How can I put the inspector back?

I pressed the double arrow symbol at the top right of the inspector and the inspector popped out, but now I can't work out how to get it back--I don't want an extra window!
I have tried hiding and re-showing the inspector and completely restarting the app, but the inspector is always this (semi) separate window.

How do I get the inspector back to where it was to begin with?
Any help would be appreciated. (Thanks in advance!)

PS. It might be worth making this more intuitive--perhaps there should be a button to put it back into the main window in the same place that the original button to pop it out was.