layout: Centralize the logic that determines whether fragments get

layers in the fragment, so that it can be activated when we're forcing
the creation of extra layers due to positioned descendants that
themselves have layers.

The newly failing tests were tests that accidentally passed due to
incorrect stacking order.

Closes servo#7281.

components/layout/block.rs

@@ -31,15 +31,17 @@ use context::LayoutContext;
 use display_list_builder::{BlockFlowDisplayListBuilding, BorderPaintingMode};
 use display_list_builder::{FragmentDisplayListBuilding};
 use floats::{ClearType, FloatKind, Floats, PlacementInfo};
-use flow::{BLOCK_POSITION_IS_STATIC, HAS_LEFT_FLOATED_DESCENDANTS, HAS_RIGHT_FLOATED_DESCENDANTS};
+use flow::{BLOCK_POSITION_IS_STATIC};
 use flow::{CLEARS_LEFT, CLEARS_RIGHT};
+use flow::{HAS_LEFT_FLOATED_DESCENDANTS, HAS_RIGHT_FLOATED_DESCENDANTS};
 use flow::{IMPACTED_BY_LEFT_FLOATS, IMPACTED_BY_RIGHT_FLOATS, INLINE_POSITION_IS_STATIC};
 use flow::{IS_ABSOLUTELY_POSITIONED};
 use flow::{ImmutableFlowUtils, MutableFlowUtils, OpaqueFlow, PreorderFlowTraversal};
 use flow::{LAYERS_NEEDED_FOR_DESCENDANTS, NEEDS_LAYER};
 use flow::{PostorderFlowTraversal, mut_base};
 use flow::{self, AbsolutePositionInfo, BaseFlow, ForceNonfloatedFlag, FlowClass, Flow};
-use fragment::{CoordinateSystem, Fragment, FragmentBorderBoxIterator, SpecificFragmentInfo};
+use fragment::{CoordinateSystem, Fragment, FragmentBorderBoxIterator, HAS_LAYER};
+use fragment::{SpecificFragmentInfo};
 use incremental::{REFLOW, REFLOW_OUT_OF_FLOW};
 use layout_debug;
 use layout_task::DISPLAY_PORT_SIZE_FACTOR;
@@ -787,6 +789,8 @@ impl BlockFlow {
                                          self.base.debug_id());
 
         if self.base.restyle_damage.contains(REFLOW) {
+            self.determine_if_layer_needed();
+
             // Our current border-box position.
             let mut cur_b = Au(0);
 
@@ -950,11 +954,6 @@ impl BlockFlow {
             }
 
             if self.base.flags.contains(IS_ABSOLUTELY_POSITIONED) {
-                // Fixed position layers get layers.
-                if self.is_fixed() {
-                    self.base.flags.insert(NEEDS_LAYER);
-                }
-
                 // Store the content block-size for use in calculating the absolute flow's
                 // dimensions later.
                 //
@@ -1564,6 +1563,36 @@ impl BlockFlow {
         }
         self.base.flags = flags
     }
+
+    fn determine_if_layer_needed(&mut self) {
+        if self.base.flags.contains(IS_ABSOLUTELY_POSITIONED) {
+            // Fixed position layers get layers.
+            if self.is_fixed() {
+                self.base.flags.insert(NEEDS_LAYER);
+                return
+            }
+        }
+
+        // This flow needs a layer if it has a 3d transform, or provides perspective
+        // to child layers. See http://dev.w3.org/csswg/css-transforms/#3d-rendering-contexts.
+        let has_3d_transform = self.transform_requires_layer();
+        let has_perspective = self.fragment.style().get_effects().perspective !=
+            LengthOrNone::None;
+
+        if has_3d_transform || has_perspective {
+            self.base.flags.insert(NEEDS_LAYER);
+            return
+        }
+
+        match (self.fragment.style().get_box().overflow_x,
+               self.fragment.style().get_box().overflow_y.0) {
+            (overflow_x::T::auto, _) | (overflow_x::T::scroll, _) |
+            (_, overflow_x::T::auto) | (_, overflow_x::T::scroll) => {
+                self.base.flags.insert(NEEDS_LAYER);
+            }
+            _ => {}
+        }
+    }
 }
 
 impl Flow for BlockFlow {
@@ -1745,6 +1774,12 @@ impl Flow for BlockFlow {
     }
 
     fn compute_absolute_position(&mut self, layout_context: &LayoutContext) {
+        if (self.base.flags.contains(IS_ABSOLUTELY_POSITIONED) &&
+                self.base.absolute_position_info.layers_needed_for_positioned_flows) ||
+                self.base.flags.contains(NEEDS_LAYER) {
+            self.fragment.flags.insert(HAS_LAYER)
+        }
+
         // FIXME (mbrubeck): Get the real container size, taking the container writing mode into
         // account.  Must handle vertical writing modes.
         let container_size = Size2D::new(self.base.block_container_inline_size, Au(0));
@@ -1754,15 +1789,7 @@ impl Flow for BlockFlow {
             self.base.stacking_relative_position_of_display_port = MAX_RECT;
         }
 
-        // This flow needs a layer if it has a 3d transform, or provides perspective
-        // to child layers. See http://dev.w3.org/csswg/css-transforms/#3d-rendering-contexts.
         let transform_style = self.fragment.style().get_used_transform_style();
-        let has_3d_transform = self.transform_requires_layer();
-        let has_perspective = self.fragment.style().get_effects().perspective != LengthOrNone::None;
-
-        if has_3d_transform || has_perspective {
-            self.base.flags.insert(NEEDS_LAYER);
-        }
 
         if self.base.flags.contains(IS_ABSOLUTELY_POSITIONED) {
             // `overflow: auto` and `overflow: scroll` force creation of layers, since we can only
@@ -1771,7 +1798,6 @@ impl Flow for BlockFlow {
                    self.fragment.style().get_box().overflow_y.0) {
                 (overflow_x::T::auto, _) | (overflow_x::T::scroll, _) |
                 (_, overflow_x::T::auto) | (_, overflow_x::T::scroll) => {
-                    self.base.flags.insert(NEEDS_LAYER);
                     self.base.clip = ClippingRegion::max();
                     self.base.stacking_relative_position_of_display_port = MAX_RECT;
                 }
@@ -1886,7 +1912,7 @@ impl Flow for BlockFlow {
         }
 
         let stacking_relative_position_of_display_port_for_children =
-            if (is_stacking_context && self.will_get_layer()) || self.is_root() {
+            if is_stacking_context || self.is_root() {
                 let visible_rect =
                     match layout_context.shared.visible_rects.get(&self.layer_id(0)) {
                         Some(visible_rect) => *visible_rect,

components/layout/display_list_builder.rs

@@ -16,7 +16,7 @@ use context::LayoutContext;
 use flex::FlexFlow;
 use flow::{self, BaseFlow, Flow, IS_ABSOLUTELY_POSITIONED, NEEDS_LAYER};
 use flow_ref;
-use fragment::{CoordinateSystem, Fragment, IframeFragmentInfo, ImageFragmentInfo};
+use fragment::{CoordinateSystem, Fragment, HAS_LAYER, IframeFragmentInfo, ImageFragmentInfo};
 use fragment::{ScannedTextFragmentInfo, SpecificFragmentInfo};
 use inline::InlineFlow;
 use list_item::ListItemFlow;
@@ -1513,7 +1513,6 @@ pub trait BlockFlowDisplayListBuilding {
                                     display_list: Box<DisplayList>,
                                     layout_context: &LayoutContext,
                                     border_painting_mode: BorderPaintingMode);
-    fn will_get_layer(&self) -> bool;
 }
 
 impl BlockFlowDisplayListBuilding for BlockFlow {
@@ -1557,35 +1556,32 @@ impl BlockFlowDisplayListBuilding for BlockFlow {
                                                border_painting_mode,
                                                background_border_level);
 
-        self.base.display_list_building_result = if self.fragment.establishes_stacking_context() {
-            if self.will_get_layer() {
-                // If we got here, then we need a new layer.
-                let scroll_policy = if self.is_fixed() {
-                    ScrollPolicy::FixedPosition
-                } else {
-                    ScrollPolicy::Scrollable
-                };
+        self.base.display_list_building_result = if self.fragment.flags.contains(HAS_LAYER) {
+            let scroll_policy = if self.is_fixed() {
+                ScrollPolicy::FixedPosition
+            } else {
+                ScrollPolicy::Scrollable
+            };
 
-                let paint_layer = PaintLayer::new(self.layer_id(0),
-                                                  color::transparent(),
-                                                  scroll_policy);
-                let layer = StackingContextLayer::Existing(paint_layer);
-                let stacking_context = self.fragment.create_stacking_context(
+            let paint_layer = PaintLayer::new(self.layer_id(0),
+                                              color::transparent(),
+                                              scroll_policy);
+            let layer = StackingContextLayer::Existing(paint_layer);
+            let stacking_context = self.fragment.create_stacking_context(
+                &self.base,
+                display_list,
+                layout_context,
+                layer,
+                StackingContextCreationMode::Normal);
+            DisplayListBuildingResult::StackingContext(stacking_context)
+        } else if self.fragment.establishes_stacking_context() {
+            DisplayListBuildingResult::StackingContext(
+                self.fragment.create_stacking_context(
                     &self.base,
                     display_list,
                     layout_context,
-                    layer,
-                    StackingContextCreationMode::Normal);
-                DisplayListBuildingResult::StackingContext(stacking_context)
-            } else {
-                DisplayListBuildingResult::StackingContext(
-                    self.fragment.create_stacking_context(
-                        &self.base,
-                        display_list,
-                        layout_context,
-                        StackingContextLayer::IfCanvas(self.layer_id(0)),
-                        StackingContextCreationMode::Normal))
-            }
+                    StackingContextLayer::IfCanvas(self.layer_id(0)),
+                    StackingContextCreationMode::Normal))
         } else {
             match self.fragment.style.get_box().position {
                 position::T::static_ => {}
@@ -1597,11 +1593,6 @@ impl BlockFlowDisplayListBuilding for BlockFlow {
         }
     }
 
-    fn will_get_layer(&self) -> bool {
-        self.base.absolute_position_info.layers_needed_for_positioned_flows ||
-            self.base.flags.contains(NEEDS_LAYER)
-    }
-
     fn build_display_list_for_absolutely_positioned_block(
             &mut self,
             mut display_list: Box<DisplayList>,
@@ -1654,23 +1645,21 @@ impl BlockFlowDisplayListBuilding for BlockFlow {
             }
         };
 
-        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 =
-                DisplayListBuildingResult::StackingContext(
-                    self.fragment.create_stacking_context(
-                        &self.base,
-                        display_list,
-                        layout_context,
-                        StackingContextLayer::IfCanvas(self.layer_id(0)),
-                        StackingContextCreationMode::Normal));
+        if !self.fragment.flags.contains(HAS_LAYER) {
+            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);
+            } else {
+                self.base.display_list_building_result =
+                    DisplayListBuildingResult::StackingContext(
+                        self.fragment.create_stacking_context(
+                            &self.base,
+                            display_list,
+                            layout_context,
+                            StackingContextLayer::IfCanvas(self.layer_id(0)),
+                            StackingContextCreationMode::Normal));
+            }
             return
         }
 

components/layout/fragment.rs

@@ -111,6 +111,9 @@ pub struct Fragment {
     /// The pseudo-element that this fragment represents.
     pub pseudo: PseudoElementType<()>,
 
+    /// Various flags for this fragment.
+    pub flags: FragmentFlags,
+
     /// A debug ID that is consistent for the life of this fragment (via transform etc).
     pub debug_id: u16,
 }
@@ -761,6 +764,7 @@ impl Fragment {
             specific: specific,
             inline_context: None,
             pseudo: node.get_pseudo_element_type().strip(),
+            flags: FragmentFlags::empty(),
             debug_id: layout_debug::generate_unique_debug_id(),
         }
     }
@@ -783,6 +787,7 @@ impl Fragment {
             specific: specific,
             inline_context: None,
             pseudo: pseudo,
+            flags: FragmentFlags::empty(),
             debug_id: layout_debug::generate_unique_debug_id(),
         }
     }
@@ -816,6 +821,7 @@ impl Fragment {
             specific: info,
             inline_context: self.inline_context.clone(),
             pseudo: self.pseudo.clone(),
+            flags: FragmentFlags::empty(),
             debug_id: self.debug_id,
         }
     }
@@ -1987,6 +1993,9 @@ impl Fragment {
 
     /// Returns true if this fragment establishes a new stacking context and false otherwise.
     pub fn establishes_stacking_context(&self) -> bool {
+        if self.flags.contains(HAS_LAYER) {
+            return true
+        }
         if self.style().get_effects().opacity != 1.0 {
             return true
         }
@@ -2373,3 +2382,11 @@ impl WhitespaceStrippingResult {
         }
     }
 }
+
+bitflags! {
+    flags FragmentFlags: u8 {
+        /// Whether this fragment has a layer.
+        const HAS_LAYER = 0x01,
+    }
+}
+

tests/ref/basic.list

@@ -257,6 +257,7 @@ flaky_cpu == linebreak_simple_a.html linebreak_simple_b.html
 # Should be == with expected failure. See #2797
 != overconstrained_block.html overconstrained_block_ref.html
 == overflow_auto.html overflow_simple_b.html
+== overflow_auto_stacking_order_a.html overflow_auto_stacking_order_ref.html
 # Should be ==?
 != overflow_position_abs_inside_normal_a.html overflow_position_abs_inside_normal_b.html
 == overflow_position_abs_simple_a.html overflow_position_abs_simple_b.html

tests/ref/overflow_auto_stacking_order_a.html

@@ -0,0 +1,22 @@
+<!DOCTYPE html>
+<style>
+div {
+    width: 100px;
+    height: 100px;
+    position: absolute;
+    top: 0;
+    left: 0;
+}
+#a {
+    background: red;
+    overflow: auto;
+}
+#b {
+    background: green;
+    top: 0;
+    left: 0;
+}
+</style>
+<div id=a></div>
+<div id=b></div>
+

tests/ref/overflow_auto_stacking_order_ref.html

@@ -0,0 +1,18 @@
+<!DOCTYPE html>
+<style>
+div {
+    width: 100px;
+    height: 100px;
+    position: absolute;
+    top: 0;
+    left: 0;
+}
+#a {
+    background: green;
+    top: 0;
+    left: 0;
+}
+</style>
+<div id=a></div>
+
+

tests/wpt/metadata-css/css21_dev/html4/abspos-overflow-010.htm

@@ -0,0 +1,4 @@
+[abspos-overflow-010.htm]
+  type: reftest
+  expected:
+    if os == "linux": FAIL

tests/wpt/metadata-css/css21_dev/html4/abspos-overflow-011.htm.ini

@@ -0,0 +1,3 @@
+[abspos-overflow-011.htm]
+  type: reftest
+  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/max-height-107.htm.ini

@@ -0,0 +1,3 @@
+[max-height-107.htm]
+  type: reftest
+  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/max-height-110.htm.ini

@@ -0,0 +1,3 @@
+[max-height-110.htm]
+  type: reftest
+  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/min-height-104.htm.ini

@@ -0,0 +1,3 @@
+[min-height-104.htm]
+  type: reftest
+  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/min-height-105.htm.ini

@@ -0,0 +1,3 @@
+[min-height-105.htm]
+  type: reftest
+  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/min-height-106.htm.ini

@@ -0,0 +1,3 @@
+[min-height-106.htm]
+  type: reftest
+  expected: FAIL

tests/wpt/metadata/html/rendering/non-replaced-elements/the-fieldset-element-0/min-width-not-important.html.ini

@@ -0,0 +1,5 @@
+[min-width-not-important.html]
+  type: reftest
+  reftype: ==
+  refurl: /html/rendering/non-replaced-elements/the-fieldset-element-0/ref.html
+  expected: FAIL