7mo falling from floor bed

After reading lots of good comments on floor beds even for under 1s, we decided to baby proof the room and start with a mattress on the floor with our almost 7mo about a week ago. At first, everything went well and my baby wasn't a wiggly one. But for the last 2 nights, we've been needing to go back to the room many times a night because she would end up with the lower half of her body off the mattress. We need to intervene because she likes rolling on her side to sleep, but when she's half way out of the mattress, she can't roll anymore, and this is disturbing everyone's sleep! For those who used / use floor beds for 6-8 mo babies, did this happen to you? Did the baby learn the boundaries of the bed? When? Any tips? She's not crawling yet. We're actually considering buying a cot... Photo of how I found her this morning when she woke up.