LibWeb: Fix position: fixed canvases/images disappearing when scrolling

This fixes the Serenity logo vanishing after scrolling on the 4th
birthday post.

The previous check did not account for any translation in the painter.
This now uses the painter's clip rect and translation to work out
if a rect is visible. It also makes use of `absolute_paint_rect()`
rather than `absolute_rect()` which can account for things like
box-shadows.

Userland/Libraries/LibWeb/Painting/CanvasPaintable.cpp

@@ -34,8 +34,8 @@ void CanvasPaintable::paint(PaintContext& context, PaintPhase phase) const
         auto canvas_rect = absolute_rect().to_rounded<int>();
         ScopedCornerRadiusClip corner_clip { context.painter(), canvas_rect, normalized_border_radii_data(ShrinkRadiiForBorders::Yes) };
 
-        // FIXME: This should be done at a different level. Also rect() does not include padding etc!
+        // FIXME: This should be done at a different level.
-        if (!context.viewport_rect().intersects(canvas_rect))
+        if (is_out_of_view(context))
             return;
 
         if (layout_box().dom_node().bitmap()) {

Userland/Libraries/LibWeb/Painting/ImagePaintable.cpp

@@ -33,8 +33,8 @@ void ImagePaintable::paint(PaintContext& context, PaintPhase phase) const
     if (!is_visible())
         return;
 
-    // FIXME: This should be done at a different level. Also rect() does not include padding etc!
+    // FIXME: This should be done at a different level.
-    if (!context.viewport_rect().intersects(enclosing_int_rect(absolute_rect())))
+    if (is_out_of_view(context))
         return;
 
     PaintableBox::paint(context, phase);

Userland/Libraries/LibWeb/Painting/PaintableBox.cpp

@@ -37,6 +37,13 @@ void PaintableBox::invalidate_stacking_context()
     m_stacking_context = nullptr;
 }
 
+bool PaintableBox::is_out_of_view(PaintContext& context) const
+{
+    return !enclosing_int_rect(absolute_paint_rect())
+                .translated(context.painter().translation())
+                .intersects(context.painter().clip_rect());
+}
+
 PaintableWithLines::PaintableWithLines(Layout::BlockContainer const& layout_box)
     : PaintableBox(layout_box)
 {

Userland/Libraries/LibWeb/Painting/PaintableBox.h

@@ -117,6 +117,8 @@ public:
 
     void invalidate_stacking_context();
 
+    bool is_out_of_view(PaintContext&) const;
+
 protected:
     explicit PaintableBox(Layout::Box const&);