LibWeb: Factor out creation of independent formatting contexts

This patch breaks FormattingContext::layout_inside() into two functions,
one that creates an independent formatting context (if needed), and
another that calls the former and then performs the inside layout within
the appropriate context.

The main goal here was to make layout_inside() return the independent
formatting context if one was created. This will allow us to defer
certain operations in child contexts until the parent context has
finished formatting the child root box.

Userland/Libraries/LibWeb/Layout/FormattingContext.cpp

@@ -75,47 +75,48 @@ bool FormattingContext::creates_block_formatting_context(const Box& box)
     return false;
 }
 
-void FormattingContext::layout_inside(Box& child_box, LayoutMode layout_mode)
+OwnPtr<FormattingContext> FormattingContext::create_independent_formatting_context_if_needed(Box& child_box)
 {
     if (!child_box.can_have_children())
-        return;
-
-    if (is<SVGSVGBox>(child_box)) {
-        SVGFormattingContext context(child_box, this);
-        context.run(child_box, layout_mode);
-        return;
-    }
+        return {};
 
     auto child_display = child_box.computed_values().display();
 
-    if (child_display.is_flex_inside()) {
-        FlexFormattingContext context(child_box, this);
-        context.run(child_box, layout_mode);
-        return;
-    }
+    if (is<SVGSVGBox>(child_box))
+        return make<SVGFormattingContext>(child_box, this);
 
-    if (creates_block_formatting_context(child_box)) {
-        BlockFormattingContext context(verify_cast<BlockContainer>(child_box), this);
-        context.run(child_box, layout_mode);
-        return;
-    }
+    if (child_display.is_flex_inside())
+        return make<FlexFormattingContext>(child_box, this);
 
-    if (child_display.is_table_inside()) {
-        TableFormattingContext context(verify_cast<TableBox>(child_box), this);
-        context.run(child_box, layout_mode);
-        return;
-    }
+    if (creates_block_formatting_context(child_box))
+        return make<BlockFormattingContext>(verify_cast<BlockContainer>(child_box), this);
 
-    VERIFY(is_block_formatting_context());
+    if (child_display.is_table_inside())
+        return make<TableFormattingContext>(verify_cast<TableBox>(child_box), this);
 
-    if (child_box.children_are_inline()) {
-        InlineFormattingContext context(verify_cast<BlockContainer>(child_box), this);
-        context.run(child_box, layout_mode);
-        return;
-    }
+    VERIFY(is_block_formatting_context());
+    if (child_box.children_are_inline())
+        return make<InlineFormattingContext>(verify_cast<BlockContainer>(child_box), this);
 
+    // The child box is a block container that doesn't create its own BFC.
+    // It will be formatted by this BFC.
     VERIFY(child_display.is_flow_inside());
-    run(child_box, layout_mode);
+    VERIFY(child_box.is_block_container());
+    return {};
+}
+
+OwnPtr<FormattingContext> FormattingContext::layout_inside(Box& child_box, LayoutMode layout_mode)
+{
+    if (!child_box.can_have_children())
+        return {};
+
+    auto independent_formatting_context = create_independent_formatting_context_if_needed(child_box);
+    if (independent_formatting_context)
+        independent_formatting_context->run(child_box, layout_mode);
+    else
+        run(child_box, layout_mode);
+
+    return independent_formatting_context;
 }
 
 static float greatest_child_width(Box const& box)

Userland/Libraries/LibWeb/Layout/FormattingContext.h

@@ -12,6 +12,8 @@ namespace Web::Layout {
 
 class FormattingContext {
 public:
+    virtual ~FormattingContext();
+
     enum class Type {
         Block,
         Inline,
@@ -38,11 +40,12 @@ public:
     static float compute_width_for_replaced_element(const ReplacedBox&);
     static float compute_height_for_replaced_element(const ReplacedBox&);
 
+    OwnPtr<FormattingContext> create_independent_formatting_context_if_needed(Box& child_box);
+
 protected:
     FormattingContext(Type, Box&, FormattingContext* parent = nullptr);
-    virtual ~FormattingContext();
 
-    void layout_inside(Box&, LayoutMode);
+    OwnPtr<FormattingContext> layout_inside(Box&, LayoutMode);
 
     struct ShrinkToFitResult {
         float preferred_width { 0 };