Bug 1342833 - WebRenderBridgeParent::ClearResources() does not clear DisplayList r=jrmuizel

gfx/layers/wr/WebRenderBridgeParent.cpp

@@ -570,6 +570,10 @@ WebRenderBridgeParent::ScheduleComposition()
 void
 WebRenderBridgeParent::ClearResources()
 {
+  if (mApi) {
+    ++mWrEpoch; // Update webrender epoch
+    mApi->ClearRootDisplayList(wr::NewEpoch(mWrEpoch), mPipelineId);
+  }
   DeleteOldImages();
   if (mCompositableHolder) {
     for (auto iter = mExternalImageIds.Iter(); !iter.Done(); iter.Next()) {

gfx/webrender_bindings/WebRenderAPI.cpp

@@ -184,6 +184,13 @@ WebRenderAPI::SetRootDisplayList(gfx::Color aBgColor,
                                  aux_size);
 }
 
+void
+WebRenderAPI::ClearRootDisplayList(Epoch aEpoch,
+                                   WrPipelineId pipeline_id)
+{
+  wr_api_clear_root_display_list(mWrApi, aEpoch, pipeline_id);
+}
+
 void
 WebRenderAPI::Readback(gfx::IntSize size,
                        uint8_t *buffer,

gfx/webrender_bindings/WebRenderAPI.h

@@ -56,6 +56,9 @@ public:
                           uint8_t *aux_data,
                           size_t aux_size);
 
+  void ClearRootDisplayList(Epoch aEpoch,
+                            WrPipelineId pipeline_id);
+
   void SetRootPipeline(wr::PipelineId aPipeline);
 
   void AddImage(wr::ImageKey aKey,

gfx/webrender_bindings/src/bindings.rs

@@ -258,6 +258,22 @@ pub unsafe extern fn wr_api_set_root_display_list(api: &mut RenderApi,
                               preserve_frame_state);
 }
 
+#[no_mangle]
+pub unsafe extern fn wr_api_clear_root_display_list(api: &mut RenderApi,
+                                                    epoch: Epoch,
+                                                    pipeline_id: PipelineId) {
+    let root_background_color = ColorF::new(0.3, 0.0, 0.0, 1.0);
+    let preserve_frame_state = true;
+    let frame_builder = WebRenderFrameBuilder::new(pipeline_id);
+
+    api.set_root_display_list(Some(root_background_color),
+                              epoch,
+                              LayoutSize::new(0.0, 0.0),
+                              frame_builder.dl_builder.finalize(),
+                              preserve_frame_state);
+}
+
+
 #[no_mangle]
 pub extern fn wr_api_generate_frame(api: &mut RenderApi) {
   api.generate_frame(None);

gfx/webrender_bindings/webrender_ffi.h

@@ -476,6 +476,10 @@ wr_api_set_root_display_list(WrAPI* api, WrEpoch epoch, float w, float h,
                              size_t aux_size)
 WR_FUNC;
 
+WR_INLINE void
+wr_api_clear_root_display_list(WrAPI* api, WrEpoch epoch, WrPipelineId pipeline_id)
+WR_FUNC;
+
 WR_INLINE void
 wr_api_generate_frame(WrAPI* api)
 WR_FUNC;