it seems to me in the latest version of IDEA 15 (143.379.11) the tool windows in the sidebar are always pinned. I have Idea 14.1.4 installed and if you unpin a window, it will slide back into the sidebar as soon as you click into the editor window. In the lastest version of Idea 15 you have to click the hide button to make the window slide back.
As a side note: it's broken in the lastest EAP of AppCode as well.
It did a quick search in YouTrack but could not find an issue. Can someone confirm the problem or provide a youtrack issue link?
Edit: I'm using OSX 10.11.1 and the custom JDK bundled versions.