LibWeb: Reorganize layout algorithm

Previously, layout recursively performed these steps (roughly):

1. Compute own width
2. Compute own position
3. Layout in-flow children
4. Compute own height
5. Layout absolutely positioned descendants

However, step (2) was pretty inconsistent. Some things computed their
own position, others had their parent do it for them, etc.
To get closer to CSS spec language, and make things easier in general,
this patch reorganizes the algorithm into:

1. Compute own width & height
2. Compute width & height of in-flow managed descendants
3. Move in-flow managed descendants to their final position
4. Layout absolutely positioned descendants

Block layout is now driven by the containing block, which will iterate
the descendants it's responsible for. There are a lot of inefficient
patterns in this logic right now, but they can easily be replaced with
better iteration functions once we settle on a long-term architecture.

Since the ICB (LayoutDocument) is at (0, 0), it doesn't rely on a
containing block to move it into place.

This code is still evolving along with my understanding of CSS layout,
so it's likely that we'll reorganize this again sooner or later. :^)

Libraries/LibWeb/Dump.cpp

@@ -146,11 +146,6 @@ void dump_tree(const LayoutNode& layout_node)
 
     if (layout_node.is_block() && static_cast<const LayoutBlock&>(layout_node).children_are_inline()) {
         auto& block = static_cast<const LayoutBlock&>(layout_node);
-        if (block.absolutely_positioned_descendant_count()) {
-            for (size_t i = 0; i < indent; ++i)
-                dbgprintf("    ");
-            dbgprintf("  %zu absolutely positioned descendant(s) tracked here\n", block.absolutely_positioned_descendant_count());
-        }
         for (size_t i = 0; i < indent; ++i)
             dbgprintf("    ");
         dbgprintf("  Line boxes (%d):\n", block.line_boxes().size());

Libraries/LibWeb/Forward.h

@@ -53,6 +53,7 @@ class LayoutBlock;
 class LayoutDocument;
 class LayoutNode;
 class LayoutNodeWithStyle;
+class LayoutReplaced;
 class LoadRequest;
 class MouseEvent;
 class Node;

Libraries/LibWeb/Layout/LayoutBlock.cpp

@@ -59,103 +59,107 @@ LayoutNode& LayoutBlock::inline_wrapper()
 void LayoutBlock::layout(LayoutMode layout_mode)
 {
     compute_width();
-
-    if (!is_inline())
-        compute_position();
-
-    layout_children(layout_mode);
-
+    layout_inside(layout_mode);
     compute_height();
 
-    if (layout_mode == LayoutMode::Default)
-        layout_absolutely_positioned_descendants();
+    layout_absolutely_positioned_descendants();
 }
 
-void LayoutBlock::layout_absolutely_positioned_descendants()
+void LayoutBlock::layout_absolutely_positioned_descendant(LayoutBox& box)
 {
-    for (auto& box : m_absolutely_positioned_descendants) {
-        box->layout(LayoutMode::Default);
-        auto& box_model = box->box_model();
-        auto& style = box->style();
-        auto zero_value = Length(0, Length::Type::Px);
-
-        auto specified_width = style.length_or_fallback(CSS::PropertyID::Width, Length(), width());
-
-        box_model.margin().top = style.length_or_fallback(CSS::PropertyID::MarginTop, {}, height());
-        box_model.margin().right = style.length_or_fallback(CSS::PropertyID::MarginRight, {}, width());
-        box_model.margin().bottom = style.length_or_fallback(CSS::PropertyID::MarginBottom, {}, height());
-        box_model.margin().left = style.length_or_fallback(CSS::PropertyID::MarginLeft, {}, width());
-
-        box_model.offset().top = style.length_or_fallback(CSS::PropertyID::Top, {}, height());
-        box_model.offset().right = style.length_or_fallback(CSS::PropertyID::Right, {}, width());
-        box_model.offset().bottom = style.length_or_fallback(CSS::PropertyID::Bottom, {}, height());
-        box_model.offset().left = style.length_or_fallback(CSS::PropertyID::Left, {}, width());
-
-        if (box_model.offset().left.is_auto() && specified_width.is_auto() && box_model.offset().right.is_auto()) {
-            if (box_model.margin().left.is_auto())
-                box_model.margin().left = zero_value;
-            if (box_model.margin().right.is_auto())
-                box_model.margin().right = zero_value;
-        }
+    box.layout(LayoutMode::Default);
+    auto& box_model = box.box_model();
+    auto& style = box.style();
+    auto zero_value = Length(0, Length::Type::Px);
 
-        Gfx::FloatPoint used_offset;
+    auto specified_width = style.length_or_fallback(CSS::PropertyID::Width, Length(), width());
 
-        float x_offset = box_model.offset().left.to_px(*box)
-            + box_model.border_box(*box).left
-            - box_model.offset().right.to_px(*box)
-            - box_model.border_box(*box).right;
+    box_model.margin().top = style.length_or_fallback(CSS::PropertyID::MarginTop, {}, height());
+    box_model.margin().right = style.length_or_fallback(CSS::PropertyID::MarginRight, {}, width());
+    box_model.margin().bottom = style.length_or_fallback(CSS::PropertyID::MarginBottom, {}, height());
+    box_model.margin().left = style.length_or_fallback(CSS::PropertyID::MarginLeft, {}, width());
 
-        float y_offset = box_model.offset().top.to_px(*box)
-            + box_model.border_box(*box).top
-            - box_model.offset().bottom.to_px(*box)
-            - box_model.border_box(*box).bottom;
+    box_model.offset().top = style.length_or_fallback(CSS::PropertyID::Top, {}, height());
+    box_model.offset().right = style.length_or_fallback(CSS::PropertyID::Right, {}, width());
+    box_model.offset().bottom = style.length_or_fallback(CSS::PropertyID::Bottom, {}, height());
+    box_model.offset().left = style.length_or_fallback(CSS::PropertyID::Left, {}, width());
 
-        if (!box_model.offset().left.is_auto()) {
-            used_offset.set_x(x_offset + box_model.margin().left.to_px(*box));
-        } else if (!box_model.offset().right.is_auto()) {
-            used_offset.set_x(width() + x_offset - box->width() - box_model.margin().right.to_px(*box));
-        }
+    if (box_model.offset().left.is_auto() && specified_width.is_auto() && box_model.offset().right.is_auto()) {
+        if (box_model.margin().left.is_auto())
+            box_model.margin().left = zero_value;
+        if (box_model.margin().right.is_auto())
+            box_model.margin().right = zero_value;
+    }
 
-        if (!box_model.offset().top.is_auto()) {
-            used_offset.set_y(y_offset + box_model.margin().top.to_px(*box));
-        } else if (!box_model.offset().bottom.is_auto()) {
-            used_offset.set_y(height() + y_offset - box->height() - box_model.margin().bottom.to_px(*box));
-        }
+    Gfx::FloatPoint used_offset;
+
+    float x_offset = box_model.offset().left.to_px(box)
+        + box_model.border_box(box).left
+        - box_model.offset().right.to_px(box)
+        - box_model.border_box(box).right;
 
-        box->set_offset(used_offset);
+    float y_offset = box_model.offset().top.to_px(box)
+        + box_model.border_box(box).top
+        - box_model.offset().bottom.to_px(box)
+        - box_model.border_box(box).bottom;
+
+    if (!box_model.offset().left.is_auto()) {
+        used_offset.set_x(x_offset + box_model.margin().left.to_px(box));
+    } else if (!box_model.offset().right.is_auto()) {
+        used_offset.set_x(width() + x_offset - box.width() - box_model.margin().right.to_px(box));
     }
-}
 
-void LayoutBlock::add_absolutely_positioned_descendant(LayoutBox& box)
-{
-    m_absolutely_positioned_descendants.set(box);
+    if (!box_model.offset().top.is_auto()) {
+        used_offset.set_y(y_offset + box_model.margin().top.to_px(box));
+    } else if (!box_model.offset().bottom.is_auto()) {
+        used_offset.set_y(height() + y_offset - box.height() - box_model.margin().bottom.to_px(box));
+    }
+
+    box.set_offset(used_offset);
 }
 
-void LayoutBlock::layout_children(LayoutMode layout_mode)
+void LayoutBlock::layout_inside(LayoutMode layout_mode)
 {
     if (children_are_inline())
         layout_inline_children(layout_mode);
     else
-        layout_block_children(layout_mode);
+        layout_contained_boxes(layout_mode);
 }
 
-void LayoutBlock::layout_block_children(LayoutMode layout_mode)
+void LayoutBlock::layout_absolutely_positioned_descendants()
+{
+    for_each_in_subtree_of_type<LayoutBox>([&](auto& box) {
+        if (box.is_absolutely_positioned() && box.containing_block() == this) {
+            layout_absolutely_positioned_descendant(box);
+        }
+        return IterationDecision::Continue;
+    });
+}
+
+void LayoutBlock::layout_contained_boxes(LayoutMode layout_mode)
 {
-    ASSERT(!children_are_inline());
     float content_height = 0;
-    for_each_child_of_type<LayoutBlock>([&](auto& child) {
-        child.layout(layout_mode);
-        if (!child.is_absolutely_positioned())
-            content_height = max(content_height, child.effective_offset().y() + child.height() + child.box_model().margin_box(*this).bottom);
+    float content_width = 0;
+    for_each_in_subtree_of_type<LayoutBox>([&](auto& box) {
+        if (box.is_absolutely_positioned() || box.containing_block() != this)
+            return IterationDecision::Continue;
+        box.layout(layout_mode);
+        if (box.is_replaced())
+            place_block_level_replaced_element_in_normal_flow(to<LayoutReplaced>(box));
+        else if (box.is_block())
+            place_block_level_non_replaced_element_in_normal_flow(to<LayoutBlock>(box));
+        else {
+            dbg() << "FIXME: How do I place a " << box.class_name() << "?";
+            ASSERT_NOT_REACHED();
+        }
+        content_height = max(content_height, box.effective_offset().y() + box.height() + box.box_model().margin_box(*this).bottom);
+        content_width = max(content_width, to<LayoutBox>(box).width());
+        return IterationDecision::Continue;
     });
-    if (layout_mode != LayoutMode::Default) {
-        float max_width = 0;
-        for_each_child([&](auto& child) {
-            if (child.is_box() && !child.is_absolutely_positioned())
-                max_width = max(max_width, to<LayoutBox>(child).width());
-        });
-        set_width(max_width);
-    }
+
+    if (layout_mode != LayoutMode::Default)
+        set_width(content_width);
+
     set_height(content_height);
 }
 
@@ -165,10 +169,8 @@ void LayoutBlock::layout_inline_children(LayoutMode layout_mode)
     m_line_boxes.clear();
     for_each_child([&](auto& child) {
         ASSERT(child.is_inline());
-        if (child.is_box() && child.is_absolutely_positioned()) {
-            const_cast<LayoutBlock*>(child.containing_block())->add_absolutely_positioned_descendant((LayoutBox&)child);
+        if (child.is_absolutely_positioned())
             return;
-        }
         child.split_into_lines(*this, layout_mode);
     });
 
@@ -376,13 +378,13 @@ void LayoutBlock::compute_width()
 
                 // Calculate the preferred width by formatting the content without breaking lines
                 // other than where explicit line breaks occur.
-                layout_children(LayoutMode::OnlyRequiredLineBreaks);
+                layout_inside(LayoutMode::OnlyRequiredLineBreaks);
                 float preferred_width = greatest_child_width();
 
                 // Also calculate the preferred minimum width, e.g., by trying all possible line breaks.
                 // CSS 2.2 does not define the exact algorithm.
 
-                layout_children(LayoutMode::AllPossibleLineBreaks);
+                layout_inside(LayoutMode::AllPossibleLineBreaks);
                 float preferred_minimum_width = greatest_child_width();
 
                 // Then the shrink-to-fit width is: min(max(preferred minimum width, available width), preferred width).
@@ -425,37 +427,58 @@ void LayoutBlock::compute_width()
     box_model().padding().right = padding_right;
 }
 
-void LayoutBlock::compute_position()
+void LayoutBlock::place_block_level_replaced_element_in_normal_flow(LayoutReplaced& box)
 {
-    if (is_absolutely_positioned()) {
-        const_cast<LayoutBlock*>(containing_block())->add_absolutely_positioned_descendant(*this);
-        return;
-    }
-
-    auto& style = this->style();
+    ASSERT(!is_absolutely_positioned());
+    auto& style = box.style();
     auto zero_value = Length(0, Length::Type::Px);
-    auto& containing_block = *this->containing_block();
+    auto& containing_block = *this;
+    auto& replaced_element_box_model = box.box_model();
+
+    replaced_element_box_model.margin().top = style.length_or_fallback(CSS::PropertyID::MarginTop, zero_value, containing_block.width());
+    replaced_element_box_model.margin().bottom = style.length_or_fallback(CSS::PropertyID::MarginBottom, zero_value, containing_block.width());
+    replaced_element_box_model.border().top = style.length_or_fallback(CSS::PropertyID::BorderTopWidth, zero_value);
+    replaced_element_box_model.border().bottom = style.length_or_fallback(CSS::PropertyID::BorderBottomWidth, zero_value);
+    replaced_element_box_model.padding().top = style.length_or_fallback(CSS::PropertyID::PaddingTop, zero_value, containing_block.width());
+    replaced_element_box_model.padding().bottom = style.length_or_fallback(CSS::PropertyID::PaddingBottom, zero_value, containing_block.width());
 
-    box_model().margin().top = style.length_or_fallback(CSS::PropertyID::MarginTop, zero_value, containing_block.width());
-    box_model().margin().bottom = style.length_or_fallback(CSS::PropertyID::MarginBottom, zero_value, containing_block.width());
-    box_model().border().top = style.length_or_fallback(CSS::PropertyID::BorderTopWidth, zero_value);
-    box_model().border().bottom = style.length_or_fallback(CSS::PropertyID::BorderBottomWidth, zero_value);
-    box_model().padding().top = style.length_or_fallback(CSS::PropertyID::PaddingTop, zero_value, containing_block.width());
-    box_model().padding().bottom = style.length_or_fallback(CSS::PropertyID::PaddingBottom, zero_value, containing_block.width());
+    float x = replaced_element_box_model.margin().left.to_px(*this)
+        + replaced_element_box_model.border().left.to_px(*this)
+        + replaced_element_box_model.padding().left.to_px(*this)
+        + replaced_element_box_model.offset().left.to_px(*this);
 
-    float position_x = box_model().margin().left.to_px(*this)
-        + box_model().border().left.to_px(*this)
-        + box_model().padding().left.to_px(*this)
-        + box_model().offset().left.to_px(*this);
+    float y = replaced_element_box_model.margin_box(*this).top + box_model().offset().top.to_px(*this);
 
-    if (parent()->is_block() && parent()->style().text_align() == CSS::ValueID::VendorSpecificCenter) {
-        position_x = (containing_block.width() / 2) - width() / 2;
+    box.set_offset(x, y);
+}
+
+void LayoutBlock::place_block_level_non_replaced_element_in_normal_flow(LayoutBlock& block)
+{
+    auto& style = block.style();
+    auto zero_value = Length(0, Length::Type::Px);
+    auto& containing_block = *this;
+    auto& box = block.box_model();
+
+    box.margin().top = style.length_or_fallback(CSS::PropertyID::MarginTop, zero_value, containing_block.width());
+    box.margin().bottom = style.length_or_fallback(CSS::PropertyID::MarginBottom, zero_value, containing_block.width());
+    box.border().top = style.length_or_fallback(CSS::PropertyID::BorderTopWidth, zero_value);
+    box.border().bottom = style.length_or_fallback(CSS::PropertyID::BorderBottomWidth, zero_value);
+    box.padding().top = style.length_or_fallback(CSS::PropertyID::PaddingTop, zero_value, containing_block.width());
+    box.padding().bottom = style.length_or_fallback(CSS::PropertyID::PaddingBottom, zero_value, containing_block.width());
+
+    float x = box.margin().left.to_px(*this)
+        + box.border().left.to_px(*this)
+        + box.padding().left.to_px(*this)
+        + box.offset().left.to_px(*this);
+
+    if (this->style().text_align() == CSS::ValueID::VendorSpecificCenter) {
+        x = (containing_block.width() / 2) - block.width() / 2;
     }
 
-    float position_y = box_model().margin_box(*this).top
-        + box_model().offset().top.to_px(*this);
+    float y = box.margin_box(*this).top
+        + box.offset().top.to_px(*this);
 
-    LayoutBlock* relevant_sibling = previous_sibling();
+    auto* relevant_sibling = block.previous_sibling();
     while (relevant_sibling != nullptr) {
         if (relevant_sibling->style().position() != CSS::Position::Absolute)
             break;
@@ -463,26 +486,26 @@ void LayoutBlock::compute_position()
     }
 
     if (relevant_sibling) {
-        auto& previous_sibling_style = relevant_sibling->box_model();
-        position_y += relevant_sibling->effective_offset().y() + relevant_sibling->height();
+        auto& sibling_box = relevant_sibling->box_model();
+        y += relevant_sibling->effective_offset().y() + relevant_sibling->height();
 
         // Collapse top margin with bottom margin of previous sibling if necessary
-        float previous_sibling_margin_bottom = previous_sibling_style.margin().bottom.to_px(*relevant_sibling);
-        float my_margin_top = box_model().margin().top.to_px(*this);
+        float previous_sibling_margin_bottom = sibling_box.margin().bottom.to_px(*relevant_sibling);
+        float my_margin_top = box.margin().top.to_px(*this);
 
         if (my_margin_top < 0 || previous_sibling_margin_bottom < 0) {
             // Negative margins present.
             float largest_negative_margin = -min(my_margin_top, previous_sibling_margin_bottom);
             float largest_positive_margin = (my_margin_top < 0 && previous_sibling_margin_bottom < 0) ? 0 : max(my_margin_top, previous_sibling_margin_bottom);
             float final_margin = largest_positive_margin - largest_negative_margin;
-            position_y += final_margin - my_margin_top;
+            y += final_margin - my_margin_top;
         } else if (previous_sibling_margin_bottom > my_margin_top) {
             // Sibling's margin is larger than mine, adjust so we use sibling's.
-            position_y += previous_sibling_margin_bottom - my_margin_top;
+            y += previous_sibling_margin_bottom - my_margin_top;
         }
     }
 
-    set_offset({ position_x, position_y });
+    block.set_offset(x, y);
 }
 
 void LayoutBlock::compute_height()

Libraries/LibWeb/Layout/LayoutBlock.h

@@ -65,26 +65,25 @@ public:
 
     virtual void split_into_lines(LayoutBlock& container, LayoutMode) override;
 
-    void add_absolutely_positioned_descendant(LayoutBox&);
-    size_t absolutely_positioned_descendant_count() const { return m_absolutely_positioned_descendants.size(); }
-
 protected:
     void compute_width();
-    void compute_position();
     void compute_height();
     void layout_absolutely_positioned_descendants();
 
 private:
     virtual bool is_block() const override { return true; }
 
+    void place_block_level_non_replaced_element_in_normal_flow(LayoutBlock&);
+    void place_block_level_replaced_element_in_normal_flow(LayoutReplaced&);
+    void layout_absolutely_positioned_descendant(LayoutBox&);
+
     NonnullRefPtr<StyleProperties> style_for_anonymous_block() const;
 
-    void layout_children(LayoutMode);
+    void layout_inside(LayoutMode);
     void layout_inline_children(LayoutMode);
-    void layout_block_children(LayoutMode);
+    void layout_contained_boxes(LayoutMode);
 
     Vector<LineBox> m_line_boxes;
-    HashTable<RefPtr<LayoutBox>> m_absolutely_positioned_descendants;
 };
 
 template<typename Callback>

Libraries/LibWeb/Layout/LayoutBox.h

@@ -71,7 +71,7 @@ protected:
     virtual void did_set_rect() { }
 
 private:
-    virtual bool is_box() const override { return true; }
+    virtual bool is_box() const final { return true; }
 
     enum class Edge {
         Top,

Libraries/LibWeb/Layout/LayoutReplaced.cpp

@@ -121,42 +121,10 @@ float LayoutReplaced::calculate_height() const
     return used_height;
 }
 
-Gfx::FloatPoint LayoutReplaced::calculate_position()
-{
-    ASSERT(!is_absolutely_positioned());
-    auto& style = this->style();
-    auto zero_value = Length(0, Length::Type::Px);
-    auto& containing_block = *this->containing_block();
-
-    box_model().margin().top = style.length_or_fallback(CSS::PropertyID::MarginTop, zero_value, containing_block.width());
-    box_model().margin().bottom = style.length_or_fallback(CSS::PropertyID::MarginBottom, zero_value, containing_block.width());
-    box_model().border().top = style.length_or_fallback(CSS::PropertyID::BorderTopWidth, zero_value);
-    box_model().border().bottom = style.length_or_fallback(CSS::PropertyID::BorderBottomWidth, zero_value);
-    box_model().padding().top = style.length_or_fallback(CSS::PropertyID::PaddingTop, zero_value, containing_block.width());
-    box_model().padding().bottom = style.length_or_fallback(CSS::PropertyID::PaddingBottom, zero_value, containing_block.width());
-
-    float x = box_model().margin().left.to_px(*this)
-        + box_model().border().left.to_px(*this)
-        + box_model().padding().left.to_px(*this)
-        + box_model().offset().left.to_px(*this);
-
-    float y = box_model().margin_box(*this).top + box_model().offset().top.to_px(*this);
-
-    return { x, y };
-}
-
-void LayoutReplaced::layout(LayoutMode layout_mode)
+void LayoutReplaced::layout(LayoutMode)
 {
     set_width(calculate_width());
     set_height(calculate_height());
-
-    LayoutBox::layout(layout_mode);
-
-    if (is_absolutely_positioned()) {
-        const_cast<LayoutBlock*>(containing_block())->add_absolutely_positioned_descendant(*this);
-    } else {
-        set_offset(calculate_position());
-    }
 }
 
 void LayoutReplaced::split_into_lines(LayoutBlock& container, LayoutMode layout_mode)

Libraries/LibWeb/Layout/LayoutReplaced.h

@@ -62,7 +62,6 @@ protected:
     virtual void split_into_lines(LayoutBlock& container, LayoutMode) override;
 
 private:
-    Gfx::FloatPoint calculate_position();
     float calculate_width() const;
     float calculate_height() const;
 

Libraries/LibWeb/Layout/LayoutTableRowGroup.cpp

@@ -57,9 +57,6 @@ void LayoutTableRowGroup::layout(LayoutMode)
 {
     compute_width();
 
-    if (!is_inline())
-        compute_position();
-
     auto column_count = this->column_count();
     Vector<float> column_widths;
     column_widths.resize(column_count);