Bug 1612440 - Discard the SpatialTree's frame state after scene swap. r=gw

The information comes with SetDisplayList messages but was applied before scene building instead of during scene swap, which breaks the transaction model and looks like a bug.

Differential Revision: https://phabricator.services.mozilla.com/D71934

gfx/wr/webrender/src/render_backend.rs

@@ -10,7 +10,7 @@
 
 use api::{ApiMsg, ClearCache, DebugCommand, DebugFlags};
 use api::{DocumentId, DocumentLayer, ExternalScrollId, FrameMsg, HitTestFlags, HitTestResult};
-use api::{IdNamespace, MemoryReport, PipelineId, RenderNotifier, SceneMsg, ScrollClamping};
+use api::{IdNamespace, MemoryReport, PipelineId, RenderNotifier, ScrollClamping};
 use api::{ScrollLocation, TransactionMsg, ResourceUpdate, BlobImageKey};
 use api::{NotificationRequest, Checkpoint, QualitySettings};
 use api::{ClipIntern, FilterDataIntern, PrimitiveKeyKind};
@@ -1015,6 +1015,12 @@ impl RenderBackend {
                     // in the update_document call below.
                     resume_rx.recv().ok();
                 }
+
+                for pipeline_id in &txn.discard_frame_state_for_pipelines {
+                    doc.scene
+                        .spatial_tree
+                        .discard_frame_state_for_pipeline(*pipeline_id);
+                }
             } else {
                 // The document was removed while we were building it, skip it.
                 // TODO: we might want to just ensure that removed documents are
@@ -1346,19 +1352,6 @@ impl RenderBackend {
                     &mut profile_counters.resources,
                 );
 
-                // TODO(nical) I believe this is wrong. We should discard this state when swapping the
-                // scene after it is built.
-                for msg in &transaction_msg.scene_ops {
-                    if let SceneMsg::SetDisplayList { preserve_frame_state: false, pipeline_id, .. } = *msg {
-                        self.documents
-                            .get_mut(&document_id)
-                            .unwrap()
-                            .scene
-                            .spatial_tree
-                            .discard_frame_state_for_pipeline(pipeline_id);
-                    }
-                }
-
                 let mut txn = Box::new(Transaction {
                     document_id,
                     blob_rasterizer: None,

gfx/wr/webrender/src/scene_builder_thread.rs

@@ -102,6 +102,7 @@ pub struct BuiltTransaction {
     pub scene_build_end_time: u64,
     pub render_frame: bool,
     pub invalidate_rendered_frame: bool,
+    pub discard_frame_state_for_pipelines: Vec<PipelineId>,
     pub timings: Option<TransactionTimings>,
 }
 
@@ -486,6 +487,7 @@ impl SceneBuilderThread {
                 blob_rasterizer: None,
                 frame_ops: Vec::new(),
                 removed_pipelines: Vec::new(),
+                discard_frame_state_for_pipelines: Vec::new(),
                 notifications: Vec::new(),
                 scene_build_start_time,
                 scene_build_end_time: precise_time_ns(),
@@ -603,6 +605,7 @@ impl SceneBuilderThread {
 
         let mut timings = None;
 
+        let mut discard_frame_state_for_pipelines = Vec::new();
         let mut removed_pipelines = Vec::new();
         let rebuild_scene = !txn.scene_ops.is_empty();
         for message in txn.scene_ops.drain(..) {
@@ -628,7 +631,7 @@ impl SceneBuilderThread {
                     content_size,
                     list_descriptor,
                     list_data,
-                    ..
+                    preserve_frame_state,
                 } => {
                     let built_display_list =
                         BuiltDisplayList::from_data(list_data, list_descriptor);
@@ -659,6 +662,10 @@ impl SceneBuilderThread {
                         blob_rasterization_end_time_ns: 0,
                         display_list_len,
                     });
+
+                    if !preserve_frame_state {
+                        discard_frame_state_for_pipelines.push(pipeline_id);
+                    }
                 }
                 SceneMsg::SetRootPipeline(pipeline_id) => {
                     scene.set_root_pipeline_id(pipeline_id);
@@ -737,6 +744,7 @@ impl SceneBuilderThread {
             blob_rasterizer: replace(&mut txn.blob_rasterizer, None),
             frame_ops: replace(&mut txn.frame_ops, Vec::new()),
             removed_pipelines,
+            discard_frame_state_for_pipelines,
             notifications: replace(&mut txn.notifications, Vec::new()),
             interner_updates,
             scene_build_start_time,