only start checking file updates when the window is loaded, otherwise we may end up with a zombie document in the file checker
only start checking file updates when the window is loaded, otherwise we may end up with a zombie document in the file checker

JackieXie168/skim,JackieXie168/skim,JackieXie168/skim,JackieXie168/skim,JackieXie168/skim