How can I permanently remove the sidebar that pops up every time I open a tab in Safari 9.1.3 on OS X El Capitan?
Open a new tab, then go to View → Hide Sidebar ⇧⌘L.
The sidebar will be hidden on new tab pages until you show it again.
Scouring the web on this one - the only thing that seems to work is putting your Dock on the left side of the screen, then disabling the auto show of the dock in the terminal.
The answer by grgarside doesn't work at all.