layout: Combine layout_2020 and layout_thread_2020 into a crate called layout (#36613)

Now that legacy layout has been removed, the name `layout_2020` doesn't
make much sense any longer, also it's 2025 now for better or worse. The
split between the "layout thread" and "layout" also doesn't make as much
sense since layout doesn't run on it's own thread. There's a possibility
that it will in the future, but that should be something that the user
of the crate controls rather than layout iself.

This is part of the larger layout interface cleanup and optimization
that
@Looriool and I are doing.

Testing: Covered by existing tests as this is just code movement.

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

components/layout/fragment_tree/positioning_fragment.rs

@@ -0,0 +1,81 @@
+/* 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 https://mozilla.org/MPL/2.0/. */
+
+use app_units::Au;
+use base::print_tree::PrintTree;
+use malloc_size_of_derive::MallocSizeOf;
+use servo_arc::Arc as ServoArc;
+use style::properties::ComputedValues;
+
+use super::{BaseFragment, BaseFragmentInfo, Fragment};
+use crate::cell::ArcRefCell;
+use crate::geom::PhysicalRect;
+
+/// Can contain child fragments with relative coordinates, but does not contribute to painting
+/// itself. [`PositioningFragment`]s may be completely anonymous, or just non-painting Fragments
+/// generated by boxes.
+#[derive(MallocSizeOf)]
+pub(crate) struct PositioningFragment {
+    pub base: BaseFragment,
+    pub rect: PhysicalRect<Au>,
+    pub children: Vec<Fragment>,
+    /// The scrollable overflow of this anonymous fragment's children.
+    pub scrollable_overflow: PhysicalRect<Au>,
+
+    /// If this fragment was created with a style, the style of the fragment.
+    #[conditional_malloc_size_of]
+    pub style: Option<ServoArc<ComputedValues>>,
+}
+
+impl PositioningFragment {
+    pub fn new_anonymous(rect: PhysicalRect<Au>, children: Vec<Fragment>) -> ArcRefCell<Self> {
+        Self::new_with_base_fragment(BaseFragment::anonymous(), None, rect, children)
+    }
+
+    pub fn new_empty(
+        base_fragment_info: BaseFragmentInfo,
+        rect: PhysicalRect<Au>,
+        style: ServoArc<ComputedValues>,
+    ) -> ArcRefCell<Self> {
+        Self::new_with_base_fragment(base_fragment_info.into(), Some(style), rect, Vec::new())
+    }
+
+    fn new_with_base_fragment(
+        base: BaseFragment,
+        style: Option<ServoArc<ComputedValues>>,
+        rect: PhysicalRect<Au>,
+        children: Vec<Fragment>,
+    ) -> ArcRefCell<Self> {
+        let content_origin = rect.origin;
+        let scrollable_overflow = children.iter().fold(PhysicalRect::zero(), |acc, child| {
+            acc.union(
+                &child
+                    .scrollable_overflow()
+                    .translate(content_origin.to_vector()),
+            )
+        });
+        ArcRefCell::new(PositioningFragment {
+            base,
+            style,
+            rect,
+            children,
+            scrollable_overflow,
+        })
+    }
+
+    pub fn print(&self, tree: &mut PrintTree) {
+        tree.new_level(format!(
+            "PositioningFragment\
+                \nbase={:?}\
+                \nrect={:?}\
+                \nscrollable_overflow={:?}",
+            self.base, self.rect, self.scrollable_overflow
+        ));
+
+        for child in &self.children {
+            child.print(tree);
+        }
+        tree.end_level();
+    }
+}