LibWeb: Include negative margins in height:auto computation for BFC root

...but never allow the resulting height to become negative. This solves
an issue seen on Acid3 where elements with negative vertical margins
expanded the size of their height:auto container instead of shrinking
it, which is the correct behavior. This now works :^)

Userland/Libraries/LibWeb/Layout/FormattingContext.cpp

@@ -286,9 +286,8 @@ float FormattingContext::compute_auto_height_for_block_formatting_context_root(F
 
             auto const& child_box_state = state.get(child_box);
 
-            // FIXME: We're ignoring negative margins here, figure out the correct thing to do.
-            float child_box_top = child_box_state.offset.y() - child_box_state.border_box_top() - max(0, child_box_state.margin_top);
-            float child_box_bottom = child_box_state.offset.y() + child_box_state.content_height + child_box_state.border_box_bottom() + max(0, child_box_state.margin_bottom);
+            float child_box_top = child_box_state.offset.y() - child_box_state.margin_box_top();
+            float child_box_bottom = child_box_state.offset.y() + child_box_state.content_height + child_box_state.margin_box_bottom();
 
             if (!top.has_value() || child_box_top < top.value())
                 top = child_box_top;
@@ -308,8 +307,7 @@ float FormattingContext::compute_auto_height_for_block_formatting_context_root(F
             return IterationDecision::Continue;
 
         auto const& child_box_state = state.get(child_box);
-        // FIXME: We're ignoring negative margins here, figure out the correct thing to do.
-        float child_box_bottom = child_box_state.offset.y() + child_box_state.content_height + child_box_state.border_box_bottom() + max(0, child_box_state.margin_bottom);
+        float child_box_bottom = child_box_state.offset.y() + child_box_state.content_height + child_box_state.margin_box_bottom();
 
         if (!bottom.has_value() || child_box_bottom > bottom.value())
             bottom = child_box_bottom;
@@ -317,7 +315,7 @@ float FormattingContext::compute_auto_height_for_block_formatting_context_root(F
         return IterationDecision::Continue;
     });
 
-    return bottom.value_or(0) - top.value_or(0);
+    return max(0, bottom.value_or(0) - top.value_or(0));
 }
 
 // 10.3.2 Inline, replaced elements, https://www.w3.org/TR/CSS22/visudet.html#inline-replaced-width