LibHTML: Implement the <blink> element

Just in time for Serenity's 1st birthday, here is the <blink> element!

This patch adds a bunch of different mechanisms to enable partial
repaints of the layout tree (LayoutNode::set_needs_display()))
It also adds LayoutNode::is_visible(), which can be toggled to prevent
a LayoutNode from rendering anything (it still takes up space though.)

Libraries/LibHTML/Layout/LayoutBlock.cpp

@@ -194,6 +194,9 @@ void LayoutBlock::compute_height()
 
 void LayoutBlock::render(RenderingContext& context)
 {
+    if (!is_visible())
+        return;
+
     LayoutNode::render(context);
 
     // FIXME: position this properly

Libraries/LibHTML/Layout/LayoutBlock.h

@@ -38,3 +38,15 @@ private:
 
     Vector<LineBox> m_line_boxes;
 };
+
+template<typename Callback>
+void LayoutNode::for_each_fragment_of_this(Callback callback)
+{
+    auto& block = *containing_block();
+    for (auto& line_box : block.line_boxes()) {
+        for (auto& fragment : line_box.fragments()) {
+            if (callback(fragment) == IterationDecision::Break)
+                return;
+        }
+    }
+}

Libraries/LibHTML/Layout/LayoutImage.cpp

@@ -34,6 +34,9 @@ void LayoutImage::layout()
 
 void LayoutImage::render(RenderingContext& context)
 {
+    if (!is_visible())
+        return;
+
     if (renders_as_alt_text()) {
         context.painter().set_font(Font::default_font());
         StylePainter::paint_frame(context.painter(), rect(), FrameShape::Container, FrameShadow::Sunken, 2);

Libraries/LibHTML/Layout/LayoutNode.cpp

@@ -1,6 +1,7 @@
 #include <LibGUI/GPainter.h>
 #include <LibHTML/DOM/Document.h>
 #include <LibHTML/DOM/Element.h>
+#include <LibHTML/Frame.h>
 #include <LibHTML/Layout/LayoutBlock.h>
 #include <LibHTML/Layout/LayoutNode.h>
 
@@ -38,6 +39,9 @@ const LayoutBlock* LayoutNode::containing_block() const
 
 void LayoutNode::render(RenderingContext& context)
 {
+    if (!is_visible())
+        return;
+
 #ifdef DRAW_BOXES_AROUND_LAYOUT_NODES
     context.painter().draw_rect(m_rect, Color::Blue);
 #endif
@@ -127,3 +131,21 @@ void LayoutNode::split_into_lines(LayoutBlock& container)
         }
     });
 }
+
+void LayoutNode::set_needs_display()
+{
+    auto* frame = document().frame();
+    ASSERT(frame);
+
+    if (!is_inline()) {
+        const_cast<Frame*>(frame)->set_needs_display(rect());
+        return;
+    }
+
+    for_each_fragment_of_this([&](auto& fragment) {
+        if (&fragment.layout_node() == this || is_ancestor_of(fragment.layout_node())) {
+            const_cast<Frame*>(frame)->set_needs_display(fragment.rect());
+        }
+        return IterationDecision::Continue;
+    });
+}

Libraries/LibHTML/Layout/LayoutNode.h

@@ -76,6 +76,16 @@ public:
 
     virtual void split_into_lines(LayoutBlock& container);
 
+    bool is_visible() const { return m_visible; }
+    void set_visible(bool visible) { m_visible = visible; }
+
+    void set_needs_display();
+
+    bool is_ancestor_of(const LayoutNode&) const;
+
+    template<typename Callback>
+    void for_each_fragment_of_this(Callback);
+
 protected:
     explicit LayoutNode(const Node*);
 
@@ -88,6 +98,7 @@ private:
     Rect m_rect;
     bool m_inline { false };
     bool m_has_style { false };
+    bool m_visible { true };
 };
 
 class LayoutNodeWithStyle : public LayoutNode {
@@ -119,3 +130,12 @@ inline const LayoutNodeWithStyle* LayoutNode::parent() const
 {
     return static_cast<const LayoutNodeWithStyle*>(TreeNode<LayoutNode>::parent());
 }
+
+inline bool LayoutNode::is_ancestor_of(const LayoutNode& other) const
+{
+    for (auto* ancestor = other.parent(); ancestor; ancestor = ancestor->parent()) {
+        if (ancestor == this)
+            return true;
+    }
+    return false;
+}

Libraries/LibHTML/Layout/LineBoxFragment.cpp

@@ -5,6 +5,11 @@
 
 void LineBoxFragment::render(RenderingContext& context)
 {
+    for (auto* ancestor = layout_node().parent(); ancestor; ancestor = ancestor->parent()) {
+        if (!ancestor->is_visible())
+            return;
+    }
+
     if (layout_node().is_text()) {
         auto& layout_text = static_cast<const LayoutText&>(layout_node());
         layout_text.render_fragment(context, *this);