Function delivered_input_serial 

pub fn delivered_input_serial() -> Option<u32>

The compositor’s serial of the newest input event (a key, a pointer button, a touch going down or up) this process’s event loop has handed the application, or that it dropped for having no window: every input the compositor sent before it has been handed over too. Input that reaches the application by another way (a desktop’s bar telling it a button of its own was pressed) names the serial it came after, and waits until this one reaches it. None until the first.