LibWeb: Stop allowing position:relative to affect layout

Relatively positioned boxes should not affect the *layout* of their
siblings. So instead of applying relative inset as a layout-time
translation on the box, we now perform the adjustment at the paintable
level instead.

This makes position:relative actually work as expected, and exposes some
new bugs we need to take care of for Acid2. :^)

Userland/Libraries/LibWeb/Layout/BlockFormattingContext.cpp

@@ -438,8 +438,7 @@ void BlockFormattingContext::place_block_level_element_in_normal_flow_vertically
 
     compute_vertical_box_model_metrics(child_box, containing_block);
 
-    float y = box_state.border_box_top()
-        + box_state.inset_top;
+    float y = box_state.border_box_top();
 
     Vector<float> collapsible_margins;
 
@@ -529,7 +528,7 @@ void BlockFormattingContext::place_block_level_element_in_normal_flow_horizontal
     if (containing_block.computed_values().text_align() == CSS::TextAlign::LibwebCenter) {
         x += (available_width_within_containing_block / 2) - box_state.content_width / 2;
     } else {
-        x += box_state.margin_box_left() + box_state.inset_left;
+        x += box_state.margin_box_left();
     }
 
     box_state.offset = Gfx::FloatPoint { x, box_state.offset.y() };
@@ -592,7 +591,7 @@ void BlockFormattingContext::layout_floating_box(Box const& box, BlockContainer
     // First we place the box normally (to get the right y coordinate.)
     // If we have a LineBuilder, we're in the middle of inline layout, otherwise this is block layout.
     if (line_builder) {
-        float y_offset = box_state.margin_box_top() + box_state.inset_top;
+        float y_offset = box_state.margin_box_top();
         line_builder->break_if_needed(layout_mode, box_state.border_box_width());
         box_state.offset.set_y(line_builder->current_y() + y_offset);
         line_builder->adjust_last_line_after_inserting_floating_box({}, box.computed_values().float_(), box_state.border_box_width());

Userland/Libraries/LibWeb/Painting/PaintableBox.cpp

@@ -59,11 +59,18 @@ void PaintableBox::set_content_size(Gfx::FloatSize const& size)
 
 Gfx::FloatPoint PaintableBox::effective_offset() const
 {
+    Gfx::FloatPoint offset;
     if (m_containing_line_box_fragment.has_value()) {
         auto const& fragment = containing_block()->paint_box()->line_boxes()[m_containing_line_box_fragment->line_box_index].fragments()[m_containing_line_box_fragment->fragment_index];
-        return fragment.offset();
+        offset = fragment.offset();
+    } else {
+        offset = m_offset;
+    }
+    if (layout_box().computed_values().position() == CSS::Position::Relative) {
+        auto const& inset = layout_box().box_model().inset;
+        offset.translate_by(inset.left, inset.top);
     }
-    return m_offset;
+    return offset;
 }
 
 Gfx::FloatRect PaintableBox::compute_absolute_rect() const