LibWeb: Take coordinate spaces into account when clearing past floats

We were forgetting to convert to and from BFC root relative coordinates
when calculating how much clearance was needed to get past floats.

This fixes the last remaining issue on Acid1, which is now perfect. :^)

Userland/Libraries/LibWeb/Layout/BlockFormattingContext.cpp

@@ -504,12 +504,25 @@ void BlockFormattingContext::place_block_level_element_in_normal_flow_vertically
 
     auto clear_floating_boxes = [&](FloatSideData& float_side) {
         if (!float_side.current_boxes.is_empty()) {
-            float clearance_y = 0;
+            // NOTE: Floating boxes are globally relevant within this BFC, *but* their offset coordinates
+            //       are relative to their containing block.
+            //       This means that we have to first convert to a root-space Y coordinate before clearing,
+            //       and then convert back to a local Y coordinate when assigning the cleared offset to
+            //       the `child_box` layout state.
+
+            // First, find the lowest margin box edge on this float side and calculate the Y offset just below it.
+            float clearance_y_in_root = 0;
             for (auto const& floating_box : float_side.current_boxes) {
-                auto const& floating_box_state = m_state.get(floating_box.box);
-                clearance_y = max(clearance_y, floating_box_state.offset.y() + floating_box_state.border_box_height());
+                auto floating_box_rect_in_root = margin_box_rect_in_ancestor_coordinate_space(floating_box.box, root(), m_state);
+                clearance_y_in_root = max(clearance_y_in_root, floating_box_rect_in_root.bottom() + 1);
             }
-            y = max(y, clearance_y);
+
+            // Then, convert the clearance Y to a coordinate relative to the containing block of `child_box`.
+            float clearance_y_in_containing_block = clearance_y_in_root;
+            for (auto* containing_block = child_box.containing_block(); containing_block && containing_block != &root(); containing_block = containing_block->containing_block())
+                clearance_y_in_containing_block -= m_state.get(*containing_block).offset.y();
+
+            y = max(y, clearance_y_in_containing_block);
             float_side.clear();
         }
     };