Comment 4 for bug 619156

well like i said the behavior you're talking about don't exist in official nautilus 2.31.6 neither. I just tried it you can resize the sidebar that's all, there no hiding magic. nautilus-elementary is based on git developpement version of nautilus.

Furthermore it would be a gtk issue because there's nothing in the code which hide a pane if you click on the separator. Or i am blind :)