LibWeb: Look at paintable directly in Element::scroll* APIs

We don't need to go via the layout tree as the element has a link
directly to its paintable where the relevant metrics are stored.

Userland/Libraries/LibWeb/DOM/Element.cpp

@@ -953,13 +953,13 @@ int Element::client_top() const
  const_cast<Document&>(document()).update_layout();
 
  // 1. If the element has no associated CSS layout box or if the CSS layout box is inline, return zero.
- if (!layout_node() || !layout_node()->is_box())
+ if (!paintable_box())
  return 0;
 
  // 2. Return the computed value of the border-top-width property
  // plus the height of any scrollbar rendered between the top padding edge and the top border edge,
  // ignoring any transforms that apply to the element and its ancestors.
- return static_cast<Layout::Box const&>(*layout_node()).computed_values().border_top().width.to_int();
+ return paintable_box()->computed_values().border_top().width.to_int();
 }
 
 // https://drafts.csswg.org/cssom-view/#dom-element-clientleft
@@ -969,13 +969,13 @@ int Element::client_left() const
  const_cast<Document&>(document()).update_layout();
 
  // 1. If the element has no associated CSS layout box or if the CSS layout box is inline, return zero.
- if (!layout_node() || !layout_node()->is_box())
+ if (!paintable_box())
  return 0;
 
  // 2. Return the computed value of the border-left-width property
  // plus the width of any scrollbar rendered between the left padding edge and the left border edge,
  // ignoring any transforms that apply to the element and its ancestors.
- return static_cast<Layout::Box const&>(*layout_node()).computed_values().border_left().width.to_int();
+ return paintable_box()->computed_values().border_left().width.to_int();
 }
 
 // https://drafts.csswg.org/cssom-view/#dom-element-clientwidth
@@ -1178,19 +1178,11 @@ double Element::scroll_top() const
  return window->scroll_y();
 
  // 8. If the element does not have any associated box, return zero and terminate these steps.
- if (!layout_node() || !is<Layout::Box>(layout_node()))
- return 0.0;
-
- // FIXME: Ideally we would stop creating a layout node for column group so that a layout node would always have
- // a paintable, but in the meantime, special case this node.
- if (layout_node()->display().is_table_column_group()) {
- VERIFY(!paintable_box());
+ if (!paintable_box())
  return 0.0;
- }
 
  // 9. Return the y-coordinate of the scrolling area at the alignment point with the top of the padding edge of the element.
  // FIXME: Is this correct?
- VERIFY(paintable_box());
  return paintable_box()->scroll_offset().y().to_double();
 }
 
@@ -1226,19 +1218,11 @@ double Element::scroll_left() const
  return window->scroll_x();
 
  // 8. If the element does not have any associated box, return zero and terminate these steps.
- if (!layout_node() || !is<Layout::Box>(layout_node()))
- return 0.0;
-
- // FIXME: Ideally we would stop creating a layout node for column group so that a layout node would always have
- // a paintable, but in the meantime, special case this node.
- if (layout_node()->display().is_table_column_group()) {
- VERIFY(!paintable_box());
+ if (!paintable_box())
  return 0.0;
- }
 
  // 9. Return the x-coordinate of the scrolling area at the alignment point with the left of the padding edge of the element.
  // FIXME: Is this correct?
- VERIFY(paintable_box());
  return paintable_box()->scroll_offset().x().to_double();
 }
 
@@ -1285,11 +1269,10 @@ void Element::set_scroll_left(double x)
  }
 
  // 10. If the element does not have any associated box, the element has no associated scrolling box, or the element has no overflow, terminate these steps.
- if (!layout_node() || !is<Layout::Box>(layout_node()))
+ if (!paintable_box())
  return;
 
- auto* box = static_cast<Layout::Box*>(layout_node());
- if (!box->is_scroll_container())
+ if (!paintable_box()->layout_box().is_scroll_container())
  return;
 
  // FIXME: or the element has no overflow.
@@ -1343,11 +1326,10 @@ void Element::set_scroll_top(double y)
  }
 
  // 10. If the element does not have any associated box, the element has no associated scrolling box, or the element has no overflow, terminate these steps.
- if (!layout_node() || !is<Layout::Box>(layout_node()))
+ if (!paintable_box())
  return;
 
- auto* box = static_cast<Layout::Box*>(layout_node());
- if (!box->is_scroll_container())
+ if (!paintable_box()->layout_box().is_scroll_container())
  return;
 
  // FIXME: or the element has no overflow.