Auto merge of #10513 - frewsxcv:is_the_html_body_element, r=Ms2ger

Extract out 'is the html body element' CSSOM concept.

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2 files changed, 12 insertions, 6 deletions

diff --git a/components/script/dom/htmlbodyelement.rs b/components/script/dom/htmlbodyelement.rs
index 8ac184ea51d..ca133b88dbb 100644
--- a/components/script/dom/htmlbodyelement.rs
+++ b/components/script/dom/htmlbodyelement.rs
@@ -47,6 +47,16 @@ impl HTMLBodyElement {
         let element = HTMLBodyElement::new_inherited(localName, prefix, document);
         Node::reflect_node(box element, document, HTMLBodyElementBinding::Wrap)
     }
+
+    /// https://drafts.csswg.org/cssom-view/#the-html-body-element
+    pub fn is_the_html_body_element(&self) -> bool {
+        let self_node = self.upcast::<Node>();
+        let root_elem = self.upcast::<Element>().root_element();
+        let root_node = root_elem.upcast::<Node>();
+        root_node.is_parent_of(self_node) &&
+            self_node.preceding_siblings().all(|n| !n.is::<HTMLBodyElement>())
+    }
+
 }
 
 impl HTMLBodyElementMethods for HTMLBodyElement {
diff --git a/components/script/dom/node.rs b/components/script/dom/node.rs
index 748b7b8249e..1e71966e4e7 100644
--- a/components/script/dom/node.rs
+++ b/components/script/dom/node.rs
@@ -594,12 +594,8 @@ impl Node {
 
         let html_element = document.GetDocumentElement();
 
-        let is_body_element = html_element.r().and_then(|root| {
-            let node = root.upcast::<Node>();
-            node.children().find(|child| { child.is::<HTMLBodyElement>() }).map(|node| {
-                *node.r() == *self
-            })
-        }).unwrap_or(false);
+        let is_body_element = self.downcast::<HTMLBodyElement>()
+                                  .map_or(false, |e| e.is_the_html_body_element());
 
         let scroll_area = window.scroll_area_query(self.to_trusted_node_address());