How to turn off Auto-show sidebar in fullscreen Safari 9
Solution 1:
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