LibWeb: Don't hit test all child stacking contexts twice

We're supposed to hit test positive z-index stacking contexts first,
and negative z-index stacking contexts later. Instead, we were hit
testing all stacking contexts both times.

This made hit testing unbearably slow on some websites.

While we're here, also add an extra comment about why stacking contexts
are traversed in reverse order. It tripped me up while looking at this,
so I'm sure it could trip someone else up too.

Regressed in 44057c94821ea5dbfa2747de7cd4053fd5fe0742.

Userland/Libraries/LibWeb/Painting/StackingContext.cpp

@@ -369,8 +369,6 @@ Optional<HitTestResult> StackingContext::hit_test(Gfx::FloatPoint const& positio
 {
     if (!m_box.is_visible())
         return {};
-    if (m_box.computed_values().z_index().value_or(0) < 0)
-        return {};
 
     auto transform_origin = this->transform_origin();
     auto transformed_position = affine_transform_matrix().inverse().value_or({}).map(position - transform_origin) + transform_origin;
@@ -385,8 +383,11 @@ Optional<HitTestResult> StackingContext::hit_test(Gfx::FloatPoint const& positio
     // https://www.w3.org/TR/CSS22/visuren.html#z-index
 
     // 7. the child stacking contexts with positive stack levels (least positive first).
+    // NOTE: Hit testing follows reverse painting order, that's why the conditions here are reversed.
     for (ssize_t i = m_children.size() - 1; i >= 0; --i) {
         auto const& child = *m_children[i];
+        if (child.m_box.computed_values().z_index().value_or(0) < 0)
+            break;
         auto result = child.hit_test(transformed_position, type);
         if (result.has_value())
             return result;
@@ -457,8 +458,11 @@ Optional<HitTestResult> StackingContext::hit_test(Gfx::FloatPoint const& positio
     }
 
     // 2. the child stacking contexts with negative stack levels (most negative first).
+    // NOTE: Hit testing follows reverse painting order, that's why the conditions here are reversed.
     for (ssize_t i = m_children.size() - 1; i >= 0; --i) {
         auto const& child = *m_children[i];
+        if (child.m_box.computed_values().z_index().value_or(0) >= 0)
+            break;
         auto result = child.hit_test(transformed_position, type);
         if (result.has_value())
             return result;