How to turn off Auto-show sidebar in fullscreen Safari 9
Simply copy and paste one or both into Terminal:

defaults write com.apple.safari ShowSidebarInTopSites bool=FALSE
defaults write com.apple.safari ShowSidebarInNewWindows bool=FALSE


If you don't want to copy two commands, then you can do both at the same time:

defaults write com.apple.safari ShowSidebarInTopSites bool=FALSE; defaults write com.apple.safari ShowSidebarInNewWindows bool=FALSE