No bug - Cherry-pick changes from Servo PR #22825. r=npotb

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

@@ -34,6 +34,17 @@ ${helpers.single_keyword(
     spec="Internal (not web-exposed)",
 )}
 
+// An internal-only property 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",

servo/components/style/properties/properties.mako.rs

@@ -3626,11 +3626,12 @@ impl<'a> StyleBuilder<'a> {
                  Position::Absolute | Position::Fixed)
     }
 
-    /// Whether this style has a top-layer style. That's implemented in Gecko
+    /// Whether this style has a top-layer style.
-    /// via the -moz-top-layer property, but servo doesn't have any concept of a
-    /// top layer (yet, it's needed for fullscreen).
     #[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")]