Auto merge of #14367 - mrobinson:scroll-fragment-point, r=pcwalton

Reimplement scrolling to fragments

<!-- Please describe your changes on the following line: -->

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #13736, #10753 (github issue number if applicable).

<!-- Either: -->
- [x] There are tests for these changes OR
- [ ] These changes do not require tests because _____

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

This reimplemntation of the feature uses ScrollRootIds to scroll
particular scrollable areas of the page.

Fixes #13736.
Fixes #10753.

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/14367)
<!-- Reviewable:end -->

components/compositing/compositor.rs

@@ -39,7 +39,7 @@ use util::geometry::ScreenPx;
 use util::opts;
 use util::prefs::PREFS;
 use webrender;
-use webrender_traits::{self, ScrollEventPhase};
+use webrender_traits::{self, ScrollEventPhase, ServoScrollRootId};
 use windowing::{self, MouseWindowEvent, WindowEvent, WindowMethods, WindowNavigateMsg};
 
 #[derive(Debug, PartialEq)]
@@ -493,9 +493,9 @@ impl<Window: WindowMethods> IOCompositor<Window> {
                 self.title_for_main_frame();
             }
 
-            (Msg::ScrollFragmentPoint(pipeline_id, point, _),
+            (Msg::ScrollFragmentPoint(pipeline_id, scroll_root_id, point, _),
              ShutdownState::NotShuttingDown) => {
-                self.scroll_fragment_to_point(pipeline_id, point);
+                self.scroll_fragment_to_point(pipeline_id, scroll_root_id, point);
             }
 
             (Msg::MoveTo(point),
@@ -761,9 +761,13 @@ impl<Window: WindowMethods> IOCompositor<Window> {
     }
 
     fn scroll_fragment_to_point(&mut self,
-                                _pipeline_id: PipelineId,
-                                _point: Point2D<f32>) {
-        println!("TODO: Support scroll_fragment_to_point again");
+                                pipeline_id: PipelineId,
+                                scroll_root_id: ScrollRootId,
+                                point: Point2D<f32>) {
+        self.webrender_api.scroll_layers_with_scroll_root_id(
+            point,
+            pipeline_id.to_webrender(),
+            ServoScrollRootId(scroll_root_id.0));
     }
 
     fn handle_window_message(&mut self, event: WindowEvent) {

components/compositing/compositor_thread.rs

@@ -8,6 +8,7 @@ use SendableFrameTree;
 use compositor::CompositingReason;
 use euclid::point::Point2D;
 use euclid::size::Size2D;
+use gfx_traits::ScrollRootId;
 use ipc_channel::ipc::IpcSender;
 use msg::constellation_msg::{Key, KeyModifiers, KeyState, PipelineId};
 use net_traits::image::base::Image;
@@ -72,7 +73,7 @@ pub enum Msg {
     ShutdownComplete,
 
     /// Scroll a page in a window
-    ScrollFragmentPoint(PipelineId, Point2D<f32>, bool),
+    ScrollFragmentPoint(PipelineId, ScrollRootId, Point2D<f32>, bool),
     /// Alerts the compositor that the current page has changed its title.
     ChangePageTitle(PipelineId, Option<String>),
     /// Alerts the compositor that the current page has changed its URL.