Bug 1591580 - Support picture caching for parent process content. r=kvark

For content that comes from the parent process (e.g. about:support)
the shape of the display list is slightly different. In these cases
ensure that we setup picture caching slice markers on the root
stacking context.

Differential Revision: https://phabricator.services.mozilla.com/D51540

--HG--
extra : moz-landing-system : lando

gfx/wr/webrender/src/picture.rs

@@ -58,6 +58,16 @@
 //! which defines the scissor rect used when replaying the tile's drawing commands and
 //! can be used for partial present.
 //!
+//! ## Display List shape
+//!
+//! WR will first look for an iframe item in the root stacking context to apply
+//! picture caching to. If that's not found, it will apply to the entire root
+//! stacking context of the display list. Apart from that, the format of the
+//! display list is not important to picture caching. Each time a new scroll root
+//! is encountered, a new picture cache slice will be created. If the display
+//! list contains more than some arbitrary number of slices (currently 8), the
+//! content will all be squashed into a single slice, in order to save GPU memory
+//! and compositing performance.
 
 use api::{MixBlendMode, PipelineId, PremultipliedColorF, FilterPrimitiveKind};
 use api::{PropertyBinding, PropertyBindingId, FilterPrimitive, FontRenderMode};

gfx/wr/webrender/src/scene_building.rs

@@ -352,9 +352,8 @@ pub struct SceneBuilder<'a> {
     /// Helper struct to map spatial nodes to external scroll offsets.
     external_scroll_mapper: ScrollOffsetMapper,
 
-    /// If true, a stacking context with create_tile_cache set to true was found
-    /// during building.
-    found_explicit_tile_cache: bool,
+    /// If true, picture caching setup has already been completed.
+    picture_caching_initialized: bool,
 
     /// The current recursion depth of iframes encountered. Used to restrict picture
     /// caching slices to only the top-level content frame.
@@ -407,7 +406,7 @@ impl<'a> SceneBuilder<'a> {
             root_pic_index: PictureIndex(0),
             rf_mapper: ReferenceFrameMapper::new(),
             external_scroll_mapper: ScrollOffsetMapper::new(),
-            found_explicit_tile_cache: false,
+            picture_caching_initialized: false,
             iframe_depth: 0,
             content_slice_count: 0,
             picture_cache_spatial_nodes: FastHashSet::default(),
@@ -503,6 +502,9 @@ impl<'a> SceneBuilder<'a> {
             return;
         }
 
+        // Ensure that setup_picture_caching has executed
+        debug_assert!(self.picture_caching_initialized);
+
         // Unconditionally insert a marker to create a picture cache slice on the
         // first cluster. This handles implicit picture caches, and also the common
         // case, by allowing the root / background primitives to be cached in a slice.
@@ -1883,7 +1885,7 @@ impl<'a> SceneBuilder<'a> {
                             self.content_slice_count = stacking_context.init_picture_caching(&self.clip_scroll_tree);
 
                             // Mark that a user supplied tile cache was specified.
-                            self.found_explicit_tile_cache = true;
+                            self.picture_caching_initialized = true;
                         }
 
                         // If the parent context primitives list is empty, it's faster
@@ -1904,6 +1906,14 @@ impl<'a> SceneBuilder<'a> {
         };
 
         if self.sc_stack.is_empty() {
+            // If we didn't encounter a content iframe, then set up picture caching slice markers
+            // on the root stacking context. This can happen in Gecko when the parent process
+            // provides the content display list (e.g. about:support, about:config etc).
+            if !self.picture_caching_initialized {
+                self.content_slice_count = stacking_context.init_picture_caching(&self.clip_scroll_tree);
+                self.picture_caching_initialized = true;
+            }
+
             self.setup_picture_caching(
                 &mut stacking_context.prim_list,
             );