LibWeb: Don't update selection if start and end node roots differ

This allows selection to work within shadow roots and prevents the
selection being cleared if the mouse moves outside of the current
document or shadow root.

Userland/Libraries/LibWeb/Page/EventHandler.cpp

@@ -546,15 +546,22 @@ bool EventHandler::handle_mousemove(CSSPixelPoint viewport_position, CSSPixelPoi
         }
         if (m_in_mouse_selection) {
             auto hit = paint_root()->hit_test(position, Painting::HitTestType::TextCursor);
+            auto should_set_cursor_position = true;
             if (start_index.has_value() && hit.has_value() && hit->dom_node()) {
-                m_navigable->set_cursor_position(DOM::Position::create(realm, *hit->dom_node(), *start_index));
                 if (auto selection = document.get_selection()) {
                     auto anchor_node = selection->anchor_node();
-                    if (anchor_node)
-                        (void)selection->set_base_and_extent(*anchor_node, selection->anchor_offset(), *hit->paintable->dom_node(), hit->index_in_node);
-                    else
+                    if (anchor_node) {
+                        if (&anchor_node->root() == &hit->dom_node()->root())
+                            (void)selection->set_base_and_extent(*anchor_node, selection->anchor_offset(), *hit->paintable->dom_node(), hit->index_in_node);
+                        else
+                            should_set_cursor_position = false;
+                    } else {
                         (void)selection->set_base_and_extent(*hit->paintable->dom_node(), hit->index_in_node, *hit->paintable->dom_node(), hit->index_in_node);
+                    }
                 }
+                if (should_set_cursor_position)
+                    m_navigable->set_cursor_position(DOM::Position::create(realm, *hit->dom_node(), *start_index));
+
                 document.navigable()->set_needs_display();
             }
         }