servo: Merge #13957 - Track overflow:scroll stacking contexts with ScrollRootId instead of StackingContextId (from mrobinson:scroll_root); r=glennw

<!-- Please describe your changes on the following line: -->

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

<!-- Either: -->
- [ ] There are tests for these changes OR
- [x] These changes do not require tests because this PR should not change behavior.

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

Source-Repo: https://github.com/servo/servo
Source-Revision: 3a3f3192a87dbcd5d05efb82a1f4dde23f9c4cf7

--HG--
rename : servo/resources/shaders/box_shadow.fs.glsl => servo/resources/shaders/cs_box_shadow.fs.glsl
rename : servo/resources/shaders/ps_box_shadow.glsl => servo/resources/shaders/cs_box_shadow.glsl

servo/components/compositing/compositor.rs

@@ -11,7 +11,7 @@ use euclid::{Point2D, Size2D};
 use euclid::point::TypedPoint2D;
 use euclid::scale_factor::ScaleFactor;
 use euclid::size::TypedSize2D;
-use gfx_traits::{DevicePixel, LayerPixel, StackingContextId};
+use gfx_traits::{DevicePixel, LayerPixel, ScrollRootId};
 use gfx_traits::{Epoch, FrameTreeId, FragmentType};
 use gleam::gl;
 use gleam::gl::types::{GLint, GLsizei};
@@ -74,13 +74,13 @@ impl ConvertPipelineIdFromWebRender for webrender_traits::PipelineId {
     }
 }
 
-trait ConvertStackingContextFromWebRender {
+trait ConvertScrollRootIdFromWebRender {
-    fn from_webrender(&self) -> StackingContextId;
+    fn from_webrender(&self) -> ScrollRootId;
 }
 
-impl ConvertStackingContextFromWebRender for webrender_traits::ServoStackingContextId {
+impl ConvertScrollRootIdFromWebRender for webrender_traits::ServoScrollRootId {
-    fn from_webrender(&self) -> StackingContextId {
+    fn from_webrender(&self) -> ScrollRootId {
-        StackingContextId::new_of_type(self.1, self.0.from_webrender())
+        ScrollRootId(self.0)
     }
 }
 
@@ -1312,7 +1312,7 @@ impl<Window: WindowMethods> IOCompositor<Window> {
         let mut stacking_context_scroll_states_per_pipeline = HashMap::new();
         for scroll_layer_state in self.webrender_api.get_scroll_layer_state() {
             let stacking_context_scroll_state = StackingContextScrollState {
-                stacking_context_id: scroll_layer_state.stacking_context_id.from_webrender(),
+                scroll_root_id: scroll_layer_state.scroll_root_id.from_webrender(),
                 scroll_offset: scroll_layer_state.scroll_offset,
             };
             let pipeline_id = scroll_layer_state.pipeline_id;

servo/components/gfx/display_list/mod.rs

@@ -20,7 +20,7 @@ use euclid::{Matrix4D, Point2D, Rect, Size2D};
 use euclid::num::{One, Zero};
 use euclid::rect::TypedRect;
 use euclid::side_offsets::SideOffsets2D;
-use gfx_traits::{ScrollPolicy, StackingContextId};
+use gfx_traits::{ScrollPolicy, ScrollRootId, StackingContextId};
 use gfx_traits::print_tree::PrintTree;
 use ipc_channel::ipc::IpcSharedMemory;
 use msg::constellation_msg::PipelineId;
@@ -215,9 +215,11 @@ impl DisplayList {
         // the DOM-side code has already translated the point for us (e.g. in
         // `Window::hit_test_query()`) by now.
         if !is_fixed && stacking_context.id != StackingContextId::root() {
-            if let Some(scroll_offset) = scroll_offsets.get(&stacking_context.id) {
+            if let Some(scroll_root_id) = stacking_context.overflow_scroll_id {
-                translated_point.x -= Au::from_f32_px(scroll_offset.x);
+                if let Some(scroll_offset) = scroll_offsets.get(&scroll_root_id) {
-                translated_point.y -= Au::from_f32_px(scroll_offset.y);
+                    translated_point.x -= Au::from_f32_px(scroll_offset.x);
+                    translated_point.y -= Au::from_f32_px(scroll_offset.y);
+                }
             }
         }
 
@@ -386,7 +388,7 @@ pub struct StackingContext {
     pub children: Vec<StackingContext>,
 
     /// If this StackingContext scrolls its overflow area, this will contain the id.
-    pub overflow_scroll_id: Option<StackingContextId>,
+    pub overflow_scroll_id: Option<ScrollRootId>,
 }
 
 impl StackingContext {
@@ -403,7 +405,7 @@ impl StackingContext {
                perspective: Matrix4D<f32>,
                establishes_3d_context: bool,
                scroll_policy: ScrollPolicy,
-               scroll_id: Option<StackingContextId>)
+               scroll_root_id: Option<ScrollRootId>)
                -> StackingContext {
         StackingContext {
             id: id,
@@ -418,7 +420,7 @@ impl StackingContext {
             establishes_3d_context: establishes_3d_context,
             scroll_policy: scroll_policy,
             children: Vec::new(),
-            overflow_scroll_id: scroll_id,
+            overflow_scroll_id: scroll_root_id,
         }
     }
 
@@ -1194,7 +1196,7 @@ impl WebRenderImageInfo {
 }
 
 /// The type of the scroll offset list. This is only populated if WebRender is in use.
-pub type ScrollOffsetMap = HashMap<StackingContextId, Point2D<f32>>;
+pub type ScrollOffsetMap = HashMap<ScrollRootId, Point2D<f32>>;
 
 
 pub trait SimpleMatrixDetection {

servo/components/gfx_traits/lib.rs

@@ -162,6 +162,58 @@ impl StackingContextId {
     }
 }
 
+/// A unique ID for every scrolling root.
+#[derive(Clone, Copy, Debug, Deserialize, Eq, Hash, HeapSizeOf, PartialEq, Serialize)]
+pub struct ScrollRootId(
+    /// The identifier for this StackingContext, derived from the Flow's memory address
+    /// and fragment type.  As a space optimization, these are combined into a single word.
+    pub usize
+);
+
+impl ScrollRootId {
+    /// Returns a new stacking context ID for a special stacking context.
+    fn next_special_id() -> usize {
+        // We shift this left by 2 to make room for the fragment type ID.
+        ((NEXT_SPECIAL_STACKING_CONTEXT_ID.fetch_add(1, Ordering::SeqCst) + 1) << 2) &
+            SPECIAL_STACKING_CONTEXT_ID_MASK
+    }
+
+    #[inline]
+    pub fn new_of_type(id: usize, fragment_type: FragmentType) -> ScrollRootId {
+        debug_assert_eq!(id & (fragment_type as usize), 0);
+        if fragment_type == FragmentType::FragmentBody {
+            ScrollRootId(id)
+        } else {
+            ScrollRootId(ScrollRootId::next_special_id() | (fragment_type as usize))
+        }
+    }
+
+    /// Returns the stacking context ID for the outer document/layout root.
+    #[inline]
+    pub fn root() -> ScrollRootId {
+        ScrollRootId(0)
+    }
+
+    /// Returns true if this is a special stacking context.
+    ///
+    /// A special stacking context is a stacking context that is one of (a) the outer stacking
+    /// context of an element with `overflow: scroll`; (b) generated content; (c) both (a) and (b).
+    #[inline]
+    pub fn is_special(&self) -> bool {
+        (self.0 & !SPECIAL_STACKING_CONTEXT_ID_MASK) == 0
+    }
+
+    #[inline]
+    pub fn id(&self) -> usize {
+        self.0 & !3
+    }
+
+    #[inline]
+    pub fn fragment_type(&self) -> FragmentType {
+        FragmentType::from_usize(self.0 & 3)
+    }
+}
+
 /// The type of fragment that a stacking context represents.
 ///
 /// This can only ever grow to maximum 4 entries. That's because we cram the value of this enum

servo/components/layout/block.rs

@@ -44,6 +44,7 @@ use flow_ref::FlowRef;
 use fragment::{CoordinateSystem, Fragment, FragmentBorderBoxIterator, Overflow};
 use fragment::SpecificFragmentInfo;
 use gfx::display_list::{ClippingRegion, StackingContext};
+use gfx_traits::ScrollRootId;
 use gfx_traits::print_tree::PrintTree;
 use layout_debug;
 use model::{CollapsibleMargins, IntrinsicISizes, MarginCollapseInfo, MaybeAuto};
@@ -2162,8 +2163,10 @@ impl Flow for BlockFlow {
         }
     }
 
-    fn collect_stacking_contexts(&mut self, parent: &mut StackingContext) {
+    fn collect_stacking_contexts(&mut self,
-        self.collect_stacking_contexts_for_block(parent);
+                                 parent: &mut StackingContext,
+                                 parent_scroll_root_id: ScrollRootId) {
+        self.collect_stacking_contexts_for_block(parent, parent_scroll_root_id);
     }
 
     fn build_display_list(&mut self, state: &mut DisplayListBuildState) {

servo/components/layout/display_list_builder.rs

@@ -28,7 +28,7 @@ use gfx::display_list::{GradientStop, IframeDisplayItem, ImageDisplayItem, WebGL
 use gfx::display_list::{LineDisplayItem, OpaqueNode};
 use gfx::display_list::{SolidColorDisplayItem, StackingContext, StackingContextType};
 use gfx::display_list::{TextDisplayItem, TextOrientation, WebRenderImageInfo};
-use gfx_traits::{ScrollPolicy, StackingContextId, color};
+use gfx_traits::{ScrollPolicy, ScrollRootId, StackingContextId, color};
 use inline::{FIRST_FRAGMENT_OF_ELEMENT, InlineFlow, LAST_FRAGMENT_OF_ELEMENT};
 use ipc_channel::ipc;
 use list_item::ListItemFlow;
@@ -78,6 +78,7 @@ pub struct DisplayListBuildState<'a> {
     pub shared_layout_context: &'a SharedLayoutContext,
     pub items: Vec<DisplayItem>,
     pub stacking_context_id_stack: Vec<StackingContextId>,
+    pub scroll_root_id_stack: Vec<ScrollRootId>,
 }
 
 impl<'a> DisplayListBuildState<'a> {
@@ -88,6 +89,7 @@ impl<'a> DisplayListBuildState<'a> {
             shared_layout_context: shared_layout_context,
             items: Vec::new(),
             stacking_context_id_stack: vec!(stacking_context_id),
+            scroll_root_id_stack: vec!(ScrollRootId::root()),
         }
     }
 
@@ -95,7 +97,7 @@ impl<'a> DisplayListBuildState<'a> {
         self.items.push(display_item);
     }
 
-    fn stacking_context_id(&self) -> StackingContextId {
+    pub fn stacking_context_id(&self) -> StackingContextId {
         self.stacking_context_id_stack.last().unwrap().clone()
     }
 
@@ -108,6 +110,19 @@ impl<'a> DisplayListBuildState<'a> {
         assert!(!self.stacking_context_id_stack.is_empty());
     }
 
+    pub fn scroll_root_id(&mut self) -> ScrollRootId {
+        self.scroll_root_id_stack.last().unwrap().clone()
+    }
+
+    pub fn push_scroll_root_id(&mut self, id: ScrollRootId) {
+        self.scroll_root_id_stack.push(id);
+    }
+
+    pub fn pop_scroll_root_id(&mut self) {
+        self.scroll_root_id_stack.pop();
+        assert!(!self.scroll_root_id_stack.is_empty());
+    }
+
     fn create_base_display_item(&self,
                                 bounds: &Rect<Au>,
                                 clip: &ClippingRegion,
@@ -299,7 +314,7 @@ pub trait FragmentDisplayListBuilding {
                                base_flow: &BaseFlow,
                                scroll_policy: ScrollPolicy,
                                mode: StackingContextCreationMode,
-                               scroll_id: Option<StackingContextId>)
+                               scroll_root_id: Option<ScrollRootId>)
                                -> StackingContext;
 
     /// Returns the 4D matrix representing this fragment's transform.
@@ -1356,7 +1371,7 @@ impl FragmentDisplayListBuilding for Fragment {
                                base_flow: &BaseFlow,
                                scroll_policy: ScrollPolicy,
                                mode: StackingContextCreationMode,
-                               scroll_id: Option<StackingContextId>)
+                               scroll_root_id: Option<ScrollRootId>)
                                -> StackingContext {
         let scrolls_overflow_area = mode == StackingContextCreationMode::ScrollWrapper;
         let border_box =
@@ -1431,7 +1446,7 @@ impl FragmentDisplayListBuilding for Fragment {
                              perspective,
                              establishes_3d_context,
                              scroll_policy,
-                             scroll_id)
+                             scroll_root_id)
     }
 
     fn adjust_clipping_region_for_children(&self,
@@ -1687,7 +1702,9 @@ impl FragmentDisplayListBuilding for Fragment {
 }
 
 pub trait BlockFlowDisplayListBuilding {
-    fn collect_stacking_contexts_for_block(&mut self, parent: &mut StackingContext);
+    fn collect_stacking_contexts_for_block(&mut self,
+                                           parent: &mut StackingContext,
+                                           parent_scroll_root_id: ScrollRootId);
     fn build_display_list_for_block(&mut self,
                                     state: &mut DisplayListBuildState,
                                     border_painting_mode: BorderPaintingMode);
@@ -1704,17 +1721,29 @@ pub trait BlockFlowDisplayListBuilding {
 }
 
 impl BlockFlowDisplayListBuilding for BlockFlow {
-    fn collect_stacking_contexts_for_block(&mut self, parent: &mut StackingContext) {
+    fn collect_stacking_contexts_for_block(&mut self,
+                                           parent: &mut StackingContext,
+                                           parent_scroll_root_id: ScrollRootId) {
         let block_stacking_context_type = self.block_stacking_context_type();
         if block_stacking_context_type == BlockStackingContextType::NonstackingContext {
             self.base.stacking_context_id = parent.id;
-            self.base.collect_stacking_contexts_for_children(parent);
+            self.base.collect_stacking_contexts_for_children(parent, parent_scroll_root_id);
             return;
         }
 
-        let has_scrolling_overflow = self.has_scrolling_overflow();
         let stacking_context_id = StackingContextId::new_of_type(self.fragment.node.id() as usize,
                                                                  self.fragment.fragment_type());
+
+        let has_scrolling_overflow = self.has_scrolling_overflow();
+        let scroll_root_id = if has_scrolling_overflow {
+            ScrollRootId::new_of_type(self.fragment.node.id() as usize,
+                                      self.fragment.fragment_type())
+        } else {
+            parent_scroll_root_id
+        };
+        self.base.scroll_root_id = scroll_root_id;
+
+
         self.base.stacking_context_id = stacking_context_id;
 
         if block_stacking_context_type == BlockStackingContextType::PseudoStackingContext {
@@ -1731,7 +1760,7 @@ impl BlockFlowDisplayListBuilding for BlockFlow {
                                                                         ScrollPolicy::Scrollable,
                                                                         creation_mode,
                                                                         None);
-            self.base.collect_stacking_contexts_for_children(&mut new_context);
+            self.base.collect_stacking_contexts_for_children(&mut new_context, scroll_root_id);
             let new_children: Vec<StackingContext> = new_context.children.drain(..).collect();
 
             let mut non_floating_children = Vec::new();
@@ -1755,10 +1784,7 @@ impl BlockFlowDisplayListBuilding for BlockFlow {
         };
 
         let (creation_mode, internal_id) = if has_scrolling_overflow {
-            (StackingContextCreationMode::ScrollWrapper,
+            (StackingContextCreationMode::ScrollWrapper, Some(self.base.scroll_root_id))
-             Some(StackingContextId::new_of_type(self.fragment.node.id() as usize,
-                                                 self.fragment.fragment_type())))
-
         } else {
             (StackingContextCreationMode::Normal, None)
         };
@@ -1769,7 +1795,7 @@ impl BlockFlowDisplayListBuilding for BlockFlow {
             scroll_policy,
             creation_mode,
             internal_id);
-        self.base.collect_stacking_contexts_for_children(&mut stacking_context);
+        self.base.collect_stacking_contexts_for_children(&mut stacking_context, scroll_root_id);
 
         parent.add_child(stacking_context);
     }
@@ -1859,7 +1885,9 @@ impl BlockFlowDisplayListBuilding for BlockFlow {
 }
 
 pub trait InlineFlowDisplayListBuilding {
-    fn collect_stacking_contexts_for_inline(&mut self, parent: &mut StackingContext);
+    fn collect_stacking_contexts_for_inline(&mut self,
+                                            parent: &mut StackingContext,
+                                            parent_scroll_root_id: ScrollRootId);
     fn build_display_list_for_inline_fragment_at_index(&mut self,
                                                        state: &mut DisplayListBuildState,
                                                        index: usize);
@@ -1867,18 +1895,21 @@ pub trait InlineFlowDisplayListBuilding {
 }
 
 impl InlineFlowDisplayListBuilding for InlineFlow {
-    fn collect_stacking_contexts_for_inline(&mut self, parent: &mut StackingContext) {
+    fn collect_stacking_contexts_for_inline(&mut self,
+                                            parent: &mut StackingContext,
+                                            parent_scroll_root_id: ScrollRootId) {
         self.base.stacking_context_id = parent.id;
+        self.base.scroll_root_id = parent_scroll_root_id;
 
         for mut fragment in self.fragments.fragments.iter_mut() {
             match fragment.specific {
                 SpecificFragmentInfo::InlineBlock(ref mut block_flow) => {
                     let block_flow = flow_ref::deref_mut(&mut block_flow.flow_ref);
-                    block_flow.collect_stacking_contexts(parent);
+                    block_flow.collect_stacking_contexts(parent, parent_scroll_root_id);
                 }
                 SpecificFragmentInfo::InlineAbsoluteHypothetical(ref mut block_flow) => {
                     let block_flow = flow_ref::deref_mut(&mut block_flow.flow_ref);
-                    block_flow.collect_stacking_contexts(parent);
+                    block_flow.collect_stacking_contexts(parent, parent_scroll_root_id);
                 }
                 _ if fragment.establishes_stacking_context() => {
                     fragment.stacking_context_id =

servo/components/layout/flex.rs

@@ -17,6 +17,7 @@ use flow::{Flow, FlowClass, ImmutableFlowUtils, OpaqueFlow};
 use flow::{INLINE_POSITION_IS_STATIC, IS_ABSOLUTELY_POSITIONED};
 use fragment::{Fragment, FragmentBorderBoxIterator, Overflow};
 use gfx::display_list::StackingContext;
+use gfx_traits::ScrollRootId;
 use layout_debug;
 use model::{Direction, IntrinsicISizes, MaybeAuto, MinMaxConstraint};
 use model::{specified, specified_or_none};
@@ -956,8 +957,10 @@ impl Flow for FlexFlow {
         self.build_display_list_for_flex(state);
     }
 
-    fn collect_stacking_contexts(&mut self, parent: &mut StackingContext) {
+    fn collect_stacking_contexts(&mut self,
-        self.block_flow.collect_stacking_contexts(parent);
+                                 parent: &mut StackingContext,
+                                 parent_scroll_root_id: ScrollRootId) {
+        self.block_flow.collect_stacking_contexts(parent, parent_scroll_root_id);
     }
 
     fn repair_style(&mut self, new_style: &Arc<ServoComputedValues>) {

servo/components/layout/flow.rs

@@ -35,7 +35,7 @@ use flow_list::{FlowList, MutFlowListIterator};
 use flow_ref::{self, FlowRef, WeakFlowRef};
 use fragment::{Fragment, FragmentBorderBoxIterator, Overflow};
 use gfx::display_list::{ClippingRegion, StackingContext};
-use gfx_traits::StackingContextId;
+use gfx_traits::{ScrollRootId, StackingContextId};
 use gfx_traits::print_tree::PrintTree;
 use inline::InlineFlow;
 use model::{CollapsibleMargins, IntrinsicISizes, MarginCollapseInfo};
@@ -223,7 +223,9 @@ pub trait Flow: fmt::Debug + Sync + Send + 'static {
         None
     }
 
-    fn collect_stacking_contexts(&mut self, _parent: &mut StackingContext);
+    fn collect_stacking_contexts(&mut self,
+                                 _parent: &mut StackingContext,
+                                 parent_scroll_root_id: ScrollRootId);
 
     /// If this is a float, places it. The default implementation does nothing.
     fn place_float_if_applicable<'a>(&mut self) {}
@@ -935,6 +937,8 @@ pub struct BaseFlow {
     /// to 0, but it assigned during the collect_stacking_contexts phase of display
     /// list construction.
     pub stacking_context_id: StackingContextId,
+
+    pub scroll_root_id: ScrollRootId,
 }
 
 impl fmt::Debug for BaseFlow {
@@ -1105,6 +1109,7 @@ impl BaseFlow {
             writing_mode: writing_mode,
             thread_id: 0,
             stacking_context_id: StackingContextId::new(0),
+            scroll_root_id: ScrollRootId::root(),
         }
     }
 
@@ -1136,9 +1141,11 @@ impl BaseFlow {
         return self as *const BaseFlow as usize;
     }
 
-    pub fn collect_stacking_contexts_for_children(&mut self, parent: &mut StackingContext) {
+    pub fn collect_stacking_contexts_for_children(&mut self,
+                                                  parent: &mut StackingContext,
+                                                  parent_scroll_root_id: ScrollRootId) {
         for kid in self.children.iter_mut() {
-            kid.collect_stacking_contexts(parent);
+            kid.collect_stacking_contexts(parent, parent_scroll_root_id);
         }
     }
 

servo/components/layout/inline.rs

@@ -19,6 +19,7 @@ use fragment::SpecificFragmentInfo;
 use gfx::display_list::{OpaqueNode, StackingContext};
 use gfx::font::FontMetrics;
 use gfx::font_context::FontContext;
+use gfx_traits::ScrollRootId;
 use gfx_traits::print_tree::PrintTree;
 use layout_debug;
 use model::IntrinsicISizesContribution;
@@ -1613,8 +1614,10 @@ impl Flow for InlineFlow {
 
     fn update_late_computed_block_position_if_necessary(&mut self, _: Au) {}
 
-    fn collect_stacking_contexts(&mut self, parent: &mut StackingContext) {
+    fn collect_stacking_contexts(&mut self,
-        self.collect_stacking_contexts_for_inline(parent);
+                                 parent: &mut StackingContext,
+                                 parent_scroll_root_id: ScrollRootId) {
+        self.collect_stacking_contexts_for_inline(parent, parent_scroll_root_id);
     }
 
     fn build_display_list(&mut self, state: &mut DisplayListBuildState) {

servo/components/layout/list_item.rs

@@ -18,6 +18,7 @@ use fragment::{CoordinateSystem, Fragment, FragmentBorderBoxIterator, GeneratedC
 use fragment::Overflow;
 use generated_content;
 use gfx::display_list::StackingContext;
+use gfx_traits::ScrollRootId;
 use inline::InlineFlow;
 use script_layout_interface::restyle_damage::RESOLVE_GENERATED_CONTENT;
 use std::sync::Arc;
@@ -145,8 +146,10 @@ impl Flow for ListItemFlow {
         self.build_display_list_for_list_item(state);
     }
 
-    fn collect_stacking_contexts(&mut self, parent: &mut StackingContext) {
+    fn collect_stacking_contexts(&mut self,
-        self.block_flow.collect_stacking_contexts(parent);
+                                 parent: &mut StackingContext,
+                                 parent_scroll_root_id: ScrollRootId) {
+        self.block_flow.collect_stacking_contexts(parent, parent_scroll_root_id);
     }
 
     fn repair_style(&mut self, new_style: &Arc<ServoComputedValues>) {

servo/components/layout/multicol.rs

@@ -17,6 +17,7 @@ use flow::{Flow, FlowClass, OpaqueFlow, mut_base, FragmentationContext};
 use flow_ref::{self, FlowRef};
 use fragment::{Fragment, FragmentBorderBoxIterator, Overflow};
 use gfx::display_list::StackingContext;
+use gfx_traits::ScrollRootId;
 use gfx_traits::print_tree::PrintTree;
 use std::cmp::{min, max};
 use std::fmt;
@@ -185,8 +186,10 @@ impl Flow for MulticolFlow {
         self.block_flow.build_display_list(state);
     }
 
-    fn collect_stacking_contexts(&mut self, parent: &mut StackingContext) {
+    fn collect_stacking_contexts(&mut self,
-        self.block_flow.collect_stacking_contexts(parent);
+                                 parent: &mut StackingContext,
+                                 parent_scroll_root_id: ScrollRootId) {
+        self.block_flow.collect_stacking_contexts(parent, parent_scroll_root_id);
     }
 
     fn repair_style(&mut self, new_style: &Arc<ServoComputedValues>) {
@@ -267,8 +270,10 @@ impl Flow for MulticolColumnFlow {
         self.block_flow.build_display_list(state);
     }
 
-    fn collect_stacking_contexts(&mut self, parent: &mut StackingContext) {
+    fn collect_stacking_contexts(&mut self,
-        self.block_flow.collect_stacking_contexts(parent);
+                                 parent: &mut StackingContext,
+                                 parent_scroll_root_id: ScrollRootId) {
+        self.block_flow.collect_stacking_contexts(parent, parent_scroll_root_id);
     }
 
     fn repair_style(&mut self, new_style: &Arc<ServoComputedValues>) {

servo/components/layout/sequential.rs

@@ -15,6 +15,7 @@ use flow::IS_ABSOLUTELY_POSITIONED;
 use fragment::FragmentBorderBoxIterator;
 use generated_content::ResolveGeneratedContent;
 use gfx::display_list::{DisplayItem, StackingContext};
+use gfx_traits::ScrollRootId;
 use script_layout_interface::restyle_damage::{REFLOW, STORE_OVERFLOW};
 use style::context::StyleContext;
 use traversal::{AssignBSizes, AssignISizes, BubbleISizes, BuildDisplayList};
@@ -78,7 +79,7 @@ pub fn build_display_list_for_subtree(flow_root: &mut Flow,
                                       root_stacking_context: &mut StackingContext,
                                       shared_layout_context: &SharedLayoutContext)
                                       -> Vec<DisplayItem> {
-    flow_root.collect_stacking_contexts(root_stacking_context);
+    flow_root.collect_stacking_contexts(root_stacking_context, ScrollRootId::root());
     let mut build_display_list = BuildDisplayList {
         state: DisplayListBuildState::new(shared_layout_context,
                                           flow::base(flow_root).stacking_context_id),

servo/components/layout/table.rs

@@ -17,6 +17,7 @@ use flow::{BaseFlow, EarlyAbsolutePositionInfo, Flow, FlowClass, ImmutableFlowUt
 use flow_list::MutFlowListIterator;
 use fragment::{Fragment, FragmentBorderBoxIterator, Overflow};
 use gfx::display_list::StackingContext;
+use gfx_traits::ScrollRootId;
 use gfx_traits::print_tree::PrintTree;
 use layout_debug;
 use model::{IntrinsicISizes, IntrinsicISizesContribution, MaybeAuto};
@@ -489,8 +490,10 @@ impl Flow for TableFlow {
         self.block_flow.build_display_list_for_block(state, border_painting_mode);
     }
 
-    fn collect_stacking_contexts(&mut self, parent: &mut StackingContext) {
+    fn collect_stacking_contexts(&mut self,
-        self.block_flow.collect_stacking_contexts(parent);
+                                 parent: &mut StackingContext,
+                                 parent_scroll_root_id: ScrollRootId) {
+        self.block_flow.collect_stacking_contexts(parent, parent_scroll_root_id);
     }
 
     fn repair_style(&mut self, new_style: &Arc<ServoComputedValues>) {

servo/components/layout/table_caption.rs

@@ -14,6 +14,7 @@ use euclid::Point2D;
 use flow::{Flow, FlowClass, OpaqueFlow};
 use fragment::{Fragment, FragmentBorderBoxIterator, Overflow};
 use gfx::display_list::StackingContext;
+use gfx_traits::ScrollRootId;
 use gfx_traits::print_tree::PrintTree;
 use std::fmt;
 use std::sync::Arc;
@@ -82,8 +83,10 @@ impl Flow for TableCaptionFlow {
         self.block_flow.build_display_list(state);
     }
 
-    fn collect_stacking_contexts(&mut self, parent: &mut StackingContext) {
+    fn collect_stacking_contexts(&mut self,
-        self.block_flow.collect_stacking_contexts(parent);
+                                 parent: &mut StackingContext,
+                                 parent_scroll_root_id: ScrollRootId) {
+        self.block_flow.collect_stacking_contexts(parent, parent_scroll_root_id);
     }
 
     fn repair_style(&mut self, new_style: &Arc<ServoComputedValues>) {

servo/components/layout/table_cell.rs

@@ -15,6 +15,7 @@ use euclid::{Point2D, Rect, SideOffsets2D, Size2D};
 use flow::{self, Flow, FlowClass, IS_ABSOLUTELY_POSITIONED, OpaqueFlow};
 use fragment::{Fragment, FragmentBorderBoxIterator, Overflow};
 use gfx::display_list::StackingContext;
+use gfx_traits::ScrollRootId;
 use gfx_traits::print_tree::PrintTree;
 use layout_debug;
 use model::MaybeAuto;
@@ -256,8 +257,10 @@ impl Flow for TableCellFlow {
         self.block_flow.build_display_list_for_block(state, border_painting_mode)
     }
 
-    fn collect_stacking_contexts(&mut self, parent: &mut StackingContext) {
+    fn collect_stacking_contexts(&mut self,
-        self.block_flow.collect_stacking_contexts(parent);
+                                 parent: &mut StackingContext,
+                                 parent_scroll_root_id: ScrollRootId) {
+        self.block_flow.collect_stacking_contexts(parent, parent_scroll_root_id);
     }
 
     fn repair_style(&mut self, new_style: &Arc<ServoComputedValues>) {

servo/components/layout/table_colgroup.rs

@@ -13,6 +13,7 @@ use euclid::Point2D;
 use flow::{BaseFlow, Flow, FlowClass, ForceNonfloatedFlag, OpaqueFlow};
 use fragment::{Fragment, FragmentBorderBoxIterator, Overflow, SpecificFragmentInfo};
 use gfx::display_list::StackingContext;
+use gfx_traits::ScrollRootId;
 use layout_debug;
 use std::cmp::max;
 use std::fmt;
@@ -95,7 +96,9 @@ impl Flow for TableColGroupFlow {
     // Table columns are invisible.
     fn build_display_list(&mut self, _: &mut DisplayListBuildState) { }
 
-    fn collect_stacking_contexts(&mut self, _parent: &mut StackingContext) { }
+    fn collect_stacking_contexts(&mut self,
+                                 _parent: &mut StackingContext,
+                                 _parent_scroll_root_id: ScrollRootId) {}
 
     fn repair_style(&mut self, _: &Arc<ServoComputedValues>) {}
 

servo/components/layout/table_row.rs

@@ -16,6 +16,7 @@ use flow::{self, EarlyAbsolutePositionInfo, Flow, FlowClass, ImmutableFlowUtils,
 use flow_list::MutFlowListIterator;
 use fragment::{Fragment, FragmentBorderBoxIterator, Overflow};
 use gfx::display_list::StackingContext;
+use gfx_traits::ScrollRootId;
 use gfx_traits::print_tree::PrintTree;
 use layout_debug;
 use model::MaybeAuto;
@@ -458,8 +459,10 @@ impl Flow for TableRowFlow {
         self.block_flow.build_display_list_for_block(state, border_painting_mode);
     }
 
-    fn collect_stacking_contexts(&mut self, parent: &mut StackingContext) {
+    fn collect_stacking_contexts(&mut self,
-        self.block_flow.collect_stacking_contexts(parent);
+                                 parent: &mut StackingContext,
+                                 parent_scroll_root_id: ScrollRootId) {
+        self.block_flow.collect_stacking_contexts(parent, parent_scroll_root_id);
     }
 
     fn repair_style(&mut self, new_style: &Arc<ServoComputedValues>) {

servo/components/layout/table_rowgroup.rs

@@ -14,6 +14,7 @@ use euclid::Point2D;
 use flow::{Flow, FlowClass, OpaqueFlow};
 use fragment::{Fragment, FragmentBorderBoxIterator, Overflow};
 use gfx::display_list::StackingContext;
+use gfx_traits::ScrollRootId;
 use gfx_traits::print_tree::PrintTree;
 use layout_debug;
 use rustc_serialize::{Encodable, Encoder};
@@ -211,8 +212,10 @@ impl Flow for TableRowGroupFlow {
         self.block_flow.build_display_list(state);
     }
 
-    fn collect_stacking_contexts(&mut self, parent: &mut StackingContext) {
+    fn collect_stacking_contexts(&mut self,
-        self.block_flow.collect_stacking_contexts(parent);
+                                 parent: &mut StackingContext,
+                                 parent_scroll_root_id: ScrollRootId) {
+        self.block_flow.collect_stacking_contexts(parent, parent_scroll_root_id);
     }
 
     fn repair_style(&mut self, new_style: &Arc<ServoComputedValues>) {

servo/components/layout/table_wrapper.rs

@@ -23,6 +23,7 @@ use floats::FloatKind;
 use flow::{Flow, FlowClass, ImmutableFlowUtils, INLINE_POSITION_IS_STATIC, OpaqueFlow};
 use fragment::{Fragment, FragmentBorderBoxIterator, Overflow};
 use gfx::display_list::StackingContext;
+use gfx_traits::ScrollRootId;
 use gfx_traits::print_tree::PrintTree;
 use model::MaybeAuto;
 use std::cmp::{max, min};
@@ -468,8 +469,10 @@ impl Flow for TableWrapperFlow {
         self.block_flow.build_display_list(state);
     }
 
-    fn collect_stacking_contexts(&mut self, parent: &mut StackingContext) {
+    fn collect_stacking_contexts(&mut self,
-        self.block_flow.collect_stacking_contexts(parent);
+                                 parent: &mut StackingContext,
+                                 parent_scroll_root_id: ScrollRootId) {
+        self.block_flow.collect_stacking_contexts(parent, parent_scroll_root_id);
     }
 
     fn repair_style(&mut self, new_style: &Arc<ServoComputedValues>) {

servo/components/layout/traversal.rs

@@ -264,10 +264,28 @@ impl<'a> BuildDisplayList<'a> {
     #[inline]
     pub fn traverse(&mut self, flow: &mut Flow) {
         if self.should_process() {
-            self.state.push_stacking_context_id(flow::base(flow).stacking_context_id);
+            let new_stacking_context =
+                flow::base(flow).stacking_context_id != self.state.stacking_context_id();
+            if new_stacking_context {
+                self.state.push_stacking_context_id(flow::base(flow).stacking_context_id);
+            }
+
+            let new_scroll_root =
+                flow::base(flow).scroll_root_id != self.state.scroll_root_id();
+            if new_scroll_root {
+                self.state.push_scroll_root_id(flow::base(flow).scroll_root_id);
+            }
+
             flow.build_display_list(&mut self.state);
             flow::mut_base(flow).restyle_damage.remove(REPAINT);
-            self.state.pop_stacking_context_id();
+
+            if new_stacking_context {
+                self.state.pop_stacking_context_id();
+            }
+
+            if new_scroll_root {
+                self.state.pop_scroll_root_id();
+            }
         }
 
         for kid in flow::child_iter_mut(flow) {

servo/components/layout/webrender_helpers.rs

@@ -13,7 +13,7 @@ use euclid::{Point2D, Rect, Size2D};
 use gfx::display_list::{BorderRadii, BoxShadowClipMode, ClippingRegion};
 use gfx::display_list::{DisplayItem, DisplayList, DisplayListTraversal};
 use gfx::display_list::{GradientStop, StackingContext, StackingContextType};
-use gfx_traits::{FragmentType, ScrollPolicy, StackingContextId};
+use gfx_traits::{FragmentType, ScrollPolicy, StackingContextId, ScrollRootId};
 use style::computed_values::{image_rendering, mix_blend_mode};
 use style::computed_values::filter::{self, Filter};
 use style::values::computed::BorderStyle;
@@ -287,18 +287,16 @@ impl WebRenderStackingContextConverter for StackingContext {
             ScrollPolicy::FixedPosition => webrender_traits::ScrollPolicy::Fixed,
         };
 
-        let webrender_stacking_context_id = self.id.convert_to_webrender();
+        let scroll_layer_id = if let Some(scroll_root_id) = self.overflow_scroll_id {
-
+            Some(frame_builder.next_scroll_layer_id(scroll_root_id))
-        let scroll_layer_id = if self.overflow_scroll_id.is_some() ||
+        } else if self.id == StackingContextId::root() {
-                                 self.id == StackingContextId::root() {
+            Some(frame_builder.next_scroll_layer_id(ScrollRootId::root()))
-            Some(frame_builder.next_scroll_layer_id())
         } else {
             None
         };
 
         let mut sc =
-            webrender_traits::StackingContext::new(webrender_stacking_context_id,
+            webrender_traits::StackingContext::new(scroll_layer_id,
-                                                   scroll_layer_id,
                                                    webrender_scroll_policy,
                                                    self.bounds.to_rectf(),
                                                    self.overflow.to_rectf(),
@@ -532,21 +530,25 @@ impl WebRenderFrameBuilder {
         id
     }
 
-    pub fn next_scroll_layer_id(&mut self) -> webrender_traits::ScrollLayerId {
+    pub fn next_scroll_layer_id(&mut self,
+                                scroll_root_id: ScrollRootId)
+                                -> webrender_traits::ScrollLayerId {
         let scroll_layer_id = self.next_scroll_layer_id;
         self.next_scroll_layer_id += 1;
-        webrender_traits::ScrollLayerId::new(self.root_pipeline_id, scroll_layer_id)
+        webrender_traits::ScrollLayerId::new(self.root_pipeline_id,
+                                             scroll_layer_id,
+                                             scroll_root_id.convert_to_webrender())
+
     }
 }
 
-trait WebRenderStackingContextIdConverter {
+trait WebRenderScrollRootIdConverter {
-    fn convert_to_webrender(&self) -> webrender_traits::ServoStackingContextId;
+    fn convert_to_webrender(&self) -> webrender_traits::ServoScrollRootId;
 }
 
-impl WebRenderStackingContextIdConverter for StackingContextId {
+impl WebRenderScrollRootIdConverter for ScrollRootId {
-    fn convert_to_webrender(&self) -> webrender_traits::ServoStackingContextId {
+    fn convert_to_webrender(&self) -> webrender_traits::ServoScrollRootId {
-        webrender_traits::ServoStackingContextId(self.fragment_type().convert_to_webrender(),
+        webrender_traits::ServoScrollRootId(self.0)
-                                                 self.id())
     }
 }
 

servo/components/layout_thread/lib.rs

@@ -57,7 +57,7 @@ use gfx::display_list::{StackingContext, StackingContextType, WebRenderImageInfo
 use gfx::font;
 use gfx::font_cache_thread::FontCacheThread;
 use gfx::font_context;
-use gfx_traits::{Epoch, FragmentType, ScrollPolicy, StackingContextId, color};
+use gfx_traits::{Epoch, FragmentType, ScrollPolicy, ScrollRootId, StackingContextId, color};
 use heapsize::HeapSizeOf;
 use ipc_channel::ipc::{self, IpcReceiver, IpcSender};
 use ipc_channel::router::ROUTER;
@@ -1285,14 +1285,13 @@ impl LayoutThread {
         let mut layout_scroll_states = HashMap::new();
         for new_scroll_state in &new_scroll_states {
             let offset = new_scroll_state.scroll_offset;
-            layout_scroll_states.insert(new_scroll_state.stacking_context_id, offset);
+            layout_scroll_states.insert(new_scroll_state.scroll_root_id, offset);
 
-            if new_scroll_state.stacking_context_id == StackingContextId::root() {
+            if new_scroll_state.scroll_root_id == ScrollRootId::root() {
                 script_scroll_states.push((UntrustedNodeAddress::from_id(0), offset))
-            } else if !new_scroll_state.stacking_context_id.is_special() &&
+            } else if !new_scroll_state.scroll_root_id.is_special() &&
-                    new_scroll_state.stacking_context_id.fragment_type() ==
+                    new_scroll_state.scroll_root_id.fragment_type() == FragmentType::FragmentBody {
-                        FragmentType::FragmentBody {
+                let id = new_scroll_state.scroll_root_id.id();
-                let id = new_scroll_state.stacking_context_id.id();
                 script_scroll_states.push((UntrustedNodeAddress::from_id(id), offset))
             }
         }

servo/components/script_traits/lib.rs

@@ -45,7 +45,7 @@ use euclid::scale_factor::ScaleFactor;
 use euclid::size::TypedSize2D;
 use gfx_traits::DevicePixel;
 use gfx_traits::Epoch;
-use gfx_traits::StackingContextId;
+use gfx_traits::ScrollRootId;
 use heapsize::HeapSizeOf;
 use hyper::header::Headers;
 use hyper::method::Method;
@@ -600,8 +600,8 @@ pub enum AnimationTickType {
 /// The scroll state of a stacking context.
 #[derive(Copy, Clone, Debug, Deserialize, Serialize)]
 pub struct StackingContextScrollState {
-    /// The ID of the stacking context.
+    /// The ID of the scroll root.
-    pub stacking_context_id: StackingContextId,
+    pub scroll_root_id: ScrollRootId,
     /// The scrolling offset of this stacking context.
     pub scroll_offset: Point2D<f32>,
 }

servo/components/servo/Cargo.lock

@@ -46,8 +46,8 @@ dependencies = [
  "util 0.0.1",
  "util_tests 0.0.1",
  "webdriver_server 0.0.1",
- "webrender 0.7.0 (git+https://github.com/servo/webrender)",
+ "webrender 0.8.0 (git+https://github.com/servo/webrender)",
- "webrender_traits 0.7.0 (git+https://github.com/servo/webrender)",
+ "webrender_traits 0.8.0 (git+https://github.com/servo/webrender)",
 ]
 
 [[package]]
@@ -233,7 +233,7 @@ dependencies = [
  "offscreen_gl_context 0.4.5 (registry+https://github.com/rust-lang/crates.io-index)",
  "plugins 0.0.1",
  "util 0.0.1",
- "webrender_traits 0.7.0 (git+https://github.com/servo/webrender)",
+ "webrender_traits 0.8.0 (git+https://github.com/servo/webrender)",
 ]
 
 [[package]]
@@ -250,7 +250,7 @@ dependencies = [
  "plugins 0.0.1",
  "serde 0.8.11 (registry+https://github.com/rust-lang/crates.io-index)",
  "serde_derive 0.8.12 (registry+https://github.com/rust-lang/crates.io-index)",
- "webrender_traits 0.7.0 (git+https://github.com/servo/webrender)",
+ "webrender_traits 0.8.0 (git+https://github.com/servo/webrender)",
 ]
 
 [[package]]
@@ -338,8 +338,8 @@ dependencies = [
  "time 0.1.35 (registry+https://github.com/rust-lang/crates.io-index)",
  "url 1.2.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "util 0.0.1",
- "webrender 0.7.0 (git+https://github.com/servo/webrender)",
+ "webrender 0.8.0 (git+https://github.com/servo/webrender)",
- "webrender_traits 0.7.0 (git+https://github.com/servo/webrender)",
+ "webrender_traits 0.8.0 (git+https://github.com/servo/webrender)",
 ]
 
 [[package]]
@@ -370,7 +370,7 @@ dependencies = [
  "style_traits 0.0.1",
  "url 1.2.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "util 0.0.1",
- "webrender_traits 0.7.0 (git+https://github.com/servo/webrender)",
+ "webrender_traits 0.8.0 (git+https://github.com/servo/webrender)",
 ]
 
 [[package]]
@@ -815,7 +815,7 @@ dependencies = [
  "unicode-script 0.1.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "url 1.2.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "util 0.0.1",
- "webrender_traits 0.7.0 (git+https://github.com/servo/webrender)",
+ "webrender_traits 0.8.0 (git+https://github.com/servo/webrender)",
  "winapi 0.2.8 (registry+https://github.com/rust-lang/crates.io-index)",
  "xi-unicode 0.0.1 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
@@ -1167,7 +1167,7 @@ dependencies = [
  "unicode-script 0.1.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "url 1.2.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "util 0.0.1",
- "webrender_traits 0.7.0 (git+https://github.com/servo/webrender)",
+ "webrender_traits 0.8.0 (git+https://github.com/servo/webrender)",
 ]
 
 [[package]]
@@ -1207,7 +1207,7 @@ dependencies = [
  "style 0.0.1",
  "url 1.2.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "util 0.0.1",
- "webrender_traits 0.7.0 (git+https://github.com/servo/webrender)",
+ "webrender_traits 0.8.0 (git+https://github.com/servo/webrender)",
 ]
 
 [[package]]
@@ -1221,7 +1221,7 @@ dependencies = [
  "profile_traits 0.0.1",
  "script_traits 0.0.1",
  "url 1.2.0 (registry+https://github.com/rust-lang/crates.io-index)",
- "webrender_traits 0.7.0 (git+https://github.com/servo/webrender)",
+ "webrender_traits 0.8.0 (git+https://github.com/servo/webrender)",
 ]
 
 [[package]]
@@ -1390,7 +1390,7 @@ dependencies = [
  "plugins 0.0.1",
  "serde 0.8.11 (registry+https://github.com/rust-lang/crates.io-index)",
  "serde_derive 0.8.12 (registry+https://github.com/rust-lang/crates.io-index)",
- "webrender_traits 0.7.0 (git+https://github.com/servo/webrender)",
+ "webrender_traits 0.8.0 (git+https://github.com/servo/webrender)",
 ]
 
 [[package]]
@@ -1428,7 +1428,7 @@ dependencies = [
  "url 1.2.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "util 0.0.1",
  "uuid 0.3.1 (registry+https://github.com/rust-lang/crates.io-index)",
- "webrender_traits 0.7.0 (git+https://github.com/servo/webrender)",
+ "webrender_traits 0.8.0 (git+https://github.com/servo/webrender)",
  "websocket 0.17.1 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
@@ -1486,7 +1486,7 @@ dependencies = [
  "url 1.2.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "util 0.0.1",
  "uuid 0.3.1 (registry+https://github.com/rust-lang/crates.io-index)",
- "webrender_traits 0.7.0 (git+https://github.com/servo/webrender)",
+ "webrender_traits 0.8.0 (git+https://github.com/servo/webrender)",
  "websocket 0.17.1 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
@@ -1983,7 +1983,7 @@ dependencies = [
  "url 1.2.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "util 0.0.1",
  "uuid 0.3.1 (registry+https://github.com/rust-lang/crates.io-index)",
- "webrender_traits 0.7.0 (git+https://github.com/servo/webrender)",
+ "webrender_traits 0.8.0 (git+https://github.com/servo/webrender)",
  "websocket 0.17.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "xml5ever 0.1.3 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
@@ -2659,8 +2659,8 @@ dependencies = [
 
 [[package]]
 name = "webrender"
-version = "0.7.0"
+version = "0.8.0"
-source = "git+https://github.com/servo/webrender#4440d1daa3d6e9630d4b164f7bae644b9dc4cb8a"
+source = "git+https://github.com/servo/webrender#d44b1ffd3ea9e9e97ef7a70c1ac0f53bbd626850"
 dependencies = [
  "app_units 0.3.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "bincode 0.6.0 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -2679,13 +2679,13 @@ dependencies = [
  "offscreen_gl_context 0.4.5 (registry+https://github.com/rust-lang/crates.io-index)",
  "rayon 0.4.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "time 0.1.35 (registry+https://github.com/rust-lang/crates.io-index)",
- "webrender_traits 0.7.0 (git+https://github.com/servo/webrender)",
+ "webrender_traits 0.8.0 (git+https://github.com/servo/webrender)",
 ]
 
 [[package]]
 name = "webrender_traits"
-version = "0.7.0"
+version = "0.8.0"
-source = "git+https://github.com/servo/webrender#4440d1daa3d6e9630d4b164f7bae644b9dc4cb8a"
+source = "git+https://github.com/servo/webrender#d44b1ffd3ea9e9e97ef7a70c1ac0f53bbd626850"
 dependencies = [
  "app_units 0.3.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "byteorder 0.5.3 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -3001,8 +3001,8 @@ dependencies = [
 "checksum wayland-sys 0.5.11 (registry+https://github.com/rust-lang/crates.io-index)" = "9633f7fe5de56544215f82eaf1b76bf1b584becf7f08b58cbef4c2c7d10e803a"
 "checksum wayland-window 0.2.3 (registry+https://github.com/rust-lang/crates.io-index)" = "309b69d3a863c9c21422d889fb7d98cf02f8a2ca054960a49243ce5b67ad884c"
 "checksum webdriver 0.14.0 (registry+https://github.com/rust-lang/crates.io-index)" = "ee2d66e90672022ced375134329c57be4db228b19b120b97b744a469c381be06"
-"checksum webrender 0.7.0 (git+https://github.com/servo/webrender)" = "<none>"
+"checksum webrender 0.8.0 (git+https://github.com/servo/webrender)" = "<none>"
-"checksum webrender_traits 0.7.0 (git+https://github.com/servo/webrender)" = "<none>"
+"checksum webrender_traits 0.8.0 (git+https://github.com/servo/webrender)" = "<none>"
 "checksum websocket 0.17.1 (registry+https://github.com/rust-lang/crates.io-index)" = "bb4a1a6ea5ed0367f32eb3d94dcc58859ef4294b5f75ba983dbf56ac314af45d"
 "checksum winapi 0.2.8 (registry+https://github.com/rust-lang/crates.io-index)" = "167dc9d6949a9b857f3451275e911c3f44255842c1f7a76f33c55103a909087a"
 "checksum winapi-build 0.1.1 (registry+https://github.com/rust-lang/crates.io-index)" = "2d315eee3b34aca4797b2da6b13ed88266e6d612562a0c46390af8299fc699bc"

servo/ports/cef/Cargo.lock

@@ -206,7 +206,7 @@ dependencies = [
  "offscreen_gl_context 0.4.5 (registry+https://github.com/rust-lang/crates.io-index)",
  "plugins 0.0.1",
  "util 0.0.1",
- "webrender_traits 0.7.0 (git+https://github.com/servo/webrender)",
+ "webrender_traits 0.8.0 (git+https://github.com/servo/webrender)",
 ]
 
 [[package]]
@@ -223,7 +223,7 @@ dependencies = [
  "plugins 0.0.1",
  "serde 0.8.11 (registry+https://github.com/rust-lang/crates.io-index)",
  "serde_derive 0.8.12 (registry+https://github.com/rust-lang/crates.io-index)",
- "webrender_traits 0.7.0 (git+https://github.com/servo/webrender)",
+ "webrender_traits 0.8.0 (git+https://github.com/servo/webrender)",
 ]
 
 [[package]]
@@ -295,8 +295,8 @@ dependencies = [
  "time 0.1.35 (registry+https://github.com/rust-lang/crates.io-index)",
  "url 1.2.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "util 0.0.1",
- "webrender 0.7.0 (git+https://github.com/servo/webrender)",
+ "webrender 0.8.0 (git+https://github.com/servo/webrender)",
- "webrender_traits 0.7.0 (git+https://github.com/servo/webrender)",
+ "webrender_traits 0.8.0 (git+https://github.com/servo/webrender)",
 ]
 
 [[package]]
@@ -327,7 +327,7 @@ dependencies = [
  "style_traits 0.0.1",
  "url 1.2.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "util 0.0.1",
- "webrender_traits 0.7.0 (git+https://github.com/servo/webrender)",
+ "webrender_traits 0.8.0 (git+https://github.com/servo/webrender)",
 ]
 
 [[package]]
@@ -731,7 +731,7 @@ dependencies = [
  "unicode-script 0.1.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "url 1.2.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "util 0.0.1",
- "webrender_traits 0.7.0 (git+https://github.com/servo/webrender)",
+ "webrender_traits 0.8.0 (git+https://github.com/servo/webrender)",
  "winapi 0.2.8 (registry+https://github.com/rust-lang/crates.io-index)",
  "xi-unicode 0.0.1 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
@@ -1074,7 +1074,7 @@ dependencies = [
  "unicode-script 0.1.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "url 1.2.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "util 0.0.1",
- "webrender_traits 0.7.0 (git+https://github.com/servo/webrender)",
+ "webrender_traits 0.8.0 (git+https://github.com/servo/webrender)",
 ]
 
 [[package]]
@@ -1107,7 +1107,7 @@ dependencies = [
  "style 0.0.1",
  "url 1.2.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "util 0.0.1",
- "webrender_traits 0.7.0 (git+https://github.com/servo/webrender)",
+ "webrender_traits 0.8.0 (git+https://github.com/servo/webrender)",
 ]
 
 [[package]]
@@ -1121,7 +1121,7 @@ dependencies = [
  "profile_traits 0.0.1",
  "script_traits 0.0.1",
  "url 1.2.0 (registry+https://github.com/rust-lang/crates.io-index)",
- "webrender_traits 0.7.0 (git+https://github.com/servo/webrender)",
+ "webrender_traits 0.8.0 (git+https://github.com/servo/webrender)",
 ]
 
 [[package]]
@@ -1290,7 +1290,7 @@ dependencies = [
  "plugins 0.0.1",
  "serde 0.8.11 (registry+https://github.com/rust-lang/crates.io-index)",
  "serde_derive 0.8.12 (registry+https://github.com/rust-lang/crates.io-index)",
- "webrender_traits 0.7.0 (git+https://github.com/servo/webrender)",
+ "webrender_traits 0.8.0 (git+https://github.com/servo/webrender)",
 ]
 
 [[package]]
@@ -1328,7 +1328,7 @@ dependencies = [
  "url 1.2.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "util 0.0.1",
  "uuid 0.3.1 (registry+https://github.com/rust-lang/crates.io-index)",
- "webrender_traits 0.7.0 (git+https://github.com/servo/webrender)",
+ "webrender_traits 0.8.0 (git+https://github.com/servo/webrender)",
  "websocket 0.17.1 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
@@ -1364,7 +1364,7 @@ dependencies = [
  "url 1.2.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "util 0.0.1",
  "uuid 0.3.1 (registry+https://github.com/rust-lang/crates.io-index)",
- "webrender_traits 0.7.0 (git+https://github.com/servo/webrender)",
+ "webrender_traits 0.8.0 (git+https://github.com/servo/webrender)",
  "websocket 0.17.1 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
@@ -1834,7 +1834,7 @@ dependencies = [
  "url 1.2.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "util 0.0.1",
  "uuid 0.3.1 (registry+https://github.com/rust-lang/crates.io-index)",
- "webrender_traits 0.7.0 (git+https://github.com/servo/webrender)",
+ "webrender_traits 0.8.0 (git+https://github.com/servo/webrender)",
  "websocket 0.17.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "xml5ever 0.1.3 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
@@ -1991,8 +1991,8 @@ dependencies = [
  "url 1.2.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "util 0.0.1",
  "webdriver_server 0.0.1",
- "webrender 0.7.0 (git+https://github.com/servo/webrender)",
+ "webrender 0.8.0 (git+https://github.com/servo/webrender)",
- "webrender_traits 0.7.0 (git+https://github.com/servo/webrender)",
+ "webrender_traits 0.8.0 (git+https://github.com/servo/webrender)",
 ]
 
 [[package]]
@@ -2517,8 +2517,8 @@ dependencies = [
 
 [[package]]
 name = "webrender"
-version = "0.7.0"
+version = "0.8.0"
-source = "git+https://github.com/servo/webrender#4440d1daa3d6e9630d4b164f7bae644b9dc4cb8a"
+source = "git+https://github.com/servo/webrender#d44b1ffd3ea9e9e97ef7a70c1ac0f53bbd626850"
 dependencies = [
  "app_units 0.3.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "bincode 0.6.0 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -2537,13 +2537,13 @@ dependencies = [
  "offscreen_gl_context 0.4.5 (registry+https://github.com/rust-lang/crates.io-index)",
  "rayon 0.4.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "time 0.1.35 (registry+https://github.com/rust-lang/crates.io-index)",
- "webrender_traits 0.7.0 (git+https://github.com/servo/webrender)",
+ "webrender_traits 0.8.0 (git+https://github.com/servo/webrender)",
 ]
 
 [[package]]
 name = "webrender_traits"
-version = "0.7.0"
+version = "0.8.0"
-source = "git+https://github.com/servo/webrender#4440d1daa3d6e9630d4b164f7bae644b9dc4cb8a"
+source = "git+https://github.com/servo/webrender#d44b1ffd3ea9e9e97ef7a70c1ac0f53bbd626850"
 dependencies = [
  "app_units 0.3.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "byteorder 0.5.3 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -2853,8 +2853,8 @@ dependencies = [
 "checksum wayland-sys 0.5.11 (registry+https://github.com/rust-lang/crates.io-index)" = "9633f7fe5de56544215f82eaf1b76bf1b584becf7f08b58cbef4c2c7d10e803a"
 "checksum wayland-window 0.2.3 (registry+https://github.com/rust-lang/crates.io-index)" = "309b69d3a863c9c21422d889fb7d98cf02f8a2ca054960a49243ce5b67ad884c"
 "checksum webdriver 0.14.0 (registry+https://github.com/rust-lang/crates.io-index)" = "ee2d66e90672022ced375134329c57be4db228b19b120b97b744a469c381be06"
-"checksum webrender 0.7.0 (git+https://github.com/servo/webrender)" = "<none>"
+"checksum webrender 0.8.0 (git+https://github.com/servo/webrender)" = "<none>"
-"checksum webrender_traits 0.7.0 (git+https://github.com/servo/webrender)" = "<none>"
+"checksum webrender_traits 0.8.0 (git+https://github.com/servo/webrender)" = "<none>"
 "checksum websocket 0.17.1 (registry+https://github.com/rust-lang/crates.io-index)" = "bb4a1a6ea5ed0367f32eb3d94dcc58859ef4294b5f75ba983dbf56ac314af45d"
 "checksum winapi 0.2.8 (registry+https://github.com/rust-lang/crates.io-index)" = "167dc9d6949a9b857f3451275e911c3f44255842c1f7a76f33c55103a909087a"
 "checksum winapi-build 0.1.1 (registry+https://github.com/rust-lang/crates.io-index)" = "2d315eee3b34aca4797b2da6b13ed88266e6d612562a0c46390af8299fc699bc"

servo/resources/shaders/cs_box_shadow.fs.glsl

@@ -0,0 +1,148 @@
+#line 1
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// See http://asciimath.org to render the equations here.
+
+// The Gaussian function used for blurring:
+//
+//     G_sigma(x) = 1/sqrt(2 pi sigma^2) e^(-x^2/(2 sigma^2))
+float gauss(float x, float sigma) {
+    float sigmaPow2 = sigma * sigma;
+    return 1.0 / sqrt(6.283185307179586 * sigmaPow2) * exp(-(x * x) / (2.0 * sigmaPow2));
+}
+
+// An approximation of the error function, which is related to the integral of the Gaussian
+// function:
+//
+//     "erf"(x) = 2/sqrt(pi) int_0^x e^(-t^2) dt
+//              ~~ 1 - 1 / (1 + a_1 x + a_2 x^2 + a_3 x^3 + a_4 x^4)^4
+//
+// where:
+//
+//     a_1 = 0.278393, a_2 = 0.230389, a_3 = 0.000972, a_4 = 0.078108
+//
+// This approximation is accurate to `5 xx 10^-4`, more than accurate enough for our purposes.
+//
+// See: https://en.wikipedia.org/wiki/Error_function#Approximation_with_elementary_functions
+float erf(float x) {
+    bool negative = x < 0.0;
+    if (negative)
+        x = -x;
+    float x2 = x * x;
+    float x3 = x2 * x;
+    float x4 = x2 * x2;
+    float denom = 1.0 + 0.278393 * x + 0.230389 * x2 + 0.000972 * x3 + 0.078108 * x4;
+    float result = 1.0 - 1.0 / (denom * denom * denom * denom);
+    return negative ? -result : result;
+}
+
+// A useful helper for calculating integrals of the Gaussian function via the error function:
+//
+//      "erf"_sigma(x) = 2 int 1/sqrt(2 pi sigma^2) e^(-x^2/(2 sigma^2)) dx
+//                     = "erf"(x/(sigma sqrt(2)))
+float erfSigma(float x, float sigma) {
+    return erf(x / (sigma * 1.4142135623730951));
+}
+
+// Returns the blurred color value from the box itself (not counting any rounded corners). `p_0` is
+// the vector distance to the top left corner of the box; `p_1` is the vector distance to its
+// bottom right corner.
+//
+//      "colorFromRect"_sigma(p_0, p_1)
+//          = int_{p_{0_y}}^{p_{1_y}} int_{p_{1_x}}^{p_{0_x}} G_sigma(y) G_sigma(x) dx dy
+//          = 1/4 ("erf"_sigma(p_{1_x}) - "erf"_sigma(p_{0_x}))
+//              ("erf"_sigma(p_{1_y}) - "erf"_sigma(p_{0_y}))
+float colorFromRect(vec2 p0, vec2 p1, float sigma) {
+    return (erfSigma(p1.x, sigma) - erfSigma(p0.x, sigma)) *
+        (erfSigma(p1.y, sigma) - erfSigma(p0.y, sigma)) / 4.0;
+}
+
+// Returns the `x` coordinate on the ellipse with the given radii for the given `y` coordinate:
+//
+//      "ellipsePoint"(y, y_0, a, b) = a sqrt(1 - ((y - y_0) / b)^2)
+float ellipsePoint(float y, float y0, vec2 radii) {
+    float bStep = (y - y0) / radii.y;
+    return radii.x * sqrt(1.0 - bStep * bStep);
+}
+
+// A helper function to compute the value that needs to be subtracted to accommodate the border
+// corners.
+//
+//     "colorCutout"_sigma(x_{0_l}, x_{0_r}, y_0, y_{min}, y_{max}, a, b)
+//          = int_{y_{min}}^{y_{max}}
+//              int_{x_{0_r} + "ellipsePoint"(y, y_0, a, b)}^{x_{0_r} + a} G_sigma(y) G_sigma(x) dx
+//              + int_{x_{0_l} - a}^{x_{0_l} - "ellipsePoint"(y, y_0, a, b)} G_sigma(y) G_sigma(x)
+//                  dx dy
+//          = int_{y_{min}}^{y_{max}} 1/2 G_sigma(y)
+//              ("erf"_sigma(x_{0_r} + a) - "erf"_sigma(x_{0_r} + "ellipsePoint"(y, y_0, a, b)) +
+//               "erf"_sigma(x_{0_l} - "ellipsePoint"(y, y_0, a, b)) - "erf"_sigma(x_{0_l} - a))
+//
+// with the outer integral evaluated numerically.
+float colorCutoutGeneral(float x0l,
+                         float x0r,
+                         float y0,
+                         float yMin,
+                         float yMax,
+                         vec2 radii,
+                         float sigma) {
+    float sum = 0.0;
+    for (float y = yMin; y <= yMax; y += 1.0) {
+        float xEllipsePoint = ellipsePoint(y, y0, radii);
+        sum += gauss(y, sigma) *
+            (erfSigma(x0r + radii.x, sigma) - erfSigma(x0r + xEllipsePoint, sigma) +
+             erfSigma(x0l - xEllipsePoint, sigma) - erfSigma(x0l - radii.x, sigma));
+    }
+    return sum / 2.0;
+}
+
+// The value that needs to be subtracted to accommodate the top border corners.
+float colorCutoutTop(float x0l, float x0r, float y0, vec2 radii, float sigma) {
+    return colorCutoutGeneral(x0l, x0r, y0, y0, y0 + radii.y, radii, sigma);
+}
+
+// The value that needs to be subtracted to accommodate the bottom border corners.
+float colorCutoutBottom(float x0l, float x0r, float y0, vec2 radii, float sigma) {
+    return colorCutoutGeneral(x0l, x0r, y0, y0 - radii.y, y0, radii, sigma);
+}
+
+// The blurred color value for the point at `pos` with the top left corner of the box at
+// `p_{0_"rect"}` and the bottom right corner of the box at `p_{1_"rect"}`.
+float color(vec2 pos, vec2 p0Rect, vec2 p1Rect, vec2 radii, float sigma) {
+    // Compute the vector distances `p_0` and `p_1`.
+    vec2 p0 = p0Rect - pos, p1 = p1Rect - pos;
+
+    // Compute the basic color `"colorFromRect"_sigma(p_0, p_1)`. This is all we have to do if
+    // the box is unrounded.
+    float cRect = colorFromRect(p0, p1, sigma);
+    if (radii.x == 0.0 || radii.y == 0.0)
+        return cRect;
+
+    // Compute the inner corners of the box, taking border radii into account: `x_{0_l}`,
+    // `y_{0_t}`, `x_{0_r}`, and `y_{0_b}`.
+    float x0l = p0.x + radii.x;
+    float y0t = p1.y - radii.y;
+    float x0r = p1.x - radii.x;
+    float y0b = p0.y + radii.y;
+
+    // Compute the final color:
+    //
+    //     "colorFromRect"_sigma(p_0, p_1) -
+    //          ("colorCutoutTop"_sigma(x_{0_l}, x_{0_r}, y_{0_t}, a, b) +
+    //           "colorCutoutBottom"_sigma(x_{0_l}, x_{0_r}, y_{0_b}, a, b))
+    float cCutoutTop = colorCutoutTop(x0l, x0r, y0t, radii, sigma);
+    float cCutoutBottom = colorCutoutBottom(x0l, x0r, y0b, radii, sigma);
+    return cRect - (cCutoutTop + cCutoutBottom);
+}
+
+void main(void) {
+    vec2 pos = vPos.xy;
+    vec2 p0Rect = vBoxShadowRect.xy, p1Rect = vBoxShadowRect.zw;
+    vec2 radii = vBorderRadii.xy;
+    float sigma = vBlurRadius / 2.0;
+    float value = color(pos, p0Rect, p1Rect, radii, sigma);
+
+    value = max(value, 0.0);
+    oFragColor = vec4(1.0, 1.0, 1.0, vInverted == 1.0 ? 1.0 - value : value);
+}

servo/resources/shaders/cs_box_shadow.glsl

@@ -0,0 +1,10 @@
+#line 1
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+varying vec2 vPos;
+flat varying vec2 vBorderRadii;
+flat varying float vBlurRadius;
+flat varying vec4 vBoxShadowRect;
+flat varying float vInverted;

servo/resources/shaders/cs_box_shadow.vs.glsl

@@ -0,0 +1,31 @@
+#line 1
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+void main(void) {
+    CachePrimitiveInstance cpi = fetch_cache_instance(gl_InstanceID);
+    RenderTaskData task = fetch_render_task(cpi.render_task_index);
+    BoxShadow bs = fetch_boxshadow(cpi.specific_prim_index);
+
+    vec2 p0 = task.data0.xy;
+    vec2 p1 = p0 + task.data0.zw;
+
+    vec2 pos = mix(p0, p1, aPosition.xy);
+
+    vBorderRadii = bs.border_radius_edge_size_blur_radius_inverted.xx;
+    vBlurRadius = bs.border_radius_edge_size_blur_radius_inverted.z;
+    vInverted = bs.border_radius_edge_size_blur_radius_inverted.w;
+    vBoxShadowRect = vec4(bs.bs_rect.xy, bs.bs_rect.xy + bs.bs_rect.zw);
+
+    // The fragment shader expects logical units, beginning at where the
+    // blur radius begins.
+    // The first path of the equation gets the virtual position in
+    // logical pixels within the patch rectangle (accounting for
+    // bilinear offset). Then we add the start position of the
+    // box shadow rect and subtract the blur radius to get the
+    // virtual coordinates that the FS expects.
+    vPos = (pos - 1.0 - p0) / uDevicePixelRatio + bs.bs_rect.xy - vec2(2.0 * vBlurRadius);
+
+    gl_Position = uTransform * vec4(pos, 0.0, 1.0);
+}

servo/resources/shaders/prim_shared.glsl

@@ -39,7 +39,7 @@ uniform sampler2DArray sCache;
 #ifdef WR_VERTEX_SHADER
 
 #define VECS_PER_LAYER             13
-#define VECS_PER_TILE               2
+#define VECS_PER_RENDER_TASK        2
 #define VECS_PER_PRIM_GEOM          2
 
 #define GRADIENT_HORIZONTAL     0
@@ -121,18 +121,33 @@ Layer fetch_layer(int index) {
     return layer;
 }
 
+struct RenderTaskData {
+    vec4 data0;
+    vec4 data1;
+};
+
+RenderTaskData fetch_render_task(int index) {
+    RenderTaskData task;
+
+    ivec2 uv = get_fetch_uv(index, VECS_PER_RENDER_TASK);
+
+    task.data0 = texelFetchOffset(sRenderTasks, uv, 0, ivec2(0, 0));
+    task.data1 = texelFetchOffset(sRenderTasks, uv, 0, ivec2(1, 0));
+
+    return task;
+}
+
 struct Tile {
     vec4 screen_origin_task_origin;
     vec4 size_target_index;
 };
 
 Tile fetch_tile(int index) {
+    RenderTaskData task = fetch_render_task(index);
+
     Tile tile;
-
+    tile.screen_origin_task_origin = task.data0;
-    ivec2 uv = get_fetch_uv(index, VECS_PER_TILE);
+    tile.size_target_index = task.data1;
-
-    tile.screen_origin_task_origin = texelFetchOffset(sRenderTasks, uv, 0, ivec2(0, 0));
-    tile.size_target_index = texelFetchOffset(sRenderTasks, uv, 0, ivec2(1, 0));
 
     return tile;
 }
@@ -259,6 +274,27 @@ PrimitiveInstance fetch_instance(int index) {
 
     return pi;
 }
+
+struct CachePrimitiveInstance {
+    int global_prim_index;
+    int specific_prim_index;
+    int render_task_index;
+};
+
+CachePrimitiveInstance fetch_cache_instance(int index) {
+    CachePrimitiveInstance cpi;
+
+    int offset = index * 1;
+
+    ivec4 data0 = int_data[offset + 0];
+
+    cpi.global_prim_index = data0.x;
+    cpi.specific_prim_index = data0.y;
+    cpi.render_task_index = data0.z;
+
+    return cpi;
+}
+
 struct Primitive {
     Layer layer;
     Tile tile;
@@ -553,7 +589,7 @@ struct BoxShadow {
     vec4 src_rect;
     vec4 bs_rect;
     vec4 color;
-    vec4 border_radii_blur_radius_inverted;
+    vec4 border_radius_edge_size_blur_radius_inverted;
 };
 
 BoxShadow fetch_boxshadow(int index) {
@@ -564,7 +600,7 @@ BoxShadow fetch_boxshadow(int index) {
     bs.src_rect = texelFetchOffset(sData64, uv, 0, ivec2(0, 0));
     bs.bs_rect = texelFetchOffset(sData64, uv, 0, ivec2(1, 0));
     bs.color = texelFetchOffset(sData64, uv, 0, ivec2(2, 0));
-    bs.border_radii_blur_radius_inverted = texelFetchOffset(sData64, uv, 0, ivec2(3, 0));
+    bs.border_radius_edge_size_blur_radius_inverted = texelFetchOffset(sData64, uv, 0, ivec2(3, 0));
 
     return bs;
 }

servo/resources/shaders/ps_box_shadow.fs.glsl

@@ -2,150 +2,8 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-/* This Source Code Form is subject to the terms of the Mozilla Public
- * License, v. 2.0. If a copy of the MPL was not distributed with this
- * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
-
-// See http://asciimath.org to render the equations here.
-
-// The Gaussian function used for blurring:
-//
-//     G_sigma(x) = 1/sqrt(2 pi sigma^2) e^(-x^2/(2 sigma^2))
-float gauss(float x, float sigma) {
-    float sigmaPow2 = sigma * sigma;
-    return 1.0 / sqrt(6.283185307179586 * sigmaPow2) * exp(-(x * x) / (2.0 * sigmaPow2));
-}
-
-// An approximation of the error function, which is related to the integral of the Gaussian
-// function:
-//
-//     "erf"(x) = 2/sqrt(pi) int_0^x e^(-t^2) dt
-//              ~~ 1 - 1 / (1 + a_1 x + a_2 x^2 + a_3 x^3 + a_4 x^4)^4
-//
-// where:
-//
-//     a_1 = 0.278393, a_2 = 0.230389, a_3 = 0.000972, a_4 = 0.078108
-//
-// This approximation is accurate to `5 xx 10^-4`, more than accurate enough for our purposes.
-//
-// See: https://en.wikipedia.org/wiki/Error_function#Approximation_with_elementary_functions
-float erf(float x) {
-    bool negative = x < 0.0;
-    if (negative)
-        x = -x;
-    float x2 = x * x;
-    float x3 = x2 * x;
-    float x4 = x2 * x2;
-    float denom = 1.0 + 0.278393 * x + 0.230389 * x2 + 0.000972 * x3 + 0.078108 * x4;
-    float result = 1.0 - 1.0 / (denom * denom * denom * denom);
-    return negative ? -result : result;
-}
-
-// A useful helper for calculating integrals of the Gaussian function via the error function:
-//
-//      "erf"_sigma(x) = 2 int 1/sqrt(2 pi sigma^2) e^(-x^2/(2 sigma^2)) dx
-//                     = "erf"(x/(sigma sqrt(2)))
-float erfSigma(float x, float sigma) {
-    return erf(x / (sigma * 1.4142135623730951));
-}
-
-// Returns the blurred color value from the box itself (not counting any rounded corners). `p_0` is
-// the vector distance to the top left corner of the box; `p_1` is the vector distance to its
-// bottom right corner.
-//
-//      "colorFromRect"_sigma(p_0, p_1)
-//          = int_{p_{0_y}}^{p_{1_y}} int_{p_{1_x}}^{p_{0_x}} G_sigma(y) G_sigma(x) dx dy
-//          = 1/4 ("erf"_sigma(p_{1_x}) - "erf"_sigma(p_{0_x}))
-//              ("erf"_sigma(p_{1_y}) - "erf"_sigma(p_{0_y}))
-float colorFromRect(vec2 p0, vec2 p1, float sigma) {
-    return (erfSigma(p1.x, sigma) - erfSigma(p0.x, sigma)) *
-        (erfSigma(p1.y, sigma) - erfSigma(p0.y, sigma)) / 4.0;
-}
-
-// Returns the `x` coordinate on the ellipse with the given radii for the given `y` coordinate:
-//
-//      "ellipsePoint"(y, y_0, a, b) = a sqrt(1 - ((y - y_0) / b)^2)
-float ellipsePoint(float y, float y0, vec2 radii) {
-    float bStep = (y - y0) / radii.y;
-    return radii.x * sqrt(1.0 - bStep * bStep);
-}
-
-// A helper function to compute the value that needs to be subtracted to accommodate the border
-// corners.
-//
-//     "colorCutout"_sigma(x_{0_l}, x_{0_r}, y_0, y_{min}, y_{max}, a, b)
-//          = int_{y_{min}}^{y_{max}}
-//              int_{x_{0_r} + "ellipsePoint"(y, y_0, a, b)}^{x_{0_r} + a} G_sigma(y) G_sigma(x) dx
-//              + int_{x_{0_l} - a}^{x_{0_l} - "ellipsePoint"(y, y_0, a, b)} G_sigma(y) G_sigma(x)
-//                  dx dy
-//          = int_{y_{min}}^{y_{max}} 1/2 G_sigma(y)
-//              ("erf"_sigma(x_{0_r} + a) - "erf"_sigma(x_{0_r} + "ellipsePoint"(y, y_0, a, b)) +
-//               "erf"_sigma(x_{0_l} - "ellipsePoint"(y, y_0, a, b)) - "erf"_sigma(x_{0_l} - a))
-//
-// with the outer integral evaluated numerically.
-float colorCutoutGeneral(float x0l,
-                         float x0r,
-                         float y0,
-                         float yMin,
-                         float yMax,
-                         vec2 radii,
-                         float sigma) {
-    float sum = 0.0;
-    for (float y = yMin; y <= yMax; y += 1.0) {
-        float xEllipsePoint = ellipsePoint(y, y0, radii);
-        sum += gauss(y, sigma) *
-            (erfSigma(x0r + radii.x, sigma) - erfSigma(x0r + xEllipsePoint, sigma) +
-             erfSigma(x0l - xEllipsePoint, sigma) - erfSigma(x0l - radii.x, sigma));
-    }
-    return sum / 2.0;
-}
-
-// The value that needs to be subtracted to accommodate the top border corners.
-float colorCutoutTop(float x0l, float x0r, float y0, vec2 radii, float sigma) {
-    return colorCutoutGeneral(x0l, x0r, y0, y0, y0 + radii.y, radii, sigma);
-}
-
-// The value that needs to be subtracted to accommodate the bottom border corners.
-float colorCutoutBottom(float x0l, float x0r, float y0, vec2 radii, float sigma) {
-    return colorCutoutGeneral(x0l, x0r, y0, y0 - radii.y, y0, radii, sigma);
-}
-
-// The blurred color value for the point at `pos` with the top left corner of the box at
-// `p_{0_"rect"}` and the bottom right corner of the box at `p_{1_"rect"}`.
-float color(vec2 pos, vec2 p0Rect, vec2 p1Rect, vec2 radii, float sigma) {
-    // Compute the vector distances `p_0` and `p_1`.
-    vec2 p0 = p0Rect - pos, p1 = p1Rect - pos;
-
-    // Compute the basic color `"colorFromRect"_sigma(p_0, p_1)`. This is all we have to do if
-    // the box is unrounded.
-    float cRect = colorFromRect(p0, p1, sigma);
-    if (radii.x == 0.0 || radii.y == 0.0)
-        return cRect;
-
-    // Compute the inner corners of the box, taking border radii into account: `x_{0_l}`,
-    // `y_{0_t}`, `x_{0_r}`, and `y_{0_b}`.
-    float x0l = p0.x + radii.x;
-    float y0t = p1.y - radii.y;
-    float x0r = p1.x - radii.x;
-    float y0b = p0.y + radii.y;
-
-    // Compute the final color:
-    //
-    //     "colorFromRect"_sigma(p_0, p_1) -
-    //          ("colorCutoutTop"_sigma(x_{0_l}, x_{0_r}, y_{0_t}, a, b) +
-    //           "colorCutoutBottom"_sigma(x_{0_l}, x_{0_r}, y_{0_b}, a, b))
-    float cCutoutTop = colorCutoutTop(x0l, x0r, y0t, radii, sigma);
-    float cCutoutBottom = colorCutoutBottom(x0l, x0r, y0b, radii, sigma);
-    return cRect - (cCutoutTop + cCutoutBottom);
-}
-
 void main(void) {
-    vec2 pos = vPos.xy;
+    vec2 uv = min(vec2(1.0), vMirrorPoint - abs(vUv.xy - vMirrorPoint));
-    vec2 p0Rect = vBoxShadowRect.xy, p1Rect = vBoxShadowRect.zw;
+    uv = mix(vCacheUvRectCoords.xy, vCacheUvRectCoords.zw, uv);
-    vec2 radii = vBorderRadii.xy;
+    oFragColor = vColor * texture(sCache, vec3(uv, vUv.z));
-    float sigma = vBlurRadius / 2.0;
-    float value = color(pos, p0Rect, p1Rect, radii, sigma);
-
-    value = max(value, 0.0);
-    oFragColor = vColor * vec4(1.0, 1.0, 1.0, vInverted == 1.0 ? 1.0 - value : value);
 }

servo/resources/shaders/ps_box_shadow.glsl

@@ -2,10 +2,8 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-varying vec2 vPos;
 flat varying vec4 vColor;
-flat varying vec2 vBorderRadii;
+
-flat varying float vBlurRadius;
+varying vec3 vUv;
-flat varying vec4 vBoxShadowRect;
+flat varying vec2 vMirrorPoint;
-flat varying vec4 vSrcRect;
+flat varying vec4 vCacheUvRectCoords;
-flat varying float vInverted;

servo/resources/shaders/ps_box_shadow.vs.glsl

@@ -13,11 +13,19 @@ void main(void) {
                                  prim.layer,
                                  prim.tile);
 
-    vPos = vi.local_clamped_pos;
+    RenderTaskData child_task = fetch_render_task(prim.user_data.z);
+    vUv.z = child_task.data1.x;
+
+    // Constant offsets to inset from bilinear filtering border.
+    vec2 patch_origin = child_task.data0.xy + vec2(1.0);
+    vec2 patch_size_device_pixels = child_task.data0.zw - vec2(2.0);
+    vec2 patch_size = patch_size_device_pixels / uDevicePixelRatio;
+
+    vUv.xy = (vi.local_clamped_pos - prim.local_rect.xy) / patch_size;
+    vMirrorPoint = 0.5 * prim.local_rect.zw / patch_size;
+
+    vec2 texture_size = vec2(textureSize(sCache, 0));
+    vCacheUvRectCoords = vec4(patch_origin, patch_origin + patch_size_device_pixels) / texture_size.xyxy;
+
     vColor = bs.color;
-    vBorderRadii = bs.border_radii_blur_radius_inverted.xy;
-    vBlurRadius = bs.border_radii_blur_radius_inverted.z;
-    vBoxShadowRect = vec4(bs.bs_rect.xy, bs.bs_rect.xy + bs.bs_rect.zw);
-    vSrcRect = vec4(bs.src_rect.xy, bs.src_rect.xy + bs.src_rect.zw);
-    vInverted = bs.border_radii_blur_radius_inverted.w;
 }