layout: Share styles to inline box children via `SharedInlineStyles` (#36896)

`TextRun`s use their parent style to render. Previously, these styles
were cloned and stored directly in the box tree `TextRun` and resulting
`TextFragment`s. This presents a problem for incremental layout.
Wrapping the style in another layer of shared ownership and mutability
will allow updating all `TextFragment`s during repaint-only incremental
layout by simply updating the box tree styles of the original text
parents.

This adds a new set of borrows when accessing text styles, but also
makes it so that during box tree block construction
`InlineFormattingContext`s are created lazily and now
`InlineFormattingContextBuilder::finish` consumes the builder, making
the API make a bit more sense. This should also improve performance of
box tree block construction slightly.

Testing: This should not change observable behavior and thus is covered
by existing WPT tests.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Oriol Brufau <obrufau@igalia.com>

1 files changed, 11 insertions, 0 deletions

diff --git a/components/layout/lib.rs b/components/layout/lib.rs
index af7d432c4d8..cd992387277 100644
--- a/components/layout/lib.rs
+++ b/components/layout/lib.rs
@@ -38,6 +38,7 @@ pub use flow::BoxTree;
 pub use fragment_tree::FragmentTree;
 pub use layout_impl::LayoutFactoryImpl;
 use malloc_size_of_derive::MallocSizeOf;
+use servo_arc::Arc as ServoArc;
 use style::logical_geometry::WritingMode;
 use style::properties::ComputedValues;
 use style::values::computed::TextDecorationLine;
@@ -45,6 +46,16 @@ use style::values::computed::TextDecorationLine;
 use crate::geom::{LogicalVec2, SizeConstraint};
 use crate::style_ext::AspectRatio;
 
+/// At times, a style is "owned" by more than one layout object. For example, text
+/// fragments need a handle on their parent inline box's style. In order to make
+/// incremental layout easier to implement, another layer of shared ownership is added via
+/// [`SharedStyle`]. This allows updating the style in originating layout object and
+/// having all "depdendent" objects update automatically.
+///
+///  Note that this is not a cost-free data structure, so should only be
+/// used when necessary.
+pub(crate) type SharedStyle = ArcRefCell<ServoArc<ComputedValues>>;
+
 /// Represents the set of constraints that we use when computing the min-content
 /// and max-content inline sizes of an element.
 pub(crate) struct ConstraintSpace {