Bug 1601865 - Fix scrollbar cache slices having extra primitives. r=n…

…ical

There's two potential cases handled by this patch:

(1) A scrollbar container followed by another scrollbar container.
    In this case, we need to ensure these are placed into separate
    clusters, even though the cluster flags otherwise match, to
    ensure that slice creation will see the two clusters.

(2) If a fixed position scroll root trails a scrollbar container.
    In this case, ensure that a scrollbar contains marks the
    cluster flags to create a slice straight after the scrollbar,
    to avoid other primitives with the same scroll root sneaking
    into the scrollbar container.

Differential Revision: https://phabricator.services.mozilla.com/D61519

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/e1a4153cfb39a6e1706e88f036d6874bd0ddc39c

webrender/src/picture.rs

@@ -3266,6 +3266,13 @@ impl PrimitiveCluster {
         spatial_node_index: SpatialNodeIndex,
         flags: ClusterFlags,
     ) -> bool {
+        // If this cluster is a scrollbar, ensure that a matching scrollbar
+        // container that follows is split up, so we don't combine the
+        // scrollbars into a single slice.
+        if self.flags.contains(ClusterFlags::SCROLLBAR_CONTAINER) {
+            return false;
+        }
+
         self.flags == flags && self.spatial_node_index == spatial_node_index
     }
 

webrender/src/scene_building.rs

@@ -3606,6 +3606,7 @@ impl FlattenedStackingContext {
         struct SliceInfo {
             cluster_index: usize,
             scroll_root: SpatialNodeIndex,
+            cluster_flags: ClusterFlags,
         }
 
         let mut content_slice_count = 0;
@@ -3621,8 +3622,19 @@ impl FlattenedStackingContext {
             // (1) This cluster is a scrollbar
             // (2) Certain conditions when the scroll root changes (see below)
             // (3) No slice exists yet
-            let create_new_slice =
-                cluster.flags.contains(ClusterFlags::SCROLLBAR_CONTAINER) ||
+            let mut cluster_flags = ClusterFlags::empty();
+
+            if cluster.flags.contains(ClusterFlags::SCROLLBAR_CONTAINER) {
+                // Scrollbar containers need to ensure that a new slice is
+                // created both before and after the scrollbar, so that no
+                // other prims with the same scroll root sneak into this slice.
+                cluster_flags.insert(
+                    ClusterFlags::CREATE_PICTURE_CACHE_PRE |
+                    ClusterFlags::CREATE_PICTURE_CACHE_POST
+                );
+            }
+
+            let create_new_slice_for_scroll_root =
                 slices.last().map(|slice| {
                     match (slice.scroll_root, scroll_root) {
                         (ROOT_SPATIAL_NODE_INDEX, ROOT_SPATIAL_NODE_INDEX) => {
@@ -3671,11 +3683,16 @@ impl FlattenedStackingContext {
                     }
                 }).unwrap_or(true);
 
+            if create_new_slice_for_scroll_root {
+                cluster_flags.insert(ClusterFlags::CREATE_PICTURE_CACHE_PRE);
+            }
+
             // Create a new slice if required
-            if create_new_slice {
+            if !cluster_flags.is_empty() {
                 slices.push(SliceInfo {
                     cluster_index,
-                    scroll_root
+                    scroll_root,
+                    cluster_flags,
                 });
             }
         }
@@ -3702,7 +3719,7 @@ impl FlattenedStackingContext {
                 content_slice_count += 1;
                 let cluster = &mut self.prim_list.clusters[slice.cluster_index];
                 // Mark that this cluster creates a picture cache slice
-                cluster.flags.insert(ClusterFlags::CREATE_PICTURE_CACHE_PRE);
+                cluster.flags.insert(slice.cluster_flags);
                 cluster.cache_scroll_root = Some(slice.scroll_root);
             }
         }