servo: Merge #7137 - layout: Make absolutely-positioned elements with `z-index: auto` not stacking contexts (from pcwalton:absolute-stacking-contexts); r=glennw

Improves many sites.

Closes #7069.

r? @glennw

Source-Repo: https://github.com/servo/servo
Source-Revision: 7ce47266acc7c23de537905746b274b4a75424fa

servo/components/gfx/display_list/mod.rs

@@ -181,25 +181,31 @@ impl DisplayList {
             for item in items.iter() {
                 match *item {
                     DisplayItem::SolidColorClass(ref solid_color) => {
-                        println!("{:?} SolidColor. {:?}", indentation, solid_color.base.bounds)
+                        println!("{} SolidColor({},{},{},{}). {:?}",
+                                 indentation,
+                                 solid_color.color.r,
+                                 solid_color.color.g,
+                                 solid_color.color.b,
+                                 solid_color.color.a,
+                                 solid_color.base.bounds)
                     }
                     DisplayItem::TextClass(ref text) => {
-                        println!("{:?} Text. {:?}", indentation, text.base.bounds)
+                        println!("{} Text. {:?}", indentation, text.base.bounds)
                     }
                     DisplayItem::ImageClass(ref image) => {
-                        println!("{:?} Image. {:?}", indentation, image.base.bounds)
+                        println!("{} Image. {:?}", indentation, image.base.bounds)
                     }
                     DisplayItem::BorderClass(ref border) => {
-                        println!("{:?} Border. {:?}", indentation, border.base.bounds)
+                        println!("{} Border. {:?}", indentation, border.base.bounds)
                     }
                     DisplayItem::GradientClass(ref gradient) => {
-                        println!("{:?} Gradient. {:?}", indentation, gradient.base.bounds)
+                        println!("{} Gradient. {:?}", indentation, gradient.base.bounds)
                     }
                     DisplayItem::LineClass(ref line) => {
-                        println!("{:?} Line. {:?}", indentation, line.base.bounds)
+                        println!("{} Line. {:?}", indentation, line.base.bounds)
                     }
                     DisplayItem::BoxShadowClass(ref box_shadow) => {
-                        println!("{:?} Box_shadow. {:?}", indentation, box_shadow.base.bounds)
+                        println!("{} Box_shadow. {:?}", indentation, box_shadow.base.bounds)
                     }
                 }
             }

servo/components/layout/display_list_builder.rs

@@ -1631,15 +1631,28 @@ impl BlockFlowDisplayListBuilding for BlockFlow {
                 Some(outer_display_list_for_overflow_scroll)
             }
             _ => {
-                self.build_display_list_for_block_base(
-                    &mut *display_list,
-                    layout_context,
-                    border_painting_mode,
-                    BackgroundAndBorderLevel::RootOfStackingContext);
+                let establishes_stacking_context = self.fragment.establishes_stacking_context();
+                let background_and_border_level = if establishes_stacking_context {
+                    BackgroundAndBorderLevel::RootOfStackingContext
+                } else {
+                    BackgroundAndBorderLevel::Block
+                };
+
+                self.build_display_list_for_block_base(&mut *display_list,
+                                                       layout_context,
+                                                       border_painting_mode,
+                                                       background_and_border_level);
                 None
             }
         };
 
+        if !self.fragment.establishes_stacking_context() {
+            display_list.form_pseudo_stacking_context_for_positioned_content();
+            self.base.display_list_building_result =
+                DisplayListBuildingResult::Normal(display_list);
+            return
+        }
+
         if !self.will_get_layer() {
             // We didn't need a layer.
             self.base.display_list_building_result =

servo/components/layout/fragment.rs

@@ -35,9 +35,9 @@ use std::fmt;
 use std::sync::{Arc, Mutex};
 use string_cache::Atom;
 use style::computed_values::content::ContentItem;
-use style::computed_values::{border_collapse, clear, mix_blend_mode, overflow_wrap, position};
-use style::computed_values::{text_align, text_decoration, white_space, word_break};
-use style::computed_values::transform_style;
+use style::computed_values::{border_collapse, clear, mix_blend_mode, overflow_wrap, overflow_x};
+use style::computed_values::{position, text_align, text_decoration, transform_style, white_space};
+use style::computed_values::{word_break, z_index};
 use style::properties::{self, ComputedValues, cascade_anonymous};
 use style::values::computed::{LengthOrPercentage, LengthOrPercentageOrAuto};
 use style::values::computed::{LengthOrPercentageOrNone};
@@ -2027,13 +2027,24 @@ impl Fragment {
             return true
         }
 
-        match self.style().get_box().position {
-            position::T::absolute | position::T::fixed => {
-                // FIXME(pcwalton): This should only establish a new stacking context when
-                // `z-index` is not `auto`. But this matches what we did before.
-                true
-            }
-            position::T::relative | position::T::static_ => {
+        // FIXME(pcwalton): Don't unconditionally form stacking contexts for `overflow_x: scroll`
+        // and `overflow_y: scroll`. This needs multiple layers per stacking context.
+        match (self.style().get_box().position,
+               self.style().get_box().z_index,
+               self.style().get_box().overflow_x,
+               self.style().get_box().overflow_y.0) {
+            (position::T::absolute,
+             z_index::T::Auto,
+             overflow_x::T::visible,
+             overflow_x::T::visible) |
+            (position::T::fixed,
+             z_index::T::Auto,
+             overflow_x::T::visible,
+             overflow_x::T::visible) => false,
+            (position::T::absolute, _, _, _) |
+            (position::T::fixed, _, _, _) => true,
+            (position::T::relative, _, _, _) |
+            (position::T::static_, _, _, _) => {
                 // FIXME(pcwalton): `position: relative` establishes a new stacking context if
                 // `z-index` is not `auto`. But this matches what we did before.
                 false