Panic in `resolve_stylist`: "invalid key" when animation node tracked but removed

[yes1688]

Version: blitz 0.3.0-alpha.2 (blitz-dom)
Platform: Linux (Vulkan/Wayland), likely cross-platform — bug is in Slab indexing logic.

Repro

use blitz_dom::{DocumentConfig, Document as _};
use blitz_html::HtmlDocument;

fn main() {
    let html = r#"
        <style>
          @keyframes pulse { 0% { opacity: 1; } 100% { opacity: 0.5; } }
          .pulse { animation: pulse 2s infinite; }
        </style>
        <div id="pulse-node" class="pulse">animated</div>
    "#;

    let mut doc = HtmlDocument::from_html(html, DocumentConfig::default());

    // First resolve — animation entry is registered for #pulse-node in animations.sets.
    doc.as_mut().resolve(0.0);

    // Remove the animated element (its slab key is now freed).
    let pulse_id = doc.inner().get_element_by_id("pulse-node");
    if let Some(id) = pulse_id {
        doc.inner_mut().mutate().remove_and_drop_node(id);
    }

    // Second resolve — panics: animations.sets still references the freed slab key.
    doc.as_mut().resolve(0.1); // panics here
}

Any DOM removal of an element with an active CSS animation: rule followed by a second resolve() reproduces it. The same panic also occurs in the GUI event loop when blitz internally recreates nodes between frames.

Panic

thread 'main' panicked at blitz-dom-0.3.0-alpha.2/src/stylo.rs:83:23:
invalid key

Root Cause

In resolve_stylist() (around stylo.rs:83):

self.nodes[node_id].set_restyle_hint(RestyleHint::RESTYLE_SELF);

Flow:

1. First resolve — traverse_dom matches CSS animations against the DOM,
   writes node_ids into animations.sets.
2. The animated node is removed from the DOM (slab key freed).
3. Second resolve — iterates animations.sets and indexes self.nodes directly.
   The slab key is stale, so the bracket index panics.

Proposed Fix

Use get_mut and skip stale entries:

let Some(node) = self.nodes.get_mut(node_id) else {
    continue; // stale animation entry — skip safely
};
node.set_restyle_hint(RestyleHint::RESTYLE_SELF);

Vendored locally with this patch — reproducer passes cleanly, GUI event loop runs cleanly through hundreds of frames.

PR

Happy to submit a PR with the 1-line fix + regression test if it matches your design intent.

---

Drafted with Claude, fix and reproduction hand-verified.