Don't make an annoyance a problem..........
Most larger slide toppers hold water, so that's that, and some ammount of water is going to dump down the side of the slide.
Many RVs require it to be level before moving slides in / out, so by lowering the jacks to drain a topper has a potential to create a slide problem; hence my opening statement.
Stopping a slide part way to drain water:
Don't stop a mechanical slide (Swintek for example) part way in or out for any reason as it can create slide motor problems; refer to opening statement.
We do stop our largest hydraulic slide part way to (mostly) drain it when appropriate. But still, the last bit of water dumps down the side of the slide (don't be putting stuff away in a bay below the slide in that situation, as it gives another meaning to "wet bay"
).