layout: Ensure abs-pos elements establish block formatting contexts

Per CSS 2.1 § 9.4.1, absolutely positioned elements establish new
block formatting contexts for their contents.

components/layout/block.rs

@@ -1429,6 +1429,9 @@ impl BlockFlow {
     /// Determines the type of formatting context this is. See the definition of
     /// `FormattingContextType`.
     pub fn formatting_context_type(&self) -> FormattingContextType {
+        if self.base.flags.contains(IS_ABSOLUTELY_POSITIONED) {
+            return FormattingContextType::Other
+        }
         let style = self.fragment.style();
         if style.get_box().float != float::T::none {
             return FormattingContextType::Other