Implemented fullscreen top layer

components/layout/display_list/builder.rs

@@ -1917,6 +1917,7 @@ impl Fragment {
             border_box.to_layout(),
             overflow.to_layout(),
             self.effective_z_index(),
+            self.style().get_box()._servo_top_layer,
             filters,
             self.style().get_effects().mix_blend_mode.to_layout(),
             self.transform_matrix(&border_box),

components/layout/display_list/items.rs

@@ -22,6 +22,7 @@ use std::cmp::Ordering;
 use std::collections::HashMap;
 use std::f32;
 use std::fmt;
+use style::computed_values::_servo_top_layer::T as InTopLayer;
 use webrender_api as wr;
 use webrender_api::{BorderRadius, ClipMode};
 use webrender_api::{ComplexClipRegion, ExternalScrollId, FilterOp};
@@ -189,6 +190,9 @@ pub struct StackingContext {
     /// The `z-index` for this stacking context.
     pub z_index: i32,
 
+    /// Whether this is the top layer.
+    pub in_top_layer: InTopLayer,
+
     /// CSS filters to be applied to this stacking context (including opacity).
     pub filters: Vec<FilterOp>,
 
@@ -220,6 +224,7 @@ impl StackingContext {
         bounds: LayoutRect,
         overflow: LayoutRect,
         z_index: i32,
+        in_top_layer: InTopLayer,
         filters: Vec<FilterOp>,
         mix_blend_mode: MixBlendMode,
         transform: Option<LayoutTransform>,
@@ -234,6 +239,7 @@ impl StackingContext {
             bounds,
             overflow,
             z_index,
+            in_top_layer,
             filters,
             mix_blend_mode,
             transform,
@@ -252,6 +258,7 @@ impl StackingContext {
             LayoutRect::zero(),
             LayoutRect::zero(),
             0,
+            InTopLayer::None,
             vec![],
             MixBlendMode::Normal,
             None,
@@ -283,6 +290,16 @@ impl StackingContext {
 
 impl Ord for StackingContext {
     fn cmp(&self, other: &Self) -> Ordering {
+        if self.in_top_layer == InTopLayer::Top {
+            if other.in_top_layer == InTopLayer::Top {
+                return Ordering::Equal;
+            } else {
+                return Ordering::Greater;
+            }
+        } else if other.in_top_layer == InTopLayer::Top {
+            return Ordering::Less;
+        }
+
         if self.z_index != 0 || other.z_index != 0 {
             return self.z_index.cmp(&other.z_index);
         }

components/style/properties/longhands/box.mako.rs

@@ -34,6 +34,17 @@ ${helpers.single_keyword(
     spec="Internal (not web-exposed)",
 )}
 
+// An internal-only attribute for elements in a top layer
+// https://fullscreen.spec.whatwg.org/#top-layer
+${helpers.single_keyword(
+    "-servo-top-layer",
+    "none top",
+    products="servo",
+    animation_value_type="none",
+    enabled_in="ua",
+    spec="Internal (not web-exposed)",
+)}
+
 ${helpers.single_keyword(
     "position",
     "static absolute relative fixed sticky",

components/style/properties/properties.mako.rs

@@ -3627,11 +3627,12 @@ impl<'a> StyleBuilder<'a> {
                  Position::Absolute | Position::Fixed)
     }
 
-    /// Whether this style has a top-layer style. That's implemented in Gecko
-    /// via the -moz-top-layer property, but servo doesn't have any concept of a
-    /// top layer (yet, it's needed for fullscreen).
+    /// Whether this style has a top-layer style.
     #[cfg(feature = "servo")]
-    pub fn in_top_layer(&self) -> bool { false }
+    pub fn in_top_layer(&self) -> bool {
+        matches!(self.get_box().clone__servo_top_layer(),
+                 longhands::_servo_top_layer::computed_value::T::Top)
+    }
 
     /// Whether this style has a top-layer style.
     #[cfg(feature = "gecko")]

resources/user-agent.css

@@ -302,6 +302,10 @@ textarea { white-space: pre-wrap; }
 
   /* intentionally not !important */
   object-fit:contain;
+
+  /* The internal-only -servo-top-layer attribute is used
+     to implement https://fullscreen.spec.whatwg.org/#top-layer */
+  -servo-top-layer: top;
 }
 
 iframe:fullscreen {

tests/wpt/mozilla/meta/mozilla/fullscreen/reftests/fullscreen-hides-others.html.ini

@@ -1,3 +0,0 @@
-[fullscreen-hides-others.html]
-  type: reftest
-  expected: FAIL