Constant xcb::ffi::xproto::XCB_ALLOW_ASYNC_POINTER

pub const XCB_ALLOW_ASYNC_POINTER: xcb_allow_t

For AsyncPointer, if the pointer is frozen by the client, pointer event processing continues normally. If the pointer is frozen twice by the client on behalf of two separate grabs, AsyncPointer thaws for both. AsyncPointer has no effect if the pointer is not frozen by the client, but the pointer need not be grabbed by the client.

TODO: rewrite this in more understandable terms.