5 files changed, 61 insertions, 6 deletions
diff --git a/components/compositing/compositor.rs b/components/compositing/compositor.rs
index 67cecd6a4a0..8eed1ed3f66 100644
--- a/components/compositing/compositor.rs
+++ b/components/compositing/compositor.rs
@@ -230,6 +230,13 @@ pub struct IOCompositor<Window: WindowMethods + ?Sized> {
     /// taken before the render is complete will not reflect the
     /// most up to date rendering.
     waiting_on_pending_frame: bool,
+
+    /// Whether to send a ReadyToPresent message to the constellation after rendering a new frame,
+    /// allowing external code to draw to the framebuffer and decide when to present the frame.
+    external_present: bool,
+
+    /// Waiting for external code to call present.
+    waiting_on_present: bool,
 }
 
 #[derive(Clone, Copy)]
@@ -382,6 +389,8 @@ impl<Window: WindowMethods + ?Sized> IOCompositor<Window> {
             exit_after_load,
             convert_mouse_to_touch,
             waiting_on_pending_frame: false,
+            external_present: false,
+            waiting_on_present: false,
         }
     }
 
@@ -1525,6 +1534,12 @@ impl<Window: WindowMethods + ?Sized> IOCompositor<Window> {
         target: CompositeTarget,
         rect: Option<Rect<f32, CSSPixel>>,
     ) -> Result<Option<Image>, UnableToComposite> {
+        if self.waiting_on_present {
+            return Err(UnableToComposite::NotReadyToPaintImage(
+                NotReadyToPaint::WaitingOnConstellation,
+            ));
+        }
+
         let size = self.embedder_coordinates.framebuffer.to_u32();
 
         if let Err(err) = self.webrender_surfman.make_gl_context_current() {
@@ -1703,8 +1718,15 @@ impl<Window: WindowMethods + ?Sized> IOCompositor<Window> {
         };
 
         // Perform the page flip. This will likely block for a while.
-        if let Err(err) = self.webrender_surfman.present() {
-            warn!("Failed to present surface: {:?}", err);
+        if self.external_present {
+            self.waiting_on_present = true;
+            let msg =
+                ConstellationMsg::ReadyToPresent(self.root_pipeline.top_level_browsing_context_id);
+            if let Err(e) = self.constellation_chan.send(msg) {
+                warn!("Sending event to constellation failed ({:?}).", e);
+            }
+        } else {
+            self.present();
         }
 
         self.composition_request = CompositionRequest::NoCompositingNecessary;
@@ -1715,6 +1737,13 @@ impl<Window: WindowMethods + ?Sized> IOCompositor<Window> {
         Ok(rv)
     }
 
+    pub fn present(&mut self) {
+        if let Err(err) = self.webrender_surfman.present() {
+            warn!("Failed to present surface: {:?}", err);
+        }
+        self.waiting_on_present = false;
+    }
+
     fn composite_if_necessary(&mut self, reason: CompositingReason) {
         if self.composition_request == CompositionRequest::NoCompositingNecessary {
             if self.is_running_problem_test {
@@ -1733,10 +1762,12 @@ impl<Window: WindowMethods + ?Sized> IOCompositor<Window> {
         let gl = &self.webrender_gl;
         self.assert_gl_framebuffer_complete();
 
-        // Make framebuffer fully transparent.
-        gl.clear_color(0.0, 0.0, 0.0, 0.0);
-        gl.clear(gleam::gl::COLOR_BUFFER_BIT);
-        self.assert_gl_framebuffer_complete();
+        if !self.external_present {
+            // Make framebuffer fully transparent.
+            gl.clear_color(0.0, 0.0, 0.0, 0.0);
+            gl.clear(gleam::gl::COLOR_BUFFER_BIT);
+            self.assert_gl_framebuffer_complete();
+        }
 
         // Make the viewport white.
         let viewport = self.embedder_coordinates.get_flipped_viewport();
@@ -1919,6 +1950,10 @@ impl<Window: WindowMethods + ?Sized> IOCompositor<Window> {
             None => eprintln!("Unable to locate path to save captures"),
         }
     }
+
+    pub fn set_external_present(&mut self, value: bool) {
+        self.external_present = value;
+    }
 }
 
 /// Why we performed a composite. This is used for debugging.
diff --git a/components/compositing/lib.rs b/components/compositing/lib.rs
index 4f29cfc1662..554f36df833 100644
--- a/components/compositing/lib.rs
+++ b/components/compositing/lib.rs
@@ -109,6 +109,8 @@ pub enum ConstellationMsg {
     ChangeBrowserVisibility(TopLevelBrowsingContextId, bool),
     /// Virtual keyboard was dismissed
     IMEDismissed,
+    /// Compositing done, but external code needs to present.
+    ReadyToPresent(TopLevelBrowsingContextId),
 }
 
 impl fmt::Debug for ConstellationMsg {
@@ -142,6 +144,7 @@ impl fmt::Debug for ConstellationMsg {
             ChangeBrowserVisibility(..) => "ChangeBrowserVisibility",
             IMEDismissed => "IMEDismissed",
             ClearCache => "ClearCache",
+            ReadyToPresent(..) => "ReadyToPresent",
         };
         write!(formatter, "ConstellationMsg::{}", variant)
     }
diff --git a/components/constellation/constellation.rs b/components/constellation/constellation.rs
index 5982f5ce76d..cf4ca3eb41b 100644
--- a/components/constellation/constellation.rs
+++ b/components/constellation/constellation.rs
@@ -1551,6 +1551,12 @@ where
             FromCompositorMsg::ChangeBrowserVisibility(top_level_browsing_context_id, visible) => {
                 self.handle_change_browser_visibility(top_level_browsing_context_id, visible);
             },
+            FromCompositorMsg::ReadyToPresent(top_level_browsing_context_id) => {
+                self.embedder_proxy.send((
+                    Some(top_level_browsing_context_id),
+                    EmbedderMsg::ReadyToPresent,
+                ));
+            },
         }
     }
 
diff --git a/components/embedder_traits/lib.rs b/components/embedder_traits/lib.rs
index 8654732735d..da117823340 100644
--- a/components/embedder_traits/lib.rs
+++ b/components/embedder_traits/lib.rs
@@ -212,6 +212,8 @@ pub enum EmbedderMsg {
     MediaSessionEvent(MediaSessionEvent),
     /// Report the status of Devtools Server with a token that can be used to bypass the permission prompt.
     OnDevtoolsStarted(Result<u16, ()>, String),
+    /// Compositing done, but external code needs to present.
+    ReadyToPresent,
 }
 
 impl Debug for EmbedderMsg {
@@ -248,6 +250,7 @@ impl Debug for EmbedderMsg {
             EmbedderMsg::MediaSessionEvent(..) => write!(f, "MediaSessionEvent"),
             EmbedderMsg::OnDevtoolsStarted(..) => write!(f, "OnDevtoolsStarted"),
             EmbedderMsg::ShowContextMenu(..) => write!(f, "ShowContextMenu"),
+            EmbedderMsg::ReadyToPresent => write!(f, "ReadyToPresent"),
         }
     }
 }
diff --git a/components/servo/lib.rs b/components/servo/lib.rs
index 51df1f0c991..91a263d450e 100644
--- a/components/servo/lib.rs
+++ b/components/servo/lib.rs
@@ -801,6 +801,14 @@ where
     pub fn deinit(self) {
         self.compositor.deinit();
     }
+
+    pub fn set_external_present(&mut self, value: bool) {
+        self.compositor.set_external_present(value)
+    }
+
+    pub fn present(&mut self) {
+        self.compositor.present();
+    }
 }
 
 fn create_embedder_channel(