.presented();
        Ok(())
    }
}

impl Drop for DrmSurface {
    fn drop(&mut self) {
        // Let the last flip retire before pulling the buffers out from under the
        // scanout engine.
        let _ = self.wait_for_flip();

        if let Some(saved) = self.saved_crtc.as_ref() {
            let _ = self.card.set_crtc(
                self.crtc,
                saved.framebuffer(),
                saved.position(),
                &[self.connector],
                saved.mode(),
            );
        }

        if let Some(cursor) = self.cursor.take() {
            // Off the CRTC before the memory goes, or the scanout engine keeps
            // compositing a freed buffer.
            #[allow(deprecated)]
            let _ = self
                .card
                .set_cursor(self.crtc, None::<&drm::control::dumbbuffer::DumbBuffer>);
            cursor.release(&self.card);
        }

        for buffer in self.buffers.drain(..) {
            // SAFETY: `ptr`/`bytes` describe exactly the mapping made in
            // `Scanout::new`, whose guard was forgotten so that this code owns it.
            // Nothing else can reference the region: the buffer has been moved out
            // of `self.buffers` and any `Frame` borrowing it is long dropped.
            unsafe {
                let _ = rustix::mm::munmap(buffer.ptr.cast::<core::ffi::c_void>(), buffer.bytes);
            }
            let _ = self.card.destroy_framebuffer(buffer.fb);
            let _ = self.card.destroy_dumb_buffer(buffer.dumb);
        }

        self.card.release_master();
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    const SCREEN: Size = Size::new(1920, 1080);

    /// A button lighting up is the case async flips exist for: the seam is a few
    /// pixels tall, in one place, and gone next frame — and the press that
    /// caused it is what the latency is measured against.
    #[test]
    fn a_small_frame_still_flips_immediately() {
        let button = [Rect::new(40, 700, 220, 48)];
        assert_eq!(
            flip_flags_for(PresentMode::Immediate, &button, SCREEN),
            PageFlipFlags::EVENT | PageFlipFlags::ASYNC
        );
    }

    /// A scrolled viewport is the case it does not: the seam crosses the text
    /// being read. This is the frame that was reported as flicker from a Pi.
    #[test]
    fn a_scrolled_viewport_waits_for_vblank() {
        let viewport = [Rect::new(320, 60, 1560, 1000)];
        assert_eq!(
            flip_flags_for(PresentMode::Immediate, &viewport, SCREEN),
            PageFlipFlags::EVENT,
            "a frame that moved everything must not tear"
        );
    }

    /// The gallery's sidebar, exactly: 300 by 1016 on a 1920x1080 panel. It is
    /// under 15% of the pixels and over 90% of the scanlines, and judging it by
    /// area sent it out async — which is the flash that was still being seen
    /// after the first version of this shipped.
    #[test]
    fn a_narrow_column_down_the_screen_is_not_a_small_frame() {
        let sidebar = [Rect::new(12, 52, 300, 1016)];
        assert_eq!(
            flip_flags_for(PresentMode::Immediate, &sidebar, SCREEN),
            PageFlipFlags::EVENT,
            "14.7% of the pixels, 94% of the rows: a tear crosses the lot"
        );
    }

    /// And the other way round, which is why this is rows and not "any big
    /// dimension": a band across the whole width can seam without anybody
    /// noticing, because the seam is as short as the band.
    #[test]
    fn a_wide_shallow_band_may_still_tear() {
        let toolbar = [Rect::new(0, 0, 1920, 40)];
        assert_eq!(
            flip_flags_for(PresentMode::Immediate, &toolbar, SCREEN),
            PageFlipFlags::EVENT | PageFlipFlags::ASYNC
        );
    }

    /// What the damage covers, not what it spans. Two specks far apart leave
    /// the rows between them untouched, and untouched rows are identical in
    /// both buffers, so the seam the beam can put there shows nothing.
    #[test]
    fn scattered_damage_is_judged_by_what_it_covers() {
        let corners = [Rect::new(0, 0, 60, 40), Rect::new(1860, 1040, 60, 40)];
        assert_eq!(
            flip_flags_for(PresentMode::Immediate, &corners, SCREEN),
            PageFlipFlags::EVENT | PageFlipFlags::ASYNC,
            "eighty rows in two places, not the thousand between them"
        );

        let neighbours = [Rect::new(40, 700, 220, 48), Rect::new(280, 700, 220, 48)];
        assert_eq!(
            flip_flags_for(PresentMode::Immediate, &neighbours, SCREEN),
            PageFlipFlags::EVENT | PageFlipFlags::ASYNC,
            "two buttons side by side are still two buttons"
        );
    }

    /// The frame this rule was costing, and the reason it was reported: the
    /// gallery's spinner sits at the top and re-damages itself every motion
    /// tick, so hovering anything below it produced a bounding box most of the
    /// screen tall. Nothing about that frame is worth a vblank.
    #[test]
    fn a_spinner_and_a_pointer_far_apart_are_two_small_things() {
        let spinner = Rect::new(736, 46, 48, 48);
        let cursor = Rect::new(910, 812, 16, 24);
        let hovered = Rect::new(820, 780, 220, 48);
        assert_eq!(
            flip_flags_for(PresentMode::Immediate, &[spinner, cursor, hovered], SCREEN),
            PageFlipFlags::EVENT | PageFlipFlags::ASYNC,
            "a spinner, a cursor and a highlight cover well under a quarter"
        );
    }

    /// A scroll damages one tall rectangle, and the whole point is that it is
    /// still caught once the count stops being a bounding box.
    #[test]
    fn coverage_still_catches_the_frames_bounds_caught() {
        let sidebar = [Rect::new(12, 52, 300, 1016)];
        assert_eq!(damaged_rows(&sidebar, SCREEN), 1016);

        let viewport = [Rect::new(320, 60, 1560, 1000)];
        assert_eq!(damaged_rows(&viewport, SCREEN), 1000);
    }

    /// Rectangles arrive in no order and may overlap. A row under two of them
    /// is still one row.
    #[test]
    fn overlapping_and_unsorted_rows_are_counted_once() {
        let stacked = [
            Rect::new(0, 300, 100, 100),
            Rect::new(0, 100, 100, 100),
            Rect::new(0, 350, 100, 100),
        ];
        assert_eq!(
            damaged_rows(&stacked, SCREEN),
            250,
            "100 at 100..200, then 150 at 300..450"
        );

        let abutting = [Rect::new(0, 100, 100, 50), Rect::new(0, 150, 100, 50)];
        assert_eq!(damaged_rows(&abutting, SCREEN), 100, "one run, not two");
    }

    /// Rows off the bottom of the panel are never scanned out, so they cannot
    /// tear and do not count.
    #[test]
    fn rows_outside_the_surface_do_not_count() {
        let overhang = [Rect::new(0, 1000, 100, 400)];
        assert_eq!(damaged_rows(&overhang, SCREEN), 80);

        let above = [Rect::new(0, -50, 100, 60)];
        assert_eq!(damaged_rows(&above, SCREEN), 10);

        let offscreen = [Rect::new(0, 1080, 100, 40)];
        assert_eq!(damaged_rows(&offscreen, SCREEN), 0);
    }

    /// More rectangles than the tracker can hold is not something this backend
    /// can reason about, and it is not going to guess in the direction that
    /// tears.
    #[test]
    fn an_oversized_list_is_treated_as_everything() {
        let many = [Rect::new(0, 0, 8, 8); MAX_DAMAGE_RECTS + 1];
        assert_eq!(damaged_rows(&many, SCREEN), SCREEN.height);
        assert_eq!(
            flip_flags_for(PresentMode::Immediate, &many, SCREEN),
            PageFlipFlags::EVENT
        );
    }

    /// A present that did not say what changed cannot be assumed to be small.
    #[test]
    fn damage_nobody_declared_is_treated_as_everything() {
        assert_eq!(
            flip_flags_for(PresentMode::Immediate, &[], SCREEN),
            PageFlipFlags::EVENT
        );
    }

    /// Asking for vsync gets vsync, whatever the damage. The mode is still a
    /// promise; it is only `Immediate` that became a preference.
    #[test]
    fn vsync_is_never_overridden() {
        for damage in [&[][..], &[Rect::new(0, 0, 4, 4)][..]] {
            assert_eq!(
                flip_flags_for(PresentMode::Vsync, damage, SCREEN),
                PageFlipFlags::EVENT
            );
        }
    }

    /// The threshold itself, from both sides, on a screen where a quarter is a
    /// round number of rows.
    #[test]
    fn the_threshold_is_a_quarter_of_the_rows() {
        let screen = Size::new(1000, 1000);
        let just_under = [Rect::new(0, 0, 8, 249)];
        let just_over = [Rect::new(0, 0, 8, 250)];
        assert_eq!(
            flip_flags_for(PresentMode::Immediate, &just_under, screen),
            PageFlipFlags::EVENT | PageFlipFlags::ASYNC
        );
        assert_eq!(
            flip_flags_for(PresentMode::Immediate, &just_over, screen),
            PageFlipFlags::EVENT
        );
    }
}