Store abspos boxes in a RefCell too

We want to mutate them when lazily computing their content sizes, but they
are behind an Arc for the hoisting infra, so it also needs its own layer
of inner mutability.

components/layout_2020/flow/construct.rs

@@ -571,9 +571,14 @@ where
                 kind,
             });
         } else {
-            let box_ = ArcRefCell::new(InlineLevelBox::OutOfFlowAbsolutelyPositionedBox(Arc::new(
-                AbsolutelyPositionedBox::construct(self.context, info, display_inside, contents),
-            )));
+            let box_ = ArcRefCell::new(InlineLevelBox::OutOfFlowAbsolutelyPositionedBox(
+                ArcRefCell::new(AbsolutelyPositionedBox::construct(
+                    self.context,
+                    info,
+                    display_inside,
+                    contents,
+                )),
+            ));
             self.current_inline_level_boxes().push(box_.clone());
             box_slot.set(LayoutBox::InlineLevel(box_))
         }
@@ -722,10 +727,11 @@ where
                 display_inside,
                 contents,
             } => {
-                let block_level_box =
-                    ArcRefCell::new(BlockLevelBox::OutOfFlowAbsolutelyPositionedBox(Arc::new(
+                let block_level_box = ArcRefCell::new(
+                    BlockLevelBox::OutOfFlowAbsolutelyPositionedBox(ArcRefCell::new(
                         AbsolutelyPositionedBox::construct(context, info, display_inside, contents),
-                    )));
+                    )),
+                );
                 (block_level_box, ContainsFloats::No)
             },
             BlockLevelCreator::OutOfFlowFloatBox {

components/layout_2020/flow/inline.rs

@@ -20,6 +20,7 @@ use crate::sizing::ContentSizes;
 use crate::style_ext::{ComputedValuesExt, Display, DisplayGeneratingBox, DisplayOutside};
 use crate::ContainingBlock;
 use app_units::Au;
+use atomic_refcell::AtomicRef;
 use gfx::text::text_run::GlyphRun;
 use servo_arc::Arc;
 use style::properties::ComputedValues;
@@ -39,7 +40,7 @@ pub(crate) struct InlineFormattingContext {
 pub(crate) enum InlineLevelBox {
     InlineBox(InlineBox),
     TextRun(TextRun),
-    OutOfFlowAbsolutelyPositionedBox(Arc<AbsolutelyPositionedBox>),
+    OutOfFlowAbsolutelyPositionedBox(ArcRefCell<AbsolutelyPositionedBox>),
     OutOfFlowFloatBox(FloatBox),
     Atomic(IndependentFormattingContext),
 }
@@ -283,8 +284,9 @@ impl InlineFormattingContext {
                     InlineLevelBox::TextRun(run) => run.layout(layout_context, &mut ifc),
                     InlineLevelBox::Atomic(a) => layout_atomic(layout_context, &mut ifc, a),
                     InlineLevelBox::OutOfFlowAbsolutelyPositionedBox(box_) => {
+                        let style = AtomicRef::map(box_.borrow(), |box_| &box_.contents.style);
                         let initial_start_corner =
-                            match Display::from(box_.contents.style.get_box().original_display) {
+                            match Display::from(style.get_box().original_display) {
                                 Display::GeneratingBox(DisplayGeneratingBox::OutsideInside {
                                     outside,
                                     inside: _,
@@ -313,7 +315,7 @@ impl InlineFormattingContext {
                             Fragment::AbsoluteOrFixedPositioned(
                                 AbsoluteOrFixedPositionedFragment {
                                     hoisted_fragment,
-                                    position: box_.contents.style.clone_position(),
+                                    position: style.clone_position(),
                                 },
                             ),
                         );

components/layout_2020/flow/mod.rs

@@ -52,7 +52,7 @@ pub(crate) enum BlockLevelBox {
         style: Arc<ComputedValues>,
         contents: BlockContainer,
     },
-    OutOfFlowAbsolutelyPositionedBox(Arc<AbsolutelyPositionedBox>),
+    OutOfFlowAbsolutelyPositionedBox(ArcRefCell<AbsolutelyPositionedBox>),
     OutOfFlowFloatBox(FloatBox),
     Independent(IndependentFormattingContext),
 }
@@ -322,7 +322,7 @@ impl BlockLevelBox {
                 positioning_context.push(hoisted_box);
                 Fragment::AbsoluteOrFixedPositioned(AbsoluteOrFixedPositionedFragment {
                     hoisted_fragment,
-                    position: box_.contents.style.clone_position(),
+                    position: box_.borrow().contents.style.clone_position(),
                 })
             },
             BlockLevelBox::OutOfFlowFloatBox(_box_) => {

components/layout_2020/flow/root.rs

@@ -207,7 +207,7 @@ impl BoxTree {
                 let contents = ReplacedContent::for_element(dirty_node)
                     .map_or(Contents::OfElement, Contents::Replaced);
                 let info = NodeAndStyleInfo::new(dirty_node, Arc::clone(&primary_style));
-                let out_of_flow_absolutely_positioned_box = Arc::new(
+                let out_of_flow_absolutely_positioned_box = ArcRefCell::new(
                     AbsolutelyPositionedBox::construct(context, &info, display_inside, contents),
                 );
                 match update_point {
@@ -267,7 +267,7 @@ fn construct_for_root_element<'dom>(
     let (contains_floats, root_box) = if box_style.position.is_absolutely_positioned() {
         (
             ContainsFloats::No,
-            BlockLevelBox::OutOfFlowAbsolutelyPositionedBox(Arc::new(
+            BlockLevelBox::OutOfFlowAbsolutelyPositionedBox(ArcRefCell::new(
                 AbsolutelyPositionedBox::construct(context, &info, display_inside, contents),
             )),
         )