servo: Merge #11398 - script: Keep the DOM-side viewport up to date when scrolling happens in WebRender (from pcwalton:webrender-viewport); r=glennw

This happens asynchronously, just as it does in non-WebRender mode.

This functionality is a prerequisite for doing proper display-list-based
hit testing in WebRender, since it moves the scroll offsets into Servo
(and, specifically, into the script thread, enabling iframe event
forwarding) instead of keeping them private to WebRender.

Requires servo/webrender_traits#55 and servo/webrender#277.

Partially addresses #11108.

r? @glennw

Source-Repo: https://github.com/servo/servo
Source-Revision: 27d1f182713077395426a53a9c91ec35c95887ee

servo/components/script/script_thread.rs

@@ -708,6 +708,11 @@ impl ScriptThread {
                         self.handle_viewport(id, rect);
                     })
                 }
+                FromConstellation(ConstellationControlMsg::SetScrollState(id, scroll_offset)) => {
+                    self.profile_event(ScriptThreadEventCategory::SetScrollState, || {
+                        self.handle_set_scroll_state(id, &scroll_offset);
+                    })
+                }
                 FromConstellation(ConstellationControlMsg::TickAllAnimations(
                         pipeline_id)) => {
                     if !animation_ticks.contains(&pipeline_id) {
@@ -850,6 +855,9 @@ impl ScriptThread {
                 ScriptThreadEventCategory::NetworkEvent => ProfilerCategory::ScriptNetworkEvent,
                 ScriptThreadEventCategory::Resize => ProfilerCategory::ScriptResize,
                 ScriptThreadEventCategory::ScriptEvent => ProfilerCategory::ScriptEvent,
+                ScriptThreadEventCategory::SetScrollState => {
+                    ProfilerCategory::ScriptSetScrollState
+                }
                 ScriptThreadEventCategory::UpdateReplacedElement => {
                     ProfilerCategory::ScriptUpdateReplacedElement
                 }
@@ -877,6 +885,8 @@ impl ScriptThread {
                 self.handle_resize_inactive_msg(id, new_size),
             ConstellationControlMsg::Viewport(..) =>
                 panic!("should have handled Viewport already"),
+            ConstellationControlMsg::SetScrollState(..) =>
+                panic!("should have handled SetScrollState already"),
             ConstellationControlMsg::Resize(..) =>
                 panic!("should have handled Resize already"),
             ConstellationControlMsg::ExitPipeline(..) =>
@@ -1077,6 +1087,19 @@ impl ScriptThread {
         panic!("Page rect message sent to nonexistent pipeline");
     }
 
+    fn handle_set_scroll_state(&self, id: PipelineId, scroll_state: &Point2D<f32>) {
+        let context = self.browsing_context.get();
+        if let Some(context) = context {
+            if let Some(inner_context) = context.find(id) {
+                let window = inner_context.active_window();
+                window.update_viewport_for_scroll(-scroll_state.x, -scroll_state.y);
+                return
+            }
+        }
+
+        panic!("Set scroll state message message sent to nonexistent pipeline: {:?}", id);
+    }
+
     fn handle_new_layout(&self, new_layout_info: NewLayoutInfo) {
         let NewLayoutInfo {
             containing_pipeline_id,