What ever is on the topper should slide off when the slide is closed. Over time they do tend to sag a little and if the original installation didn't mount the rail high enough on the rig they can pool rain water (I have this problem) Some folks place a beach ball or pool noodle under the topper to keep the rain from pooling.
Jake my sidekick (yellow Lab)
2017 RAM 2500 CC 4X4 Cummins Diesel
2016 Flagstaff 26 FKWS
AF&AM & El Korah Shrine of Idaho