LibWeb: Include siblings+descendants when invalidating style

When an element is invalidated, it's possible for any subsequent sibling
or any of their descendants to also need invalidation. (Due to the CSS
sibling combinators, `+` and `~`)

For DOM node insertion/removal, we must also invalidate preceding
siblings, since they could be affected by :first-child, :last-child or
:nth-child() selectors.

This increases the amount of invalidation we do, but it's more correct.

In the future, we will implement optimizations that drastically reduce
the number of elements invalidated.

Userland/Libraries/LibWeb/DOM/Node.cpp

@@ -405,18 +405,42 @@ void Node::invalidate_style(StyleInvalidationReason reason)
         return;
     }
 
-    for_each_in_inclusive_subtree([&](Node& node) {
-        node.m_needs_style_update = true;
-        if (node.has_children())
-            node.m_child_needs_style_update = true;
-        if (auto shadow_root = node.is_element() ? static_cast<DOM::Element&>(node).shadow_root() : nullptr) {
-            node.m_child_needs_style_update = true;
-            shadow_root->m_needs_style_update = true;
-            if (shadow_root->has_children())
-                shadow_root->m_child_needs_style_update = true;
+    // When invalidating style for a node, we actually invalidate:
+    // - the node itself
+    // - all of its descendants
+    // - all of its preceding siblings and their descendants (only on DOM insert/remove)
+    // - all of its subsequent siblings and their descendants
+    // FIXME: This is a lot of invalidation and we should implement more sophisticated invalidation to do less work!
+
+    auto invalidate_entire_subtree = [&](Node& subtree_root) {
+        subtree_root.for_each_in_inclusive_subtree([&](Node& node) {
+            node.m_needs_style_update = true;
+            if (node.has_children())
+                node.m_child_needs_style_update = true;
+            if (auto shadow_root = node.is_element() ? static_cast<DOM::Element&>(node).shadow_root() : nullptr) {
+                node.m_child_needs_style_update = true;
+                shadow_root->m_needs_style_update = true;
+                if (shadow_root->has_children())
+                    shadow_root->m_child_needs_style_update = true;
+            }
+            return TraversalDecision::Continue;
+        });
+    };
+
+    invalidate_entire_subtree(*this);
+
+    if (reason == StyleInvalidationReason::NodeInsertBefore || reason == StyleInvalidationReason::NodeRemove) {
+        for (auto* sibling = previous_sibling(); sibling; sibling = sibling->previous_sibling()) {
+            if (sibling->is_element())
+                invalidate_entire_subtree(*sibling);
         }
-        return TraversalDecision::Continue;
-    });
+    }
+
+    for (auto* sibling = next_sibling(); sibling; sibling = sibling->next_sibling()) {
+        if (sibling->is_element())
+            invalidate_entire_subtree(*sibling);
+    }
+
     for (auto* ancestor = parent_or_shadow_host(); ancestor; ancestor = ancestor->parent_or_shadow_host())
         ancestor->m_child_needs_style_update = true;
     document().schedule_style_update();