LibWeb: When collapsing margins, consider border box heights

Empty boxes should be fully collapsed, but a box with border and/or
padding is not empty.

This fixes an issue where <hr> elements were getting weirdly collapsed
since they have zero content height (but some border height.)

Libraries/LibWeb/Layout/BlockFormattingContext.cpp

@@ -548,7 +548,7 @@ void BlockFormattingContext::place_block_level_non_replaced_element_in_normal_fl
     while (relevant_sibling != nullptr) {
         if (!relevant_sibling->is_absolutely_positioned() && !relevant_sibling->is_floating()) {
             collapsed_bottom_margin_of_preceding_siblings = max(collapsed_bottom_margin_of_preceding_siblings, relevant_sibling->box_model().margin.bottom);
-            if (relevant_sibling->height() > 0)
+            if (relevant_sibling->border_box_height() > 0)
                 break;
         }
         relevant_sibling = relevant_sibling->previous_sibling();

Libraries/LibWeb/Layout/Box.h

@@ -58,6 +58,12 @@ public:
         return width() + border_box.left + border_box.right;
     }
 
+    float border_box_height() const
+    {
+        auto border_box = box_model().border_box();
+        return height() + border_box.top + border_box.bottom;
+    }
+
     Gfx::FloatRect content_box_as_relative_rect() const
     {
         return { m_offset, m_size };