layout: Simplify PositioningContext by having it hold a single Vec (#36795)

`PositioningContext` held two vectors, one inside an `Option`, to
differentiate between the version used for a containing block for all
descendants (including `position: absolute` and `position: fixed`) or
only for `position: absolute` descendants. This distinction was really
hard to reason about and required a lot of bookkeeping about what kind
of `PositioningContext` a layout box's parent expected. In addition, it
led to a lot of mistakes.

This change simplifies things so that `PositioningContext` only holds a
single vector. When it comes time to lay out hoisted absolutely
positioned
fragments, the code then:
 - lays out all of them (in the case of a `PositioningContext` for all
   descendants), or
 - only lays out the `position: absolute` descendants and preserves the
   `position: fixed` descendants (in the case the `PositioningContext`
   is only for `position: absolute`.), or
- lays out none of them if the `PositioningContext` was created for
  box that did not establish a containing block for absolutes.

It's possible that this way of dealing with hoisted absolutes is a bit
less efficient, but, the number of these descendants is typically quite
small, so it should not be significant. In addition, this decreases the
size in memory of all `PositioningContexts` which are created in more
situations as time goes on.

Testing: There is a new WPT test with this change.
Fixes: #36696.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/layout/flexbox/layout.rs

@@ -49,7 +49,6 @@ use crate::{
 struct FlexContext<'a> {
     config: FlexContainerConfig,
     layout_context: &'a LayoutContext<'a>,
-    positioning_context: &'a mut PositioningContext,
     containing_block: &'a ContainingBlock<'a>, // For items
     container_inner_size_constraint: FlexRelativeVec2<SizeConstraint>,
 }
@@ -657,7 +656,6 @@ impl FlexContainer {
         let mut flex_context = FlexContext {
             config: self.config.clone(),
             layout_context,
-            positioning_context,
             containing_block,
             // https://drafts.csswg.org/css-flexbox/#definite-sizes
             container_inner_size_constraint: self.config.flex_axis.vec2_to_flex_relative(
@@ -1775,16 +1773,7 @@ impl FlexItem<'_> {
     ) -> Option<FlexItemLayoutResult> {
         let containing_block = flex_context.containing_block;
         let independent_formatting_context = &self.box_.independent_formatting_context;
-        let mut positioning_context = independent_formatting_context
-            .new_positioning_context()
-            .unwrap_or_else(|| {
-                PositioningContext::new_for_subtree(
-                    flex_context
-                        .positioning_context
-                        .collects_for_nearest_positioned_ancestor(),
-                )
-            });
-
+        let mut positioning_context = PositioningContext::default();
         let item_writing_mode = independent_formatting_context.style().writing_mode;
         let item_is_horizontal = item_writing_mode.is_horizontal();
         let flex_axis = flex_context.config.flex_axis;
@@ -2617,17 +2606,7 @@ impl FlexItemBox {
         cross_size_stretches_to_container_size: bool,
         intrinsic_sizing_mode: IntrinsicSizingMode,
     ) -> Au {
-        let mut positioning_context = self
-            .independent_formatting_context
-            .new_positioning_context()
-            .unwrap_or_else(|| {
-                PositioningContext::new_for_subtree(
-                    flex_context
-                        .positioning_context
-                        .collects_for_nearest_positioned_ancestor(),
-                )
-            });
-
+        let mut positioning_context = PositioningContext::default();
         let style = self.independent_formatting_context.style();
         match &self.independent_formatting_context.contents {
             IndependentFormattingContextContents::Replaced(replaced) => {