LibWeb: Place right-side floats relative to their containing block

We were incorrectly placing them relative to the BFC root, but CSS2
says they are relative to their own containing block.

Userland/Libraries/LibWeb/Layout/BlockFormattingContext.cpp

@@ -55,19 +55,17 @@ void BlockFormattingContext::parent_context_did_dimension_child_root_box()
 {
  m_was_notified_after_parent_dimensioned_my_root_box = true;
 
- // Now that we know the width of our root box, we can position floats horizontally.
- auto root_width = m_state.get(root()).content_width;
-
  // Left-side floats: offset_from_edge is from left edge (0) to left content edge of floating_box.
  for (auto& floating_box : m_left_floats.all_boxes) {
  auto& box_state = m_state.get_mutable(floating_box->box);
  box_state.offset.set_x(floating_box->offset_from_edge);
  }
 
- // Right-side floats: offset_from_edge is from right edge (root_width) to the left content edge of floating_box.
+ // Right-side floats: offset_from_edge is from right edge (float_containing_block_width) to the left content edge of floating_box.
  for (auto& floating_box : m_right_floats.all_boxes) {
+ auto float_containing_block_width = m_state.get(*floating_box->box.containing_block()).content_width;
  auto& box_state = m_state.get_mutable(floating_box->box);
- box_state.offset.set_x(root_width - floating_box->offset_from_edge);
+ box_state.offset.set_x(float_containing_block_width - floating_box->offset_from_edge);
  }
 
  // We can also layout absolutely positioned boxes within this BFC.