Add initial support for scrollable overflow in layout_2020

This still isn't totally correct and non-root scrolling is not handled
at all, but the root frame now scrolls.

components/layout_2020/flow/inline.rs

@@ -339,11 +339,12 @@ impl Lines {
             block: line_block_size,
         };
         self.next_line_block_position += size.block;
-        self.fragments.push(Fragment::Anonymous(AnonymousFragment {
+        self.fragments
-            children: line_contents,
+            .push(Fragment::Anonymous(AnonymousFragment::new(
-            rect: Rect { start_corner, size },
+                Rect { start_corner, size },
-            mode: containing_block.style.writing_mode,
+                line_contents,
-        }))
+                containing_block.style.writing_mode,
+            )))
     }
 }
 
@@ -402,22 +403,24 @@ impl<'box_tree> PartialInlineBoxFragment<'box_tree> {
         inline_position: &mut Length,
         at_line_break: bool,
     ) {
-        let mut fragment = BoxFragment {
+        let content_rect = Rect {
-            tag: self.tag,
-            style: self.style.clone(),
-            children: std::mem::take(&mut nesting_level.fragments_so_far),
-            content_rect: Rect {
             size: Vec2 {
                 inline: *inline_position - self.start_corner.inline,
                 block: nesting_level.max_block_size_of_fragments_so_far,
             },
             start_corner: self.start_corner.clone(),
-            },
-            padding: self.padding.clone(),
-            border: self.border.clone(),
-            margin: self.margin.clone(),
-            block_margins_collapsed_with_children: CollapsedBlockMargins::zero(),
         };
+
+        let mut fragment = BoxFragment::new(
+            self.tag,
+            self.style.clone(),
+            std::mem::take(&mut nesting_level.fragments_so_far),
+            content_rect,
+            self.padding.clone(),
+            self.border.clone(),
+            self.margin.clone(),
+            CollapsedBlockMargins::zero(),
+        );
         let last_fragment = self.last_box_tree_fragment && !at_line_break;
         if last_fragment {
             *inline_position += fragment.padding.inline_end +
@@ -470,16 +473,16 @@ fn layout_atomic<'box_tree>(
             let size = replaced.used_size_as_if_inline_element(ifc.containing_block, &atomic.style);
             let fragments = replaced.make_fragments(&atomic.style, size.clone());
             let content_rect = Rect { start_corner, size };
-            BoxFragment {
+            BoxFragment::new(
-                tag: atomic.tag,
+                atomic.tag,
-                style: atomic.style.clone(),
+                atomic.style.clone(),
-                children: fragments,
+                fragments,
                 content_rect,
                 padding,
                 border,
                 margin,
-                block_margins_collapsed_with_children: CollapsedBlockMargins::zero(),
+                CollapsedBlockMargins::zero(),
-            }
+            )
         },
         Err(non_replaced) => {
             let box_size = atomic.style.box_size();
@@ -545,16 +548,16 @@ fn layout_atomic<'box_tree>(
                     inline: inline_size,
                 },
             };
-            BoxFragment {
+            BoxFragment::new(
-                tag: atomic.tag,
+                atomic.tag,
-                style: atomic.style.clone(),
+                atomic.style.clone(),
-                children: independent_layout.fragments,
+                independent_layout.fragments,
                 content_rect,
                 padding,
                 border,
                 margin,
-                block_margins_collapsed_with_children: CollapsedBlockMargins::zero(),
+                CollapsedBlockMargins::zero(),
-            }
+            )
         },
     };
 

components/layout_2020/flow/mod.rs

@@ -495,16 +495,16 @@ fn layout_in_flow_non_replaced_block_level<'a>(
             inline: inline_size,
         },
     };
-    BoxFragment {
+    BoxFragment::new(
         tag,
-        style: style.clone(),
+        style.clone(),
-        children: fragments,
+        fragments,
         content_rect,
         padding,
         border,
         margin,
         block_margins_collapsed_with_children,
-    }
+    )
 }
 
 /// https://drafts.csswg.org/css2/visudet.html#block-replaced-width
@@ -545,16 +545,17 @@ fn layout_in_flow_replaced_block_level<'a>(
         },
         size,
     };
-    BoxFragment {
+    let block_margins_collapsed_with_children = CollapsedBlockMargins::from_margin(&margin);
+    BoxFragment::new(
         tag,
-        style: style.clone(),
+        style.clone(),
-        children: fragments,
+        fragments,
         content_rect,
         padding,
         border,
-        block_margins_collapsed_with_children: CollapsedBlockMargins::from_margin(&margin),
         margin,
-    }
+        block_margins_collapsed_with_children,
+    )
 }
 
 fn solve_inline_margins_for_in_flow_block_level(

components/layout_2020/flow/root.rs

@@ -11,12 +11,14 @@ use crate::formatting_contexts::IndependentFormattingContext;
 use crate::fragments::Fragment;
 use crate::geom;
 use crate::geom::flow_relative::Vec2;
+use crate::geom::physical;
 use crate::positioned::AbsolutelyPositionedBox;
 use crate::positioned::PositioningContext;
 use crate::replaced::ReplacedContent;
 use crate::sizing::ContentSizesRequest;
 use crate::style_ext::{Display, DisplayGeneratingBox, DisplayInside};
 use crate::DefiniteContainingBlock;
+use euclid::Size2D;
 use gfx_traits::print_tree::PrintTree;
 use script_layout_interface::wrapper_traits::LayoutNode;
 use servo_arc::Arc;
@@ -26,7 +28,14 @@ use style::Zero;
 use style_traits::CSSPixel;
 
 pub struct BoxTreeRoot(BlockFormattingContext);
-pub struct FragmentTreeRoot(Vec<Fragment>);
+
+pub struct FragmentTreeRoot {
+    /// The children of the root of the fragment tree.
+    children: Vec<Fragment>,
+
+    /// The scrollable overflow of the root of the fragment tree.
+    scrollable_overflow: physical::Rect<Length>,
+}
 
 impl BoxTreeRoot {
     pub fn construct<'dom, Node>(context: &LayoutContext, root_element: Node) -> Self
@@ -131,7 +140,35 @@ impl BoxTreeRoot {
             &mut independent_layout.fragments,
         );
 
-        FragmentTreeRoot(independent_layout.fragments)
+        // FIXME(mrobinson, bug 25564): We should be using the containing block
+        // here to properly convert scrollable overflow to physical geometry.
+        let scrollable_overflow =
+            independent_layout
+                .fragments
+                .iter()
+                .fold(physical::Rect::zero(), |acc, child| {
+                    let child_overflow = child.scrollable_overflow();
+
+                    // 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 = physical::Rect {
+                        top_left: physical::Vec2::zero(),
+                        size: physical::Vec2 {
+                            x: child_overflow.size.x + child_overflow.top_left.x,
+                            y: child_overflow.size.y + child_overflow.top_left.y,
+                        },
+                    };
+                    acc.axis_aligned_bounding_box(&child_overflow)
+                });
+
+        FragmentTreeRoot {
+            children: independent_layout.fragments,
+            scrollable_overflow,
+        }
     }
 }
 
@@ -151,15 +188,22 @@ impl FragmentTreeRoot {
                 y: Length::new(viewport_size.height),
             },
         };
-        for fragment in &self.0 {
+        for fragment in &self.children {
             fragment.build_display_list(builder, &containing_block)
         }
     }
 
     pub fn print(&self) {
         let mut print_tree = PrintTree::new("Fragment Tree".to_string());
-        for fragment in &self.0 {
+        for fragment in &self.children {
             fragment.print(&mut print_tree);
         }
     }
+
+    pub fn scrollable_overflow(&self) -> webrender_api::units::LayoutSize {
+        webrender_api::units::LayoutSize::from_untyped(Size2D::new(
+            self.scrollable_overflow.size.x.px(),
+            self.scrollable_overflow.size.y.px(),
+        ))
+    }
 }

components/layout_2020/fragments.rs

@@ -3,6 +3,7 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
 use crate::geom::flow_relative::{Rect, Sides, Vec2};
+use crate::geom::physical;
 use gfx::text::glyph::GlyphStore;
 use gfx_traits::print_tree::PrintTree;
 use servo_arc::Arc as ServoArc;
@@ -36,6 +37,9 @@ pub(crate) struct BoxFragment {
     pub margin: Sides<Length>,
 
     pub block_margins_collapsed_with_children: CollapsedBlockMargins,
+
+    /// The scrollable overflow of this box fragment.
+    pub scrollable_overflow: physical::Rect<Length>,
 }
 
 pub(crate) struct CollapsedBlockMargins {
@@ -55,6 +59,9 @@ pub(crate) struct AnonymousFragment {
     pub rect: Rect<Length>,
     pub children: Vec<Fragment>,
     pub mode: WritingMode,
+
+    /// The scrollable overflow of this anonymous fragment's children.
+    pub scrollable_overflow: physical::Rect<Length>,
 }
 
 pub(crate) struct TextFragment {
@@ -90,6 +97,21 @@ impl Fragment {
             Fragment::Image(fragment) => fragment.print(tree),
         }
     }
+
+    pub fn scrollable_overflow(&self) -> physical::Rect<Length> {
+        // FIXME(mrobinson, bug 25564): We should be using the containing block
+        // here to properly convert scrollable overflow to physical geometry.
+        match self {
+            Fragment::Box(fragment) => fragment.scrollable_overflow.clone(),
+            Fragment::Anonymous(fragment) => fragment.scrollable_overflow.clone(),
+            Fragment::Text(fragment) => fragment
+                .rect
+                .to_physical(fragment.parent_style.writing_mode, &physical::Rect::zero()),
+            Fragment::Image(fragment) => fragment
+                .rect
+                .to_physical(fragment.style.writing_mode, &physical::Rect::zero()),
+        }
+    }
 }
 
 impl AnonymousFragment {
@@ -98,6 +120,21 @@ impl AnonymousFragment {
             children: vec![],
             rect: Rect::zero(),
             mode,
+            scrollable_overflow: physical::Rect::zero(),
+        }
+    }
+
+    pub fn new(rect: Rect<Length>, children: Vec<Fragment>, mode: WritingMode) -> Self {
+        // FIXME(mrobinson, bug 25564): We should be using the containing block
+        // here to properly convert scrollable overflow to physical geometry.
+        let scrollable_overflow = children.iter().fold(physical::Rect::zero(), |acc, child| {
+            acc.axis_aligned_bounding_box(&child.scrollable_overflow())
+        });
+        AnonymousFragment {
+            rect,
+            children,
+            mode,
+            scrollable_overflow,
         }
     }
 
@@ -116,6 +153,37 @@ impl AnonymousFragment {
 }
 
 impl BoxFragment {
+    pub fn new(
+        tag: OpaqueNode,
+        style: ServoArc<ComputedValues>,
+        children: Vec<Fragment>,
+        content_rect: Rect<Length>,
+        padding: Sides<Length>,
+        border: Sides<Length>,
+        margin: Sides<Length>,
+        block_margins_collapsed_with_children: CollapsedBlockMargins,
+    ) -> BoxFragment {
+        // FIXME(mrobinson, bug 25564): We should be using the containing block
+        // here to properly convert scrollable overflow to physical geometry.
+        let scrollable_overflow = children.iter().fold(
+            content_rect
+                .inflate(&border)
+                .to_physical(style.writing_mode, &physical::Rect::zero()),
+            |acc, child| acc.axis_aligned_bounding_box(&child.scrollable_overflow()),
+        );
+        BoxFragment {
+            tag,
+            style,
+            children,
+            content_rect,
+            padding,
+            border,
+            margin,
+            block_margins_collapsed_with_children,
+            scrollable_overflow,
+        }
+    }
+
     pub fn padding_rect(&self) -> Rect<Length> {
         self.content_rect.inflate(&self.padding)
     }

components/layout_2020/geom.rs

@@ -60,6 +60,15 @@ pub(crate) mod flow_relative {
     }
 }
 
+impl<T: Zero> physical::Vec2<T> {
+    pub fn zero() -> Self {
+        Self {
+            x: T::zero(),
+            y: T::zero(),
+        }
+    }
+}
+
 impl<T: fmt::Debug> fmt::Debug for physical::Vec2<T> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         // Not using f.debug_struct on purpose here, to keep {:?} output somewhat compact
@@ -387,6 +396,33 @@ impl<T> physical::Rect<T> {
     }
 }
 
+impl physical::Rect<Length> {
+    pub fn axis_aligned_bounding_box(&self, other: &Self) -> Self {
+        let top_left = physical::Vec2 {
+            x: self.top_left.x.min(other.top_left.x),
+            y: self.top_left.y.min(other.top_left.y),
+        };
+
+        let bottom_corner_x = (self.top_left.x + self.size.x).max(other.top_left.x + other.size.x);
+        let bottom_corner_y = (self.top_left.y + self.size.y).max(other.top_left.y + other.size.y);
+        let size = physical::Vec2 {
+            x: bottom_corner_x - top_left.x,
+            y: bottom_corner_y - top_left.y,
+        };
+
+        Self { top_left, size }
+    }
+}
+
+impl<T: Zero> physical::Rect<T> {
+    pub fn zero() -> Self {
+        Self {
+            top_left: physical::Vec2::zero(),
+            size: physical::Vec2::zero(),
+        }
+    }
+}
+
 impl From<physical::Rect<Length>> for Rect<CSSPixel> {
     fn from(r: physical::Rect<Length>) -> Self {
         Rect {

components/layout_2020/positioned.rs

@@ -290,11 +290,11 @@ impl<'box_tree> PositioningContext<'box_tree> {
             );
             positioned_box_fragment
                 .children
-                .push(Fragment::Anonymous(AnonymousFragment {
+                .push(Fragment::Anonymous(AnonymousFragment::new(
+                    padding_rect,
                     children,
-                    rect: padding_rect,
+                    positioned_box_fragment.style.writing_mode,
-                    mode: positioned_box_fragment.style.writing_mode,
+                )))
-                }))
         }
     }
 }
@@ -470,16 +470,16 @@ impl<'box_tree> HoistedAbsolutelyPositionedBox<'box_tree> {
                 size,
             };
 
-            BoxFragment {
+            BoxFragment::new(
-                tag: self.absolutely_positioned_box.contents.tag,
+                self.absolutely_positioned_box.contents.tag,
-                style: style.clone(),
+                style.clone(),
-                children: fragments,
+                fragments,
                 content_rect,
                 padding,
                 border,
                 margin,
-                block_margins_collapsed_with_children: CollapsedBlockMargins::zero(),
+                CollapsedBlockMargins::zero(),
-            }
+            )
         })
     }
 }

components/layout_thread_2020/lib.rs

@@ -1374,12 +1374,16 @@ impl LayoutThread {
             document.will_paint();
         }
 
+        let mut display_list = DisplayListBuilder::new(
+            self.id.to_webrender(),
+            context,
+            fragment_tree.scrollable_overflow(),
+        );
+
         let viewport_size = webrender_api::units::LayoutSize::from_untyped(Size2D::new(
             self.viewport_size.width.to_f32_px(),
             self.viewport_size.height.to_f32_px(),
         ));
-        let mut display_list =
-            DisplayListBuilder::new(self.id.to_webrender(), context, viewport_size);
         fragment_tree.build_display_list(&mut display_list, viewport_size);
 
         if self.dump_flow_tree {