servo: Merge #16010 - Properly coalesce whitespace when given a refer…

…ence child (fixes #15979) (from nox:h5e); r=KiChjang

Source-Repo: https://github.com/servo/servo
Source-Revision: 32784c5fa8f30f007a45122dae5ae16d06d1733d

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : ddf9716a214137691d0bd6ef8d1ca080f8dc5644

servo/components/script/dom/servoparser/mod.rs

@@ -657,7 +657,12 @@ fn insert(parent: &Node, reference_child: Option<&Node>, child: NodeOrText<JS<No
             parent.InsertBefore(&n, reference_child).unwrap();
         },
         NodeOrText::AppendText(t) => {
-            if let Some(text) = parent.GetLastChild().and_then(Root::downcast::<Text>) {
+            let text = reference_child
+                .and_then(Node::GetPreviousSibling)
+                .or_else(|| parent.GetLastChild())
+                .and_then(Root::downcast::<Text>);
+
+            if let Some(text) = text {
                 text.upcast::<CharacterData>().append_data(&t);
             } else {
                 let text = Text::new(String::from(t).into(), &parent.owner_doc());