Allow inline elements to create reference frames

This is important so that transforms applied to elements actually apply
to the display items created for those elements.

components/layout/display_list/builder.rs

@@ -576,6 +576,12 @@ pub trait FragmentDisplayListBuilding {
         state: &mut StackingContextCollectionState,
     ) -> bool;
 
+    fn create_stacking_context_for_inline_block(
+        &mut self,
+        base: &BaseFlow,
+        state: &mut StackingContextCollectionState,
+    ) -> bool;
+
     /// Adds the display items necessary to paint the background of this fragment to the display
     /// list if necessary.
     fn build_display_list_for_background_if_applicable(
@@ -816,6 +822,35 @@ impl FragmentDisplayListBuilding for Fragment {
         }
     }
 
+    fn create_stacking_context_for_inline_block(
+        &mut self,
+        base: &BaseFlow,
+        state: &mut StackingContextCollectionState,
+    ) -> bool {
+        self.stacking_context_id = state.allocate_stacking_context_info(StackingContextType::Real);
+
+        let established_reference_frame = if self.can_establish_reference_frame() {
+            // WebRender currently creates reference frames automatically, so just add
+            // a placeholder node to allocate a ClipScrollNodeIndex for this reference frame.
+            self.established_reference_frame =
+                Some(state.add_clip_scroll_node(ClipScrollNode::placeholder()));
+            self.established_reference_frame
+        } else {
+            None
+        };
+
+        let current_stacking_context_id = state.current_stacking_context_id;
+        let stacking_context = self.create_stacking_context(
+            self.stacking_context_id,
+            &base,
+            StackingContextType::Real,
+            established_reference_frame,
+            state.current_clipping_and_scrolling,
+        );
+        state.add_stacking_context(current_stacking_context_id, stacking_context);
+        true
+    }
+
     fn build_display_list_for_background_if_applicable(
         &self,
         state: &mut DisplayListBuildState,
@@ -1562,14 +1597,21 @@ impl FragmentDisplayListBuilding for Fragment {
         display_list_section: DisplayListSection,
         clip: Rect<Au>,
     ) {
+        let previous_clipping_and_scrolling = state.current_clipping_and_scrolling;
+        if let Some(index) = self.established_reference_frame {
+            state.current_clipping_and_scrolling = ClippingAndScrolling::simple(index);
+        }
+
         self.restyle_damage.remove(ServoRestyleDamage::REPAINT);
         self.build_display_list_no_damage(
             state,
             stacking_relative_border_box,
             border_painting_mode,
             display_list_section,
             clip,
-        )
+        );
+
+        state.current_clipping_and_scrolling = previous_clipping_and_scrolling;
     }
 
     fn build_display_list_no_damage(
@@ -2861,8 +2903,11 @@ impl InlineFlowDisplayListBuilding for InlineFlow {
             .cloned()
             .unwrap_or_else(Rect::max_rect);
 
+        let previous_cb_clipping_and_scrolling = state.containing_block_clipping_and_scrolling;
+
         for fragment in self.fragments.fragments.iter_mut() {
-            let previous_cb_clipping_and_scrolling = state.containing_block_clipping_and_scrolling;
+            state.containing_block_clipping_and_scrolling = previous_cb_clipping_and_scrolling;
+
             if establishes_containing_block_for_absolute(
                 StackingContextCollectionFlags::empty(),
                 fragment.style.get_box().position,
@@ -2871,26 +2916,20 @@ impl InlineFlowDisplayListBuilding for InlineFlow {
                     state.current_clipping_and_scrolling;
             }
 
-            if !fragment.collect_stacking_contexts_for_blocklike_fragment(state) {
-                if fragment.establishes_stacking_context() {
-                    fragment.stacking_context_id =
-                        state.allocate_stacking_context_info(StackingContextType::Real);
-
-                    let current_stacking_context_id = state.current_stacking_context_id;
-                    let stacking_context = fragment.create_stacking_context(
-                        fragment.stacking_context_id,
-                        &self.base,
-                        StackingContextType::Real,
-                        None,
-                        state.current_clipping_and_scrolling,
-                    );
+            // We clear this here, but it might be set again if we create a stacking context for
+            // this fragment.
+            fragment.established_reference_frame = None;
 
-                    state.add_stacking_context(current_stacking_context_id, stacking_context);
-                } else {
+            if !fragment.collect_stacking_contexts_for_blocklike_fragment(state) {
+                if !fragment.establishes_stacking_context() {
                     fragment.stacking_context_id = state.current_stacking_context_id;
+                } else {
+                    fragment.create_stacking_context_for_inline_block(&self.base, state);
                 }
             }
 
+            // Reset the containing block clipping and scrolling before each loop iteration,
+            // so we don't pollute subsequent fragments.
             state.containing_block_clipping_and_scrolling = previous_cb_clipping_and_scrolling;
         }
     }

components/layout/fragment.rs

@@ -11,7 +11,7 @@ use app_units::Au;
 use canvas_traits::canvas::{CanvasMsg, CanvasId};
 use context::{LayoutContext, with_thread_local_font_context};
 use display_list::ToLayout;
-use display_list::items::{BLUR_INFLATION_FACTOR, OpaqueNode};
+use display_list::items::{BLUR_INFLATION_FACTOR, ClipScrollNodeIndex, OpaqueNode};
 use euclid::{Point2D, Vector2D, Rect, Size2D};
 use floats::ClearType;
 use flow::{GetBaseFlow, ImmutableFlowUtils};
@@ -152,6 +152,11 @@ pub struct Fragment {
     /// to 0, but it assigned during the collect_stacking_contexts phase of display
     /// list construction.
     pub stacking_context_id: StackingContextId,
+
+    /// The indices of this Fragment's ClipScrollNode. If this fragment doesn't have a
+    /// `established_reference_frame` assigned, it will use the `clipping_and_scrolling` of the
+    /// parent block.
+    pub established_reference_frame: Option<ClipScrollNodeIndex>,
 }
 
 impl Serialize for Fragment {
@@ -633,6 +638,7 @@ impl Fragment {
             flags: FragmentFlags::empty(),
             debug_id: DebugId::new(),
             stacking_context_id: StackingContextId::root(),
+            established_reference_frame: None,
         }
     }
 
@@ -662,6 +668,7 @@ impl Fragment {
             flags: FragmentFlags::empty(),
             debug_id: DebugId::new(),
             stacking_context_id: StackingContextId::root(),
+            established_reference_frame: None,
         }
     }
 
@@ -687,6 +694,7 @@ impl Fragment {
             flags: FragmentFlags::empty(),
             debug_id: DebugId::new(),
             stacking_context_id: StackingContextId::root(),
+            established_reference_frame: None,
         }
     }
 
@@ -715,6 +723,7 @@ impl Fragment {
             flags: FragmentFlags::empty(),
             debug_id: self.debug_id.clone(),
             stacking_context_id: StackingContextId::root(),
+            established_reference_frame: None,
         }
     }
 

components/layout/tests/size_of.rs

@@ -10,5 +10,5 @@ extern crate layout;
 use layout::Fragment;
 use layout::SpecificFragmentInfo;
 
-size_of_test!(test_size_of_fragment, Fragment, 160);
+size_of_test!(test_size_of_fragment, Fragment, 176);
 size_of_test!(test_size_of_specific_fragment_info, SpecificFragmentInfo, 24);