From 996f3228a2e518fb5562a4b2f91743ba435df9c3 Mon Sep 17 00:00:00 2001
From: Andreas Kling <kling@serenityos.org>
Date: Mon, 21 Mar 2022 12:12:50 +0100
Subject: [PATCH] LibWeb: Fix O(n^2) traversal in hit testing

We already walk the entire paint tree within each stacking context in
the main hit testing function (StackingContext::hit_test()), so there's
no need for each individual paintable to walk its own children again.

By not doing that, we remove a source of O(n^2) traversal which made hit
testing on deeply nested web pages unbearably slow.
---
 .../Libraries/LibWeb/Painting/PaintableBox.cpp    | 11 ++---------
 .../Libraries/LibWeb/Painting/StackingContext.cpp | 15 ++++++---------
 2 files changed, 8 insertions(+), 18 deletions(-)

diff --git a/Userland/Libraries/LibWeb/Painting/PaintableBox.cpp b/Userland/Libraries/LibWeb/Painting/PaintableBox.cpp
index 8e7f59d797..4dbcf2349a 100644
--- a/Userland/Libraries/LibWeb/Painting/PaintableBox.cpp
+++ b/Userland/Libraries/LibWeb/Painting/PaintableBox.cpp
@@ -535,16 +535,9 @@ Optional<HitTestResult> PaintableBox::hit_test(Gfx::FloatPoint const& position,
         return stacking_context()->hit_test(position, type);
     }
 
-    Optional<HitTestResult> result;
     if (absolute_border_box_rect().contains(position.x(), position.y()))
-        result = HitTestResult { *this };
-    for_each_child_in_paint_order([&](auto& child) {
-        if (child.paintable()) {
-            if (auto child_result = child.paintable()->hit_test(position, type); child_result.has_value())
-                result = move(child_result);
-        }
-    });
-    return result;
+        return HitTestResult { *this };
+    return {};
 }
 
 Optional<HitTestResult> PaintableWithLines::hit_test(const Gfx::FloatPoint& position, HitTestType type) const
diff --git a/Userland/Libraries/LibWeb/Painting/StackingContext.cpp b/Userland/Libraries/LibWeb/Painting/StackingContext.cpp
index 2cf8aa97e3..c8349e4fd3 100644
--- a/Userland/Libraries/LibWeb/Painting/StackingContext.cpp
+++ b/Userland/Libraries/LibWeb/Painting/StackingContext.cpp
@@ -297,9 +297,8 @@ Optional<HitTestResult> StackingContext::hit_test(Gfx::FloatPoint const& positio
     // 6. the child stacking contexts with stack level 0 and the positioned descendants with stack level 0.
     m_box.for_each_in_subtree_of_type<Layout::Box>([&](Layout::Box const& box) {
         if (box.is_positioned() && !box.paint_box()->stacking_context()) {
-            result = box.paint_box()->hit_test(transformed_position, type);
-            if (result.has_value())
-                return IterationDecision::Break;
+            if (auto candidate = box.paint_box()->hit_test(transformed_position, type); candidate.has_value())
+                result = move(candidate);
         }
         return IterationDecision::Continue;
     });
@@ -316,9 +315,8 @@ Optional<HitTestResult> StackingContext::hit_test(Gfx::FloatPoint const& positio
     // 4. the non-positioned floats.
     m_box.for_each_in_subtree_of_type<Layout::Box>([&](Layout::Box const& box) {
         if (box.is_floating()) {
-            result = box.paint_box()->hit_test(transformed_position, type);
-            if (result.has_value())
-                return IterationDecision::Break;
+            if (auto candidate = box.paint_box()->hit_test(transformed_position, type); candidate.has_value())
+                result = move(candidate);
         }
         return IterationDecision::Continue;
     });
@@ -329,9 +327,8 @@ Optional<HitTestResult> StackingContext::hit_test(Gfx::FloatPoint const& positio
     if (!m_box.children_are_inline()) {
         m_box.for_each_in_subtree_of_type<Layout::Box>([&](Layout::Box const& box) {
             if (!box.is_absolutely_positioned() && !box.is_floating()) {
-                result = box.paint_box()->hit_test(transformed_position, type);
-                if (result.has_value())
-                    return IterationDecision::Break;
+                if (auto candidate = box.paint_box()->hit_test(transformed_position, type); candidate.has_value())
+                    result = move(candidate);
             }
             return IterationDecision::Continue;
         });