Auto merge of #27425 - jdm:preserve-draw-buffer, r=asajeffrey

Support preserveDrawingBuffer

Depends on https://github.com/asajeffrey/surfman-chains/pull/9 and https://github.com/servo/webxr/pull/190.

---
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #24604
- [x] There are tests for these changes

components/canvas/Cargo.toml

@@ -40,7 +40,7 @@ style = { path = "../style" }
 style_traits = { path = "../style_traits" }
 # NOTE: the sm-angle feature only enables angle on windows, not other platforms!
 surfman = { version = "0.3", features = ["sm-angle","sm-angle-default"] }
-surfman-chains = "0.4"
+surfman-chains = "0.5"
 surfman-chains-api = "0.2"
 time = { version = "0.1.0", optional = true }
 webrender = { git = "https://github.com/servo/webrender" }

components/canvas/webgl_thread.rs

@@ -827,22 +827,30 @@ impl WebGLThread {
 
             debug!("Swapping {:?}", context_id);
             swap_chain
-                .swap_buffers(&mut self.device, &mut data.ctx)
+                .swap_buffers(
+                    &mut self.device,
+                    &mut data.ctx,
+                    if data.attributes.preserve_drawing_buffer {
+                        surfman_chains::PreserveBuffer::Yes(&*data.gl)
+                    } else {
+                        surfman_chains::PreserveBuffer::No
+                    },
+                )
                 .unwrap();
             debug_assert_eq!(data.gl.get_error(), gl::NO_ERROR);
 
-            // TODO: if preserveDrawingBuffer is true, then blit the front buffer to the back buffer
-            // https://github.com/servo/servo/issues/24604
-            debug!("Clearing {:?}", context_id);
-            let alpha = data
-                .state
-                .requested_flags
-                .contains(ContextAttributeFlags::ALPHA);
-            let clear_color = [0.0, 0.0, 0.0, !alpha as i32 as f32];
-            swap_chain
-                .clear_surface(&mut self.device, &mut data.ctx, &*data.gl, clear_color)
-                .unwrap();
-            debug_assert_eq!(data.gl.get_error(), gl::NO_ERROR);
+            if !data.attributes.preserve_drawing_buffer {
+                debug!("Clearing {:?}", context_id);
+                let alpha = data
+                    .state
+                    .requested_flags
+                    .contains(ContextAttributeFlags::ALPHA);
+                let clear_color = [0.0, 0.0, 0.0, !alpha as i32 as f32];
+                swap_chain
+                    .clear_surface(&mut self.device, &mut data.ctx, &*data.gl, clear_color)
+                    .unwrap();
+                debug_assert_eq!(data.gl.get_error(), gl::NO_ERROR);
+            }
 
             // Rebind framebuffers as appropriate.
             debug!("Rebinding {:?}", context_id);

components/webrender_surfman/Cargo.toml

@@ -13,5 +13,5 @@ path = "lib.rs"
 [dependencies]
 euclid = "0.20"
 surfman = "0.3"
-surfman-chains = "0.4"
+surfman-chains = "0.5"
 

components/webrender_surfman/lib.rs

@@ -147,7 +147,7 @@ impl WebrenderSurfman {
         let ref mut device = self.0.device.borrow_mut();
         let ref mut context = self.0.context.borrow_mut();
         if let Some(ref swap_chain) = self.0.swap_chain {
-            return swap_chain.swap_buffers(device, context);
+            return swap_chain.swap_buffers(device, context, surfman_chains::PreserveBuffer::No);
         }
         let mut surface = device.unbind_surface_from_context(context)?.unwrap();
         device.present_surface(context, &mut surface)?;