From 7fcf61be358180365fb950fe7ba4f68314e39cad Mon Sep 17 00:00:00 2001
From: Andreas Kling <kling@serenityos.org>
Date: Fri, 5 Jun 2020 16:54:28 +0200
Subject: [PATCH] LibWeb: Improve computation of a layout node's containing
 block

In particular, we now compute the containing block of boxes with
position:absolute and position:fixed (more) correctly.
---
 Libraries/LibWeb/Layout/LayoutDocument.h |  2 ++
 Libraries/LibWeb/Layout/LayoutNode.cpp   | 39 +++++++++++++++++++++---
 Libraries/LibWeb/Layout/LayoutNode.h     |  7 ++++-
 3 files changed, 43 insertions(+), 5 deletions(-)

diff --git a/Libraries/LibWeb/Layout/LayoutDocument.h b/Libraries/LibWeb/Layout/LayoutDocument.h
index c89353b8ca..542b415818 100644
--- a/Libraries/LibWeb/Layout/LayoutDocument.h
+++ b/Libraries/LibWeb/Layout/LayoutDocument.h
@@ -45,6 +45,8 @@ public:
 
     void did_set_viewport_rect(Badge<Frame>, const Gfx::Rect&);
 
+    virtual bool is_root() const override { return true; }
+
 private:
     LayoutRange m_selection;
 };
diff --git a/Libraries/LibWeb/Layout/LayoutNode.cpp b/Libraries/LibWeb/Layout/LayoutNode.cpp
index 678e49b8b9..cbcfd4d7f6 100644
--- a/Libraries/LibWeb/Layout/LayoutNode.cpp
+++ b/Libraries/LibWeb/Layout/LayoutNode.cpp
@@ -29,7 +29,9 @@
 #include <LibWeb/DOM/Element.h>
 #include <LibWeb/Frame.h>
 #include <LibWeb/Layout/LayoutBlock.h>
+#include <LibWeb/Layout/LayoutDocument.h>
 #include <LibWeb/Layout/LayoutNode.h>
+#include <LibWeb/Layout/LayoutReplaced.h>
 
 namespace Web {
 
@@ -53,13 +55,36 @@ void LayoutNode::layout(LayoutMode layout_mode)
     });
 }
 
+bool LayoutNode::can_contain_boxes_with_position_absolute() const
+{
+    return style().position() != CSS::Position::Static || is_root();
+}
+
 const LayoutBlock* LayoutNode::containing_block() const
 {
-    for (auto* ancestor = parent(); ancestor; ancestor = ancestor->parent()) {
-        if (is<LayoutBlock>(*ancestor))
-            return to<LayoutBlock>(ancestor);
+    if (is_text()) {
+        auto* ancestor = parent();
+        while (ancestor && ((ancestor->is_inline() && !is<LayoutReplaced>(*ancestor)) || !is<LayoutBlock>(*ancestor)))
+            ancestor = ancestor->parent();
+        return to<LayoutBlock>(ancestor);
     }
-    return nullptr;
+
+    if (is_absolutely_positioned()) {
+        auto* ancestor = parent();
+        while (ancestor && !ancestor->can_contain_boxes_with_position_absolute())
+            ancestor = ancestor->parent();
+        while (ancestor && (!is<LayoutBlock>(ancestor) || ancestor->is_anonymous()))
+            ancestor = ancestor->containing_block();
+        return to<LayoutBlock>(ancestor);
+    }
+
+    if (style().position() == CSS::Position::Fixed)
+        return &root();
+
+    auto* ancestor = parent();
+    while (ancestor && ((ancestor->is_inline() && !is<LayoutReplaced>(*ancestor)) || !is<LayoutBlock>(*ancestor)))
+        ancestor = ancestor->parent();
+    return to<LayoutBlock>(ancestor);
 }
 
 void LayoutNode::render(RenderingContext& context)
@@ -155,4 +180,10 @@ Gfx::FloatPoint LayoutNode::box_type_agnostic_position() const
     return position;
 }
 
+bool LayoutNode::is_absolutely_positioned() const
+{
+    return style().position() == CSS::Position::Absolute;
+}
+
+
 }
diff --git a/Libraries/LibWeb/Layout/LayoutNode.h b/Libraries/LibWeb/Layout/LayoutNode.h
index b9a5adbb13..8c31244e9c 100644
--- a/Libraries/LibWeb/Layout/LayoutNode.h
+++ b/Libraries/LibWeb/Layout/LayoutNode.h
@@ -81,7 +81,8 @@ public:
             callback(*node);
     }
 
-    virtual const char* class_name() const { return "LayoutNode"; }
+    virtual const char* class_name() const = 0;
+    virtual bool is_root() const { return false; }
     virtual bool is_text() const { return false; }
     virtual bool is_block() const { return false; }
     virtual bool is_replaced() const { return false; }
@@ -108,8 +109,12 @@ public:
     virtual void layout(LayoutMode);
     virtual void render(RenderingContext&);
 
+    bool is_absolutely_positioned() const;
+
     const LayoutBlock* containing_block() const;
 
+    bool can_contain_boxes_with_position_absolute() const;
+
     virtual LayoutNode& inline_wrapper() { return *this; }
 
     const StyleProperties& style() const;