From c875cdae64c9448c1e73a040054400ce30c77697 Mon Sep 17 00:00:00 2001
From: Aliaksandr Kalenik <kalenik.aliaksandr@gmail.com>
Date: Mon, 16 Sep 2024 21:56:40 +0200
Subject: [PATCH] LibWeb: Avoid layout run to calculate auto height of FC child
 in BFC

Before this change, each BFC child that established an FC root was laid
out at least twice: the first time to perform a normal layout, and the
second time to perform an intrinsic layout to determine the automatic
content height. With this change, we avoid the second run by querying
the formatting context for the height it used after performing the
normal layout.
---
 .../LibWeb/Layout/BlockFormattingContext.cpp         | 12 ++++++++----
 .../Libraries/LibWeb/Layout/BlockFormattingContext.h |  2 +-
 2 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/Userland/Libraries/LibWeb/Layout/BlockFormattingContext.cpp b/Userland/Libraries/LibWeb/Layout/BlockFormattingContext.cpp
index 83302988146..4497a0d794f 100644
--- a/Userland/Libraries/LibWeb/Layout/BlockFormattingContext.cpp
+++ b/Userland/Libraries/LibWeb/Layout/BlockFormattingContext.cpp
@@ -408,7 +408,7 @@ void BlockFormattingContext::compute_width_for_block_level_replaced_element_in_n
     box_state.padding_right = padding_right;
 }
 
-void BlockFormattingContext::compute_height(Box const& box, AvailableSpace const& available_space)
+void BlockFormattingContext::compute_height(Box const& box, AvailableSpace const& available_space, FormattingContext const* box_formatting_context)
 {
     auto const& computed_values = box.computed_values();
     auto& box_used_values = m_state.get_mutable(box);
@@ -419,7 +419,11 @@ void BlockFormattingContext::compute_height(Box const& box, AvailableSpace const
         height = compute_height_for_replaced_element(box, available_space);
     } else {
         if (should_treat_height_as_auto(box, available_space)) {
-            height = compute_auto_height_for_block_level_element(box, m_state.get(box).available_inner_space_or_constraints_from(available_space));
+            if (box_formatting_context) {
+                height = box_formatting_context->automatic_content_height();
+            } else {
+                height = compute_auto_height_for_block_level_element(box, m_state.get(box).available_inner_space_or_constraints_from(available_space));
+            }
         } else {
             height = calculate_inner_height(box, available_space.height, computed_values.height());
         }
@@ -698,7 +702,7 @@ void BlockFormattingContext::layout_block_level_box(Box const& box, BlockContain
     // Tables already set their height during the independent formatting context run. When multi-line text cells are involved, using different
     // available space here than during the independent formatting context run can result in different line breaks and thus a different height.
     if (!box.display().is_table_inside()) {
-        compute_height(box, available_space);
+        compute_height(box, available_space, independent_formatting_context);
     }
 
     if (independent_formatting_context || !margins_collapse_through(box, m_state)) {
@@ -941,7 +945,7 @@ void BlockFormattingContext::layout_floating_box(Box const& box, BlockContainer
         compute_height(box, available_space);
 
     auto independent_formatting_context = layout_inside(box, m_layout_mode, box_state.available_inner_space_or_constraints_from(available_space));
-    compute_height(box, available_space);
+    compute_height(box, available_space, independent_formatting_context);
 
     // First we place the box normally (to get the right y coordinate.)
     // If we have a LineBuilder, we're in the middle of inline layout, otherwise this is block layout.
diff --git a/Userland/Libraries/LibWeb/Layout/BlockFormattingContext.h b/Userland/Libraries/LibWeb/Layout/BlockFormattingContext.h
index 0b183451252..aae4039046f 100644
--- a/Userland/Libraries/LibWeb/Layout/BlockFormattingContext.h
+++ b/Userland/Libraries/LibWeb/Layout/BlockFormattingContext.h
@@ -37,7 +37,7 @@ public:
 
     virtual void parent_context_did_dimension_child_root_box() override;
 
-    void compute_height(Box const&, AvailableSpace const&);
+    void compute_height(Box const&, AvailableSpace const&, FormattingContext const* box_formatting_context = nullptr);
 
     SpaceUsedAndContainingMarginForFloats space_used_and_containing_margin_for_floats(CSSPixels y) const;
     [[nodiscard]] SpaceUsedByFloats intrusion_by_floats_into_box(Box const&, CSSPixels y_in_box) const;