LibWeb: Separate layout tree rendering into phases

CSS defines a very specific paint order. This patch starts steering us
towards respecting that by introducing the PaintPhase enum with values:

- Background
- Border
- Foreground
- Overlay (internal overlays used by inspector)

Basically, to get the right visual result, we have to render the page
multiple times, going one phase at a time.

Libraries/LibWeb/Layout/LayoutBlock.cpp

@@ -600,19 +600,22 @@ void LayoutBlock::compute_height()
     }
 }
 
-void LayoutBlock::render(RenderingContext& context)
+void LayoutBlock::render(RenderingContext& context, PaintPhase phase)
 {
     if (!is_visible())
         return;
 
-    LayoutBox::render(context);
+    LayoutBox::render(context, phase);
 
-    if (children_are_inline()) {
-        for (auto& line_box : m_line_boxes) {
-            for (auto& fragment : line_box.fragments()) {
-                if (context.should_show_line_box_borders())
-                    context.painter().draw_rect(enclosing_int_rect(fragment.absolute_rect()), Color::Green);
-                fragment.render(context);
+    // FIXME: Inline backgrounds etc.
+    if (phase == PaintPhase::Foreground) {
+        if (children_are_inline()) {
+            for (auto& line_box : m_line_boxes) {
+                for (auto& fragment : line_box.fragments()) {
+                    if (context.should_show_line_box_borders())
+                        context.painter().draw_rect(enclosing_int_rect(fragment.absolute_rect()), Color::Green);
+                    fragment.render(context);
+                }
             }
         }
     }

Libraries/LibWeb/Layout/LayoutBlock.h

@@ -41,7 +41,7 @@ public:
     virtual const char* class_name() const override { return "LayoutBlock"; }
 
     virtual void layout(LayoutMode = LayoutMode::Default) override;
-    virtual void render(RenderingContext&) override;
+    virtual void render(RenderingContext&, PaintPhase) override;
 
     virtual LayoutNode& inline_wrapper() override;
 

Libraries/LibWeb/Layout/LayoutBox.cpp

@@ -31,9 +31,6 @@
 #include <LibWeb/Layout/LayoutBlock.h>
 #include <LibWeb/Layout/LayoutBox.h>
 
-//#define DRAW_BOXES_AROUND_LAYOUT_NODES
-//#define DRAW_BOXES_AROUND_HOVERED_NODES
-
 namespace Web {
 
 void LayoutBox::paint_border(RenderingContext& context, Edge edge, const Gfx::FloatRect& rect, CSS::PropertyID style_property_id, CSS::PropertyID color_property_id, CSS::PropertyID width_property_id)
@@ -189,7 +186,7 @@ void LayoutBox::paint_border(RenderingContext& context, Edge edge, const Gfx::Fl
     }
 }
 
-void LayoutBox::render(RenderingContext& context)
+void LayoutBox::render(RenderingContext& context, PaintPhase phase)
 {
     if (!is_visible())
         return;
@@ -198,21 +195,14 @@ void LayoutBox::render(RenderingContext& context)
     if (is_fixed_position())
         context.painter().translate(context.scroll_offset());
 
-#ifdef DRAW_BOXES_AROUND_LAYOUT_NODES
-    context.painter().draw_rect(m_rect, Color::Blue);
-#endif
-#ifdef DRAW_BOXES_AROUND_HOVERED_NODES
-    if (!is_anonymous() && node() == document().hovered_node())
-        context.painter().draw_rect(m_rect, Color::Red);
-#endif
-
     Gfx::FloatRect padded_rect;
     padded_rect.set_x(absolute_x() - box_model().padding().left.to_px(*this));
     padded_rect.set_width(width() + box_model().padding().left.to_px(*this) + box_model().padding().right.to_px(*this));
     padded_rect.set_y(absolute_y() - box_model().padding().top.to_px(*this));
     padded_rect.set_height(height() + box_model().padding().top.to_px(*this) + box_model().padding().bottom.to_px(*this));
 
-    if (!is_body()) {
+    if (phase == PaintPhase::Background && !is_body()) {
+        // FIXME: We should paint the body here too, but that currently happens at the view layer.
         auto bgcolor = style().property(CSS::PropertyID::BackgroundColor);
         if (bgcolor.has_value() && bgcolor.value()->is_color()) {
             context.painter().fill_rect(enclosing_int_rect(padded_rect), bgcolor.value()->to_color(document()));
@@ -227,20 +217,22 @@ void LayoutBox::render(RenderingContext& context)
         }
     }
 
-    Gfx::FloatRect bordered_rect;
-    bordered_rect.set_x(padded_rect.x() - box_model().border().left.to_px(*this));
-    bordered_rect.set_width(padded_rect.width() + box_model().border().left.to_px(*this) + box_model().border().right.to_px(*this));
-    bordered_rect.set_y(padded_rect.y() - box_model().border().top.to_px(*this));
-    bordered_rect.set_height(padded_rect.height() + box_model().border().top.to_px(*this) + box_model().border().bottom.to_px(*this));
+    if (phase == PaintPhase::Border) {
+        Gfx::FloatRect bordered_rect;
+        bordered_rect.set_x(padded_rect.x() - box_model().border().left.to_px(*this));
+        bordered_rect.set_width(padded_rect.width() + box_model().border().left.to_px(*this) + box_model().border().right.to_px(*this));
+        bordered_rect.set_y(padded_rect.y() - box_model().border().top.to_px(*this));
+        bordered_rect.set_height(padded_rect.height() + box_model().border().top.to_px(*this) + box_model().border().bottom.to_px(*this));
 
-    paint_border(context, Edge::Left, bordered_rect, CSS::PropertyID::BorderLeftStyle, CSS::PropertyID::BorderLeftColor, CSS::PropertyID::BorderLeftWidth);
-    paint_border(context, Edge::Right, bordered_rect, CSS::PropertyID::BorderRightStyle, CSS::PropertyID::BorderRightColor, CSS::PropertyID::BorderRightWidth);
-    paint_border(context, Edge::Top, bordered_rect, CSS::PropertyID::BorderTopStyle, CSS::PropertyID::BorderTopColor, CSS::PropertyID::BorderTopWidth);
-    paint_border(context, Edge::Bottom, bordered_rect, CSS::PropertyID::BorderBottomStyle, CSS::PropertyID::BorderBottomColor, CSS::PropertyID::BorderBottomWidth);
+        paint_border(context, Edge::Left, bordered_rect, CSS::PropertyID::BorderLeftStyle, CSS::PropertyID::BorderLeftColor, CSS::PropertyID::BorderLeftWidth);
+        paint_border(context, Edge::Right, bordered_rect, CSS::PropertyID::BorderRightStyle, CSS::PropertyID::BorderRightColor, CSS::PropertyID::BorderRightWidth);
+        paint_border(context, Edge::Top, bordered_rect, CSS::PropertyID::BorderTopStyle, CSS::PropertyID::BorderTopColor, CSS::PropertyID::BorderTopWidth);
+        paint_border(context, Edge::Bottom, bordered_rect, CSS::PropertyID::BorderBottomStyle, CSS::PropertyID::BorderBottomColor, CSS::PropertyID::BorderBottomWidth);
+    }
 
-    LayoutNodeWithStyleAndBoxModelMetrics::render(context);
+    LayoutNodeWithStyleAndBoxModelMetrics::render(context, phase);
 
-    if (node() && document().inspected_node() == node())
+    if (phase == PaintPhase::Overlay && node() && document().inspected_node() == node())
         context.painter().draw_rect(enclosing_int_rect(absolute_rect()), Color::Magenta);
 }
 

Libraries/LibWeb/Layout/LayoutBox.h

@@ -67,7 +67,7 @@ public:
     void set_stacking_context(NonnullOwnPtr<StackingContext> context) { m_stacking_context = move(context); }
     StackingContext* enclosing_stacking_context();
 
-    virtual void render(RenderingContext&) override;
+    virtual void render(RenderingContext&, PaintPhase) override;
 
 protected:
     LayoutBox(const Node* node, NonnullRefPtr<StyleProperties> style)

Libraries/LibWeb/Layout/LayoutCanvas.cpp

@@ -49,18 +49,21 @@ void LayoutCanvas::layout(LayoutMode layout_mode)
     LayoutReplaced::layout(layout_mode);
 }
 
-void LayoutCanvas::render(RenderingContext& context)
+void LayoutCanvas::render(RenderingContext& context, PaintPhase phase)
 {
     if (!is_visible())
         return;
 
-    // FIXME: This should be done at a different level. Also rect() does not include padding etc!
-    if (!context.viewport_rect().intersects(enclosing_int_rect(absolute_rect())))
-        return;
+    LayoutReplaced::render(context, phase);
 
-    if (node().bitmap())
-        context.painter().draw_scaled_bitmap(enclosing_int_rect(absolute_rect()), *node().bitmap(), node().bitmap()->rect());
-    LayoutReplaced::render(context);
+    if (phase == PaintPhase::Foreground) {
+        // FIXME: This should be done at a different level. Also rect() does not include padding etc!
+        if (!context.viewport_rect().intersects(enclosing_int_rect(absolute_rect())))
+            return;
+
+        if (node().bitmap())
+            context.painter().draw_scaled_bitmap(enclosing_int_rect(absolute_rect()), *node().bitmap(), node().bitmap()->rect());
+    }
 }
 
 }

Libraries/LibWeb/Layout/LayoutCanvas.h

@@ -39,7 +39,7 @@ public:
     virtual ~LayoutCanvas() override;
 
     virtual void layout(LayoutMode = LayoutMode::Default) override;
-    virtual void render(RenderingContext&) override;
+    virtual void render(RenderingContext&, PaintPhase) override;
 
     const HTMLCanvasElement& node() const { return static_cast<const HTMLCanvasElement&>(LayoutReplaced::node()); }
 

Libraries/LibWeb/Layout/LayoutDocument.cpp

@@ -100,9 +100,17 @@ void LayoutDocument::did_set_viewport_rect(Badge<Frame>, const Gfx::IntRect& a_v
     });
 }
 
-void LayoutDocument::render(RenderingContext& context)
+void LayoutDocument::paint_all_phases(RenderingContext& context)
 {
-    stacking_context()->render(context);
+    render(context, PaintPhase::Background);
+    render(context, PaintPhase::Border);
+    render(context, PaintPhase::Foreground);
+    render(context, PaintPhase::Overlay);
+}
+
+void LayoutDocument::render(RenderingContext& context, PaintPhase phase)
+{
+    stacking_context()->render(context, phase);
 }
 
 }

Libraries/LibWeb/Layout/LayoutDocument.h

@@ -40,7 +40,9 @@ public:
     virtual const char* class_name() const override { return "LayoutDocument"; }
     virtual void layout(LayoutMode = LayoutMode::Default) override;
 
-    virtual void render(RenderingContext&) override;
+    void paint_all_phases(RenderingContext&);
+
+    virtual void render(RenderingContext&, PaintPhase) override;
 
     const LayoutRange& selection() const { return m_selection; }
     LayoutRange& selection() { return m_selection; }

Libraries/LibWeb/Layout/LayoutFrame.cpp

@@ -59,28 +59,30 @@ void LayoutFrame::layout(LayoutMode layout_mode)
     LayoutReplaced::layout(layout_mode);
 }
 
-void LayoutFrame::render(RenderingContext& context)
+void LayoutFrame::render(RenderingContext& context, PaintPhase phase)
 {
-    LayoutReplaced::render(context);
+    LayoutReplaced::render(context, phase);
 
-    auto* hosted_document = node().hosted_document();
-    if (!hosted_document)
-        return;
-    auto* hosted_layout_tree = hosted_document->layout_node();
-    if (!hosted_layout_tree)
-        return;
+    if (phase == PaintPhase::Foreground) {
+        auto* hosted_document = node().hosted_document();
+        if (!hosted_document)
+            return;
+        auto* hosted_layout_tree = hosted_document->layout_node();
+        if (!hosted_layout_tree)
+            return;
 
-    context.painter().save();
-    auto old_viewport_rect = context.viewport_rect();
+        context.painter().save();
+        auto old_viewport_rect = context.viewport_rect();
 
-    context.painter().add_clip_rect(enclosing_int_rect(absolute_rect()));
-    context.painter().translate(absolute_x(), absolute_y());
+        context.painter().add_clip_rect(enclosing_int_rect(absolute_rect()));
+        context.painter().translate(absolute_x(), absolute_y());
 
-    context.set_viewport_rect({ {}, node().hosted_frame()->size() });
-    const_cast<LayoutDocument*>(hosted_layout_tree)->render(context);
+        context.set_viewport_rect({ {}, node().hosted_frame()->size() });
+        const_cast<LayoutDocument*>(hosted_layout_tree)->paint_all_phases(context);
 
-    context.set_viewport_rect(old_viewport_rect);
-    context.painter().restore();
+        context.set_viewport_rect(old_viewport_rect);
+        context.painter().restore();
+    }
 }
 
 void LayoutFrame::did_set_rect()

Libraries/LibWeb/Layout/LayoutFrame.h

@@ -36,7 +36,7 @@ public:
     LayoutFrame(const Element&, NonnullRefPtr<StyleProperties>);
     virtual ~LayoutFrame() override;
 
-    virtual void render(RenderingContext&) override;
+    virtual void render(RenderingContext&, PaintPhase) override;
     virtual void layout(LayoutMode) override;
 
     const HTMLIFrameElement& node() const { return static_cast<const HTMLIFrameElement&>(LayoutReplaced::node()); }

Libraries/LibWeb/Layout/LayoutImage.cpp

@@ -77,7 +77,7 @@ void LayoutImage::layout(LayoutMode layout_mode)
     LayoutReplaced::layout(layout_mode);
 }
 
-void LayoutImage::render(RenderingContext& context)
+void LayoutImage::render(RenderingContext& context, PaintPhase phase)
 {
     if (!is_visible())
         return;
@@ -86,18 +86,20 @@ void LayoutImage::render(RenderingContext& context)
     if (!context.viewport_rect().intersects(enclosing_int_rect(absolute_rect())))
         return;
 
-    LayoutReplaced::render(context);
+    LayoutReplaced::render(context, phase);
 
-    if (renders_as_alt_text()) {
-        auto& image_element = to<HTMLImageElement>(node());
-        context.painter().set_font(Gfx::Font::default_font());
-        Gfx::StylePainter::paint_frame(context.painter(), enclosing_int_rect(absolute_rect()), context.palette(), Gfx::FrameShape::Container, Gfx::FrameShadow::Sunken, 2);
-        auto alt = image_element.alt();
-        if (alt.is_empty())
-            alt = image_element.src();
-        context.painter().draw_text(enclosing_int_rect(absolute_rect()), alt, Gfx::TextAlignment::Center, style().color_or_fallback(CSS::PropertyID::Color, document(), Color::Black), Gfx::TextElision::Right);
-    } else if (m_image_loader.bitmap()) {
-        context.painter().draw_scaled_bitmap(enclosing_int_rect(absolute_rect()), *m_image_loader.bitmap(), m_image_loader.bitmap()->rect());
+    if (phase == PaintPhase::Foreground) {
+        if (renders_as_alt_text()) {
+            auto& image_element = to<HTMLImageElement>(node());
+            context.painter().set_font(Gfx::Font::default_font());
+            Gfx::StylePainter::paint_frame(context.painter(), enclosing_int_rect(absolute_rect()), context.palette(), Gfx::FrameShape::Container, Gfx::FrameShadow::Sunken, 2);
+            auto alt = image_element.alt();
+            if (alt.is_empty())
+                alt = image_element.src();
+            context.painter().draw_text(enclosing_int_rect(absolute_rect()), alt, Gfx::TextAlignment::Center, style().color_or_fallback(CSS::PropertyID::Color, document(), Color::Black), Gfx::TextElision::Right);
+        } else if (m_image_loader.bitmap()) {
+            context.painter().draw_scaled_bitmap(enclosing_int_rect(absolute_rect()), *m_image_loader.bitmap(), m_image_loader.bitmap()->rect());
+        }
     }
 }
 

Libraries/LibWeb/Layout/LayoutImage.h

@@ -39,7 +39,7 @@ public:
     virtual ~LayoutImage() override;
 
     virtual void layout(LayoutMode = LayoutMode::Default) override;
-    virtual void render(RenderingContext&) override;
+    virtual void render(RenderingContext&, PaintPhase) override;
 
     const Element& node() const { return static_cast<const Element&>(LayoutReplaced::node()); }
 

Libraries/LibWeb/Layout/LayoutListItemMarker.cpp

@@ -38,8 +38,10 @@ LayoutListItemMarker::~LayoutListItemMarker()
 {
 }
 
-void LayoutListItemMarker::render(RenderingContext& context)
+void LayoutListItemMarker::render(RenderingContext& context, PaintPhase phase)
 {
+    if (phase != PaintPhase::Foreground)
+        return;
     Gfx::IntRect bullet_rect { 0, 0, 4, 4 };
     bullet_rect.center_within(enclosing_int_rect(absolute_rect()));
     // FIXME: It would be nicer to not have to go via the parent here to get our inherited style.

Libraries/LibWeb/Layout/LayoutListItemMarker.h

@@ -35,7 +35,7 @@ public:
     LayoutListItemMarker();
     virtual ~LayoutListItemMarker() override;
 
-    virtual void render(RenderingContext&) override;
+    virtual void render(RenderingContext&, PaintPhase) override;
 
 private:
     virtual const char* class_name() const override { return "LayoutListItemMarker"; }

Libraries/LibWeb/Layout/LayoutNode.cpp

@@ -89,7 +89,7 @@ const LayoutBlock* LayoutNode::containing_block() const
     return nearest_block_ancestor();
 }
 
-void LayoutNode::render(RenderingContext& context)
+void LayoutNode::render(RenderingContext& context, PaintPhase phase)
 {
     if (!is_visible())
         return;
@@ -97,7 +97,7 @@ void LayoutNode::render(RenderingContext& context)
     for_each_child([&](auto& child) {
         if (child.is_box() && to<LayoutBox>(child).stacking_context())
             return;
-        child.render(context);
+        child.render(context, phase);
     });
 }
 

Libraries/LibWeb/Layout/LayoutNode.h

@@ -171,7 +171,14 @@ public:
     };
 
     virtual void layout(LayoutMode);
-    virtual void render(RenderingContext&);
+
+    enum class PaintPhase {
+        Background,
+        Border,
+        Foreground,
+        Overlay,
+    };
+    virtual void render(RenderingContext&, PaintPhase);
 
     bool is_absolutely_positioned() const;
     bool is_fixed_position() const;

Libraries/LibWeb/Layout/LayoutWidget.cpp

@@ -65,9 +65,4 @@ void LayoutWidget::update_widget()
     widget().move_to(adjusted_widget_position);
 }
 
-void LayoutWidget::render(RenderingContext& context)
-{
-    LayoutReplaced::render(context);
-}
-
 }

Libraries/LibWeb/Layout/LayoutWidget.h

@@ -35,8 +35,6 @@ public:
     LayoutWidget(const Element&, GUI::Widget&);
     virtual ~LayoutWidget() override;
 
-    virtual void render(RenderingContext&) override;
-
     GUI::Widget& widget() { return m_widget; }
     const GUI::Widget& widget() const { return m_widget; }
 

Libraries/LibWeb/Layout/StackingContext.cpp

@@ -47,17 +47,17 @@ StackingContext::StackingContext(LayoutBox& box, StackingContext* parent)
     }
 }
 
-void StackingContext::render(RenderingContext& context)
+void StackingContext::render(RenderingContext& context, LayoutNode::PaintPhase phase)
 {
     if (!m_box.is_root()) {
-        m_box.render(context);
+        m_box.render(context, phase);
     } else {
         // NOTE: LayoutDocument::render() merely calls StackingContext::render()
         //       so we call its base class instead.
-        to<LayoutDocument>(m_box).LayoutBlock::render(context);
+        to<LayoutDocument>(m_box).LayoutBlock::render(context, phase);
     }
     for (auto* child : m_children) {
-        child->render(context);
+        child->render(context, phase);
     }
 }
 

Libraries/LibWeb/Layout/StackingContext.h

@@ -27,7 +27,7 @@
 #pragma once
 
 #include <AK/Vector.h>
-#include <LibWeb/Forward.h>
+#include <LibWeb/Layout/LayoutNode.h>
 
 namespace Web {
 
@@ -40,7 +40,7 @@ public:
     StackingContext* parent() { return m_parent; }
     const StackingContext* parent() const { return m_parent; }
 
-    void render(RenderingContext&);
+    void render(RenderingContext&, LayoutNode::PaintPhase);
 
     void dump(int indent = 0) const;
 

Libraries/LibWeb/PageView.cpp

@@ -210,7 +210,7 @@ void PageView::paint_event(GUI::PaintEvent& event)
     RenderingContext context(painter, palette(), { horizontal_scrollbar().value(), vertical_scrollbar().value() });
     context.set_should_show_line_box_borders(m_should_show_line_box_borders);
     context.set_viewport_rect(viewport_rect_in_content_coordinates());
-    layout_root()->render(context);
+    layout_root()->paint_all_phases(context);
 }
 
 void PageView::mousemove_event(GUI::MouseEvent& event)

Services/WebContent/PageHost.cpp

@@ -88,7 +88,7 @@ void PageHost::paint(const Gfx::IntRect& content_rect, Gfx::Bitmap& target)
 
     Web::RenderingContext context(painter, palette(), Gfx::IntPoint());
     context.set_viewport_rect(content_rect);
-    layout_root->render(context);
+    layout_root->paint_all_phases(context);
 }
 
 void PageHost::set_viewport_rect(const Gfx::IntRect& rect)