LibWeb: Try to work out the intrinsic size of <svg> elements

If the `width` and `height` attributes are provided, we derive the
intrinsic size and ratio from them.

Otherwise, we trace a rectangle around the geometry elements inside
the SVG and use the size of that as the intrinsic size.

This is definitely far from correct, but is still a much better guess
at the intrinsic size than nothing.

Userland/Libraries/LibWeb/Layout/SVGSVGBox.cpp

@@ -1,10 +1,13 @@
 /*
  * Copyright (c) 2020, Matthew Olsson <mattco@serenityos.org>
+ * Copyright (c) 2022, Andreas Kling <kling@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include <LibWeb/HTML/Parser/HTMLParser.h>
 #include <LibWeb/Layout/ReplacedBox.h>
+#include <LibWeb/Layout/SVGGeometryBox.h>
 #include <LibWeb/Painting/SVGSVGPaintable.h>
 
 namespace Web::Layout {
@@ -19,4 +22,72 @@ RefPtr<Painting::Paintable> SVGSVGBox::create_paintable() const
     return Painting::SVGSVGPaintable::create(*this);
 }
 
+void SVGSVGBox::prepare_for_replaced_layout()
+{
+    if (dom_node().has_attribute(HTML::AttributeNames::width) && dom_node().has_attribute(HTML::AttributeNames::height)) {
+        Optional<float> w;
+        Optional<float> h;
+        if (auto width = HTML::parse_dimension_value(dom_node().attribute(HTML::AttributeNames::width))) {
+            if (width->has_length())
+                w = width->to_length().to_px(*this);
+        }
+        if (auto height = HTML::parse_dimension_value(dom_node().attribute(HTML::AttributeNames::height))) {
+            if (height->has_length())
+                h = height->to_length().to_px(*this);
+        }
+        if (w.has_value() && h.has_value()) {
+            set_intrinsic_width(*w);
+            set_intrinsic_height(*h);
+            set_intrinsic_aspect_ratio(*w / *h);
+            return;
+        }
+    }
+
+    Optional<Gfx::FloatRect> united_rect;
+
+    auto add_to_united_rect = [&](Gfx::FloatRect const& rect) {
+        if (united_rect.has_value())
+            united_rect = united_rect->united(rect);
+        else
+            united_rect = rect;
+    };
+
+    for_each_in_subtree_of_type<SVGGeometryBox>([&](SVGGeometryBox const& geometry_box) {
+        auto& dom_node = const_cast<SVG::SVGGeometryElement&>(geometry_box.dom_node());
+        if (dom_node.has_attribute(HTML::AttributeNames::width) && dom_node.has_attribute(HTML::AttributeNames::height)) {
+            Gfx::FloatRect rect;
+            // FIXME: Allow for relative lengths here
+            rect.set_width(computed_values().width().resolved(*this, CSS::Length::make_px(0)).to_px(*this));
+            rect.set_height(computed_values().height().resolved(*this, CSS::Length::make_px(0)).to_px(*this));
+            add_to_united_rect(rect);
+            return IterationDecision::Continue;
+        }
+
+        auto& path = dom_node.get_path();
+        auto path_bounding_box = path.bounding_box();
+
+        // Stroke increases the path's size by stroke_width/2 per side.
+        auto stroke_width = geometry_box.dom_node().stroke_width().value_or(0);
+        path_bounding_box.inflate(stroke_width, stroke_width);
+
+        auto& maybe_view_box = this->dom_node().view_box();
+
+        if (maybe_view_box.has_value()) {
+            auto view_box = maybe_view_box.value();
+            Gfx::FloatRect rect(view_box.min_x, view_box.min_y, view_box.width, view_box.height);
+            add_to_united_rect(rect);
+            return IterationDecision::Continue;
+        }
+
+        add_to_united_rect(path_bounding_box);
+        return IterationDecision::Continue;
+    });
+
+    if (united_rect.has_value()) {
+        set_intrinsic_width(united_rect->width());
+        set_intrinsic_height(united_rect->height());
+        set_intrinsic_aspect_ratio(united_rect->width() / united_rect->height());
+    }
+}
+
 }

Userland/Libraries/LibWeb/Layout/SVGSVGBox.h

@@ -21,6 +21,8 @@ public:
     virtual bool can_have_children() const override { return true; }
 
     virtual RefPtr<Painting::Paintable> create_paintable() const override;
+
+    virtual void prepare_for_replaced_layout() override;
 };
 
 }