LibWeb: Make tiled backgrounds scroll with content

Previously the page background was always draw relative to the viewport
instead of following with the content. This should eventually become
an opt-in mode (via CSS "background-attachment") but for now let's have
the default behavior be that backgrounds scroll with content.

Also take this opportunity to move the background painting code from
the two web views to a shared location in InitialContainingBlockBox.

Userland/Libraries/LibWeb/InProcessWebView.cpp

@@ -262,14 +262,7 @@ void InProcessWebView::paint_event(GUI::PaintEvent& event)
         return;
     }
 
-    painter.fill_rect(event.rect(), document()->background_color(palette()));
-
-    if (auto background_bitmap = document()->background_image()) {
-        painter.draw_tiled_bitmap(event.rect(), *background_bitmap);
-    }
-
     painter.translate(frame_thickness(), frame_thickness());
-    painter.translate(-horizontal_scrollbar().value(), -vertical_scrollbar().value());
 
     PaintContext context(painter, palette(), { horizontal_scrollbar().value(), vertical_scrollbar().value() });
     context.set_should_show_line_box_borders(m_should_show_line_box_borders);

Userland/Libraries/LibWeb/Layout/InitialContainingBlockBox.cpp

@@ -24,6 +24,7 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+#include <LibGfx/Painter.h>
 #include <LibWeb/Dump.h>
 #include <LibWeb/Layout/InitialContainingBlockBox.h>
 #include <LibWeb/Page/Frame.h>
@@ -61,8 +62,26 @@ void InitialContainingBlockBox::build_stacking_context_tree()
     });
 }
 
+void InitialContainingBlockBox::paint_document_background(PaintContext& context)
+{
+    context.painter().fill_rect(Gfx::IntRect { {}, context.viewport_rect().size() }, document().background_color(context.palette()));
+    context.painter().translate(-context.viewport_rect().location());
+
+    if (auto background_bitmap = document().background_image()) {
+        Gfx::IntRect background_rect {
+            0,
+            0,
+            context.viewport_rect().x() + context.viewport_rect().width(),
+            context.viewport_rect().y() + context.viewport_rect().height()
+        };
+        context.painter().blit_tiled(background_rect, *background_bitmap, background_bitmap->rect());
+    }
+}
+
 void InitialContainingBlockBox::paint_all_phases(PaintContext& context)
 {
+    paint_document_background(context);
+
     paint(context, PaintPhase::Background);
     paint(context, PaintPhase::Border);
     paint(context, PaintPhase::Foreground);

Userland/Libraries/LibWeb/Layout/InitialContainingBlockBox.h

@@ -41,6 +41,8 @@ public:
     void paint_all_phases(PaintContext&);
     virtual void paint(PaintContext&, PaintPhase) override;
 
+    void paint_document_background(PaintContext&);
+
     virtual HitTestResult hit_test(const Gfx::IntPoint&, HitTestType) const override;
 
     const LayoutRange& selection() const { return m_selection; }

Userland/Services/WebContent/PageHost.cpp

@@ -84,14 +84,6 @@ void PageHost::paint(const Gfx::IntRect& content_rect, Gfx::Bitmap& target)
         return;
     }
 
-    painter.fill_rect(bitmap_rect, layout_root->document().background_color(palette()));
-
-    if (auto background_bitmap = layout_root->document().background_image()) {
-        painter.draw_tiled_bitmap(bitmap_rect, *background_bitmap);
-    }
-
-    painter.translate(-content_rect.x(), -content_rect.y());
-
     Web::PaintContext context(painter, palette(), content_rect.top_left());
     context.set_should_show_line_box_borders(m_should_show_line_box_borders);
     context.set_viewport_rect(content_rect);