Return non-nil if the frame and buffer state appears to have changed.
VARIABLE is a variable name whose value is either nil or a state vector
that will be updated to contain all frames and buffers,
aside from buffers whose names start with space,
along with the buffers' read-only and modified flags. This allows a fast
check to see whether buffer menus might need to be recomputed.
If this function returns non-nil, it updates the internal vector to reflect
the current state.
If VARIABLE is nil, an internal variable is used. Users should not
pass nil for VARIABLE.
(fn &optional VARIABLE)