From 723ecc70b617a1365b8ad107857b85e1aac29149 Mon Sep 17 00:00:00 2001
From: Nicolas Silva <nsilva@mozilla.com>
Date: Thu, 14 May 2020 12:40:49 +0000
Subject: [PATCH] Bug 1637300 - Further reduce unnecessary scene building. r=gw

Differential Revision: https://phabricator.services.mozilla.com/D74881
---
 gfx/wr/webrender/src/scene_builder_thread.rs | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/gfx/wr/webrender/src/scene_builder_thread.rs b/gfx/wr/webrender/src/scene_builder_thread.rs
index 3adf4a27d34a..b24dca653fb5 100644
--- a/gfx/wr/webrender/src/scene_builder_thread.rs
+++ b/gfx/wr/webrender/src/scene_builder_thread.rs
@@ -633,7 +633,6 @@ impl SceneBuilderThread {
                     list_data,
                     preserve_frame_state,
                 } => {
-                    rebuild_scene = true;
                     let built_display_list =
                         BuiltDisplayList::from_data(list_data, list_descriptor);
                     let display_list_len = built_display_list.data().len();
@@ -645,6 +644,11 @@ impl SceneBuilderThread {
                         continue;
                     }
 
+                    // Note: We could further reduce the amount of unnecessary scene
+                    // building by keeping track of which pipelines are used by the
+                    // scene (bug 1490751).
+                    rebuild_scene = true;
+
                     scene.set_display_list(
                         pipeline_id,
                         epoch,
@@ -669,8 +673,10 @@ impl SceneBuilderThread {
                     }
                 }
                 SceneMsg::SetRootPipeline(pipeline_id) => {
-                    rebuild_scene = true;
-                    scene.set_root_pipeline_id(pipeline_id);
+                    if scene.root_pipeline_id != Some(pipeline_id) {
+                        rebuild_scene = true;
+                        scene.set_root_pipeline_id(pipeline_id);
+                    }
                 }
                 SceneMsg::RemovePipeline(pipeline_id) => {
                     scene.remove_pipeline(pipeline_id);