script: Generate only a single frame during "update the rendering" (#38858)

Instead of generating a frame for every display list, which might be one
rendered frame per `<iframe>`, generate only a single frame per call to
"update the rendering." This should make rendering more efficient when
there are `<iframe>`s present and also open up optimizations for
non-display list frames.

Testing: This could potentially reduce flashing of content during
rendering
updates, but that is very difficult to test.

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

components/shared/compositing/lib.rs

@@ -111,6 +111,9 @@ pub enum CompositorMsg {
         /// An [ipc::IpcBytesReceiver] used to send the raw data of the display list.
         display_list_receiver: ipc::IpcBytesReceiver,
     },
+    /// Ask the renderer to generate a frame for the current set of display lists that
+    /// have been sent to the renderer.
+    GenerateFrame,
     /// Create a new image key. The result will be returned via the
     /// provided channel sender.
     GenerateImageKey(IpcSender<ImageKey>),
@@ -244,6 +247,13 @@ impl CrossProcessCompositorApi {
         }
     }
 
+    /// Ask the Servo renderer to generate a new frame after having new display lists.
+    pub fn generate_frame(&self) {
+        if let Err(error) = self.0.send(CompositorMsg::GenerateFrame) {
+            warn!("Error generating frame: {error}");
+        }
+    }
+
     /// Create a new image key. Blocks until the key is available.
     pub fn generate_image_key_blocking(&self) -> Option<ImageKey> {
         let (sender, receiver) = ipc::channel().unwrap();

components/shared/layout/lib.rs

@@ -441,6 +441,15 @@ bitflags! {
         const BuiltStackingContextTree = 1 << 2;
         const BuiltDisplayList = 1 << 3;
         const UpdatedScrollNodeOffset = 1 << 4;
+        const UpdatedCanvasContents = 1 << 5;
+    }
+}
+
+impl ReflowPhasesRun {
+    pub fn needs_frame(&self) -> bool {
+        self.intersects(
+            Self::BuiltDisplayList | Self::UpdatedScrollNodeOffset | Self::UpdatedCanvasContents,
+        )
     }
 }