LibWeb: Add PaintableBox::absolute_paint_rect()

This method returns the total area this element will paint to.
Currently, this just means accounting for box-shadows, though
there are likely more effects that need to be accounted for here.

Userland/Libraries/LibWeb/Painting/PaintableBox.cpp

@@ -13,7 +13,6 @@
 #include <LibWeb/Painting/BackgroundPainting.h>
 #include <LibWeb/Painting/FilterPainting.h>
 #include <LibWeb/Painting/PaintableBox.h>
-#include <LibWeb/Painting/ShadowPainting.h>
 #include <LibWeb/Painting/StackingContext.h>
 #include <LibWeb/Platform/FontPlugin.h>
 
@@ -92,6 +91,28 @@ Gfx::FloatRect PaintableBox::absolute_rect() const
     return *m_absolute_rect;
 }
 
+Gfx::FloatRect PaintableBox::compute_absolute_paint_rect() const
+{
+    // FIXME: This likely incomplete:
+    auto rect = absolute_border_box_rect();
+    auto resolved_box_shadow_data = resolve_box_shadow_data();
+    for (auto const& shadow : resolved_box_shadow_data) {
+        if (shadow.placement == ShadowPlacement::Inner)
+            continue;
+        auto inflate = shadow.spread_distance + shadow.blur_radius;
+        auto shadow_rect = rect.inflated(inflate, inflate, inflate, inflate).translated(shadow.offset_x, shadow.offset_y);
+        rect = rect.united(shadow_rect);
+    }
+    return rect;
+}
+
+Gfx::FloatRect PaintableBox::absolute_paint_rect() const
+{
+    if (!m_absolute_paint_rect.has_value())
+        m_absolute_paint_rect = compute_absolute_paint_rect();
+    return *m_absolute_paint_rect;
+}
+
 void PaintableBox::set_containing_line_box_fragment(Optional<Layout::LineBoxFragmentCoordinate> fragment_coordinate)
 {
     m_containing_line_box_fragment = fragment_coordinate;
@@ -235,11 +256,11 @@ void PaintableBox::paint_background(PaintContext& context) const
     Painting::paint_background(context, layout_box(), background_rect, background_color, computed_values().image_rendering(), background_layers, normalized_border_radii_data());
 }
 
-void PaintableBox::paint_box_shadow(PaintContext& context) const
+Vector<ShadowData> PaintableBox::resolve_box_shadow_data() const
 {
     auto box_shadow_data = computed_values().box_shadow();
     if (box_shadow_data.is_empty())
-        return;
+        return {};
 
     Vector<ShadowData> resolved_box_shadow_data;
     resolved_box_shadow_data.ensure_capacity(box_shadow_data.size());
@@ -252,6 +273,15 @@ void PaintableBox::paint_box_shadow(PaintContext& context) const
             static_cast<int>(layer.spread_distance.to_px(layout_box())),
             layer.placement == CSS::ShadowPlacement::Outer ? ShadowPlacement::Outer : ShadowPlacement::Inner);
     }
+
+    return resolved_box_shadow_data;
+}
+
+void PaintableBox::paint_box_shadow(PaintContext& context) const
+{
+    auto resolved_box_shadow_data = resolve_box_shadow_data();
+    if (resolved_box_shadow_data.is_empty())
+        return;
     Painting::paint_box_shadow(context, absolute_border_box_rect().to_rounded<int>(), normalized_border_radii_data(), resolved_box_shadow_data);
 }
 

Userland/Libraries/LibWeb/Painting/PaintableBox.h

@@ -9,6 +9,7 @@
 #include <LibWeb/Painting/BorderPainting.h>
 #include <LibWeb/Painting/BorderRadiusCornerClipper.h>
 #include <LibWeb/Painting/Paintable.h>
+#include <LibWeb/Painting/ShadowPainting.h>
 
 namespace Web::Painting {
 
@@ -68,6 +69,8 @@ public:
         return rect;
     }
 
+    Gfx::FloatRect absolute_paint_rect() const;
+
     float border_box_width() const
     {
         auto border_box = box_model().border_box();
@@ -123,6 +126,7 @@ protected:
     virtual void paint_box_shadow(PaintContext&) const;
 
     virtual Gfx::FloatRect compute_absolute_rect() const;
+    virtual Gfx::FloatRect compute_absolute_paint_rect() const;
 
     enum class ShrinkRadiiForBorders {
         Yes,
@@ -131,6 +135,8 @@ protected:
 
     Painting::BorderRadiiData normalized_border_radii_data(ShrinkRadiiForBorders shrink = ShrinkRadiiForBorders::No) const;
 
+    Vector<ShadowData> resolve_box_shadow_data() const;
+
 private:
     Optional<OverflowData> m_overflow_data;
 
@@ -143,6 +149,7 @@ private:
     OwnPtr<Painting::StackingContext> m_stacking_context;
 
     Optional<Gfx::FloatRect> mutable m_absolute_rect;
+    Optional<Gfx::FloatRect> mutable m_absolute_paint_rect;
 
     mutable bool m_clipping_overflow { false };
     Optional<BorderRadiusCornerClipper> mutable m_overflow_corner_radius_clipper;