static POOL_HWND_CACHE: OnceLock<Mutex<HashMap<String, usize>>>Expand description
HWND cache for pool windows. Populated at on_after_created
(register_pool_window) and consulted at promote_pool_window as
the source of truth — BrowserHost::window_handle() returns null
once the page loads even though the underlying Win32 window is
alive (verified by IsWindow — see
SPEC_POOL_WINDOW_HWND_NULL_2026_05_06.md §4.1 diagnostic run).
Entries are removed on pool-window destruction
(on_pool_window_destroyed) so the map can’t leak across the
process lifetime. The HWND is stored as usize so this state is
Send + Sync without unsafe; callers cast back to HWND /
*mut c_void at use site.