compositing: Combine webrender_traits and compositing_traits (#36372)

These two traits both exposed different parts of the compositing API,
but now that the compositor doesn't depend directly on `script` any
longer and the `script_traits` crate has been split into the
`constellation_traits` crate, this can be finally be cleaned up without
causing circular dependencies. In addition, some unit tests for the
`IOPCompositor`'s scroll node tree are also moved into
`compositing_traits` as well.

Testing: This just combines two crates, so no new tests are necessary.
Fixes: #35984.
Signed-off-by: Martin Robinson <mrobinson@igalia.com>

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/shared/compositing/rendering_context.rs

@@ -0,0 +1,887 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+#![deny(unsafe_code)]
+
+use std::cell::{Cell, RefCell, RefMut};
+use std::num::NonZeroU32;
+use std::rc::Rc;
+use std::sync::Arc;
+
+use dpi::PhysicalSize;
+use euclid::default::{Rect, Size2D as UntypedSize2D};
+use euclid::{Point2D, Size2D};
+use gleam::gl::{self, Gl};
+use glow::NativeFramebuffer;
+use image::RgbaImage;
+use log::{debug, trace, warn};
+use raw_window_handle::{DisplayHandle, WindowHandle};
+pub use surfman::Error;
+use surfman::chains::{PreserveBuffer, SwapChain};
+use surfman::{
+    Adapter, Connection, Context, ContextAttributeFlags, ContextAttributes, Device, GLApi,
+    NativeContext, NativeWidget, Surface, SurfaceAccess, SurfaceInfo, SurfaceTexture, SurfaceType,
+};
+use webrender_api::units::{DeviceIntRect, DevicePixel};
+
+/// The `RenderingContext` trait defines a set of methods for managing
+/// an OpenGL or GLES rendering context.
+/// Implementors of this trait are responsible for handling the creation,
+/// management, and destruction of the rendering context and its associated
+/// resources.
+pub trait RenderingContext {
+    /// Prepare this [`RenderingContext`] to be rendered upon by Servo. For instance,
+    /// by binding a framebuffer to the current OpenGL context.
+    fn prepare_for_rendering(&self) {}
+    /// Read the contents of this [`Renderingcontext`] into an in-memory image. If the
+    /// image cannot be read (for instance, if no rendering has taken place yet), then
+    /// `None` is returned.
+    ///
+    /// In a double-buffered [`RenderingContext`] this is expected to read from the back
+    /// buffer. That means that once Servo renders to the context, this should return those
+    /// results, even before [`RenderingContext::present`] is called.
+    fn read_to_image(&self, source_rectangle: DeviceIntRect) -> Option<RgbaImage>;
+    /// Get the current size of this [`RenderingContext`].
+    fn size(&self) -> PhysicalSize<u32>;
+    /// Get the current size of this [`RenderingContext`] as [`Size2D`].
+    fn size2d(&self) -> Size2D<u32, DevicePixel> {
+        let size = self.size();
+        Size2D::new(size.width, size.height)
+    }
+    /// Resizes the rendering surface to the given size.
+    fn resize(&self, size: PhysicalSize<u32>);
+    /// Presents the rendered frame to the screen. In a double-buffered context, this would
+    /// swap buffers.
+    fn present(&self);
+    /// Makes the context the current OpenGL context for this thread.
+    /// After calling this function, it is valid to use OpenGL rendering
+    /// commands.
+    fn make_current(&self) -> Result<(), Error>;
+    /// Returns the `gleam` version of the OpenGL or GLES API.
+    fn gleam_gl_api(&self) -> Rc<dyn gleam::gl::Gl>;
+    /// Returns the OpenGL or GLES API.
+    fn glow_gl_api(&self) -> Arc<glow::Context>;
+    /// Creates a texture from a given surface and returns the surface texture,
+    /// the OpenGL texture object, and the size of the surface. Default to `None`.
+    fn create_texture(
+        &self,
+        _surface: Surface,
+    ) -> Option<(SurfaceTexture, u32, UntypedSize2D<i32>)> {
+        None
+    }
+    /// Destroys the texture and returns the surface. Default to `None`.
+    fn destroy_texture(&self, _surface_texture: SurfaceTexture) -> Option<Surface> {
+        None
+    }
+    /// The connection to the display server for WebGL. Default to `None`.
+    fn connection(&self) -> Option<Connection> {
+        None
+    }
+}
+
+/// A rendering context that uses the Surfman library to create and manage
+/// the OpenGL context and surface. This struct provides the default implementation
+/// of the `RenderingContext` trait, handling the creation, management, and destruction
+/// of the rendering context and its associated resources.
+///
+/// The `SurfmanRenderingContext` struct encapsulates the necessary data and methods
+/// to interact with the Surfman library, including creating surfaces, binding surfaces,
+/// resizing surfaces, presenting rendered frames, and managing the OpenGL context state.
+struct SurfmanRenderingContext {
+    gleam_gl: Rc<dyn Gl>,
+    glow_gl: Arc<glow::Context>,
+    device: RefCell<Device>,
+    context: RefCell<Context>,
+}
+
+impl Drop for SurfmanRenderingContext {
+    fn drop(&mut self) {
+        let device = &mut self.device.borrow_mut();
+        let context = &mut self.context.borrow_mut();
+        let _ = device.destroy_context(context);
+    }
+}
+
+impl SurfmanRenderingContext {
+    fn new(connection: &Connection, adapter: &Adapter) -> Result<Self, Error> {
+        let mut device = connection.create_device(adapter)?;
+
+        let flags = ContextAttributeFlags::ALPHA |
+            ContextAttributeFlags::DEPTH |
+            ContextAttributeFlags::STENCIL;
+        let gl_api = connection.gl_api();
+        let version = match &gl_api {
+            GLApi::GLES => surfman::GLVersion { major: 3, minor: 0 },
+            GLApi::GL => surfman::GLVersion { major: 3, minor: 2 },
+        };
+        let context_descriptor =
+            device.create_context_descriptor(&ContextAttributes { flags, version })?;
+        let context = device.create_context(&context_descriptor, None)?;
+
+        #[allow(unsafe_code)]
+        let gleam_gl = {
+            match gl_api {
+                GLApi::GL => unsafe {
+                    gl::GlFns::load_with(|func_name| device.get_proc_address(&context, func_name))
+                },
+                GLApi::GLES => unsafe {
+                    gl::GlesFns::load_with(|func_name| device.get_proc_address(&context, func_name))
+                },
+            }
+        };
+
+        #[allow(unsafe_code)]
+        let glow_gl = unsafe {
+            glow::Context::from_loader_function(|function_name| {
+                device.get_proc_address(&context, function_name)
+            })
+        };
+
+        Ok(SurfmanRenderingContext {
+            gleam_gl,
+            glow_gl: Arc::new(glow_gl),
+            device: RefCell::new(device),
+            context: RefCell::new(context),
+        })
+    }
+
+    fn create_surface(&self, surface_type: SurfaceType<NativeWidget>) -> Result<Surface, Error> {
+        let device = &mut self.device.borrow_mut();
+        let context = &self.context.borrow();
+        device.create_surface(context, SurfaceAccess::GPUOnly, surface_type)
+    }
+
+    fn bind_surface(&self, surface: Surface) -> Result<(), Error> {
+        let device = &self.device.borrow();
+        let context = &mut self.context.borrow_mut();
+        device
+            .bind_surface_to_context(context, surface)
+            .map_err(|(err, mut surface)| {
+                let _ = device.destroy_surface(context, &mut surface);
+                err
+            })?;
+        Ok(())
+    }
+
+    fn create_attached_swap_chain(&self) -> Result<SwapChain<Device>, Error> {
+        let device = &mut self.device.borrow_mut();
+        let context = &mut self.context.borrow_mut();
+        SwapChain::create_attached(device, context, SurfaceAccess::GPUOnly)
+    }
+
+    fn resize_surface(&self, size: PhysicalSize<u32>) -> Result<(), Error> {
+        let size = Size2D::new(size.width as i32, size.height as i32);
+        let device = &mut self.device.borrow_mut();
+        let context = &mut self.context.borrow_mut();
+
+        let mut surface = device.unbind_surface_from_context(context)?.unwrap();
+        device.resize_surface(context, &mut surface, size)?;
+        device
+            .bind_surface_to_context(context, surface)
+            .map_err(|(err, mut surface)| {
+                let _ = device.destroy_surface(context, &mut surface);
+                err
+            })
+    }
+
+    fn present_bound_surface(&self) -> Result<(), Error> {
+        let device = &self.device.borrow();
+        let context = &mut self.context.borrow_mut();
+
+        let mut surface = device.unbind_surface_from_context(context)?.unwrap();
+        device.present_surface(context, &mut surface)?;
+        device
+            .bind_surface_to_context(context, surface)
+            .map_err(|(err, mut surface)| {
+                let _ = device.destroy_surface(context, &mut surface);
+                err
+            })
+    }
+
+    #[allow(dead_code)]
+    fn native_context(&self) -> NativeContext {
+        let device = &self.device.borrow();
+        let context = &self.context.borrow();
+        device.native_context(context)
+    }
+
+    fn framebuffer(&self) -> Option<NativeFramebuffer> {
+        let device = &self.device.borrow();
+        let context = &self.context.borrow();
+        device
+            .context_surface_info(context)
+            .unwrap_or(None)
+            .and_then(|info| info.framebuffer_object)
+    }
+
+    fn prepare_for_rendering(&self) {
+        let framebuffer_id = self
+            .framebuffer()
+            .map_or(0, |framebuffer| framebuffer.0.into());
+        self.gleam_gl
+            .bind_framebuffer(gleam::gl::FRAMEBUFFER, framebuffer_id);
+    }
+
+    fn read_to_image(&self, source_rectangle: DeviceIntRect) -> Option<RgbaImage> {
+        let framebuffer_id = self
+            .framebuffer()
+            .map_or(0, |framebuffer| framebuffer.0.into());
+        Framebuffer::read_framebuffer_to_image(&self.gleam_gl, framebuffer_id, source_rectangle)
+    }
+
+    fn make_current(&self) -> Result<(), Error> {
+        let device = &self.device.borrow();
+        let context = &mut self.context.borrow();
+        device.make_context_current(context)
+    }
+
+    fn create_texture(
+        &self,
+        surface: Surface,
+    ) -> Option<(SurfaceTexture, u32, UntypedSize2D<i32>)> {
+        let device = &self.device.borrow();
+        let context = &mut self.context.borrow_mut();
+        let SurfaceInfo {
+            id: front_buffer_id,
+            size,
+            ..
+        } = device.surface_info(&surface);
+        debug!("... getting texture for surface {:?}", front_buffer_id);
+        let surface_texture = device.create_surface_texture(context, surface).unwrap();
+        let gl_texture = device
+            .surface_texture_object(&surface_texture)
+            .map(|tex| tex.0.get())
+            .unwrap_or(0);
+        Some((surface_texture, gl_texture, size))
+    }
+
+    fn destroy_texture(&self, surface_texture: SurfaceTexture) -> Option<Surface> {
+        let device = &self.device.borrow();
+        let context = &mut self.context.borrow_mut();
+        device
+            .destroy_surface_texture(context, surface_texture)
+            .map_err(|(error, _)| error)
+            .ok()
+    }
+
+    fn connection(&self) -> Option<Connection> {
+        Some(self.device.borrow().connection())
+    }
+}
+
+/// A software rendering context that uses a software OpenGL implementation to render
+/// Servo. This will generally have bad performance, but can be used in situations where
+/// it is more convenient to have consistent, but slower display output.
+///
+/// The results of the render can be accessed via [`RenderingContext::read_to_image`].
+pub struct SoftwareRenderingContext {
+    size: Cell<PhysicalSize<u32>>,
+    surfman_rendering_info: SurfmanRenderingContext,
+    swap_chain: SwapChain<Device>,
+}
+
+impl SoftwareRenderingContext {
+    pub fn new(size: PhysicalSize<u32>) -> Result<Self, Error> {
+        let connection = Connection::new()?;
+        let adapter = connection.create_software_adapter()?;
+        let surfman_rendering_info = SurfmanRenderingContext::new(&connection, &adapter)?;
+
+        let surfman_size = Size2D::new(size.width as i32, size.height as i32);
+        let surface =
+            surfman_rendering_info.create_surface(SurfaceType::Generic { size: surfman_size })?;
+        surfman_rendering_info.bind_surface(surface)?;
+        surfman_rendering_info.make_current()?;
+
+        let swap_chain = surfman_rendering_info.create_attached_swap_chain()?;
+        Ok(SoftwareRenderingContext {
+            size: Cell::new(size),
+            surfman_rendering_info,
+            swap_chain,
+        })
+    }
+}
+
+impl Drop for SoftwareRenderingContext {
+    fn drop(&mut self) {
+        let device = &mut self.surfman_rendering_info.device.borrow_mut();
+        let context = &mut self.surfman_rendering_info.context.borrow_mut();
+        let _ = self.swap_chain.destroy(device, context);
+    }
+}
+
+impl RenderingContext for SoftwareRenderingContext {
+    fn prepare_for_rendering(&self) {
+        self.surfman_rendering_info.prepare_for_rendering();
+    }
+
+    fn read_to_image(&self, source_rectangle: DeviceIntRect) -> Option<RgbaImage> {
+        self.surfman_rendering_info.read_to_image(source_rectangle)
+    }
+
+    fn size(&self) -> PhysicalSize<u32> {
+        self.size.get()
+    }
+
+    fn resize(&self, size: PhysicalSize<u32>) {
+        if self.size.get() == size {
+            return;
+        }
+
+        self.size.set(size);
+
+        let device = &mut self.surfman_rendering_info.device.borrow_mut();
+        let context = &mut self.surfman_rendering_info.context.borrow_mut();
+        let size = Size2D::new(size.width as i32, size.height as i32);
+        let _ = self.swap_chain.resize(device, context, size);
+    }
+
+    fn present(&self) {
+        let device = &mut self.surfman_rendering_info.device.borrow_mut();
+        let context = &mut self.surfman_rendering_info.context.borrow_mut();
+        let _ = self
+            .swap_chain
+            .swap_buffers(device, context, PreserveBuffer::No);
+    }
+
+    fn make_current(&self) -> Result<(), Error> {
+        self.surfman_rendering_info.make_current()
+    }
+
+    fn gleam_gl_api(&self) -> Rc<dyn gleam::gl::Gl> {
+        self.surfman_rendering_info.gleam_gl.clone()
+    }
+
+    fn glow_gl_api(&self) -> Arc<glow::Context> {
+        self.surfman_rendering_info.glow_gl.clone()
+    }
+
+    fn create_texture(
+        &self,
+        surface: Surface,
+    ) -> Option<(SurfaceTexture, u32, UntypedSize2D<i32>)> {
+        self.surfman_rendering_info.create_texture(surface)
+    }
+
+    fn destroy_texture(&self, surface_texture: SurfaceTexture) -> Option<Surface> {
+        self.surfman_rendering_info.destroy_texture(surface_texture)
+    }
+
+    fn connection(&self) -> Option<Connection> {
+        self.surfman_rendering_info.connection()
+    }
+}
+
+/// A [`RenderingContext`] that uses the `surfman` library to render to a
+/// `raw-window-handle` identified window. `surfman` will attempt to create an
+/// OpenGL context and surface for this window. This is a simple implementation
+/// of the [`RenderingContext`] crate, but by default it paints to the entire window
+/// surface.
+///
+/// If you would like to paint to only a portion of the window, consider using
+/// [`OffscreenRenderingContext`] by calling [`WindowRenderingContext::offscreen_context`].
+pub struct WindowRenderingContext {
+    size: Cell<PhysicalSize<u32>>,
+    surfman_context: SurfmanRenderingContext,
+}
+
+impl WindowRenderingContext {
+    pub fn new(
+        display_handle: DisplayHandle,
+        window_handle: WindowHandle,
+        size: PhysicalSize<u32>,
+    ) -> Result<Self, Error> {
+        let connection = Connection::from_display_handle(display_handle)?;
+        let adapter = connection.create_adapter()?;
+        let surfman_context = SurfmanRenderingContext::new(&connection, &adapter)?;
+
+        let native_widget = connection
+            .create_native_widget_from_window_handle(
+                window_handle,
+                Size2D::new(size.width as i32, size.height as i32),
+            )
+            .expect("Failed to create native widget");
+
+        let surface = surfman_context.create_surface(SurfaceType::Widget { native_widget })?;
+        surfman_context.bind_surface(surface)?;
+        surfman_context.make_current()?;
+
+        Ok(Self {
+            size: Cell::new(size),
+            surfman_context,
+        })
+    }
+
+    pub fn offscreen_context(
+        self: &Rc<Self>,
+        size: PhysicalSize<u32>,
+    ) -> OffscreenRenderingContext {
+        OffscreenRenderingContext::new(self.clone(), size)
+    }
+
+    /// Stop rendering to the window that was used to create this `WindowRenderingContext`
+    /// or last set with [`Self::set_window`].
+    ///
+    /// TODO: This should be removed once `WebView`s can replace their `RenderingContext`s.
+    pub fn take_window(&self) -> Result<(), Error> {
+        let device = self.surfman_context.device.borrow_mut();
+        let mut context = self.surfman_context.context.borrow_mut();
+        let mut surface = device.unbind_surface_from_context(&mut context)?.unwrap();
+        device.destroy_surface(&mut context, &mut surface)?;
+        Ok(())
+    }
+
+    /// Replace the window that this [`WindowRenderingContext`] renders to and give it a new
+    /// size.
+    ///
+    /// TODO: This should be removed once `WebView`s can replace their `RenderingContext`s.
+    pub fn set_window(
+        &self,
+        window_handle: WindowHandle,
+        size: PhysicalSize<u32>,
+    ) -> Result<(), Error> {
+        let mut device = self.surfman_context.device.borrow_mut();
+        let mut context = self.surfman_context.context.borrow_mut();
+
+        let native_widget = device
+            .connection()
+            .create_native_widget_from_window_handle(
+                window_handle,
+                Size2D::new(size.width as i32, size.height as i32),
+            )
+            .expect("Failed to create native widget");
+
+        let surface_access = SurfaceAccess::GPUOnly;
+        let surface_type = SurfaceType::Widget { native_widget };
+        let surface = device.create_surface(&context, surface_access, surface_type)?;
+
+        device
+            .bind_surface_to_context(&mut context, surface)
+            .map_err(|(err, mut surface)| {
+                let _ = device.destroy_surface(&mut context, &mut surface);
+                err
+            })?;
+        device.make_context_current(&context)?;
+        Ok(())
+    }
+
+    pub fn surfman_details(&self) -> (RefMut<Device>, RefMut<Context>) {
+        (
+            self.surfman_context.device.borrow_mut(),
+            self.surfman_context.context.borrow_mut(),
+        )
+    }
+}
+
+impl RenderingContext for WindowRenderingContext {
+    fn prepare_for_rendering(&self) {
+        self.surfman_context.prepare_for_rendering();
+    }
+
+    fn read_to_image(&self, source_rectangle: DeviceIntRect) -> Option<RgbaImage> {
+        self.surfman_context.read_to_image(source_rectangle)
+    }
+
+    fn size(&self) -> PhysicalSize<u32> {
+        self.size.get()
+    }
+
+    fn resize(&self, size: PhysicalSize<u32>) {
+        match self.surfman_context.resize_surface(size) {
+            Ok(..) => self.size.set(size),
+            Err(error) => warn!("Error resizing surface: {error:?}"),
+        }
+    }
+
+    fn present(&self) {
+        if let Err(error) = self.surfman_context.present_bound_surface() {
+            warn!("Error presenting surface: {error:?}");
+        }
+    }
+
+    fn make_current(&self) -> Result<(), Error> {
+        self.surfman_context.make_current()
+    }
+
+    fn gleam_gl_api(&self) -> Rc<dyn gleam::gl::Gl> {
+        self.surfman_context.gleam_gl.clone()
+    }
+
+    fn glow_gl_api(&self) -> Arc<glow::Context> {
+        self.surfman_context.glow_gl.clone()
+    }
+
+    fn create_texture(
+        &self,
+        surface: Surface,
+    ) -> Option<(SurfaceTexture, u32, UntypedSize2D<i32>)> {
+        self.surfman_context.create_texture(surface)
+    }
+
+    fn destroy_texture(&self, surface_texture: SurfaceTexture) -> Option<Surface> {
+        self.surfman_context.destroy_texture(surface_texture)
+    }
+
+    fn connection(&self) -> Option<Connection> {
+        self.surfman_context.connection()
+    }
+}
+
+struct Framebuffer {
+    gl: Rc<dyn Gl>,
+    framebuffer_id: gl::GLuint,
+    renderbuffer_id: gl::GLuint,
+    texture_id: gl::GLuint,
+}
+
+impl Framebuffer {
+    fn bind(&self) {
+        trace!("Binding FBO {}", self.framebuffer_id);
+        self.gl
+            .bind_framebuffer(gl::FRAMEBUFFER, self.framebuffer_id)
+    }
+}
+
+impl Drop for Framebuffer {
+    fn drop(&mut self) {
+        self.gl.bind_framebuffer(gl::FRAMEBUFFER, 0);
+        self.gl.delete_textures(&[self.texture_id]);
+        self.gl.delete_renderbuffers(&[self.renderbuffer_id]);
+        self.gl.delete_framebuffers(&[self.framebuffer_id]);
+    }
+}
+
+impl Framebuffer {
+    fn new(gl: Rc<dyn Gl>, size: PhysicalSize<u32>) -> Self {
+        let framebuffer_ids = gl.gen_framebuffers(1);
+        gl.bind_framebuffer(gl::FRAMEBUFFER, framebuffer_ids[0]);
+
+        let texture_ids = gl.gen_textures(1);
+        gl.bind_texture(gl::TEXTURE_2D, texture_ids[0]);
+        gl.tex_image_2d(
+            gl::TEXTURE_2D,
+            0,
+            gl::RGBA as gl::GLint,
+            size.width as gl::GLsizei,
+            size.height as gl::GLsizei,
+            0,
+            gl::RGBA,
+            gl::UNSIGNED_BYTE,
+            None,
+        );
+        gl.tex_parameter_i(
+            gl::TEXTURE_2D,
+            gl::TEXTURE_MAG_FILTER,
+            gl::NEAREST as gl::GLint,
+        );
+        gl.tex_parameter_i(
+            gl::TEXTURE_2D,
+            gl::TEXTURE_MIN_FILTER,
+            gl::NEAREST as gl::GLint,
+        );
+
+        gl.framebuffer_texture_2d(
+            gl::FRAMEBUFFER,
+            gl::COLOR_ATTACHMENT0,
+            gl::TEXTURE_2D,
+            texture_ids[0],
+            0,
+        );
+
+        gl.bind_texture(gl::TEXTURE_2D, 0);
+
+        let renderbuffer_ids = gl.gen_renderbuffers(1);
+        let depth_rb = renderbuffer_ids[0];
+        gl.bind_renderbuffer(gl::RENDERBUFFER, depth_rb);
+        gl.renderbuffer_storage(
+            gl::RENDERBUFFER,
+            gl::DEPTH_COMPONENT24,
+            size.width as gl::GLsizei,
+            size.height as gl::GLsizei,
+        );
+        gl.framebuffer_renderbuffer(
+            gl::FRAMEBUFFER,
+            gl::DEPTH_ATTACHMENT,
+            gl::RENDERBUFFER,
+            depth_rb,
+        );
+
+        Self {
+            gl,
+            framebuffer_id: *framebuffer_ids
+                .first()
+                .expect("Guaranteed by GL operations"),
+            renderbuffer_id: *renderbuffer_ids
+                .first()
+                .expect("Guaranteed by GL operations"),
+            texture_id: *texture_ids.first().expect("Guaranteed by GL operations"),
+        }
+    }
+
+    fn read_to_image(&self, source_rectangle: DeviceIntRect) -> Option<RgbaImage> {
+        Self::read_framebuffer_to_image(&self.gl, self.framebuffer_id, source_rectangle)
+    }
+
+    fn read_framebuffer_to_image(
+        gl: &Rc<dyn Gl>,
+        framebuffer_id: u32,
+        source_rectangle: DeviceIntRect,
+    ) -> Option<RgbaImage> {
+        gl.bind_framebuffer(gl::FRAMEBUFFER, framebuffer_id);
+
+        // For some reason, OSMesa fails to render on the 3rd
+        // attempt in headless mode, under some conditions.
+        // I think this can only be some kind of synchronization
+        // bug in OSMesa, but explicitly un-binding any vertex
+        // array here seems to work around that bug.
+        // See https://github.com/servo/servo/issues/18606.
+        gl.bind_vertex_array(0);
+
+        let mut pixels = gl.read_pixels(
+            source_rectangle.min.x,
+            source_rectangle.min.y,
+            source_rectangle.width(),
+            source_rectangle.height(),
+            gl::RGBA,
+            gl::UNSIGNED_BYTE,
+        );
+        let gl_error = gl.get_error();
+        if gl_error != gl::NO_ERROR {
+            warn!("GL error code 0x{gl_error:x} set after read_pixels");
+        }
+
+        // flip image vertically (texture is upside down)
+        let source_rectangle = source_rectangle.to_usize();
+        let orig_pixels = pixels.clone();
+        let stride = source_rectangle.width() * 4;
+        for y in 0..source_rectangle.height() {
+            let dst_start = y * stride;
+            let src_start = (source_rectangle.height() - y - 1) * stride;
+            let src_slice = &orig_pixels[src_start..src_start + stride];
+            pixels[dst_start..dst_start + stride].clone_from_slice(&src_slice[..stride]);
+        }
+
+        RgbaImage::from_raw(
+            source_rectangle.width() as u32,
+            source_rectangle.height() as u32,
+            pixels,
+        )
+    }
+}
+
+pub struct OffscreenRenderingContext {
+    parent_context: Rc<WindowRenderingContext>,
+    size: Cell<PhysicalSize<u32>>,
+    framebuffer: RefCell<Framebuffer>,
+}
+
+type RenderToParentCallback = Box<dyn Fn(&glow::Context, Rect<i32>) + Send + Sync>;
+
+impl OffscreenRenderingContext {
+    fn new(parent_context: Rc<WindowRenderingContext>, size: PhysicalSize<u32>) -> Self {
+        let framebuffer = RefCell::new(Framebuffer::new(parent_context.gleam_gl_api(), size));
+        Self {
+            parent_context,
+            size: Cell::new(size),
+            framebuffer,
+        }
+    }
+
+    pub fn parent_context(&self) -> &WindowRenderingContext {
+        &self.parent_context
+    }
+
+    pub fn render_to_parent_callback(&self) -> Option<RenderToParentCallback> {
+        // Don't accept a `None` context for the source framebuffer.
+        let front_framebuffer_id =
+            NonZeroU32::new(self.framebuffer.borrow().framebuffer_id).map(NativeFramebuffer)?;
+        let parent_context_framebuffer_id = self.parent_context.surfman_context.framebuffer();
+        let size = self.size.get();
+        let size = Size2D::new(size.width as i32, size.height as i32);
+        Some(Box::new(move |gl, target_rect| {
+            Self::blit_framebuffer(
+                gl,
+                Rect::new(Point2D::origin(), size.to_i32()),
+                front_framebuffer_id,
+                target_rect,
+                parent_context_framebuffer_id,
+            );
+        }))
+    }
+
+    #[allow(unsafe_code)]
+    fn blit_framebuffer(
+        gl: &glow::Context,
+        source_rect: Rect<i32>,
+        source_framebuffer_id: NativeFramebuffer,
+        target_rect: Rect<i32>,
+        target_framebuffer_id: Option<NativeFramebuffer>,
+    ) {
+        use glow::HasContext as _;
+        unsafe {
+            gl.clear_color(0.0, 0.0, 0.0, 0.0);
+            gl.scissor(
+                target_rect.origin.x,
+                target_rect.origin.y,
+                target_rect.width(),
+                target_rect.height(),
+            );
+            gl.enable(gl::SCISSOR_TEST);
+            gl.clear(gl::COLOR_BUFFER_BIT);
+            gl.disable(gl::SCISSOR_TEST);
+
+            gl.bind_framebuffer(gl::READ_FRAMEBUFFER, Some(source_framebuffer_id));
+            gl.bind_framebuffer(gl::DRAW_FRAMEBUFFER, target_framebuffer_id);
+
+            gl.blit_framebuffer(
+                source_rect.origin.x,
+                source_rect.origin.y,
+                source_rect.origin.x + source_rect.width(),
+                source_rect.origin.y + source_rect.height(),
+                target_rect.origin.x,
+                target_rect.origin.y,
+                target_rect.origin.x + target_rect.width(),
+                target_rect.origin.y + target_rect.height(),
+                gl::COLOR_BUFFER_BIT,
+                gl::NEAREST,
+            );
+            gl.bind_framebuffer(gl::FRAMEBUFFER, target_framebuffer_id);
+        }
+    }
+}
+
+impl RenderingContext for OffscreenRenderingContext {
+    fn size(&self) -> PhysicalSize<u32> {
+        self.size.get()
+    }
+
+    fn resize(&self, new_size: PhysicalSize<u32>) {
+        let old_size = self.size.get();
+        if old_size == new_size {
+            return;
+        }
+
+        let gl = self.parent_context.gleam_gl_api();
+        let new_framebuffer = Framebuffer::new(gl.clone(), new_size);
+
+        let old_framebuffer =
+            std::mem::replace(&mut *self.framebuffer.borrow_mut(), new_framebuffer);
+        self.size.set(new_size);
+
+        let blit_size = new_size.min(old_size);
+        let rect = Rect::new(
+            Point2D::origin(),
+            Size2D::new(blit_size.width, blit_size.height),
+        )
+        .to_i32();
+
+        let Some(old_framebuffer_id) =
+            NonZeroU32::new(old_framebuffer.framebuffer_id).map(NativeFramebuffer)
+        else {
+            return;
+        };
+        let new_framebuffer_id =
+            NonZeroU32::new(self.framebuffer.borrow().framebuffer_id).map(NativeFramebuffer);
+        Self::blit_framebuffer(
+            &self.glow_gl_api(),
+            rect,
+            old_framebuffer_id,
+            rect,
+            new_framebuffer_id,
+        );
+    }
+
+    fn prepare_for_rendering(&self) {
+        self.framebuffer.borrow().bind();
+    }
+
+    fn present(&self) {}
+
+    fn make_current(&self) -> Result<(), surfman::Error> {
+        self.parent_context.make_current()
+    }
+
+    fn gleam_gl_api(&self) -> Rc<dyn gleam::gl::Gl> {
+        self.parent_context.gleam_gl_api()
+    }
+
+    fn glow_gl_api(&self) -> Arc<glow::Context> {
+        self.parent_context.glow_gl_api()
+    }
+
+    fn create_texture(
+        &self,
+        surface: Surface,
+    ) -> Option<(SurfaceTexture, u32, UntypedSize2D<i32>)> {
+        self.parent_context.create_texture(surface)
+    }
+
+    fn destroy_texture(&self, surface_texture: SurfaceTexture) -> Option<Surface> {
+        self.parent_context.destroy_texture(surface_texture)
+    }
+
+    fn connection(&self) -> Option<Connection> {
+        self.parent_context.connection()
+    }
+
+    fn read_to_image(&self, source_rectangle: DeviceIntRect) -> Option<RgbaImage> {
+        self.framebuffer.borrow().read_to_image(source_rectangle)
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use dpi::PhysicalSize;
+    use euclid::{Box2D, Point2D, Size2D};
+    use gleam::gl;
+    use image::Rgba;
+    use surfman::{Connection, ContextAttributeFlags, ContextAttributes, Error, GLApi, GLVersion};
+
+    use super::Framebuffer;
+
+    #[test]
+    #[allow(unsafe_code)]
+    fn test_read_pixels() -> Result<(), Error> {
+        let connection = Connection::new()?;
+        let adapter = connection.create_software_adapter()?;
+        let mut device = connection.create_device(&adapter)?;
+        let context_descriptor = device.create_context_descriptor(&ContextAttributes {
+            version: GLVersion::new(3, 0),
+            flags: ContextAttributeFlags::empty(),
+        })?;
+        let mut context = device.create_context(&context_descriptor, None)?;
+
+        let gl = match connection.gl_api() {
+            GLApi::GL => unsafe { gl::GlFns::load_with(|s| device.get_proc_address(&context, s)) },
+            GLApi::GLES => unsafe {
+                gl::GlesFns::load_with(|s| device.get_proc_address(&context, s))
+            },
+        };
+
+        device.make_context_current(&context)?;
+
+        {
+            const SIZE: u32 = 16;
+            let framebuffer = Framebuffer::new(gl, PhysicalSize::new(SIZE, SIZE));
+            framebuffer.bind();
+            framebuffer
+                .gl
+                .clear_color(12.0 / 255.0, 34.0 / 255.0, 56.0 / 255.0, 78.0 / 255.0);
+            framebuffer.gl.clear(gl::COLOR_BUFFER_BIT);
+
+            let rect = Box2D::from_origin_and_size(Point2D::zero(), Size2D::new(SIZE, SIZE));
+            let img = framebuffer
+                .read_to_image(rect.to_i32())
+                .expect("Should have been able to read back image.");
+            assert_eq!(img.width(), SIZE);
+            assert_eq!(img.height(), SIZE);
+
+            let expected_pixel: Rgba<u8> = Rgba([12, 34, 56, 78]);
+            assert!(img.pixels().all(|&p| p == expected_pixel));
+        }
+
+        device.destroy_context(&mut context)?;
+
+        Ok(())
+    }
+}