Auto merge of #25862 - mrobinson:transforms-containing-block, r=pcwalton,nox

Have transforms and filters be CBs for all descendants in layout_2020

This is a feature that was never properly implemented in the previous
layout system. We still need to preserve their in-tree order in the
display list though.

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

---
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- [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)

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- [x] There are tests for these changes OR
- [ ] These changes do not require tests because ___

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components/layout_2020/display_list/mod.rs

@@ -37,7 +37,12 @@ type ItemTag = (u64, u16);
 type HitInfo = Option<ItemTag>;
 
 pub struct DisplayListBuilder<'a> {
+    /// The current SpatialId and ClipId information for this `DisplayListBuilder`.
     current_space_and_clip: wr::SpaceAndClipInfo,
+
+    /// The id of the nearest ancestor reference frame for this `DisplayListBuilder`.
+    nearest_reference_frame: wr::SpatialId,
+
     pub context: &'a LayoutContext<'a>,
     pub wr: wr::DisplayListBuilder,
 
@@ -56,6 +61,7 @@ impl<'a> DisplayListBuilder<'a> {
     ) -> Self {
         Self {
             current_space_and_clip: wr::SpaceAndClipInfo::root_scroll(pipeline_id),
+            nearest_reference_frame: wr::SpatialId::root_reference_frame(pipeline_id),
             is_contentful: false,
             context,
             wr: wr::DisplayListBuilder::new(pipeline_id, viewport_size),
@@ -68,9 +74,14 @@ impl<'a> DisplayListBuilder<'a> {
     }
 
     fn clipping_and_scrolling_scope<R>(&mut self, f: impl FnOnce(&mut Self) -> R) -> R {
-        let previous = self.current_space_and_clip;
+        let previous_space_and_clip = self.current_space_and_clip;
+        let previous_nearest_reference_frame = self.nearest_reference_frame;
+
         let result = f(self);
-        self.current_space_and_clip = previous;
+
+        self.current_space_and_clip = previous_space_and_clip;
+        self.nearest_reference_frame = previous_nearest_reference_frame;
+
         result
     }
 }

components/layout_2020/display_list/stacking_context.rs

@@ -6,6 +6,7 @@ use crate::display_list::conversions::ToWebRender;
 use crate::display_list::DisplayListBuilder;
 use crate::fragments::{AnonymousFragment, BoxFragment, Fragment};
 use crate::geom::PhysicalRect;
+use crate::style_ext::ComputedValuesExt;
 use euclid::default::Rect;
 use gfx_traits::{combine_id_with_fragment_type, FragmentType};
 use std::cmp::Ordering;
@@ -14,7 +15,6 @@ use style::computed_values::float::T as ComputedFloat;
 use style::computed_values::mix_blend_mode::T as ComputedMixBlendMode;
 use style::computed_values::overflow_x::T as ComputedOverflow;
 use style::computed_values::position::T as ComputedPosition;
-use style::computed_values::transform_style::T as ComputedTransformStyle;
 use style::values::computed::Length;
 use style::values::generics::box_::Perspective;
 use style::values::generics::transform;
@@ -96,7 +96,7 @@ impl<'a> StackingContext<'a> {
 
     fn z_index(&self) -> i32 {
         match self.initializing_fragment {
-            Some(fragment) => fragment.effective_z_index(),
+            Some(fragment) => fragment.style.effective_z_index(),
             None => 0,
         }
     }
@@ -259,7 +259,7 @@ impl Fragment {
 
 impl BoxFragment {
     fn get_stacking_context_type(&self) -> Option<StackingContextType> {
-        if self.establishes_stacking_context() {
+        if self.style.establishes_stacking_context() {
             return Some(StackingContextType::Real);
         }
 
@@ -291,68 +291,6 @@ impl BoxFragment {
         StackingContextSection::BlockBackgroundsAndBorders
     }
 
-    /// Returns true if this fragment establishes a new stacking context and false otherwise.
-    fn establishes_stacking_context(&self) -> bool {
-        let effects = self.style.get_effects();
-        if effects.opacity != 1.0 {
-            return true;
-        }
-
-        if effects.mix_blend_mode != ComputedMixBlendMode::Normal {
-            return true;
-        }
-
-        if self.has_transform_or_perspective() {
-            return true;
-        }
-
-        if !self.style.get_effects().filter.0.is_empty() {
-            return true;
-        }
-
-        if self.style.get_box().transform_style == ComputedTransformStyle::Preserve3d ||
-            self.style.overrides_transform_style()
-        {
-            return true;
-        }
-
-        // Fixed position and sticky position always create stacking contexts.
-        // TODO(mrobinson): We need to handle sticky positioning here when we support it.
-        if self.style.get_box().position == ComputedPosition::Fixed {
-            return true;
-        }
-
-        // Statically positioned fragments don't establish stacking contexts if the previous
-        // conditions are not fulfilled. Furthermore, z-index doesn't apply to statically
-        // positioned fragments.
-        if self.style.get_box().position == ComputedPosition::Static {
-            return false;
-        }
-
-        // For absolutely and relatively positioned fragments we only establish a stacking
-        // context if there is a z-index set.
-        // See https://www.w3.org/TR/CSS2/visuren.html#z-index
-        !self.style.get_position().z_index.is_auto()
-    }
-
-    // Get the effective z-index of this fragment. Z-indices only apply to positioned element
-    // per CSS 2 9.9.1 (http://www.w3.org/TR/CSS2/visuren.html#z-index), so this value may differ
-    // from the value specified in the style.
-    fn effective_z_index(&self) -> i32 {
-        match self.style.get_box().position {
-            ComputedPosition::Static => {},
-            _ => return self.style.get_position().z_index.integer_or(0),
-        }
-
-        0
-    }
-
-    /// Returns true if this fragment has a transform, or perspective property set.
-    fn has_transform_or_perspective(&self) -> bool {
-        !self.style.get_box().transform.0.is_empty() ||
-            self.style.get_box().perspective != Perspective::None
-    }
-
     fn build_stacking_context_tree<'a>(
         &'a self,
         fragment: &'a Fragment,
@@ -452,8 +390,7 @@ impl BoxFragment {
         // TODO(mrobinson): Eventually this should use the spatial id of the reference
         // frame that is the parent of this one once we have full support for stacking
         // contexts and transforms.
-        builder.current_space_and_clip.spatial_id =
-            wr::SpatialId::root_reference_frame(builder.wr.pipeline_id);
+        builder.current_space_and_clip.spatial_id = builder.nearest_reference_frame;
     }
 
     fn build_scroll_frame_if_necessary(
@@ -505,7 +442,7 @@ impl BoxFragment {
         builder: &mut DisplayListBuilder,
         border_rect: &PhysicalRect<Length>,
     ) -> bool {
-        if !self.has_transform_or_perspective() {
+        if !self.style.has_transform_or_perspective() {
             return false;
         }
         let untyped_border_rect = border_rect.to_untyped();
@@ -535,6 +472,7 @@ impl BoxFragment {
             wr::PropertyBinding::Value(reference_frame_transform),
             reference_frame_kind,
         );
+        builder.nearest_reference_frame = builder.current_space_and_clip.spatial_id;
         true
     }
 

components/layout_2020/flow/mod.rs

@@ -223,7 +223,8 @@ fn layout_block_level_children<'a>(
                 })
                 .collect()
         } else {
-            let has_positioned_ancestor = positioning_context.has_positioned_ancestor();
+            let collects_for_nearest_positioned_ancestor =
+                positioning_context.collects_for_nearest_positioned_ancestor();
             let mut fragments = child_boxes
                 .par_iter()
                 .enumerate()
@@ -238,7 +239,7 @@ fn layout_block_level_children<'a>(
                             /* float_context = */ None,
                         )
                     },
-                    || PositioningContext::new_for_rayon(has_positioned_ancestor),
+                    || PositioningContext::new_for_rayon(collects_for_nearest_positioned_ancestor),
                     PositioningContext::append,
                 )
                 .collect();
@@ -275,7 +276,7 @@ impl BlockLevelBox {
                 tag,
                 style,
                 contents,
-            } => Fragment::Box(positioning_context.for_maybe_position_relative(
+            } => Fragment::Box(positioning_context.layout_maybe_position_relative_fragment(
                 layout_context,
                 containing_block,
                 style,
@@ -293,7 +294,7 @@ impl BlockLevelBox {
                 },
             )),
             BlockLevelBox::Independent(contents) => {
-                Fragment::Box(positioning_context.for_maybe_position_relative(
+                Fragment::Box(positioning_context.layout_maybe_position_relative_fragment(
                     layout_context,
                     containing_block,
                     &contents.style,
@@ -326,8 +327,7 @@ impl BlockLevelBox {
                 ))
             },
             BlockLevelBox::OutOfFlowFloatBox(_box_) => {
-                // FIXME: call for_maybe_position_relative here
-                // TODO
+                // FIXME: call layout_maybe_position_relative_fragment here
                 Fragment::Anonymous(AnonymousFragment::no_op(
                     containing_block.style.writing_mode,
                 ))

components/layout_2020/flow/root.rs

@@ -137,7 +137,8 @@ impl BoxTreeRoot {
         };
 
         let dummy_tree_rank = 0;
-        let mut positioning_context = PositioningContext::new_for_initial_containing_block();
+        let mut positioning_context =
+            PositioningContext::new_for_containing_block_for_all_descendants();
         let mut independent_layout = self.0.layout(
             layout_context,
             &mut positioning_context,
@@ -145,7 +146,7 @@ impl BoxTreeRoot {
             dummy_tree_rank,
         );
 
-        positioning_context.layout_in_initial_containing_block(
+        positioning_context.layout_initial_containing_block_children(
             layout_context,
             &initial_containing_block,
             &mut independent_layout.fragments,

components/layout_2020/positioned.rs

@@ -25,7 +25,12 @@ pub(crate) struct AbsolutelyPositionedBox {
 
 pub(crate) struct PositioningContext<'box_tree> {
     for_nearest_positioned_ancestor: Option<Vec<HoistedAbsolutelyPositionedBox<'box_tree>>>,
-    for_initial_containing_block: Vec<HoistedAbsolutelyPositionedBox<'box_tree>>,
+
+    // For nearest `containing block for all descendants` as defined by the CSS transforms
+    // spec.
+    // https://www.w3.org/TR/css-transforms-1/#containing-block-for-all-descendants
+    for_nearest_containing_block_for_all_descendants:
+        Vec<HoistedAbsolutelyPositionedBox<'box_tree>>,
 }
 
 #[derive(Debug)]
@@ -127,62 +132,158 @@ impl AbsolutelyPositionedBox {
 }
 
 impl<'box_tree> PositioningContext<'box_tree> {
-    pub(crate) fn new_for_initial_containing_block() -> Self {
+    pub(crate) fn new_for_containing_block_for_all_descendants() -> Self {
         Self {
             for_nearest_positioned_ancestor: None,
-            for_initial_containing_block: Vec::new(),
+            for_nearest_containing_block_for_all_descendants: Vec::new(),
         }
     }
 
-    pub(crate) fn new_for_rayon(has_positioned_ancestor: bool) -> Self {
+    pub(crate) fn new_for_rayon(collects_for_nearest_positioned_ancestor: bool) -> Self {
         Self {
-            for_nearest_positioned_ancestor: if has_positioned_ancestor {
+            for_nearest_positioned_ancestor: if collects_for_nearest_positioned_ancestor {
                 Some(Vec::new())
             } else {
                 None
             },
-            for_initial_containing_block: Vec::new(),
+            for_nearest_containing_block_for_all_descendants: Vec::new(),
         }
     }
 
-    pub(crate) fn has_positioned_ancestor(&self) -> bool {
+    pub(crate) fn collects_for_nearest_positioned_ancestor(&self) -> bool {
         self.for_nearest_positioned_ancestor.is_some()
     }
 
-    pub(crate) fn for_maybe_position_relative(
+    /// Given `fragment_layout_fn`, a closure which lays out a fragment in a provided
+    /// `PositioningContext`, create a new positioning context if necessary for the fragment and
+    /// lay out the fragment and all its children. Returns the newly created `BoxFragment`.
+    pub(crate) fn layout_maybe_position_relative_fragment(
         &mut self,
         layout_context: &LayoutContext,
         containing_block: &ContainingBlock,
         style: &ComputedValues,
-        f: impl FnOnce(&mut Self) -> BoxFragment,
+        fragment_layout_fn: impl FnOnce(&mut Self) -> BoxFragment,
     ) -> BoxFragment {
+        debug_assert!(style.clone_position() != Position::Fixed);
+        debug_assert!(style.clone_position() != Position::Absolute);
+
+        if style.establishes_containing_block_for_all_descendants() {
+            let mut fragment = Self::layout_containing_block_for_all_descendants(
+                layout_context,
+                fragment_layout_fn,
+            );
             if style.clone_position() == Position::Relative {
-            let mut fragment =
-                // Establing a containing block for absolutely positioned descendants
-                Self::for_positioned(layout_context, &mut self.for_initial_containing_block, f);
+                fragment.content_rect.start_corner +=
+                    &relative_adjustement(style, containing_block);
+            }
+            return fragment;
+        }
 
+        if style.clone_position() == Position::Relative {
+            let mut fragment = Self::create_and_layout_positioned(
+                layout_context,
+                style,
+                &mut self.for_nearest_containing_block_for_all_descendants,
+                fragment_layout_fn,
+            );
             fragment.content_rect.start_corner += &relative_adjustement(style, containing_block);
-            fragment
-        } else {
-            f(self)
-        }
+            return fragment;
         }
 
-    fn for_positioned(
+        // We don't need to create a new PositioningContext for this Fragment, so
+        // we pass in the current one to the fragment layout closure.
+        fragment_layout_fn(self)
+    }
+
+    /// Given `fragment_layout_fn`, a closure which lays out a fragment in a provided
+    /// `PositioningContext`, create a positioning context a positioned fragment and lay out the
+    /// fragment and all its children. Returns the resulting `BoxFragment`.
+    fn create_and_layout_positioned(
         layout_context: &LayoutContext,
-        for_initial_containing_block: &mut Vec<HoistedAbsolutelyPositionedBox<'box_tree>>,
-        f: impl FnOnce(&mut Self) -> BoxFragment,
+        style: &ComputedValues,
+        for_nearest_containing_block_for_all_descendants: &mut Vec<
+            HoistedAbsolutelyPositionedBox<'box_tree>,
+        >,
+        fragment_layout_fn: impl FnOnce(&mut Self) -> BoxFragment,
     ) -> BoxFragment {
+        if style.establishes_containing_block_for_all_descendants() {
+            return Self::layout_containing_block_for_all_descendants(
+                layout_context,
+                fragment_layout_fn,
+            );
+        }
+
         let mut new = Self {
             for_nearest_positioned_ancestor: Some(Vec::new()),
-            for_initial_containing_block: std::mem::take(for_initial_containing_block),
+            for_nearest_containing_block_for_all_descendants: std::mem::take(
+                for_nearest_containing_block_for_all_descendants,
+            ),
         };
-        let mut positioned_box_fragment = f(&mut new);
-        new.layout_in_positioned_ancestor(layout_context, &mut positioned_box_fragment);
-        *for_initial_containing_block = new.for_initial_containing_block;
+        let mut positioned_box_fragment = fragment_layout_fn(&mut new);
+        new.layout_positioned_fragment_children(layout_context, &mut positioned_box_fragment);
+        *for_nearest_containing_block_for_all_descendants =
+            new.for_nearest_containing_block_for_all_descendants;
         positioned_box_fragment
     }
 
+    /// Given `fragment_layout_fn`, a closure which lays out a fragment in a provided
+    /// `PositioningContext`, create a positioning context for a fragment that establishes a
+    /// containing block for all descendants and lay out the fragment and all its children using
+    /// the new positioning context. Returns the resulting `BoxFragment`.
+    fn layout_containing_block_for_all_descendants(
+        layout_context: &LayoutContext,
+        fragment_layout_fn: impl FnOnce(&mut Self) -> BoxFragment,
+    ) -> BoxFragment {
+        let mut containing_block_for_all_descendants =
+            Self::new_for_containing_block_for_all_descendants();
+        debug_assert!(containing_block_for_all_descendants
+            .for_nearest_positioned_ancestor
+            .is_none());
+
+        let mut new_fragment = fragment_layout_fn(&mut containing_block_for_all_descendants);
+
+        let padding_rect = Rect {
+            size: new_fragment.content_rect.size.clone(),
+            // Ignore the content rect’s position in its own containing block:
+            start_corner: Vec2::zero(),
+        }
+        .inflate(&new_fragment.padding);
+        let containing_block = DefiniteContainingBlock {
+            size: padding_rect.size.clone(),
+            style: &new_fragment.style,
+        };
+
+        // Loop because it’s possible that we discover (the static position of)
+        // more absolutely-positioned boxes while doing layout for others.
+        let mut new_child_fragments = Vec::new();
+        while !containing_block_for_all_descendants
+            .for_nearest_containing_block_for_all_descendants
+            .is_empty()
+        {
+            HoistedAbsolutelyPositionedBox::layout_many(
+                layout_context,
+                &std::mem::take(
+                    &mut containing_block_for_all_descendants
+                        .for_nearest_containing_block_for_all_descendants,
+                ),
+                &mut new_child_fragments,
+                &mut containing_block_for_all_descendants
+                    .for_nearest_containing_block_for_all_descendants,
+                &containing_block,
+            )
+        }
+
+        new_fragment
+            .children
+            .push(Fragment::Anonymous(AnonymousFragment::new(
+                padding_rect,
+                new_child_fragments,
+                new_fragment.style.writing_mode,
+            )));
+
+        new_fragment
+    }
+
     pub(crate) fn push(&mut self, box_: HoistedAbsolutelyPositionedBox<'box_tree>) {
         if let Some(nearest) = &mut self.for_nearest_positioned_ancestor {
             match box_
@@ -196,13 +297,14 @@ impl<'box_tree> PositioningContext<'box_tree> {
                 Position::Static | Position::Relative => unreachable!(),
             }
         }
-        self.for_initial_containing_block.push(box_)
+        self.for_nearest_containing_block_for_all_descendants
+            .push(box_)
     }
 
     pub(crate) fn append(&mut self, other: Self) {
         vec_append_owned(
-            &mut self.for_initial_containing_block,
-            other.for_initial_containing_block,
+            &mut self.for_nearest_containing_block_for_all_descendants,
+            other.for_nearest_containing_block_for_all_descendants,
         );
         match (
             self.for_nearest_positioned_ancestor.as_mut(),
@@ -219,7 +321,8 @@ impl<'box_tree> PositioningContext<'box_tree> {
         tree_rank_in_parent: usize,
         f: impl FnOnce(&mut Self) -> Vec<Fragment>,
     ) -> Vec<Fragment> {
-        let for_icb_so_far = self.for_initial_containing_block.len();
+        let for_containing_block_for_all_descendants =
+            self.for_nearest_containing_block_for_all_descendants.len();
         let for_nearest_so_far = self
             .for_nearest_positioned_ancestor
             .as_ref()
@@ -228,7 +331,8 @@ impl<'box_tree> PositioningContext<'box_tree> {
         let fragments = f(self);
 
         adjust_static_positions(
-            &mut self.for_initial_containing_block[for_icb_so_far..],
+            &mut self.for_nearest_containing_block_for_all_descendants
+                [for_containing_block_for_all_descendants..],
             &fragments,
             tree_rank_in_parent,
         );
@@ -242,7 +346,7 @@ impl<'box_tree> PositioningContext<'box_tree> {
         fragments
     }
 
-    pub(crate) fn layout_in_initial_containing_block(
+    pub(crate) fn layout_initial_containing_block_children(
         &mut self,
         layout_context: &LayoutContext,
         initial_containing_block: &DefiniteContainingBlock,
@@ -252,18 +356,21 @@ impl<'box_tree> PositioningContext<'box_tree> {
 
         // Loop because it’s possible that we discover (the static position of)
         // more absolutely-positioned boxes while doing layout for others.
-        while !self.for_initial_containing_block.is_empty() {
+        while !self
+            .for_nearest_containing_block_for_all_descendants
+            .is_empty()
+        {
             HoistedAbsolutelyPositionedBox::layout_many(
                 layout_context,
-                &std::mem::take(&mut self.for_initial_containing_block),
+                &std::mem::take(&mut self.for_nearest_containing_block_for_all_descendants),
                 fragments,
-                &mut self.for_initial_containing_block,
+                &mut self.for_nearest_containing_block_for_all_descendants,
                 initial_containing_block,
             )
         }
     }
 
-    fn layout_in_positioned_ancestor(
+    fn layout_positioned_fragment_children(
         &mut self,
         layout_context: &LayoutContext,
         positioned_box_fragment: &mut BoxFragment,
@@ -285,7 +392,7 @@ impl<'box_tree> PositioningContext<'box_tree> {
                 layout_context,
                 &for_here,
                 &mut children,
-                &mut self.for_initial_containing_block,
+                &mut self.for_nearest_containing_block_for_all_descendants,
                 &containing_block,
             );
             positioned_box_fragment
@@ -304,16 +411,18 @@ impl<'box_tree> HoistedAbsolutelyPositionedBox<'box_tree> {
         layout_context: &LayoutContext,
         boxes: &[Self],
         fragments: &mut Vec<Fragment>,
-        for_initial_containing_block: &mut Vec<HoistedAbsolutelyPositionedBox<'box_tree>>,
+        for_nearest_containing_block_for_all_descendants: &mut Vec<
+            HoistedAbsolutelyPositionedBox<'box_tree>,
+        >,
         containing_block: &DefiniteContainingBlock,
     ) {
         if layout_context.use_rayon {
             fragments.par_extend(boxes.par_iter().mapfold_reduce_into(
-                for_initial_containing_block,
-                |for_initial_containing_block, box_| {
+                for_nearest_containing_block_for_all_descendants,
+                |for_nearest_containing_block_for_all_descendants, box_| {
                     Fragment::Box(box_.layout(
                         layout_context,
-                        for_initial_containing_block,
+                        for_nearest_containing_block_for_all_descendants,
                         containing_block,
                     ))
                 },
@@ -324,7 +433,7 @@ impl<'box_tree> HoistedAbsolutelyPositionedBox<'box_tree> {
             fragments.extend(boxes.iter().map(|box_| {
                 Fragment::Box(box_.layout(
                     layout_context,
-                    for_initial_containing_block,
+                    for_nearest_containing_block_for_all_descendants,
                     containing_block,
                 ))
             }))
@@ -334,7 +443,9 @@ impl<'box_tree> HoistedAbsolutelyPositionedBox<'box_tree> {
     pub(crate) fn layout(
         &self,
         layout_context: &LayoutContext,
-        for_initial_containing_block: &mut Vec<HoistedAbsolutelyPositionedBox<'box_tree>>,
+        for_nearest_containing_block_for_all_descendants: &mut Vec<
+            HoistedAbsolutelyPositionedBox<'box_tree>,
+        >,
         containing_block: &DefiniteContainingBlock,
     ) -> BoxFragment {
         let style = &self.absolutely_positioned_box.contents.style;
@@ -396,8 +507,13 @@ impl<'box_tree> HoistedAbsolutelyPositionedBox<'box_tree> {
             block_end: block_axis.margin_end,
         };
 
-        let for_icb = for_initial_containing_block;
-        PositioningContext::for_positioned(layout_context, for_icb, |positioning_context| {
+        let for_containing_block_for_all_descendants =
+            for_nearest_containing_block_for_all_descendants;
+        PositioningContext::create_and_layout_positioned(
+            layout_context,
+            style,
+            for_containing_block_for_all_descendants,
+            |positioning_context| {
                 let size;
                 let fragments;
                 match self.absolutely_positioned_box.contents.as_replaced() {
@@ -416,7 +532,9 @@ impl<'box_tree> HoistedAbsolutelyPositionedBox<'box_tree> {
                                 Anchor::Start(start) => {
                                     cbis - start - pb.inline_sum() - margin.inline_sum()
                                 },
-                            Anchor::End(end) => cbis - end - pb.inline_sum() - margin.inline_sum(),
+                                Anchor::End(end) => {
+                                    cbis - end - pb.inline_sum() - margin.inline_sum()
+                                },
                             };
                             self.absolutely_positioned_box
                                 .contents
@@ -455,7 +573,9 @@ impl<'box_tree> HoistedAbsolutelyPositionedBox<'box_tree> {
 
                 let inline_start = match inline_axis.anchor {
                     Anchor::Start(start) => start + pb.inline_start + margin.inline_start,
-                Anchor::End(end) => cbis - end - pb.inline_end - margin.inline_end - size.inline,
+                    Anchor::End(end) => {
+                        cbis - end - pb.inline_end - margin.inline_end - size.inline
+                    },
                 };
                 let block_start = match block_axis.anchor {
                     Anchor::Start(start) => start + pb.block_start + margin.block_start,
@@ -480,7 +600,8 @@ impl<'box_tree> HoistedAbsolutelyPositionedBox<'box_tree> {
                     margin,
                     CollapsedBlockMargins::zero(),
                 )
-        })
+            },
+        )
     }
 }
 

components/layout_2020/style_ext.rs

@@ -3,9 +3,13 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
 use crate::geom::{flow_relative, PhysicalSides, PhysicalSize};
+use style::computed_values::mix_blend_mode::T as ComputedMixBlendMode;
+use style::computed_values::position::T as ComputedPosition;
+use style::computed_values::transform_style::T as ComputedTransformStyle;
 use style::properties::ComputedValues;
 use style::values::computed::{Length, LengthPercentage, LengthPercentageOrAuto};
 use style::values::computed::{NonNegativeLengthPercentage, Size};
+use style::values::generics::box_::Perspective;
 use style::values::generics::length::MaxSize;
 use style::values::specified::box_ as stylo;
 
@@ -49,6 +53,10 @@ pub(crate) trait ComputedValuesExt {
     fn padding(&self) -> flow_relative::Sides<LengthPercentage>;
     fn border_width(&self) -> flow_relative::Sides<Length>;
     fn margin(&self) -> flow_relative::Sides<LengthPercentageOrAuto>;
+    fn has_transform_or_perspective(&self) -> bool;
+    fn effective_z_index(&self) -> i32;
+    fn establishes_stacking_context(&self) -> bool;
+    fn establishes_containing_block_for_all_descendants(&self) -> bool;
 }
 
 impl ComputedValuesExt for ComputedValues {
@@ -165,6 +173,80 @@ impl ComputedValuesExt for ComputedValues {
             self.writing_mode,
         )
     }
+
+    /// Returns true if this style has a transform, or perspective property set.
+    fn has_transform_or_perspective(&self) -> bool {
+        !self.get_box().transform.0.is_empty() || self.get_box().perspective != Perspective::None
+    }
+
+    /// Get the effective z-index of this fragment. Z-indices only apply to positioned elements
+    /// per CSS 2 9.9.1 (http://www.w3.org/TR/CSS2/visuren.html#z-index), so this value may differ
+    /// from the value specified in the style.
+    fn effective_z_index(&self) -> i32 {
+        match self.get_box().position {
+            ComputedPosition::Static => 0,
+            _ => self.get_position().z_index.integer_or(0),
+        }
+    }
+
+    /// Returns true if this fragment establishes a new stacking context and false otherwise.
+    fn establishes_stacking_context(&self) -> bool {
+        let effects = self.get_effects();
+        if effects.opacity != 1.0 {
+            return true;
+        }
+
+        if effects.mix_blend_mode != ComputedMixBlendMode::Normal {
+            return true;
+        }
+
+        if self.has_transform_or_perspective() {
+            return true;
+        }
+
+        if !self.get_effects().filter.0.is_empty() {
+            return true;
+        }
+
+        if self.get_box().transform_style == ComputedTransformStyle::Preserve3d ||
+            self.overrides_transform_style()
+        {
+            return true;
+        }
+
+        // Fixed position and sticky position always create stacking contexts.
+        // TODO(mrobinson): We need to handle sticky positioning here when we support it.
+        if self.get_box().position == ComputedPosition::Fixed {
+            return true;
+        }
+
+        // Statically positioned fragments don't establish stacking contexts if the previous
+        // conditions are not fulfilled. Furthermore, z-index doesn't apply to statically
+        // positioned fragments.
+        if self.get_box().position == ComputedPosition::Static {
+            return false;
+        }
+
+        // For absolutely and relatively positioned fragments we only establish a stacking
+        // context if there is a z-index set.
+        // See https://www.w3.org/TR/CSS2/visuren.html#z-index
+        !self.get_position().z_index.is_auto()
+    }
+
+    /// Returns true if this style establishes a containing block for all descendants
+    /// including fixed and absolutely positioned ones.
+    fn establishes_containing_block_for_all_descendants(&self) -> bool {
+        if self.has_transform_or_perspective() {
+            return true;
+        }
+
+        if !self.get_effects().filter.0.is_empty() {
+            return true;
+        }
+
+        // TODO: We need to handle CSS Contain here.
+        false
+    }
 }
 
 impl From<stylo::Display> for Display {

tests/wpt/metadata-layout-2020/css/css-transforms/preserve3d-button.html.ini

@@ -1,2 +0,0 @@
-[preserve3d-button.html]
-  expected: FAIL

tests/wpt/metadata-layout-2020/css/css-transforms/transform-abspos-001.html.ini

@@ -1,2 +0,0 @@
-[transform-abspos-001.html]
-  expected: FAIL

tests/wpt/metadata-layout-2020/css/css-transforms/transform-abspos-002.html.ini

@@ -1,2 +0,0 @@
-[transform-abspos-002.html]
-  expected: FAIL

tests/wpt/metadata-layout-2020/css/css-transforms/transform-abspos-003.html.ini

@@ -1,2 +0,0 @@
-[transform-abspos-003.html]
-  expected: FAIL

tests/wpt/metadata-layout-2020/css/css-transforms/transform-abspos-004.html.ini

@@ -1,2 +0,0 @@
-[transform-abspos-004.html]
-  expected: FAIL

tests/wpt/metadata-layout-2020/css/css-transforms/transform-abspos-006.html.ini

@@ -1,2 +0,0 @@
-[transform-abspos-006.html]
-  expected: FAIL

tests/wpt/metadata-layout-2020/css/css-transforms/transform-abspos-007.html.ini

@@ -1,2 +0,0 @@
-[transform-abspos-007.html]
-  expected: FAIL

tests/wpt/metadata-layout-2020/css/filter-effects/filtered-inline-is-container.html.ini

@@ -0,0 +1,2 @@
+[filtered-inline-is-container.html]
+  expected: FAIL

tests/wpt/mozilla/meta-layout-2020/css/transform_stacking_context_a.html.ini

@@ -1,2 +0,0 @@
-[transform_stacking_context_a.html]
-  expected: FAIL

tests/wpt/mozilla/meta-layout-2020/css/translate_clip_nested.html.ini

@@ -1,2 +0,0 @@
-[translate_clip_nested.html]
-  expected: FAIL