Don't persist styles on elements not in the document.

components/script/layout_wrapper.rs

@@ -199,6 +199,10 @@ impl<'ln> TNode for ServoLayoutNode<'ln> {
             self.node.parent_node_ref().map(|node| self.new_with_this_lifetime(&node))
         }
     }
+
+    fn is_in_doc(&self) -> bool {
+        unsafe { (*self.node.unsafe_get()).is_in_doc() }
+    }
 }
 
 pub struct ServoChildrenIterator<'a> {

components/style/dom.rs

@@ -99,6 +99,8 @@ pub trait TNode : Sized + Copy + Clone + Debug + NodeInfo {
     unsafe fn set_can_be_fragmented(&self, value: bool);
 
     fn parent_node(&self) -> Option<Self>;
+
+    fn is_in_doc(&self) -> bool;
 }
 
 /// Wrapper to output the ElementData along with the node when formatting for

components/style/gecko/wrapper.rs

@@ -157,6 +157,10 @@ impl<'ln> TNode for GeckoNode<'ln> {
         unsafe { bindings::Gecko_GetParentNode(self.0).map(GeckoNode) }
     }
 
+    fn is_in_doc(&self) -> bool {
+        unsafe { bindings::Gecko_IsInDocument(self.0) }
+    }
+
     fn needs_dirty_on_viewport_size_changed(&self) -> bool {
         // Gecko's node doesn't have the DIRTY_ON_VIEWPORT_SIZE_CHANGE flag,
         // so we force them to be dirtied on viewport size change, regardless if

components/style/gecko_bindings/bindings.rs

@@ -267,6 +267,9 @@ extern "C" {
 extern "C" {
     pub fn Gecko_NodeIsElement(node: RawGeckoNodeBorrowed) -> bool;
 }
+extern "C" {
+    pub fn Gecko_IsInDocument(node: RawGeckoNodeBorrowed) -> bool;
+}
 extern "C" {
     pub fn Gecko_GetParentNode(node: RawGeckoNodeBorrowed)
      -> RawGeckoNodeBorrowedOrNull;

components/style/traversal.rs

@@ -324,16 +324,23 @@ pub fn resolve_style<E, F, G, H>(context: &StyleContext<E>, element: E,
     // on the Element.
     callback(element.borrow_data().unwrap().styles());
 
-    // Clear any styles in display:none subtrees to leave the tree in a valid state.
-    if let Some(root) = display_none_root {
+    // Clear any styles in display:none subtrees or subtrees not in the document,
+    // to leave the tree in a valid state.  For display:none subtrees, we leave
+    // the styles on the display:none root, but for subtrees not in the document,
+    // we clear styles all the way up to the root of the disconnected subtree.
+    let in_doc = element.as_node().is_in_doc();
+    if !in_doc || display_none_root.is_some() {
         let mut curr = element;
         loop {
             unsafe { curr.unset_dirty_descendants(); }
-            if curr == root {
+            if in_doc && curr == display_none_root.unwrap() {
                 break;
             }
             clear_data(curr);
-            curr = curr.parent_element().unwrap();
+            curr = match curr.parent_element() {
+                Some(parent) => parent,
+                None => break,
+            };
         }
     }
 }