Fix stale node references after node removal

[joeleaver]

Problem

When nodes are removed from the document via remove_node or remove_and_drop_node, several pieces of internal state can retain references to the removed node IDs:

1. hover_node_id - If a node is removed while it's being hovered, hover_node_id still points to the deleted node ID
2. snapshots - If a snapshot was created for a node (e.g., during hover state change or attribute modification), the snapshot persists after the node is removed
3. layout_children - The parent's cached layout children list may still contain the removed node's ID

These stale references cause panics when later operations try to access the deleted nodes:

• traversal.rs - node_layout_ancestors() panics with "invalid key" when accessing self.nodes[stale_id]
• selector_parser.rs - Stylo's style invalidation code panics when processing snapshots for deleted nodes

When This Happens

This issue is triggered when:

1. A node has hover state or a snapshot attached to it
2. The node is removed from the document
3. Before the next resolve() call, either:
   • The mouse moves (triggering set_hover_to which calls maybe_node_layout_ancestors with the stale hover_node_id)
   • resolve() is called (Stylo tries to process the stale snapshot)

This is most likely to occur in applications that:

• Perform direct DOM manipulation (as opposed to virtual DOM diffing)
• Remove nodes that are currently hovered or have recently had attributes modified
• Have the incremental feature enabled for style invalidation

Solution

Clean up all internal references when nodes are removed. In process_removed_subtree:

1. Clear hover state if the removed node was being hovered:

if doc.hover_node_id == Some(node_id) {
    doc.hover_node_id = None;
    doc.hover_node_is_text = false;
}

2. Remove snapshots for removed nodes:

if node.has_snapshot {
    let opaque_id = style::dom::TNode::opaque(&&*node);
    doc.snapshots.remove(&opaque_id);
    node.has_snapshot = false;
}

3. Clear parent's layout_children cache in removal functions:

*parent.layout_children.borrow_mut() = None;

Changes

• mutator.rs:

  • remove_node() - Clear parent's layout_children cache
  • remove_and_drop_node() - Clear parent's layout_children cache
  • remove_and_drop_all_children() - Clear parent's layout_children cache
  • process_removed_subtree() - Clear hover state and remove snapshots for removed nodes

• document.rs:

  • snapshot_node() - Use empty Vec instead of None for non-element node attrs (prevents panic in Stylo's get_attr())
  • Added unit tests for the fixes

Tests

Added three new tests:

• removing_hovered_node_clears_hover_state - Verifies hover state is cleared when hovered node is removed
• removing_node_clears_snapshot - Verifies snapshots are removed when node is removed
• stale_layout_children_do_not_panic - Verifies traversal handles stale layout_children gracefully

All existing tests continue to pass.

[nicoburns]

The snapshot and hover_node_id changes make sense to me. active_node_id has the same issue as hover_node_id (although it's less likely to be triggered). So we should probably handle that too.

The layout_children does not make sense sense to me. The function that is described as panicking does not access layout_children (it does use layout_parent however). The test probably passes, but I think it probably passes on main too. What was your reasoning for making this change?