Avoid re-building the frame when we only need the hit tester.

webrender/src/render_backend.rs

@@ -348,14 +348,12 @@ impl Document {
 
 struct DocumentOps {
     scroll: bool,
-    build_frame: bool,
 }
 
 impl DocumentOps {
     fn nop() -> Self {
         DocumentOps {
             scroll: false,
-            build_frame: false,
         }
     }
 }
@@ -612,7 +610,6 @@ impl RenderBackend {
                             replace(&mut txn.resource_updates, Vec::new()),
                             replace(&mut txn.frame_ops, Vec::new()),
                             replace(&mut txn.notifications, Vec::new()),
-                            txn.build_frame,
                             txn.render_frame,
                             &mut frame_counter,
                             &mut profile_counters,
@@ -865,7 +862,6 @@ impl RenderBackend {
             rasterized_blobs: Vec::new(),
             notifications: transaction_msg.notifications,
             set_root_pipeline: None,
-            build_frame: transaction_msg.generate_frame,
             render_frame: transaction_msg.generate_frame,
         });
 
@@ -900,7 +896,6 @@ impl RenderBackend {
                 replace(&mut txn.resource_updates, Vec::new()),
                 replace(&mut txn.frame_ops, Vec::new()),
                 replace(&mut txn.notifications, Vec::new()),
-                txn.build_frame,
                 txn.render_frame,
                 frame_counter,
                 profile_counters,
@@ -937,7 +932,6 @@ impl RenderBackend {
         resource_updates: Vec<ResourceUpdate>,
         mut frame_ops: Vec<FrameMsg>,
         mut notifications: Vec<NotificationRequest>,
-        mut build_frame: bool,
         mut render_frame: bool,
         frame_counter: &mut u32,
         profile_counters: &mut BackendProfileCounters,
@@ -950,7 +944,7 @@ impl RenderBackend {
         // fiddle with things after a potentially long scene build, but just
         // before rendering. This is useful for rendering with the latest
         // async transforms.
-        if build_frame {
+        if requested_frame {
             if let Some(ref sampler) = self.sampler {
                 frame_ops.append(&mut sampler.sample());
             }
@@ -964,7 +958,6 @@ impl RenderBackend {
         for frame_msg in frame_ops {
             let _timer = profile_counters.total_time.timer();
             let op = doc.process_frame_msg(frame_msg);
-            build_frame |= op.build_frame;
             scroll |= op.scroll;
         }
 
@@ -979,30 +972,23 @@ impl RenderBackend {
             &mut profile_counters.resources,
         );
 
-        // After applying the new scene we need to
-        // rebuild the hit-tester, so we trigger a frame generation
-        // step.
-        //
-        // TODO: We could avoid some the cost of building the frame by only
-        // building the information required for hit-testing (See #2807).
-        build_frame |= has_built_scene;
-
         if doc.dynamic_properties.flush_pending_updates() {
             doc.frame_is_valid = false;
             doc.hit_tester_is_valid = false;
-            build_frame = true;
         }
 
         if !doc.can_render() {
             // TODO: this happens if we are building the first scene asynchronously and
             // scroll at the same time. we should keep track of the fact that we skipped
             // composition here and do it as soon as we receive the scene.
-            build_frame = false;
             render_frame = false;
         }
 
-        if doc.frame_is_valid {
-            build_frame = false;
+        // Avoid re-building the frame if the current built frame is still valid.
+        let build_frame = render_frame && !doc.frame_is_valid;
+
+        if !doc.hit_tester_is_valid && !build_frame {
+            doc.rebuild_hit_tester();
         }
 
         let mut frame_build_time = None;

webrender/src/scene_builder.rs

@@ -33,7 +33,6 @@ pub struct Transaction {
     pub frame_ops: Vec<FrameMsg>,
     pub notifications: Vec<NotificationRequest>,
     pub set_root_pipeline: Option<PipelineId>,
-    pub build_frame: bool,
     pub render_frame: bool,
 }
 
@@ -66,7 +65,6 @@ pub struct BuiltTransaction {
     pub notifications: Vec<NotificationRequest>,
     pub scene_build_start_time: u64,
     pub scene_build_end_time: u64,
-    pub build_frame: bool,
     pub render_frame: bool,
 }
 
@@ -246,7 +244,6 @@ impl SceneBuilder {
 
             let txn = Box::new(BuiltTransaction {
                 document_id: item.document_id,
-                build_frame: true,
                 render_frame: item.build_frame,
                 built_scene,
                 resource_updates: Vec::new(),
@@ -334,7 +331,6 @@ impl SceneBuilder {
 
         Box::new(BuiltTransaction {
             document_id: txn.document_id,
-            build_frame: txn.build_frame || built_scene.is_some(),
             render_frame: txn.render_frame,
             built_scene,
             rasterized_blobs,