Ignore listener registrations for removed views, and prune dead ids before layout broadcasts

[criccomini]

Problem

A view can be removed by an earlier update message in the same batch as its own RegisterListener message. The EditorView hits this easily because it calls id.register_listener(LayoutChanged::listener_key()) when it is built, and an editor inside nested dyn_containers can be swapped out before that message is processed.

When that happens, the RegisterListener handler still pushes the dead ViewId into window_state.listeners, and records the key on the shared stale ViewState rather than the real one. remove_view has already run, so nothing ever cleans the id out. On the next commit_box_tree, get_layout on the dead id returns Some (the stale state owns a live taffy node), and get_layout_window_origin then calls ViewStorage::box_tree, which panics:

thread 'main' panicked at src/view/storage.rs:134:14:
all view ids are created with a root
   3: <floem::view::storage::ViewStorage>::box_tree
   4: ... <floem::view::id::ViewId>::get_layout_window_origin ...
   6: <floem::window::handle::WindowHandle>::commit_box_tree

Fix

• RegisterListener skips ids whose view no longer exists.
• commit_box_tree prunes invalid ids out of the listener lists before the LayoutChanged and VisualChanged broadcasts.

🤖 Generated with Claude Code

[criccomini]

(This was Fable low, but the crash and debugging did happen :)