LibWeb: Move slotted style invalidation into a helper

Slottable.cpp still handled the style invalidation fallout from slot
assignment changes directly. Move that ::slotted()-related policy into
CSS::Invalidation::SlotInvalidator.

Slot assignment remains DOM bookkeeping. The helper now owns the
choice to dirty element slottables when they gain or lose assignment
to a slot.

Libraries/LibWeb/CMakeLists.txt

@@ -186,6 +186,7 @@ set(SOURCES
     CSS/Invalidation/NodeInvalidator.cpp
     CSS/Invalidation/PartInvalidator.cpp
     CSS/Invalidation/PseudoClassInvalidator.cpp
+    CSS/Invalidation/SlotInvalidator.cpp
     CSS/Invalidation/StyleInvalidator.cpp
     CSS/Invalidation/StructuralMutationInvalidator.cpp
     CSS/Interpolation.cpp

Libraries/LibWeb/CSS/Invalidation/SlotInvalidator.cpp

@@ -0,0 +1,23 @@
+/*
+ * Copyright (c) 2026-present, the Ladybird developers
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibWeb/CSS/Invalidation/SlotInvalidator.h>
+#include <LibWeb/DOM/Element.h>
+#include <LibWeb/DOM/Slottable.h>
+#include <LibWeb/DOM/Text.h>
+
+namespace Web::CSS::Invalidation {
+
+void invalidate_style_after_slottable_assignment_change(DOM::Slottable const& slottable)
+{
+    slottable.visit(
+        [](GC::Ref<DOM::Element> const& element) {
+            element->set_needs_style_update(true);
+        },
+        [](GC::Ref<DOM::Text> const&) {});
+}
+
+}

Libraries/LibWeb/CSS/Invalidation/SlotInvalidator.h

@@ -0,0 +1,15 @@
+/*
+ * Copyright (c) 2026-present, the Ladybird developers
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibWeb/DOM/Slottable.h>
+
+namespace Web::CSS::Invalidation {
+
+void invalidate_style_after_slottable_assignment_change(DOM::Slottable const&);
+
+}

Libraries/LibWeb/DOM/Slottable.cpp

@@ -6,6 +6,7 @@
  */
 
 #include <LibWeb/Bindings/MainThreadVM.h>
+#include <LibWeb/CSS/Invalidation/SlotInvalidator.h>
 #include <LibWeb/DOM/Element.h>
 #include <LibWeb/DOM/Node.h>
 #include <LibWeb/DOM/ShadowRoot.h>
@@ -202,14 +203,6 @@ void assign_slottables(GC::Ref<HTML::HTMLSlotElement> slot)
     if (slottables != slot->assigned_nodes_internal())
         signal_a_slot_change(slot);
 
-    auto invalidate_for_slottable_change = [](Slottable const& slottable) {
-        slottable.visit(
-            [](GC::Ref<Element> const& element) {
-                element->set_needs_style_update(true);
-            },
-            [](GC::Ref<Text> const&) {});
-    };
-
     // AD-HOC: Clear the assigned slot for slottables that are no longer assigned to this slot.
     //         This must happen before setting the new assigned slots to avoid stale references
     //         during style computation.
@@ -219,7 +212,7 @@ void assign_slottables(GC::Ref<HTML::HTMLSlotElement> slot)
         });
         // ::slotted(...) rules in the slot's shadow scope no longer apply to a slottable that just lost its
         // assignment, so its style must be recomputed.
-        invalidate_for_slottable_change(old_slottable);
+        CSS::Invalidation::invalidate_style_after_slottable_assignment_change(old_slottable);
     }
 
     // 4. For each slottable in slottables, set slottable’s assigned slot to slot.
@@ -229,7 +222,7 @@ void assign_slottables(GC::Ref<HTML::HTMLSlotElement> slot)
         });
         // Newly-assigned slottables become subject to ::slotted(...) rules in the slot's shadow scope, so their style
         // needs to be recomputed.
-        invalidate_for_slottable_change(slottable);
+        CSS::Invalidation::invalidate_style_after_slottable_assignment_change(slottable);
     }
 
     // 3. Set slot’s assigned nodes to slottables.