The following:

Doesn't take into account that the user may want to forget the image before it's loaded. Here:

or here:

The following would be a better alternative (untested):

let mut cache = cache.lock();
if cache.contains(&uri) {
  cache.insert(uri.clone(), Poll::Ready(result));
  ctx.request_repaint();
  log::trace!("finished loading {uri:?}");
}
else {
  log::trace!("cancelled loading {uri:?}");
}

This assert was recently added by @bircni and @lucasmerlin in 58b2ac8. It's possible similar assertions were added in other loaders.