LibWeb: Hack BFC to always remember to handle position:absolute elements

Normally we don't layout position:absolute elements until after the
parent formatting context has assigned dimensions to the current
formatting context's root box.

However, some of our parent contexts (especially FFC) don't do this
reliably, which makes position:absolute children have 0x0 dimensions.

Hack this for now by making ~BFC() pretend that the parent assigned
dimensions if it hadn't done it already.

Userland/Libraries/LibWeb/Layout/BlockFormattingContext.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2020-2021, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2020-2022, Andreas Kling <kling@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -25,6 +25,12 @@ BlockFormattingContext::BlockFormattingContext(BlockContainer& root, FormattingC
 
 BlockFormattingContext::~BlockFormattingContext()
 {
+    if (!m_was_notified_after_parent_dimensioned_my_root_box) {
+        // HACK: The parent formatting context never notified us after assigning dimensions to our root box.
+        //       Pretend that it did anyway, to make sure absolutely positioned children get laid out.
+        // FIXME: Get rid of this hack once parent contexts behave properly.
+        parent_context_did_dimension_child_root_box();
+    }
 }
 
 bool BlockFormattingContext::is_initial() const
@@ -47,6 +53,8 @@ void BlockFormattingContext::run(Box&, LayoutMode layout_mode)
 
 void BlockFormattingContext::parent_context_did_dimension_child_root_box()
 {
+    m_was_notified_after_parent_dimensioned_my_root_box = true;
+
     for (auto& box : m_absolutely_positioned_boxes)
         layout_absolutely_positioned_element(box);
 

Userland/Libraries/LibWeb/Layout/BlockFormattingContext.h

@@ -76,6 +76,8 @@ private:
     FloatSideData m_right_floats;
 
     Vector<Box&> m_absolutely_positioned_boxes;
+
+    bool m_was_notified_after_parent_dimensioned_my_root_box { false };
 };
 
 }