layout_2020: Paint hoisted positioned fragments in tree order

Instead of painting hoisted position fragments in the order to which
they are hoisted, paint them in tree order and properly incorporate them
into the stacking context.

We do this by creating a placeholder fragment in the original tree position
of hoisted fragments. The ghost fragment contains an atomic id which
links back to the hoisted fragment in the containing block.

While building the stacking context, we keep track of containing blocks
and their children. When encountering a placeholder fragment we look at
the containing block's hoisted children in order to properly paint the
hoisted fragment.

One notable design modification in this change is that hoisted fragments
no longer need an AnonymousFragment as their parent. Instead they are
now direct children of the fragment that establishes their containing block.

1 files changed, 42 insertions, 5 deletions

diff --git a/components/layout_2020/fragments.rs b/components/layout_2020/fragments.rs
index fcf0bf21d84..70c65dd470d 100644
--- a/components/layout_2020/fragments.rs
+++ b/components/layout_2020/fragments.rs
@@ -2,10 +2,11 @@
  * 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 crate::geom::flow_relative::{Rect, Sides, Vec2};
+use crate::geom::flow_relative::{Rect, Sides};
 use crate::geom::{PhysicalPoint, PhysicalRect};
 #[cfg(debug_assertions)]
 use crate::layout_debug;
+use crate::positioned::HoistedFragmentId;
 use gfx::text::glyph::GlyphStore;
 use gfx_traits::print_tree::PrintTree;
 #[cfg(not(debug_assertions))]
@@ -24,11 +25,15 @@ use webrender_api::{FontInstanceKey, ImageKey};
 pub(crate) enum Fragment {
     Box(BoxFragment),
     Anonymous(AnonymousFragment),
+    AbsoluteOrFixedPositioned(AbsoluteOrFixedPositionedFragment),
     Text(TextFragment),
     Image(ImageFragment),
 }
 
 #[derive(Serialize)]
+pub(crate) struct AbsoluteOrFixedPositionedFragment(pub HoistedFragmentId);
+
+#[derive(Serialize)]
 pub(crate) struct BoxFragment {
     pub tag: OpaqueNode,
     pub debug_id: DebugId,
@@ -49,6 +54,9 @@ pub(crate) struct BoxFragment {
 
     /// The scrollable overflow of this box fragment.
     pub scrollable_overflow_from_children: PhysicalRect<Length>,
+
+    /// XXX Add thsi
+    pub hoisted_fragment_id: Option<HoistedFragmentId>,
 }
 
 #[derive(Serialize)]
@@ -100,18 +108,22 @@ pub(crate) struct ImageFragment {
 }
 
 impl Fragment {
-    pub fn position_mut(&mut self) -> &mut Vec2<Length> {
-        match self {
+    pub fn offset_inline(&mut self, offset: &Length) {
+        let position = match self {
             Fragment::Box(f) => &mut f.content_rect.start_corner,
+            Fragment::AbsoluteOrFixedPositioned(_) => return,
             Fragment::Anonymous(f) => &mut f.rect.start_corner,
             Fragment::Text(f) => &mut f.rect.start_corner,
             Fragment::Image(f) => &mut f.rect.start_corner,
-        }
+        };
+
+        position.inline += *offset;
     }
 
     pub fn print(&self, tree: &mut PrintTree) {
         match self {
             Fragment::Box(fragment) => fragment.print(tree),
+            Fragment::AbsoluteOrFixedPositioned(fragment) => fragment.print(tree),
             Fragment::Anonymous(fragment) => fragment.print(tree),
             Fragment::Text(fragment) => fragment.print(tree),
             Fragment::Image(fragment) => fragment.print(tree),
@@ -124,6 +136,7 @@ impl Fragment {
         let containing_block = PhysicalRect::zero();
         match self {
             Fragment::Box(fragment) => fragment.scrollable_overflow_for_parent(&containing_block),
+            Fragment::AbsoluteOrFixedPositioned(_) => PhysicalRect::zero(),
             Fragment::Anonymous(fragment) => fragment.scrollable_overflow.clone(),
             Fragment::Text(fragment) => fragment
                 .rect
@@ -133,6 +146,26 @@ impl Fragment {
                 .to_physical(fragment.style.writing_mode, &containing_block),
         }
     }
+
+    pub fn is_hoisted(&self) -> bool {
+        match self {
+            Fragment::Box(fragment) if fragment.hoisted_fragment_id.is_some() => true,
+            _ => false,
+        }
+    }
+
+    pub fn hoisted_fragment_id(&self) -> Option<&HoistedFragmentId> {
+        match self {
+            Fragment::Box(fragment) => fragment.hoisted_fragment_id.as_ref(),
+            _ => None,
+        }
+    }
+}
+
+impl AbsoluteOrFixedPositionedFragment {
+    pub fn print(&self, tree: &mut PrintTree) {
+        tree.add_item(format!("AbsoluteOrFixedPositionedFragment({:?})", self.0));
+    }
 }
 
 impl AnonymousFragment {
@@ -189,6 +222,7 @@ impl BoxFragment {
         border: Sides<Length>,
         margin: Sides<Length>,
         block_margins_collapsed_with_children: CollapsedBlockMargins,
+        hoisted_fragment_id: Option<HoistedFragmentId>,
     ) -> BoxFragment {
         let scrollable_overflow_from_children =
             children.iter().fold(PhysicalRect::zero(), |acc, child| {
@@ -205,6 +239,7 @@ impl BoxFragment {
             margin,
             block_margins_collapsed_with_children,
             scrollable_overflow_from_children,
+            hoisted_fragment_id,
         }
     }
 
@@ -242,7 +277,8 @@ impl BoxFragment {
                 \nborder rect={:?}\
                 \nscrollable_overflow={:?}\
                 \noverflow={:?} / {:?}\
-                \nstyle={:p}",
+                \nstyle={:p}\
+                \nhoisted_id={:?}",
             self.content_rect,
             self.padding_rect(),
             self.border_rect(),
@@ -250,6 +286,7 @@ impl BoxFragment {
             self.style.get_box().overflow_x,
             self.style.get_box().overflow_y,
             self.style,
+            self.hoisted_fragment_id,
         ));
 
         for child in &self.children {