I'm looking for information about what is safe to put in a message handler registered with GUIRegisterMsg(), because the help file contains the warning: "blocking of running user functions which executes window messages with commands such as "Msgbox()" can lead to unexpected behavior, the return to the system should be as fast as possible !!!", but doesn't exactly say what "blocking" is or what the "unexpected behaviour" is. (And "unexpected" is not necessarily bad or buggy if you know what it