Function external_properties_pending 

pub fn external_properties_pending() -> bool

Returns true when the cross-thread typed-property channel currently holds undrained writes.

Non-destructive: peeks the receiver’s queue length without consuming any entries. The window backend calls this after App::tick() so it can self-schedule a follow-up frame when a write is still sitting in the bus (e.g. a background thread pushed after this tick’s drain_external_properties already ran) - otherwise the value would wait for the next unrelated OS event to wake the loop.

Returns false when the channel was never initialised, is empty, or its lock is poisoned.