LibWeb: Special case SVG masks during layout

Rather than try to lay out masks normally, this updates the TreeBuilder
to create layout nodes for masks as a child of their user (i.e. the
masked element). This allows each use of a mask to be laid out
differently, which makes supporting `maskContentUnits=objectBoundingBox`
fairly easy.

The `SVGFormattingContext` is then updated to lay out masks last (as
their sizing may depend on their parent), and treats them like
viewports.

This is pretty ad-hoc, but the SVG specification does not give any
guidance on how to actually implement this.

Userland/Libraries/LibWeb/SVG/SVGDefsElement.cpp

@@ -27,10 +27,4 @@ void SVGDefsElement::initialize(JS::Realm& realm)
     set_prototype(&Bindings::ensure_web_prototype<Bindings::SVGDefsElementPrototype>(realm, "SVGDefsElement"_fly_string));
 }
 
-JS::GCPtr<Layout::Node> SVGDefsElement::create_layout_node(NonnullRefPtr<CSS::StyleProperties> style)
-{
-    // FIXME: We need this layout node so any <mask>s inside this element get layout computed.
-    return heap().allocate_without_realm<Layout::SVGBox>(document(), *this, move(style));
-}
-
 }

Userland/Libraries/LibWeb/SVG/SVGDefsElement.h

@@ -17,7 +17,10 @@ class SVGDefsElement final : public SVGGraphicsElement {
 public:
     virtual ~SVGDefsElement();
 
-    virtual JS::GCPtr<Layout::Node> create_layout_node(NonnullRefPtr<CSS::StyleProperties>) override;
+    virtual JS::GCPtr<Layout::Node> create_layout_node(NonnullRefPtr<CSS::StyleProperties>) override
+    {
+        return nullptr;
+    }
 
 private:
     SVGDefsElement(DOM::Document&, DOM::QualifiedName);

Userland/Libraries/LibWeb/SVG/SVGMaskElement.cpp

@@ -7,7 +7,6 @@
 
 #include <LibWeb/Bindings/SVGMaskElementPrototype.h>
 #include <LibWeb/DOM/Document.h>
-#include <LibWeb/Layout/SVGGraphicsBox.h>
 #include <LibWeb/SVG/AttributeNames.h>
 #include <LibWeb/SVG/SVGMaskElement.h>
 
@@ -28,9 +27,10 @@ void SVGMaskElement::initialize(JS::Realm& realm)
     set_prototype(&Bindings::ensure_web_prototype<Bindings::SVGMaskElementPrototype>(realm, "SVGMaskElement"_fly_string));
 }
 
-JS::GCPtr<Layout::Node> SVGMaskElement::create_layout_node(NonnullRefPtr<CSS::StyleProperties> style)
+JS::GCPtr<Layout::Node> SVGMaskElement::create_layout_node(NonnullRefPtr<CSS::StyleProperties>)
 {
-    return heap().allocate_without_realm<Layout::SVGGraphicsBox>(document(), *this, move(style));
+    // Masks are handled as a special case in the TreeBuilder.
+    return nullptr;
 }
 
 void SVGMaskElement::attribute_changed(FlyString const& name, Optional<String> const& value)

Userland/Libraries/LibWeb/SVG/SVGMaskElement.h

@@ -8,16 +8,34 @@
 
 #include <LibWeb/SVG/AttributeParser.h>
 #include <LibWeb/SVG/SVGGraphicsElement.h>
+#include <LibWeb/SVG/SVGViewport.h>
 
 namespace Web::SVG {
 
-class SVGMaskElement final : public SVGGraphicsElement {
+class SVGMaskElement final : public SVGGraphicsElement
+    , public SVGViewport {
+
     WEB_PLATFORM_OBJECT(SVGMaskElement, SVGGraphicsElement);
     JS_DECLARE_ALLOCATOR(SVGMaskElement);
 
 public:
     virtual ~SVGMaskElement() override;
 
+    virtual Optional<ViewBox> view_box() const override
+    {
+        // maskContentUnits = objectBoundingBox acts like the mask is sized to the bounding box
+        // of the target element, with a viewBox of "0 0 1 1".
+        if (mask_content_units() == MaskContentUnits::ObjectBoundingBox)
+            return ViewBox { 0, 0, 1, 1 };
+        return {};
+    }
+
+    virtual Optional<PreserveAspectRatio> preserve_aspect_ratio() const override
+    {
+        // preserveAspectRatio = none (allow mask to be scaled in both x and y to match target size)
+        return PreserveAspectRatio { PreserveAspectRatio::Align::None, {} };
+    }
+
     virtual void attribute_changed(FlyString const& name, Optional<String> const& value) override;
 
     virtual JS::GCPtr<Layout::Node> create_layout_node(NonnullRefPtr<CSS::StyleProperties>) override;

Userland/Libraries/LibWeb/SVG/SVGStopElement.cpp

@@ -6,8 +6,7 @@
 
 #include <LibWeb/Bindings/Intrinsics.h>
 #include <LibWeb/CSS/Parser/Parser.h>
-#include <LibWeb/CSS/StyleValues/IdentifierStyleValue.h>
-#include <LibWeb/Layout/BlockContainer.h>
+#include <LibWeb/CSS/StyleProperties.h>
 #include <LibWeb/SVG/AttributeNames.h>
 #include <LibWeb/SVG/AttributeParser.h>
 #include <LibWeb/SVG/SVGStopElement.h>