layout: Split overflow calculation after fragment tree construction (#37203)

Instead of computing scrollable overflow while constructing the fragment
tree, we will now do it later. In the future this will also allow to
only recalculate the overflow without rebuilding the tree when transform
properties change, but that's left for a follow-up.

Stylo PR: https://github.com/servo/stylo/pull/194

Testing: One test is now passing (more investigation is needed), but
otherwise this isn't expected to have any effect.

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

components/layout/fragment_tree/box_fragment.rs

@@ -89,7 +89,7 @@ pub(crate) struct BoxFragment {
     block_margins_collapsed_with_children: Option<Box<CollapsedBlockMargins>>,
 
     /// The scrollable overflow of this box fragment.
-    pub scrollable_overflow_from_children: PhysicalRect<Au>,
+    scrollable_overflow: Option<PhysicalRect<Au>>,
 
     /// The resolved box insets if this box is `position: sticky`. These are calculated
     /// during `StackingContextTree` construction because they rely on the size of the
@@ -114,11 +114,6 @@ impl BoxFragment {
         margin: PhysicalSides<Au>,
         clearance: Option<Au>,
     ) -> BoxFragment {
-        let scrollable_overflow_from_children =
-            children.iter().fold(PhysicalRect::zero(), |acc, child| {
-                acc.union(&child.scrollable_overflow_for_parent())
-            });
-
         BoxFragment {
             base: base_fragment_info.into(),
             style,
@@ -131,7 +126,7 @@ impl BoxFragment {
             clearance,
             baselines: Baselines::default(),
             block_margins_collapsed_with_children: None,
-            scrollable_overflow_from_children,
+            scrollable_overflow: None,
             resolved_sticky_insets: AtomicRefCell::default(),
             background_mode: BackgroundMode::Normal,
             specific_layout_info: None,
@@ -203,13 +198,23 @@ impl BoxFragment {
     /// Get the scrollable overflow for this [`BoxFragment`] relative to its
     /// containing block.
     pub fn scrollable_overflow(&self) -> PhysicalRect<Au> {
+        self.scrollable_overflow
+            .expect("Should only call `scrollable_overflow()` after calculating overflow")
+    }
+
+    pub(crate) fn calculate_scrollable_overflow(&mut self) {
+        let scrollable_overflow_from_children = self
+            .children
+            .iter()
+            .fold(PhysicalRect::zero(), |acc, child| {
+                acc.union(&child.calculate_scrollable_overflow_for_parent())
+            });
         let physical_padding_rect = self.padding_rect();
         let content_origin = self.content_rect.origin.to_vector();
-        physical_padding_rect.union(
-            &self
-                .scrollable_overflow_from_children
-                .translate(content_origin),
-        )
+        self.scrollable_overflow = Some(
+            physical_padding_rect
+                .union(&scrollable_overflow_from_children.translate(content_origin)),
+        );
     }
 
     pub(crate) fn set_containing_block(&mut self, containing_block: &PhysicalRect<Au>) {
@@ -275,7 +280,12 @@ impl BoxFragment {
         tree.end_level();
     }
 
-    pub fn scrollable_overflow_for_parent(&self) -> PhysicalRect<Au> {
+    pub(crate) fn scrollable_overflow_for_parent(&self) -> PhysicalRect<Au> {
+        // TODO: Properly handle absolutely positioned fragments.
+        if self.style.get_box().position.is_absolutely_positioned() {
+            return PhysicalRect::zero();
+        }
+
         let mut overflow = self.border_rect();
         if !self.style.establishes_scroll_container(self.base.flags) {
             // https://www.w3.org/TR/css-overflow-3/#scrollable
@@ -328,7 +338,7 @@ impl BoxFragment {
     ///
     /// Return the clipped the scrollable overflow based on its scroll origin, determined by overflow direction.
     /// For an element, the clip rect is the padding rect and for viewport, it is the initial containing block.
-    pub fn clip_unreachable_scrollable_overflow_region(
+    pub(crate) fn clip_unreachable_scrollable_overflow_region(
         &self,
         scrollable_overflow: PhysicalRect<Au>,
         clipping_rect: PhysicalRect<Au>,
@@ -362,7 +372,7 @@ impl BoxFragment {
     ///
     /// Return the clipped the scrollable overflow based on its scroll origin, determined by overflow direction.
     /// This will coincides with the scrollport if the fragment is a scroll container.
-    pub fn reachable_scrollable_overflow_region(&self) -> PhysicalRect<Au> {
+    pub(crate) fn reachable_scrollable_overflow_region(&self) -> PhysicalRect<Au> {
         self.clip_unreachable_scrollable_overflow_region(
             self.scrollable_overflow(),
             self.padding_rect(),
@@ -421,9 +431,7 @@ impl BoxFragment {
             return convert_to_au_or_auto(PhysicalSides::new(top, right, bottom, left));
         }
 
-        debug_assert!(
-            position == ComputedPosition::Fixed || position == ComputedPosition::Absolute
-        );
+        debug_assert!(position.is_absolutely_positioned());
 
         let margin_rect = self.margin_rect();
         let (top, bottom) = match (&insets.top, &insets.bottom) {

components/layout/fragment_tree/fragment.rs

@@ -183,19 +183,36 @@ impl Fragment {
         }
     }
 
-    pub fn scrollable_overflow_for_parent(&self) -> PhysicalRect<Au> {
+    pub(crate) fn scrollable_overflow_for_parent(&self) -> PhysicalRect<Au> {
         match self {
             Fragment::Box(fragment) | Fragment::Float(fragment) => {
-                fragment.borrow().scrollable_overflow_for_parent()
+                return fragment.borrow().scrollable_overflow_for_parent();
             },
             Fragment::AbsoluteOrFixedPositioned(_) => PhysicalRect::zero(),
-            Fragment::Positioning(fragment) => fragment.borrow().scrollable_overflow,
+            Fragment::Positioning(fragment) => fragment.borrow().scrollable_overflow_for_parent(),
             Fragment::Text(fragment) => fragment.borrow().rect,
             Fragment::Image(fragment) => fragment.borrow().rect,
             Fragment::IFrame(fragment) => fragment.borrow().rect,
         }
     }
 
+    pub(crate) fn calculate_scrollable_overflow_for_parent(&self) -> PhysicalRect<Au> {
+        self.calculate_scrollable_overflow();
+        self.scrollable_overflow_for_parent()
+    }
+
+    pub(crate) fn calculate_scrollable_overflow(&self) {
+        match self {
+            Fragment::Box(fragment) | Fragment::Float(fragment) => {
+                fragment.borrow_mut().calculate_scrollable_overflow()
+            },
+            Fragment::Positioning(fragment) => {
+                fragment.borrow_mut().calculate_scrollable_overflow()
+            },
+            _ => {},
+        }
+    }
+
     pub(crate) fn cumulative_border_box_rect(&self) -> Option<PhysicalRect<Au>> {
         match self {
             Fragment::Box(fragment) | Fragment::Float(fragment) => {

components/layout/fragment_tree/fragment_tree.rs

@@ -2,14 +2,14 @@
  * 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 std::cell::Cell;
+
 use app_units::Au;
 use base::print_tree::PrintTree;
 use compositing_traits::display_list::AxesScrollSensitivity;
-use euclid::default::Size2D;
 use fxhash::FxHashSet;
 use malloc_size_of_derive::MallocSizeOf;
 use style::animation::AnimationSetKey;
-use webrender_api::units;
 
 use super::{BoxFragment, ContainingBlockManager, Fragment};
 use crate::ArcRefCell;
@@ -30,7 +30,7 @@ pub struct FragmentTree {
 
     /// The scrollable overflow rectangle for the entire tree
     /// <https://drafts.csswg.org/css-overflow/#scrollable>
-    pub(crate) scrollable_overflow: PhysicalRect<Au>,
+    scrollable_overflow: Cell<Option<PhysicalRect<Au>>>,
 
     /// The containing block used in the layout of this fragment tree.
     pub(crate) initial_containing_block: PhysicalRect<Au>,
@@ -43,13 +43,12 @@ impl FragmentTree {
     pub(crate) fn new(
         layout_context: &LayoutContext,
         root_fragments: Vec<Fragment>,
-        scrollable_overflow: PhysicalRect<Au>,
         initial_containing_block: PhysicalRect<Au>,
         viewport_scroll_sensitivity: AxesScrollSensitivity,
     ) -> Self {
         let fragment_tree = Self {
             root_fragments,
-            scrollable_overflow,
+            scrollable_overflow: Cell::default(),
             initial_containing_block,
             viewport_scroll_sensitivity,
         };
@@ -97,11 +96,35 @@ impl FragmentTree {
         }
     }
 
-    pub fn scrollable_overflow(&self) -> units::LayoutSize {
-        units::LayoutSize::from_untyped(Size2D::new(
-            self.scrollable_overflow.size.width.to_f32_px(),
-            self.scrollable_overflow.size.height.to_f32_px(),
-        ))
+    pub(crate) fn scrollable_overflow(&self) -> PhysicalRect<Au> {
+        self.scrollable_overflow
+            .get()
+            .expect("Should only call `scrollable_overflow()` after calculating overflow")
+    }
+
+    pub(crate) fn calculate_scrollable_overflow(&self) {
+        self.scrollable_overflow
+            .set(Some(self.root_fragments.iter().fold(
+                PhysicalRect::zero(),
+                |acc, child| {
+                    let child_overflow = child.calculate_scrollable_overflow_for_parent();
+
+                    // https://drafts.csswg.org/css-overflow/#scrolling-direction
+                    // We want to clip scrollable overflow on box-start and inline-start
+                    // sides of the scroll container.
+                    //
+                    // FIXME(mrobinson, bug 25564): This should take into account writing
+                    // mode.
+                    let child_overflow = PhysicalRect::new(
+                        euclid::Point2D::zero(),
+                        euclid::Size2D::new(
+                            child_overflow.size.width + child_overflow.origin.x,
+                            child_overflow.size.height + child_overflow.origin.y,
+                        ),
+                    );
+                    acc.union(&child_overflow)
+                },
+            )));
     }
 
     pub(crate) fn find<T>(

components/layout/fragment_tree/positioning_fragment.rs

@@ -22,7 +22,7 @@ pub(crate) struct PositioningFragment {
     pub children: Vec<Fragment>,
 
     /// The scrollable overflow of this anonymous fragment's children.
-    pub scrollable_overflow: PhysicalRect<Au>,
+    scrollable_overflow: Option<PhysicalRect<Au>>,
 
     /// The style of the fragment.
     pub style: ServoArc<ComputedValues>,
@@ -55,20 +55,12 @@ impl PositioningFragment {
         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_for_parent()
-                    .translate(content_origin.to_vector()),
-            )
-        });
         ArcRefCell::new(PositioningFragment {
             base,
             style,
             rect,
             children,
-            scrollable_overflow,
+            scrollable_overflow: None,
             cumulative_containing_block_rect: PhysicalRect::zero(),
         })
     }
@@ -81,6 +73,25 @@ impl PositioningFragment {
         rect.translate(self.cumulative_containing_block_rect.origin.to_vector())
     }
 
+    pub(crate) fn calculate_scrollable_overflow(&mut self) {
+        self.scrollable_overflow = Some(self.children.iter().fold(
+            PhysicalRect::zero(),
+            |acc, child| {
+                acc.union(
+                    &child
+                        .calculate_scrollable_overflow_for_parent()
+                        .translate(self.rect.origin.to_vector()),
+                )
+            },
+        ));
+    }
+
+    pub(crate) fn scrollable_overflow_for_parent(&self) -> PhysicalRect<Au> {
+        self.scrollable_overflow.expect(
+            "Should only call `scrollable_overflow_for_parent()` after calculating overflow",
+        )
+    }
+
     pub fn print(&self, tree: &mut PrintTree) {
         tree.new_level(format!(
             "PositioningFragment\